Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2015 Pressure Drag Reduction on Patterned Cylindrical Models Inspired by Biomimicry Larissa Mendes Ferreira Follow this and additional works at the FSU Digital Library. For more information, please contact lib-ir@fsu.edu
FLORIDA STATE UNIVERSITY COLLEGE OF ENGINEERING PRESSURE DRAG REDUCTION ON PATTERNED CYLINDRICAL MODELS INSPIRED BY BIOMIMICRY By LARISSA MENDES FERREIRA A Thesis submitted to the Department of Civil and Environmental Engineering in partial fulfillment of the requirements for the degree of Master of Science Degree Awarded: Spring Semester, 2015
Larissa M. Ferreira defended this thesis on April 17, 2015. The members of the supervisory committee were: Sungmoon Jung Professor Directing Thesis Michelle Rambo-Roddenberry Committee Member Lisa Spainhour Committee Member The Graduate School has verified and approved the above-named committee members, and certifies that the thesis has been approved in accordance with university requirements. ii
I dedicate this manuscript to my mother and father. Thank you for always giving me the chance to make you proud. iii
ACKNOWLEDGMENTS I would like to thank the following people for helping me through this challenging, yet incredibly rewarding, experience: Dr. Sungmoon Jung Gholamreza Amirinia Tufan Guha Jeremy Phillips James Gillman John Strike The above mentioned have dedicated countless amounts of time to helping me complete my research project in a timely and honorable manner. Many of them did not have any obligation to assist me aside from their generosity and genuine care for other people. While some of these people may not realize their impact on my work, even the smallest word of advice helped tremendously. Thank you. This project was funded in part by the 2013 Space Grant Fellowship Program provided through the NASA Florida Space Grant Consortium. iv
TABLE OF CONTENTS List of Figures... vii List of Tables... x Abstract... xi 1. Introduction... 1 2. Literature Review... 3 2.1 General Wind Behavior... 3 2.1.1 Laminar Flow and Turbulent Flow... 3 2.1.2 Flow Separation... 4 2.1.3 Reynolds Number... 5 2.1.4 Pressure Coefficients... 5 2.1.5 Drag Coefficients... 6 2.2 Simulating Atmospheric Boundary Layer... 6 2.2.1 Characteristics of Empty Wind Tunnel... 7 2.2.2 Spire and Roughness Element Design... 8 2.3 Flow Distribution Patterns... 9 2.3.1 Obtaining Pressure Data... 9 2.3.2 Roof Flow Patterns... 10 2.3.3 Flow Patterns around a Cylinder... 10 2.4 Full Scale vs. Model Scale Testing... 11 2.4.1 General Model Scale Characteristics... 12 2.4.2 Differences between Pressure Coefficients... 12 2.5 Passive Mechanisms and Previous Research... 13 2.5.1 Passive vs. Active Mechanisms... 13 2.5.2 Previous Research on Low-Rise Structures... 13 2.5.3 Previous Research on Cylindrical Structures... 15 2.6 Biological Inspiration... 16 3. Experimental Setup... 18 3.1 Part One: Setup for Obtaining Wind Tunnel Characteristics... 19 3.1.1 Design of ABL Simulator... 20 v
3.2 Part Two: Setup for Specimen Testing... 23 3.2.1 Specimen Manufacturing... 24 3.2.2 Pressure Tap Locations... 25 3.2.3 Force-Moment Sensor... 29 4. Results and Discussion... 31 4.1 Part One: Wind Tunnel Characteristics... 31 4.1.1 Velocity Profiles... 31 4.1.2 Turbulence Intensities... 34 4.1.3 Power Spectral Densities... 37 4.2 Part Two: Specimen Testing... 40 4.2.1 Drag Coefficients... 40 4.2.2 Pressure Coefficients... 43 5. Conclusions... 50 5.1 Future Work... 51 Appendix... 52 References... 71 Biographical Sketch... 74 vi
LIST OF FIGURES 2-1 Attached (left) and separated (right) flow around a smooth cylinder 4 2-2 Drag coefficient for smooth, dimpled, and roughened cylinders 11 2-3 Separated flow region along windward edge of roof 14 2-4 Architectural elements used in pressure reduction testing 15 2-5 Close up of sharks skin to show dermal denticles, or riblets 16 2-6 Humpback whale flipper: leading edge tubercles 17 3-1 Side view of test section setup with ABL simulator 18 3-2 Holder for hot-wire anemometer probe 19 3-3 Windward view of ABL simulator 22 3-4 Leeward view of ABL simulator 22 3-5 Smooth (left), U-grooved (center), and V-grooved (right) models, assembled 23 3-6 Cylinder halves 24 3-7 Top and bottom lids 24 3-8 Specimen angle orientation based on top view 25 3-9 ZOC22B/32Px input arrangement 27 3-10 LabVIEW front panel for specimen data acquisition 28 3-11 Force-moment sensor converter (left) and holder (right) 29 3-12 Complete force-moment sensor mounting mechanism 30 4-1 60 m/s velocity profiles at specimen location 31 4-2 Experimental velocity profile compared to ASCE 7-10 exposure categories 32 4-3 ABL height variation along test section length 33 4-4 60 m/s turbulence intensity profiles at specimen location 34 4-5 Experimental turbulence intensity compared to ASCE 7-10 exposure categories 35 vii
4-6 Turbulence intensities at a constant height of 15 cm at specimen location 36 4-7 40 m/s turbulence intensity profile at specimen location with ABL simulator 37 4-8 60 m/s power spectral density at 15 cm height with ABL simulator 38 4-9 60 m/s power spectral density at 30 cm height with ABL simulator 39 4-10 40 m/s power spectral density at 30 cm height with ABL simulator 39 4-11 Drag coefficient variations among cylinder types without ABL simulator 41 4-12 Drag coefficient variations among cylinder types with ABL simulator 42 4-13 60 m/s pressure coefficient variations without ABL simulator 43 4-14 60 m/s pressure coefficient variations with ABL simulator 44 4-15 Adjusted 60 m/s pressure coefficient variations with ABL simulator 45 4-16 60 m/s smooth cylinder pressure contour with (left) and without (right) ABL 46 simulator 4-17 60 m/s U-grooved cylinder pressure contour with (left) and without (right) 47 ABL simulator 4-18 60 m/s V-grooved cylinder pressure contour with (left) and without (right) 49 ABL simulator A-1 Surface patterns on smooth (left), U-grooved (center), and V-grooved (right) 52 cylinders A-2 Sketch of tap locations 53 A-3 LabVIEW block diagram for specimen data acquisition 54 A-4 20 m/s velocity profiles at specimen location 54 A-5 40 m/s velocity profiles at specimen location 55 A-6 40 m/s velocity profiles 60 cm from beginning of test section 55 A-7 40 m/s velocity profiles 125 cm from beginning of test section 56 A-8 20 m/s turbulence intensity profiles at specimen location 56 A-9 40 m/s velocity profiles at specimen location 57 viii
A-10 20 m/s power spectral density at 15 cm height without ABL simulator 57 A-11 20 m/s power spectral density at 30 cm height without ABL simulator 58 A-12 20 m/s power spectral density at 15 cm height with ABL simulator 58 A-13 20 m/s power spectral density at 30 cm height with ABL simulator 59 A-14 40 m/s power spectral density at 15 cm height without ABL simulator 59 A-15 40 m/s power spectral density at 30 cm height without ABL simulator 60 A-16 40 m/s power spectral density at 30 cm height with ABL simulator 60 A-17 60 m/s power spectral density at 15 cm height without ABL simulator 61 A-18 60 m/s power spectral density at 30 cm height without ABL simulator 61 A-19 20 m/s pressure coefficient variations without ABL simulator 62 A-20 20 m/s pressure coefficient variations with ABL simulator 62 A-21 40 m/s pressure coefficient variations without ABL simulator 63 A-22 40 m/s pressure coefficient variations with ABL simulator 63 A-23 Adjusted 60 m/s pressure coefficient variations with ABL simulator 64 A-24 20 m/s smooth cylinder pressure contour with (left) and without (right) 65 ABL simulator A-25 20 m/s U-grooved cylinder pressure contour with (left) and without (right) 66 ABL simulator A-26 20 m/s V-grooved cylinder pressure contour with (left) and without (right) 67 ABL simulator A-27 40 m/s smooth cylinder pressure contour with (left) and without (right) 68 ABL simulator A-28 40 m/s U-grooved cylinder pressure contour with (left) and without (right) 69 ABL simulator A-29 40 m/s V-grooved cylinder pressure contour with (left) and without (right) 70 ABL simulator ix
LIST OF TABLES 3-1 Experiment part one: data sets 21 3-2 Pressure tap locations (Sorted by angle) 26 3-3 Arrangement patterns for pneumatic input access 27 x
ABSTRACT Extensive research has been previously conducted on cylindrical models with different surface patterns. These experiments were generally performed under uniform flow and laminar flow conditions. However, high-rise buildings are subjected to different wind conditions. They are large enough to be affected by the Earth s atmospheric boundary layer (ABL) and often in conjunction with turbulent flow conditions. Incorporating knowledge from this, the present research aims to quantify the effectiveness of surface patterns in reducing pressure drag under a simulated ABL condition. The surface patterns selected are originally inspired from marine animal anatomy. V-grooved riblets mimic the miniscule patterns found on a sharks skin, which aid sharks in reducing drag while propelling forward in the water. U-grooved riblets (ie: bumps) mimic the tubercles located on the leading edge of a humpback whale s pectoral fins, which serve to increase maximum lift and reduce drag. In addition to patterned cylinders, pressure tests will also be conducted on a smooth-surfaced cylinder, serving as the control of the experiment. The length of the wind tunnel test section is considerably short in comparison to the lengths regularly used in this type of testing, having a height, width, and length dimension of 0.61m x 0.61m x 2m, respectively. As a result, there needs to be an array of roughness elements placed at the upstream end of the test section. These elements will serve to induce a thicker ABL within the test section before interacting with the test specimen. After a series of experimental tests, this project successfully generated an ABL in a short wind tunnel, allowing for a detailed study on the effects of surface patterns on scaled-down versions of high-rise structures. The results indicated that the cylinder covered in V-grooved riblets was most effective in reducing pressure drag when subjected to a turbulent flow characteristics. xi
CHAPTER 1 INTRODUCTION High-rise buildings, categorized as any structure requiring the use of a mechanical vertical transportation system (ie: elevators), can vary greatly in size; from a common 10-story office building to the 828 m tall Burj Khalifa in Dubai. While each structure faces their own design challenges, one great determining factor is common among them all: wind. High-rise structures all around the world literally face wind forces and are heavily designed to withstand these lateral forces with intricate steel framing and cross-bracing. The scope of the research presented here focuses specifically on cylindrical high-rise structures and draws inspiration from the biology of marine mammals and previous research conducted, mainly, by aerospace engineers. Experiments, explained in great detail in the upcoming chapters, will be carried out in a closed loop subsonic wind tunnel on three different test specimen. With the exception of the control specimen, which is simply a smooth cylinder, the test models are uniformly patterned all along their surfaces. V-shaped riblets, also referred to as grooves, inspired from the miniscule riblets found on a sharks skin, will cover the surface of one model. U-shaped riblets (ie: bumps), inspired from the tubercles found on the leading edge of a humpback whales pectoral fin, will cover the surface of another. The idea in question is whether tiny surface patterns on scaled-down high-rise cylindrical models will aid in decreasing drag forces caused by wind. This idea has already been tested under laminar flow several times with a variety surface patterns, ergo the previously mentioned inspiration from aerospace engineers. The difference in the present research stems from lack of civil engineering applications in studying laminar flow alone. In order for this drag reduction mechanism to be of significance in the civil engineering field, it must be successful under turbulent flow, especially under ABL wind conditions. This corresponds directly with higher wind speeds. The wind tunnel in which testing will take place has a 0.61 m x 0.61 m x 2 m test section. Typically, wind tunnel experiments on scaled-down versions of structures are conducted in much larger test sections to allow easier development of the atmospheric boundary layer. Due to the very limited wind tunnel test section length available, an array of triangular spires and roughness elements will be 1
constructed and placed at the upstream end of the test section in order to simulate the appropriate wind velocity and turbulence intensity profiles. A series of highly sensitive sensors will be used in data collection and exported into a data acquisition software for analyzing and presenting the acquired data. To allow for a better understanding of the purpose behind each major experimental task, the wind tunnel experimental data will be presented in two parts. Part 1 will only discuss the findings related to recreating the atmospheric boundary layer within the wind tunnel test section. Part 2 will deal with the main portion of the research; its data will be thoroughly analyzed and applied in arriving at a conclusion on the effects of groove-like patterns on cylindrical structures. 2
CHAPTER 2 LITERATURE REVIEW Prior to beginning any physical experiments on a wind-tunnel model of a structure, thorough research was required to create a suitable wind tunnel environment. Additional useful information included an overview of wind behavior, the differences between full-scale testing and wind-tunnel modeling, residential roof and cylindrical high-rise pressure distribution patterns, previously tested passive mechanisms, and biological inspirations for the passive mechanisms tested in this particular project. 2.1 General Wind Behavior The most general definition of wind is described as the perceptible natural movement of air. This fluid movement is the result of differences in atmospheric pressure between two points, forcing the air to move from high pressure areas to low pressure areas. The rate in which these masses of air move is dependent on the magnitude of this difference in atmospheric pressure. This natural phenomena is known to exert significant amounts of force when it interacts with structures and must therefore be thoroughly understood and heavily considered in design. 2.1.1 Laminar Flow and Turbulent Flow Wind can be categorized into three types of flow: laminar flow, transitional flow, and turbulent flow. Laminar flow, which can also be referred to as streamline flow, occurs when the wind flows in parallel layers, with little to no disruption between them. This type of flow, where viscous forces are dominant over inertial forces, generally occurs at lower velocities. On the other hand, turbulent flow is a chaotic flow, where vortices and alternating eddies form. In this flow, which tends to occur at higher velocities, inertial forces dominate over viscous forces. As the velocity of wind fluctuates, the type of flow must also alternate between laminar and turbulent. For this change to occur, the wind must transition; this transitional flow is known as a type of flow in itself. During this stage, the fluid flow contains characteristics of both laminar and turbulent flow. Understanding, and potentially manipulating, the point at which laminar flow transitions to turbulent flow is the key to mitigating the harsh effects of wind on structures. 3
2.1.2 Flow Separation When wind strikes a structure from any direction, the wind must be temporarily displaced from its natural location to get around obstacles in order to continue on its natural path. When this occurs, there is what is known as a separation point. This is defined as the point in which the flow breaks away from the surface of an obstacle. Once this separation occurs, the previously smooth flow of wind becomes dominated by unsteady, recirculating vortices. The vortices that form along the downstream end of an object interacting with a fluid flow is known as a wake. Figure 2-1 depicts the flow behavior that occurs behind a smooth-surfaced sphere. In a case where the flow is separated, shown in the right side of the figure, the windward facing direction of the sphere experiences a positive pressure (similar to a push) while the leeward facing direction experiences a negative pressure, often referred to as suction (similar to a pull). As a result, in most cases, flow separation causes an increase in pressure drag; this is the form of drag caused by the pressure differences between the front (windward) and rear (leeward) surface of a structure. Figure 2-1. Attached (left) and separated (right) flow around a smooth sphere In order to reduce pressure drag, the separation point should be delayed as much as possible. The separation point of wind flow against a structure can be delayed if the laminar flow of the wind is transitioned to turbulent flow sooner rather than later. When the flow is laminar, it is unable to adjust to the increasing pressure on the surface and separates as a result. However, when the flow is turbulent, there is a sufficient amount of momentum in the flow to overcome a larger adverse pressure gradient. This occurs when the flow is forced to decelerate quickly and is often graphically represented as a pressure increase in conjunction with downstream movement. The flow is then able to separate after 4
the boundary layer speed relative to an object falls almost to zero. While the flow cannot be kept from separating entirely, it can be delayed to occur much further downstream. 2.1.3 Reynolds Number The Reynolds number is the most common way to quantify whether a flow is laminar, turbulent, or transitioning. Recall, from Section 2.1.1, that laminar flows are dominated by viscous forces and turbulent flows are dominated by inertial forces. Therefore, the Reynolds number is explained as a dimensionless number that measures the ratio of inertial forces to viscous forces. It is also an important parameter that is used to relate the size of an object to the flow conditions it experiences. (2-1) Where U is the free-stream velocity, D is the characteristic length of the object (eg: cylinder, sphere, etc.), and ν is the viscosity of the fluid. The characteristic length of an object in question is simply the length of the widest point in which the fluid must go around. In the case of a cylinder or sphere, the characteristic length would be its diameter. Based on Equation 2-1, and assuming that a fluids viscosity remains constant, a low Reynolds number represents a laminar flow-type. In this range, the free-stream velocity represented in the numerator is low and the flow is dependent mainly on the viscosity of the fluid, represented in the denominator. A high Reynolds number represents a turbulent flow-type, where the free-stream velocity is significantly larger while the viscosity remains constant. The Reynolds number in which a flow transitions from laminar to turbulent is known as the critical Reynolds number. Therefore, it can be deduced that by reducing the critical Reynolds number, the transition between laminar and turbulent flow occurs more rapidly, delaying flow separation. 2.1.4 Pressure Coefficients Any technical paper discussing the wind load pressure on a structure will present pressure data in terms of the pressure coefficient, C p. This is a useful coefficient because it presents pressure data relative to the pressure obtained at a reference point. The most common types of pressure coefficients are the peak pressure coefficient, mean pressure coefficient, and the root mean square (rms) pressure coefficient. These coefficients can be calculated using the following formula: 5
. (2-2) Where ρ is the ambient air density, U is the mean reference velocity, P 0 is the pressure at the reference location, and P tap is the pressure measured at the model (or full-scale) tap [Cochran & English, 1997]. Note that for peak, mean, and rms pressure coefficients, P tap must correspond to the peak pressure, the mean pressure, and the rms pressure at that particular model (or full-scale) tap, respectively. 2.1.5 Drag Coefficients Drag forces are a result of the opposite and relative motion of an object with respect to a surrounding fluid, such as air or water. Although there are several types of drag, the most relevant ones for the presented research are known as pressure drag and skin friction drag. Briefly mentioned in Section 2.1.1, pressure (or form) drag arises because of the shape of an object; bodies with larger apparent cross sections will sustain larger drag forces than objects with smaller ones. Skin friction drag arises from the resistance of a fluid against the surface of an object. The combination of these drag types follows the drag equation, which introduces the drag coefficient term, C D. (2-3) Where F D is the drag force, ρ is the fluid s density, U is the flow velocity, and A is the frontal area of the object. This coefficient is often used in data analysis to quantify the total resistance experienced by an object. Through the use of a force sensor, the drag forces, F D, can be recorded at several velocities within the wind tunnel and analyses on the potential drag reduction techniques can be made. 2.2 Simulating Atmospheric Boundary Layer In a realistic setting, wind is greatly influenced by its height relative to the ground as well as the surrounding topography. This makes it unreliable to test a scaled down version of a cylindrical highrise structure in a wind tunnel that only produces a uniform wind flow. To simulate the realistic wind characteristics that such a structure is subjected to, it is essential that an atmospheric boundary layer (ABL) be replicated within the wind tunnel. Past research has led to the discovery that an ABL can be reproduced within a wind tunnel with the proper combination of spires and roughness elements spread across the test section. 6
2.2.1 Characteristics of Empty Wind Tunnel The calculations involved in designing the spires and roughness elements for the simulation of the ABL require an initial study of the characteristics of the empty wind tunnel. These characteristics include: 1) mean velocity profile 2) turbulence intensity and 3) power spectral density. By running a uniform flow of air through the test section, wind speeds at a set distance can be measured at a variety of heights in order to obtain the thickness of the boundary layer, δ. This thickness is located at the height in which the velocity of the wind, U, is over 99% of the wind velocity at an undisturbed height, U δ [Lopes et al., 2008]. Knowing δ, the mean velocity profile can be created using: Ū Ū (2-4) Where z is defined as the height in which U is being measured and α is the power law exponent, given in Table 26.9.1 of the ASCE 7-10 Building Code [American Society of Civil Engineers (ASCE), 2010]. The mean velocity profile should be plotted with the height on the y-axis and the U/U δ ratio on the x-axis. Following the mean velocity profile of the empty wind tunnel is forming a graph representing the wind tunnel s turbulence intensity, I. It is defined as the fluctuating velocity component of wind flow as can be calculated using: Ū (2-5) Where σ U is the standard deviation of U. The turbulence intensity should be plotted with the height on the y-axis and the turbulence intensity on the x-axis. The final empty wind tunnel characteristic, the power spectral density, serves to show the distribution of frequencies throughout a particular time series. For this plot, the power spectral density function, R N, is plotted on the y-axis against the non-dimensional frequency, f L, plotted on the x-axis. (2-6) Ū (2-7) 7
Where z is the height from the bottom of the test section in meters, Ū(z) is the mean velocity of a particular time series, n is the frequency of a particular wind speed, S UU (z, n) is the power spectrum 2 found through the use of the Fourier Transform, and σ U is the variance of that particular wind speed. The design of spires and roughness elements may be designed in conjunction with the turbulence intensity and the power spectral density; their design relies mainly on the boundary layer thickness, δ, and the power law exponent, α. 2.2.2 Spire and Roughness Element Design The physical elements required in simulating an ABL in a short wind tunnel, such as the one used for this experiment, include a combination of triangular spires and an array of roughness elements spread throughout the test section of the tunnel. The spires, which are to be placed at the beginning of the test section, serve to encourage the air flow to run mostly along the upper part of the wind tunnel. Essentially a result of encouraging higher flow speeds towards the top of the test section, the spires help induce vortices and turbulence within the wind tunnel [Lopes et al., 2008]. For this experiment, the design of the spires is based on the Irwin method, which was created specifically to cater to wind tunnels with short test section lengths. It is important to note that this method does not eliminate the difficulty in producing the desired turbulence intensity profile or power spectrum. Several simple equations have been generated to assist in obtaining proper spire dimensions based on individual wind tunnel characteristics [Irwin, 1981]: The height of the spires, h, is calculated using the following equation:. (2-8) Where δ is the thickness of the boundary layer and α is the power law exponent. Assuming a lateral spacing between spires of h/2, the base-to-height ratio (b/h) is given through the following formula: 1 (2-9) 8
Where. (2-10) (2-11) and H is the test section height. Using Equation 2-9, the base of the spires can be calculated. These expressions produce spire dimensions that are expected to create a simulated boundary layer at a distance, 4.5h, downstream of the spire array [Irwin, 1981]. 2.3 Flow Distribution Patterns The distribution of flow along low-rise roof systems and cylindrical high-rise structures as a result of wind loading has been studied countless times in the past. One common denominator in dealing with the direct interaction of wind on any given structure is the importance of measuring the pressure distribution along its surface. In order to obtain this data, the use of a specific sensor, known as a pressure scanner, is necessary. This section will briefly discuss how a pressure scanner works in addition to relevant flow patterns on different surfaces. 2.3.1 Obtaining Pressure Data The pressure scanner used in this experiment is the Scanivalve ZOC22B/32Px, which is equipped with 32 pneumatic inputs and a reference input, among others. Each pneumatic input will be connected to a pressure tap. These pressure taps are small holes that are drilled perpendicular to the surface in question; in this case, the surfaces will be the smooth cylinder and the two patterned cylinders. Due to the tendency of wind flowing tangential to a surface, some of the taps will be perpendicular to the flow direction and will record pressures resulting from the random component in air velocity. These pressures, known as static pressures, will be recorded by the transducer which will then measure the difference in pressure between the static pressures (obtained from the taps) and a reference pressure (obtained from a reference point). The reference pressure is obtained from any arbitrary location and is decided on by the experimenter. Generally, reference pressures are obtained from either an external free-stream location, a point inside the wind tunnel test section (usually upstream from the test model), or a point on the model itself. 9
For the research presented here, the reference input was located inside the test section near the upstream end. The reference pressure and the static pressures obtained from the transducer are then used to calculate the pressure coefficient, C p. 2.3.2 Roof Flow Patterns The combinations of roof flow distributions on low-rise structures are endless as parameters such as wind velocity, building dimensions and geometry, and roof angle change from structure to structure. However, there is one particular parameter that has resulted in the worst roof pressures regardless of structural characteristics: the angle of attack of the wind. Data has shown that when wind strikes a structure such that its corner is the first thing to come into contact with the wind, the highest suctions and uplift pressures are obtained [Gerhardt & Kramer, 1992]. Although this effect is strongest from cornering winds, it is also present from most other wind angles. When the wind separates along the edges, corners, and ridges of a roof, the local flow speed increases and causes a negative pressure (suction) underneath this separated flow [Banks et al., 2001]. This negative pressure is often the cause of roof failures on low-rise structures, especially when subjected to extremely high winds such as those of a hurricane or tornado. Knowing the tendencies of roof failure, as well as its causes, highlights the importance of researching ways to mitigate these high pressures. 2.3.3 Flow Patterns around a Cylinder Unlike the flow pattern of a low-rise structure, the flow around a cylindrical structure is independent of the angle of attack due to its symmetrical nature. Still, there is a high level of predictability based on its Reynolds number value. During subcritical flows, which include low Reynolds numbers and laminar characteristics, drag forces around a cylinder are dominated by skin friction drag forces. Conversely, during supercritical flows, which include high Reynolds numbers and turbulent characteristics, drag forces are dominated by pressure drag forces. Previous research has indicated that the critical Reynolds number for a smooth cylinder is approximately 2 x 10 5 [Quintavalla et al., 2013]. This critical Reynolds number is graphically represented as the point in which the drag coefficient, C p, begins to decrease, as shown in Figure 2-2. The area following the abrupt end to the rapid drag reduction is considered the supercritical region. The slight increase that occurs in that area is a result of a shift in the separation line toward the upstream end of the cylinder [Butt et al., 2014]. It is also 10
known that in laminar flows, roughness and surface patterns have proven to reduce the critical Reynolds number and, consequently, the total drag around a cylinder. Figure 2-2. Drag coefficient for smooth, dimpled, and roughened cylinders [Butt et al., 2014] 2.4 Full Scale vs. Model Scale Testing Wind engineering research is most often conducted either through full-scale testing or model-scale testing in a wind tunnel; each with their own advantages and disadvantages. The ideal form of testing is full-scale testing because it provides the most realistic simulation of how wind interacts with a structure. As a result, the pressure distribution data collected through full-scale tests are dependable as they correlate well with real-life flow patterns and behaviors. It is, however, relatively expensive to construct and maintain a full-scale facility in comparison to the construction and maintenance of a wind tunnel. Although, full scale testing is known to give more accurate results in wind-related experiments, the more common practice involves wind tunnel testing, with some experiments incorporating both forms of testing for a broader range of data and results. Wind tunnel testing requires a scaled down model of a structure, or structural component, and a simulation of an ABL (Refer to Section 2.2), which can 11
be hard to reproduce. This has resulted in the reputation that wind tunnel testing is more often a matching process than a representation of truly independent simulations [Surry, 1991], where the results of experiments are significantly varied until reasonable agreement in obtained. 2.4.1 General Model Scale Characteristics Relevant papers covered throughout this literature review involving wind tunnel testing on scaled models of structures utilized scales ranging from 1:20 to 1:500. In addition, the wind tunnels themselves, even when considered small or short, had significantly longer fetches, or testing sections, than the wind tunnel at Florida State University s (FSU) Main Campus. For instance, one particular source, which explains how to simulate the ABL in a short wind tunnel, uses a wind tunnel with a testing section approximately 5 meters long; the wind tunnel located at FSU s Main Campus, however, has a test section of only 2 meters. Therefore, for this experiment the scale was 1:667, where 1 cm represented an actual length of 6.67 m. 2.4.2 Differences between Pressure Coefficients Similar to several aspects of wind engineering, the differences in pressure coefficients between fullscale data and model-scale data cannot be concluded with absolute certainty. However, observations leading to a potential pattern can still be made from previous experimental data. One particular research experiment [Surry, 1991] had its intentions set on comparing data between both scales for verification. The experiment was carried out using a 1:100 scale model of the full-scale building located at Texas Tech University, along with the actual building itself. Results showed that with each wind direction, the pressure coefficients were slightly higher for the full-scale building than for the model. For example, the mean pressure coefficient for the full scale was about -1.3 while the mean pressure coefficient for the model scale was -1.0 [Surry, 1991]. Another experiment supporting this pattern tested a set of architectural elements on a model scale and on a full-scale structure for mitigation of roof and wall corner suctions [Bitsuamlak et al., 2013]. Comparing results between the two original structures (without any architectural features) shows the same pattern mentioned above; the full scale data results in higher pressure coefficients than the model scale data. It is important to note, however, that although this difference in pressure coefficients appears to be consistent, there are also cases that show reasonable agreement between the two scales. An experiment 12
conducted at Concordia University in Canada perfectly demonstrates this exception. Full-scale and wind tunnel tests were carried out on a series of configurations involving panel and rainscreen combinations with hopes of improving the design guidelines of pressure-equalized rainscreen (PER) walls [Kala et al., 2008]. Pressure coefficient results show that for all four configurations tested, the wind tunnel data fell well within the range of the field data. Taking all this into consideration, it is safe to say that at the very least wind tunnel pressure coefficients often provide higher values than full-scale values, which result in conservative conclusions. 2.5 Passive Mechanisms and Previous Research The idea that drag forces exerted on a structure may be reduced is nothing new to engineers and related scientists. Countless experiments have been conducted in the past, by researchers all over the world, with the same goal in mind. This section will serve to explain the difference between certain mechanisms and also briefly review some of major findings associated with passive mechanisms. 2.5.1 Passive vs. Active Mechanisms All experiments related to reducing drag forces on a structure can be separated into two major categories; the mechanisms in which the force reduction is achieved can be either (1) passive or (2) active. A passive mechanism is one that is related to the anatomy of a structure; where features are intentionally incorporated into a design in order to dictate fluid flow around a particular surface. Oppositely, an active mechanism is generally the more intricate and expensive flow control method. This type of mechanism requires that external energy and consequently, additional mass, be added into a particular system. Considering the civil engineering relevance of both options, the presented information will deal strictly with passive mechanisms. 2.5.2 Previous Research on Low-Rise Structures When subjected to hurricane force winds, the roofs of low-rise structures have a high tendency of failing, especially upon the breach of the building envelope. As shown in Figure 2-3, this particular kind of damage is a result of the extreme negative pressure (suction) exerted on the envelope by vortices that form amidst the flow separation region along the edges, corners and ridges of low-rise building roofs [Banks et al., 2001]. 13
Figure 2-3. Separated flow region along windward edge of roof [Banks et al., 2001]. Knowing the typical locations that have the highest risks of failure, researchers began exploring ways to mitigate these pressures through local geometric modifications. The goals for pressure reduction were generally achieved by eliminating sharp edges, displacing formed vortices, and disrupting additional vortices formation [Surry & Lin, 1995]. Some of the successful pressure reduction techniques include several variations of parapets, splitters, and circular projections along the roof edges. Other techniques, shown in Figure 2-4, involved architectural elements that were more aesthetically obvious such as (a) trellises, (b) gable end and ridgeline extensions, and (c) wall extensions [Bitsuamlak et al., 2013]. Among the previous research conducted on low-rise residential structures, the most effective pressure reduction mechanism was the porous parapet. Showing an approximate 70% reduction in roof pressures, the porous parapet is successful for the following reasons: (1) the porous screen disrupts vortex formation along roof edges and corners, (2) the parapet absorbs some of the flow energy upon contact, and (3) the vortices that do form are displaced enough as to have their negative effects significantly reduced [Suaris & Irwin, 2010]. 14
Figure 2-4. Architectural elements used in pressure reduction testing [Bitsuamlak et al., 2013] 2.5.3 Previous Research on Cylindrical Structures The majority of pressure reduction techniques researched with cylindrical structures involves surface manipulation, such as added roughness or patterns. The main reason for this is that most of this research is related to aerospace, not structural, applications. Nonetheless, there are several lessons to be learned from past experiments. 15
Focusing mainly on surface patterns, which tend to be more effective in reducing drag forces than surface roughness, the following ideas have been previously tested: U-shaped grooves, V-shaped grooves, and hexagonal bumps and dimples, among others. Hexagonal bumps were found to induce a larger drag coefficient reduction than hexagonal dimples. The drag coefficient for the smooth cylinder in this case was obtained as 1.17 at a Reynolds number of 2 x 10 5 while the bumps resulted in drag coefficients of about 0.77 and 0.73, at angles of attack equal to 0 and 90, respectively [Butt et al., 2014]. Additionally, U-grooved and V-grooved cylinder consistently showed drag reduction effects in comparison to smooth cylinders with equal dimensions. One particular experiment showed U-grooves were able to reduce the drag coefficient by 18.6% at a Reynolds number of 1.4 x 10 5 [Lim & Lee, 2003]. It is important to note that most of the previously conducted research was conducted in laminar, not turbulent, flow. Despite the research suggesting that ribleted surfaces are consistently successful in reducing drag on cylinders, there has also been research to challenge this conclusion. One particular thesis [Horsten, 2005] presents findings that support the idea that riblets actually increase drag forces in laminar flow and reduce drag in turbulent flow. This research discusses a possible cause to be the substantially larger surface area coming into contact with the wind in comparison to that of a smooth cylinder. It is argued that if the riblets on a cylinder are unable to compensate by keeping vortices out of the riblet valleys, the drag reduction effects are lost, and in some cases, inverted. Therefore, while successful drag force reduction results provide hope and purpose for the focus of this thesis, there is still plenty that remains unknown. 2.6 Biological Inspiration Figure 2-5. Close up of sharks skin to show dermal denticles, or riblets [Bechert et al., 2000] 16
The process of evolution has taught us that every natural entity must adapt to changes in its environment in order to survive. Therefore, when thinking about ways to reduce drag reduction on a man-made structure, it makes sense to explore beings that must constantly battle against such forces, such as a bird in flight or a dolphin racing in the water. The research presented in the following chapters began as a quest in combining biological characteristics into physical design. In particular, the inspiration for U-shaped and V-shaped grooves on a cylinder were inspired from the riblets, or dermal denticles, found on a sharks skin and tubercles located on the leading edge of a humpback whales pectoral fin. Dermal riblets, shown in Figure 2-5, are defined as miniscule stream-wise grooves and serve to break up vortices and reduce surface shear stress and momentum loss. Additionally, leading edge tubercles, shown in Figure 2-6, work by exciting the surrounding flow in order to create vortices. This delays the stall angle and aids in maintaining lift [Fish & Lauder, 2006]. Figure 2-6. Humpback whale flipper: leading edge tubercles [Fish & Lauder, 2006] 17
CHAPTER 3 EXPERIMENTAL SETUP Figure 3-1. Side view of test section setup with ABL simulator All experiments were conducted inside a closed loop subsonic wind tunnel with a 61 cm x 61 cm x 2.0 m (24 in. x 24 in. x 79 in.) test section. The test section is made up of wooden panels throughout the first 50.8 cm (20 in.) and 2.5 cm (1 in.) thick Plexiglas for the remainder of its length, as depicted in Figure 3-1. Please refer back to this figure for the horizontal distances regarding the use of the hotwire anemometer presented in Section 3.1. Also note how the ABL simulator is placed at the very beginning of the test section to ensure proper development of the boundary layer effect. The wind tunnel was capable of generating wind speeds up to 70 m/s (about 157 mph), although it was not necessary for this particular project. The experiments were broken up into two parts in order to better organize the data collected: the first part of the experiment focuses strictly on the wind tunnel characteristics within the test section (both empty and with the ABL simulator) while the second part focuses on the main research topic. The following sections will cover the experimental setup of each in further detail. 18
3.1 Part One: Setup for Obtaining Wind Tunnel Characteristics Figure 3-2. Holder for hot-wire anemometer probe In order to obtain the characteristics of the wind tunnel, data must be collected while the test section is completely empty as well as while the device used in simulating the ABL within the tunnel is installed. Beginning with an empty test section, a hot-wire anemometer was placed at three different locations along the center of the test section. From the beginning of the test section, the distances are the following: 60 cm (23.6 in.), 125 cm (49.2 in.), and 190 cm (74.8 in.). At each of these horizontal locations, the anemometer measured wind speeds at several vertical locations and at several different wind speeds. Figure 3-2 shows the probe holder that was manufactured in order to achieve the exact measurement locations The vertical locations, measured from the bottom of the test section, included the following heights: 0.7 cm (0.3 in.), 2.0 cm (0.8 in.), 3.0 cm (1.2 in.), 4.0 cm (1.6 in.), 6.0 cm (2.4 in.), 8.0 cm (3.2 in.), 10.0 cm (3.9 in.), 15.0 cm (5.9 in.), 20.0 cm (7.9 in.), 25.0 cm (9.8 in.), 30.0 cm (11.8 in.), 40.0 cm (15.7 in.), and 50.0 cm (19.7 in.). The wind speeds included were 20 m/s (45 mph), 40 m/s (89 mph), 60 m/s (134 mph). Table 3-1 conveniently depicts which measurements (vertical and horizontal distances and corresponding wind speeds) were taken in this part of the experiment, each denoted by an X. For each of the 65 points selected, a 30 second time-series of data was collected using LabVIEW for data acquisition, at a sampling rate of 1667 Hz, for a total of 50,000 samples per measurement location. 19
In addition to collecting the data mentioned above, which mainly served in generating the velocity profile of the wind tunnel, an additional set of tests were conducted to find the tunnels turbulence intensity. At the horizontal distance of 190 cm and at a test section height of 15 cm, representing an actual height of 100 m, data was collected at several wind speeds; starting at 10 m/s up to 60 m/s, in increments of 5 m/s, for a total of 11 time series. Each time series was analyzed using Excel and MATLAB to create the velocity profile, turbulence intensity profile, and power spectra of this particular wind tunnel test section. All tests were repeated a second time with the ABL simulator inside the test section. 3.1.1 Design of ABL Simulator The main determining factor in the design of the triangular spires and roughness elements for the ABL simulator was the length of the wind tunnel test section. Because the test section is very short in relation to most wind tunnels used in this type of experiment, it was crucial that the simulator be designed as large as possible to allow for an optimal specimen size. Allowing 1.8 m (71 in.) inside the test section for the ABL to fully develop before encountering the models meant that the maximum height of the spire would be 40 cm (15.7 in.). After obtaining the power law exponent for Exposure D from Table 26.9.1 of the ASCE 7-10 Building Code [ASCE, 2010] as 1/11.5 = 0.087, Equation 2-8 was used to calculate the ABL height of 30 cm (12 in.). Equations 2-9 through 2-11 were subsequently used to calculate the remaining dimensions. These dimensions were slightly adjusted to make manufacturing simpler. The final dimensions for the bases of the spires and the roughness element cubes were 4.3 cm (1.7 in.) and 0.64 cm (0.25 in.), respectively. Four triangular spires were welded to the windward end of a 0.95 cm (3/8 ) thick aluminum plate and an additional 139 cubes were punched and sealed onto the same plate using an epoxy adhesive, as shown in Figure 3-3 and Figure 3-4. The spires were spaced 17.5 cm (6.9 in.) apart with the array of roughness elements spread throughout the remaining length of the plate in a staggered pattern. Roughness elements on the same row were spaced 7 cm (2.8 in.) apart and each row was spaced (3.0 in.) apart. Every row was horizontally staggered 3.5 cm (1.4 in.) in relation to neighboring rows to ensure maximum surface turbulence simulation. 20
Horizontal Dist. (cm) 60 125 190 ABL Within ABL Outside ABL Within ABL Outside ABL Within ABL Outside ABL Table 3-1. Experiment part one: Data sets Vertical Dist. (cm) Actual Height (m) Wind Speed (m/s) 20 40 60 0.7 4.67 X 2.0 13.33 X 3.0 20.00 X 4.0 26.67 X 6.0 40.00 X 8.0 53.33 X 10.0 66.67 X 15.0 100.00 X 20.0 133.33 X 25.0 166.67 X 30.0 200.00 X 40.0 266.67 X 50.0 333.34 X 0.7 4.67 X 2.0 13.33 X 3.0 20.00 X 4.0 26.67 X 6.0 40.00 X 8.0 53.33 X 10.0 66.67 X 15.0 100.00 X 20.0 133.33 X 25.0 166.67 X 30.0 200.00 X 40.0 266.67 X 50.0 333.34 X 0.7 4.67 X X X 2.0 13.33 X X X 3.0 20.00 X X X 4.0 26.67 X X X 6.0 40.00 X X X 8.0 53.33 X X X 10.0 66.67 X X X 15.0 100.00 X X X 20.0 133.33 X X X 25.0 166.67 X X X 30.0 200.00 X X X 40.0 266.67 X X X 50.0 333.34 X X X 21
Figure 3-3. Windward view of ABL simulator Figure 3-4. Leeward view of ABL simulator 22
3.2 Part Two: Setup for Specimen Testing This part of the experiment involves acquiring the pressure and force distribution for three different cylindrical models. The pressures at predetermined points along the models surfaces, discussed further in Section 3.2.2, were collected using a Scanivalve ZOC22B/32Px pressure scanner and the forces were collected using a JR3 30E15 6-axis force-moment sensor LabVIEW program. The test models were mounted along the center of the test section 190 cm (74.8 in.) from the upstream end. For future referencing, whenever the specimen location is mentioned, it is referring to this distance. The cylindrical models tested in this experiment have the same general dimensions but have varying surface patterns. Up-close images of the surface patterns are shown in Figure A-1 of the Appendix. The V-grooves are triangular protrusions with a height of 0.60 mm (0.02 in.) and a peak-to-peak spacing of 0.90 mm (0.04 in.). The U-grooves are essentially semi-circular lumps with alternating concavity with a height of 1.00 mm (0.04 in.) and a spacing, or diameter, of 1.90 mm (0.07 in.). Each cylinder, regardless of surface pattern, has a total height of 25.4 cm (10 in.) and an outer diameter of 6.35 cm (2.5 in.). They are each made up of two cylindrical halves and two lids. The cylindrical halves were manufactured from a 6.35 cm (2.5 in.) outer diameter aluminum tube and the lids from 0.64 cm (0.25 in.) aluminum stock. The assembled cylindrical models are shown in Figure 3-5. Figure 3-5. Smooth (left), U-grooved (center), and V-grooved (right) models, assembled 23
3.2.1 Specimen Manufacturing In order to make each cylinder half, two 24.1 cm (9.5 in.) length pieces of aluminum tubing were cut. At this point, both tubes were strategically cut in half, longitudinally. Due to the width of the saw blade, known as the kerf, one of the halves of each of the pieces of tubing would have to be slightly smaller than half of the original cylinder. These smaller halves were scrapped and the two remaining exact halves, shown in Figure 3-6, were used in forming one perfectly circular cylinder. Each cylinder then had two threaded holes drilled on each end (four holes total on each half), separated by 90, suitable for #6-32 screws. These holes, when used in conjunction with the top and bottom lids shown in Figure 3-7, served to secure all the parts together. Figure 3-6. Cylinder halves Figure 3-7. Top and bottom lids 24
In manufacturing each lid, a water jet cutter was used. The diameter for the lids was 6.35 cm (2.5 in.). Both the top and bottom lids each had 4 clearance holes, separated by 90, to allow for the #6-32 screws that would hold all the manufactured parts together. These screws had a flat head which meant that the holes on the lids needed to be chamfered to avoid any protrusion from the ends of the cylinders. Unlike the top lid, which is entirely solid with the exception of the four clearance holes, the bottom lid also had two threaded 0.64 cm (0.25 in.) diameter holes, separated by 180, at points along an imaginary 3.8 cm (1.5 in.) diameter circle. These holes served to mount the models onto the wind tunnel test section. The bottom lids also had a 2.5 cm (1 in.) hole cut out of its center to allow the pressure tubes to exit the model and connect to the transducer. 3.2.2 Pressure Tap Locations As mentioned in Section 2.3.1, the Scanivalve ZOC22B/32Px pressure scanner has 32 pneumatic inputs and one reference input, among a few others. All 32 pressure inputs were strategically inserted into holes along the surface of the cylinders to show the most relevant pressure data. Table 3-2 and Figure A-2, shown in the Appendix, show the exact locations in which the taps were placed. The heights specified in the tables consider the bottom of the cylinder to be zero and the angles are orientated as shown in Figure 3-8. The reference input was located at a predetermined location based on the permanent setup of the wind tunnel facility. This input is located 30 cm (11.8 in.) from the beginning of the test section (upstream end), 13 cm (5.11 in.) from the side, and 33 cm (13 in.) from the bottom. Figure 3-8. Specimen angle orientation based on top view 25
Table 3-2. Pressure tap locations (Sorted by angle) Height, cm Angle (in.) Actual Height, m 15 15 (5.91) 100 30 15 (5.91) 100 45 15 (5.91) 100 60 15 (5.91) 100 3 (1.18) 20 6 (2.36) 40 70 9 (3.54) 60 12 (4.72) 80 15 (5.91) 100 3 (1.18) 20 6 (2.36) 40 80 9 (3.54) 60 12 (4.72) 80 15 (5.91) 100 3 (1.18) 20 6 (2.36) 40 90 9 (3.54) 60 12 (4.72) 80 15 (5.91) 100 3 (1.18) 20 6 (2.36) 40 100 9 (3.54) 60 12 (4.72) 80 15 (5.91) 100 3 (1.18) 20 6 (2.36) 40 110 9 (3.54) 60 12 (4.72) 80 15 (5.91) 100 120 15 (5.91) 100 150 15 (5.91) 100 180 15 (5.91) 100 In order to have the pressure scanner on Operate mode, a control pressure ranging between 55 psi and 65 psi needed to be applied to it. For this, a portable air compressor with an applied pressure of 60 psi was connected to the calibration control (Cal Ctl) tube of the pressure scanner, shown in the upper right area of Figure 3-9. 26
Figure 3-9. ZOC22B/32Px input arrangement [From Scanivalve Corp. Manual] For data acquisition, an NI 9472 digital output (DO) module was used. Five of the eight available channels (DO0 through DO4) were necessary for obtaining readings from all 32 pneumatic inputs found on the pressure scanner. A set of arrangements were created, based on a binary address system, to access each input individually, shown in Table 3-3. The green blocks represent the NI module channels that required voltage signals in order to access the particular physical input. The LabVIEW program, shown in Figure 3-10, required that the green buttons be pushed in accordance to the patterns in Tables 3-3 prior to acquiring a new time series of data. Table 3-3. Arrangement patterns for pneumatic input access Physical Channel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Arrangement DO0 DO1 DO2 DO3 DO4 27
Physical Channel 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Table 3-3 - Continued Arrangement DO0 DO1 DO2 DO3 DO4 Figure 3-10. LabVIEW front panel for specimen data acquisition 28
3.2.3 Force-Moment Sensor The JR3 30E15 6-axis force-moment sensor is capable of reading forces and moments in the x-, y-, and z-direction. The orientation of the sensor was such that the x-axis was along the direction of flow, the y-axis was perpendicular to the flow along the width of the cylinders, and the z-axis was also perpendicular to the flow but along the height of the cylinders. The outputs obtained from the LabVIEW data acquisition system are provided as voltages and must be converted with a calibration matrix provided in the product s specification sheet. Since the force-moment sensor will remain outside the test section, while the test models are placed inside it, an intricate mounting mechanism was designed to ensure proper readings were obtained. Two parts were designed and manufactured for the mounting of the force-moment sensor: the converter and the sensor holder, both shown in Figure 3-11. The converter serves to connect the model, located inside the test section, to the force-moment sensor on the outside. The bottom of the converter will interact with the top of the sensor through four equally spaced, threaded holes suitable for M6x1 bolts drilled along an imaginary circle with a 4.0 cm (1.57 in.) diameter. The top of the converter will interact with the bottom of the test models through two clearance holes suitable for ¼- 20 bolts. These bolts, drilled on opposite sides of a 3.8 cm (1.5 in.) diameter circle, will be bolted to the converter, placed through holes in the bottom Plexiglas piece, and screwed into the threaded holes on the bottom lids of the cylinders, which were mentioned in Section 3.2.1. The holes in the Plexiglas must be significantly larger than necessary for the bolts to pass through in order to avoid any force dissipation from the bolts resulting from contact with the bottom piece of the test section. Figure 3-11. Force-moment sensor converter (left) and holder (right) 29
The sensor holder serves to secure the force-moment sensor beneath the bottom of the test section Plexiglas. Similar to the converter, the bottom of the holder will have four equally spaced, clearance holes suitable for M6x1 bolts drill along an imaginary circle with a 4.0 cm (1.57 in.) diameter; this will secure the sensor to the holder. Additionally, there will be two threaded holes suitable for ¼-20 bolts, drilled on opposite sides of an imaginary circle with a 10.2 cm (4.0 in.) diameter. These holes will correspond to holes drilled through the Plexiglas and will allow the holder to be secured to the outside of the test section. The complete force-moment sensor mounting mechanism can be seen in Figure 3-12. This figure also shows how the pressure scanner tubes exit the model and the test section. Figure 3-12. Complete force-moment sensor mounting mechanism 30
CHAPTER 4 RESULTS AND DISCUSSION This chapter will discuss important points obtained through the wind tunnel experiments explained in Chapter 3. The results and discussions presented here will also be broken up into two parts. The wind tunnel characteristics data will be presented first, followed by the specimen testing data. 4.1 Part One: Wind Tunnel Characteristics This section will serve to compare the velocity profile, turbulence intensity, and power spectral densities of the wind tunnel test section with and without the presence of the ABL simulator. 4.1.1 Velocity Profiles The velocity profiles were obtained for several horizontal distances along the test section, as shown in Table 3-1. Although all remaining figures are shown in Figure A-4 through Figure A-7 of the Appendix, the profiles discussed here will include only those located at the specimen location. Figure 4-1. 60 m/s velocity profiles at specimen location 31
Figure 4-1 shows the velocity profile of the test section at a freestream velocity of 60 m/s (45 mph) with and without the ABL simulator. This plot was obtained by dividing the average velocity from the time series at each height by the freestream velocity. The freestream velocity was taken as the average of the wind speeds, on a non-normalized velocity profile, in which the profile became leveled. For the 60 m/s case including the ABL, this wind speed was 52.4 m/s. The blue points, representing data that includes the ABL simulator, show a slow and consistent developing boundary layer whereas the red points, representing an empty test section, show a rapid and suddenly developed boundary layer. The estimated boundary layer thicknesses based on Figure 4-1 including and excluding the ABL simulator are 40 cm (15.7 in.) and 10 cm (3.1 in.), respectively. Figure 4-2. Experimental velocity profile compared to ASCE 7-10 exposure categories The 60 m/s velocity profile in the case including the ABL simulator was also compared to the ASCE 7-10 velocity profiles for exposures B, C, and D. Despite the fact that the design of the spires and roughness elements in the ABL simulator incorporated values for Exposure D, the velocity profile exists somewhere between Exposures C and D, as shown in Figure 4-2. The ASCE 7-10 profiles were calculated using full-scale heights and were plotted alongside the representative full-scale heights of 32
the wind tunnel test section, mentioned in Section 2.4.1, for comparison. Equation 4-1 was used in creating the profiles for the different exposures. Ū (4-1) Where h is the full-scale height of a model-scale height along the test section, and b and α z are values taken from Table 26.9-1 for all exposure categories. In order to show the ABL growth along the distance of the test section and compare both cases to each other, Figure 4-3 shows the ABL heights plotted against every measured distance. Overall, the velocity profile results came out as expected. Based on the ABL simulator design calculations discussed in Section 2.2.2 and Section 3.1.1, the ABL simulator was successful in increasing the boundary layer height to four times the original boundary layer height. It is worth noting that although the ABL simulator design calculations intended for a 30 cm (12 in.) boundary layer, the actual simulated boundary layer ended up measuring 40 cm (15.7 in.), 33% thicker than planned. Figure 4-3. ABL height variation along test section length 33
4.1.2 Turbulence Intensities This section will mainly discuss the profiles for 60 m/s (134 mph) data sets and/or those including the ABL simulator. All turbulence intensity profiles that are not shown in this section can be seen in Figure A-8 and Figure A-9 of the Appendix. Turbulence intensity calculations were made for all data sets collected at the specimen location. These calculations were made by taking the average and the standard deviation of each time series within a data set and implementing Equation 2-5 discussed in Section 2.2.1. The turbulence intensity profiles at a freestream velocity of 60 m/s (134 mph) with and without the ABL simulator are shown in Figure 4-4. While both profiles display reasonable trends, the data set including the ABL simulator has turbulence values averaging 13% higher than the data set without the ABL simulator. As intended, the ABL simulator successfully manipulated the flow inside the wind tunnel to generate a larger amount of turbulence, similar to what we observe from the atmospheric boundary layer. This ABL wind effect will be necessary to investigate the pressure distribution results along the surfaces of the cylindrical models. Figure 4-4. 60 m/s turbulence intensity profiles at specimen location 34
The profiles shown in Figure 4-5 compares the turbulence intensity inside the test section containing the ABL simulator to the turbulence intensities for all exposure categories in the ASCE 7-10 Building Code. The points represent the experimental data and the smooth lines represent the turbulence intensity profiles provided by Equation 26.9-7, shown here as Equation 4-2. (4-2) Where c represents the turbulence intensity factor for the exposure categories given in Table 26.9-1 of the ASCE 7-10 Building Code [ASCE, 2010], and z represents the height, in meters, from the bottom of the test section. Figure 4-5. Experimental turbulence intensity compared to ASCE 7-10 exposure categories Although the velocity profile compared in Figure 4-2 of Section 4.1.1 shows agreement between Exposures C and D of the Building Code, the turbulence intensity profile for the same data set shows a better agreement to that of Exposure B. This may indicate a lack of particular frequency signal ranges in the turbulent characteristics produced by the short wind tunnel test section. While this issue does not affect any results presented in later sections, it is discussed further in Section 4.1.3. 35
Data sets were also collected at a constant height of 15 cm (5.91 in.) at varying wind velocities, as discussed in Section 3.1. The purpose of these results was to better quantify the magnitude of the difference in turbulence levels between the case of the empty test section and the case including the ABL simulator. Figure 4-6 shows that the ABL simulator increased the turbulence intensity of the wind tunnel by an average of about 20% at the 15 cm height. While the levels of increase change along the heights of the test section, Figure 4-4 helps support the claim that the turbulence intensity levels are consistently larger in the case of test section with the ABL simulator. Figure 4-6. Turbulence intensities at a constant height of 15 cm at specimen location Unlike the data presented in Figure 4-4, unexpected results were obtained for data sets including the ABL simulator at freestream velocities 20 m/s (45 mph) and 40 m/s (89 mph). Shown in Figure 4-7, between the 15 cm and 30 cm heights, the turbulence levels are significantly larger than the remaining points on the profile. This area of drastically large turbulence is most likely the result of the ongoing formation of the turbulent boundary layer behind the ABL simulator. This results in unsteady and broad wake formations, discussed briefly in Section 2.1.2 and shown in the right side of Figure 2-1. Referring back to Figure 4-4 and Figure 4-5, it appears that the atmospheric boundary layer is fully 36
simulated by 60 m/s (134 mph), at a Reynolds number of 2.55 x 10 5. This suggests that the critical Reynolds number occurs at some point between the 40 m/s and 60 m/s wind speeds. Figure 4-7. 40 m/s turbulence intensity profile at specimen location with ABL simulator 4.1.3 Power Spectral Densities Power spectral densities (PSD) were obtained for all three wind speeds, with and without the ABL simulator, at 15 cm (5.91 in.) and 30 cm (11.81 in.) heights. The experimental PSD s were then compared to the Kaimal spectrum given by Equation 4-3, where f L is the non-dimensional frequency of the respective experimental data set obtained from the pwelch function in MATLAB. The goal of the experimental PSD is to reasonably match the Kaimal spectrum; as a good match indicates a good representation of wind turbulence within the data.,. (4-3) The PSD s for the 60 m/s free-stream velocity including the ABL simulator, seen in Figure 4-8 and Figure 4-9, show the best agreement with the Kaimal spectrum out of all the experimental data sets. 37
These results indicate that the wind turbulence generated inside the wind tunnel test section is mainly composed of high frequency signals, shown by the denser portion of the blue-line plot toward the right side of the figures. Conversely, there is a very low representation of low frequency signals in any of the data sets. Past research [Burton, 2001 and Lopes et al. 2008] has indicated the difficulty in producing reliable PSD s within short wind tunnels, so it is not unusual that Figure 4-8 and Figure 4-9 display a matching trend without any numerical agreement, considering the exceedingly small dimensions of the test section. Figure 4-8. 60 m/s power spectral density at 15 cm height with ABL simulator Similar to the PSD s for all remaining data sets, shown in Figure A-10 through Figure A-18 of the Appendix, Figure 4-10 displays a poor match to the Kaimal spectrum. The concentrations of sinusoidal signals in the high frequency range differ from the ones presented in Figure 4-8 and Figure 4-9 in the way that it does not level off at any point before dropping in value. Additionally, the R N values displayed on the y-axis are significantly larger in most of the analyzed data sets. 38
Figure 4-9. 60 m/s power spectral density at 30 cm height with ABL simulator Figure 4-10. 40 m/s power spectral density at 15 cm height with ABL simulator 39
While a power spectrum composed mainly of high frequency signals was expected from data sets without the ABL simulator, it was unexpected that similar results be obtained from data sets including the ABL simulator. Fortunately, for the purposes of the drag reduction component of this research, a poor representation of a power spectrum will not affect the average force and pressure distributions discussed in Section 4.2. However, it is important to understand that any analysis on dynamic simulations on a structural model under the power spectrum conditions presented in this section will not be accurate. 4.2 Part Two: Specimen Testing This section will present the drag coefficient and pressure coefficient results obtained through the use of the force-moment sensor and the pressure scanner, respectively. Results will be compared in two ways: (a) from cylinder to cylinder and (b) with and without the presence of the ABL simulator. 4.2.1 Drag Coefficients As discussed in Section 2.1.5, the drag coefficient, C D, is a dimensionless number used to quantify the resistance experienced by an object in a fluid environment. More specifically, the drag coefficient is an overall quantity that combines the effects of the two main types of drag: skin friction drag and pressure drag. Calculations for the drag coefficients presented in this section were obtained using Equation 2-3. Although forces along the x-axis (direction of flow) were directly obtained through the force-moment sensor, the values did not appear to be correct (possibly due to error in the sensor setup or wiring) leading to the following alternative calculation option. The drag forces, F D, were obtained using the moments about the y-axis (direction perpendicular to flow) provided by the sensor.. (4-4) The moment arm, 0.625H, was taken as five-eighths of the height of the cylinder since the air flow is assumed to be distributed in a parabolic fashion along the height of the model, as confirmed in the velocity profiles in Section 4.1.1. Using Equation 4-4, along with a moment arm of 15.9 cm (6.3 in.), the forces along the x-axis (direction of flow) were calculated. 40
Figure 4-11. Drag coefficient variations among cylinder types without ABL simulator Figure 4-11 and Figure 4-12 show the drag coefficients for all three cylinders with increasing Reynolds numbers with and without the ABL simulator. The Reynolds numbers were calculated using Equation 2-1; where the diameter used was 6.35 cm (2.5 in.) and the kinematic viscosity of air used was 14.970 x 10-6 m 2 /s (representing an environmental temperature of 70 F). The Reynolds numbers calculated through the free-stream velocities 20 m/s, 40 m/s, and 60 m/s were 8.48 x 10 4, 1.70 x 10 5, and 2.55 x 10 5, respectively. In the case excluding the ABL simulator, the smooth cylinder has the highest resistance under the lowest Reynolds number tested while the U- and V-grooved cylinders have practically equal drag coefficients. Interestingly, the drag coefficient of the smooth cylinder drops drastically between the 20 m/s and 40 m/s wind velocities and continues to drop steadily until the 60 m/s velocity until all three cylinders are very close in value; having drag coefficients ranging from 0.49 to 0.61. Similar to the results obtained from the data sets without the ABL simulator, Figure 4-12 shows that the smooth cylinder has a significantly larger drag coefficient than the other two models. Unlike the data sets without the ABL simulator, however, the U-groove and V-groove drag coefficients do not match; the U-groove coefficient is about 31.5% larger than the V-groove coefficient. Note how the trend in the cases shown in Figure 4-12 remains consistent throughout all Reynolds number values: 41
the smooth and V-grooved cylinder have the highest and lowest drag coefficients in every case, respectively. Figure 4-12. Drag coefficient variations among cylinder types with ABL simulator The overall decrease in drag coefficients between Figure 4-11 and Figure 4-12 can be attributed to the presence of the ABL simulator at the beginning of the test section. Mimicking the effects of surrounding infrastructure, the ABL simulator reduces the wind force exerted on the cylinders by passively altering the flow velocities, and therefore, also reducing the associated drag coefficients. In addition, it is worth noting the lack of drag reduction produced by the V-grooved cylinder in the case without the ABL. Aside from the point corresponding to the lowest Reynolds number, the V-groove drag coefficient is either greater than or equal to the smooth cylinder coefficients. This may be the result of a flaw in the riblet dimensions, as discussed briefly in Section 2.5.3. Since the V-grooved cylinder appears to have successfully reduced drag in turbulent flow (including ABL simulator) and not laminar flow (not including ABL simulator), it can be argued that perhaps the riblets were unable to prevent vortex formation within its valleys [Horsten, 2005]. This inability to obstruct the formation of vortices, which is suspected to be the main drag reduction mechanism in the use of riblets, can be 42
resolved by adjusting the peak-to-peak spacing. Judging by the success of the U-groove results under laminar flow, the V-groove peak-to-peak spacing adjustment may likely have to approach that of the U-grooved cylinder. 4.2.2 Pressure Coefficients This section will discuss the pressure coefficients obtained between the three cylinder models in cases including and excluding the ABL simulator. All data was obtained from the pressure taps discussed in Section 3.2.2 and calculated using Equation 2-2. The pressure coefficients along the perimeter of each cylinder at the 15 cm (5.91 in.) height and 60 m/s (134 mph) freestream velocity are shown in Figure 4-13 and Figure 4-14. Included in these figures is the experimental data obtained at a Reynolds number of 1.1 x 10 5 for comparison [Roshko, 1961]. The remaining figures for the 20 m/s (45 mph) and 40 m/s (89 mph) wind speeds can be seen in Figure A-19 through Figure A-22 of the Appendix. The U- grooved cylinder pressure coefficients for 45 and 60 have been removed from the figures below due to an experimental error which showed large disagreement with the remaining data. Inaccurate values for the U-grooved cylinder plots in the Appendix have also been omitted. Recall from Figure 3-8 that 0 and 180 represent the windward-most and leeward-most points on the cylinder, respectively. Figure 4-13. 60 m/s pressure coefficient variations without ABL simulator 43
Figure 4-14. 60 m/s pressure coefficient variations with ABL simulator By plotting the pressure coefficient against the cylinder angle, it becomes possible to identify the points in which the flow is separating from the surface of the cylinders. On the windward side of the cylinder (approximately between 0 and 90 ), the flow continues to accelerate, as it has to go further than the surrounding fluid, producing a drop in the pressure. The flow then decelerates and may separate as the pressure increases, which is termed as the adverse pressure gradient. As discussed in Section 2.1.2, overcoming the adverse pressure gradient is an indicator of flow separation. In other words, the flow has to overcome the increasing pressure as well as the friction to prevent the separation. This is represented among Figure 4-13 and Figure 4-14 as the points where the slope of each curve is positive. In the case without the ABL simulator, the angle in which the smooth cylinder begins to overcome the adverse pressure gradient is 60, the earliest out of all the model types. Under the same conditions, the V- and U-grooved cylinder begin overcoming this pressure gradient at 70 and 90, respectively. In the case with the ABL simulator, the smooth cylinder is able to delay the beginning of the separation process to 80, while the V- and U-grooved points remained the same. The ends of the adverse pressure gradients for the case without the ABL simulator, as shown in Figure 4-13, are approximately 90, 110, and 90, for the smooth, U-grooved, and V-grooved cylinders, respectively. For the case 44
including the ABL simulator, shown in Figure 4-14, they are 100, 110, and 90, respectively. The separation points for both cases indicate that a delay of separation occurs only with the smooth cylinder once the ABL simulator is introduced. The U-groove cylinder is able to delay separation in either case, the latest of all the model types. Note that unlike all the cases including the ABL simulator, shown Figure 4-14, the V-grooved cylinder consistently shows larger negative pressures. This finding supports the claim presented in Section 4.2.1, stating that perhaps the V-grooves in this experiment are actually counter-productive under laminar flow. While the trend of the pressure coefficients compares well to Roshko s experimental data, the values shown in Figure 4-13 and Figure 4-14 are significantly more negative. This is likely the result of improper implementation of the reference pressure, discussed in Section 2.3.1 and shown in Equation 2-2. The pressures were likely obstructed, resulting in skewed net pressure values. Accounting for this, the data was manipulated to better match the typical pressure coefficient variation data as shown in Figure 4-15 and Figure A-23 of the Appendix. Past research [Roshko, 1961] has indicated that pressure coefficient for a smooth cylinder should approach -1 at about 100. Knowing this, the 100 pressure coefficient became the reference pressure for the observed data. Upon normalizing this data, it can be seen in Figure 4-15 that there is a good match between the expected results and the adjusted results. Figure 4-15. Adjusted 60 m/s pressure coefficient variations with ABL simulator 45
Pressure contours were also obtained for all data sets along all the heights and between 70 and 110. Figure 4-16 through 4-18 show the pressure contours for all the 60 m/s (134 mph) data sets. Refer to Figure A-24 through Figure A-29 of the Appendix for remaining contour plots. Figure 4-16. 60 m/s smooth cylinder pressure contour with (left) and without (right) ABL simulator 46
Figure 4-17. 60 m/s U-grooved cylinder pressure contour with (left) and without (right) ABL simulator The pressure contours between the cases including and excluding the ABL simulator for each cylinder type show similar patterns with consistently smaller negative pressure coefficients. Based on the 47
average pressure contours for the cases not including the ABL simulator, the increases in pressure coefficients (or decrease in suction) are 11.78% between the smooth cylinder and the U-grooved cylinder and 21.10% between the smooth cylinder and the V-grooved cylinder. Similarly, the increases in pressure coefficients for the cases including the ABL simulator are 4.99% between the smooth and the U-grooved cylinder and 5.63% between the smooth and the V-grooved cylinder. For all figures including the ABL simulator, the effect of the adverse pressure gradient discussed previously is visible throughout the pressure contours. Among these figures, the contour lines run more vertically and have less negative pressure coefficients as the angle increases, which supports the findings discussed between Figure 4-13 and Figure 4-14. To compare the effectiveness of the surface patterns, the following discussion will focus mainly on the figures including the ABL simulator. Based on Figure 4-16 and Figure 4-18, a similar contour pattern is shared between the smooth and V-grooved cylinder. The contour pattern for the U-grooved cylinder, shown in Figure 4-17, is also similar to the other cylinders with the exception of the high pressure coefficient area occurring between 70 and 90 angles and 3 cm (1.18 in.) and 9 cm (3.54 cm) heights. This low pressure area, which is consistent throughout both plots in Figure 4-17, may be attributed to the physical pressure tap locations at that height. These particular taps are found along the peak of a U-groove, unlike the tap locations at all other heights, which are located along its valleys. This was an unintended difference and should be corrected in the event of further testing. These pressure contours suggest that the V-grooved cylinder has the lowest negative pressures among the cylinder types. Despite the finding that the flow separates relatively early in the cases with the ABL simulator, as discussed earlier in this section, the pressure coefficient values are consistently less negative. Additionally, while the U-grooved cylinder showed later flow separation at about 110, it shows negative pressure coefficient values between those of the smooth and V-grooved cylinders. This implies that delaying flow separation is not the sole determining factor in concluding which surface pattern is more effective in reducing drag forces on a cylindrical structure. This factor, coupled with relative pressure coefficients, are what truly allows a conclusion to be made. 48
Figure 4-18. 60 m/s V-grooved cylinder pressure contour with (left) and without (right) ABL simulator 49
CHAPTER 5 CONCLUSIONS Thorough analysis and discussion, presented in Chapter 4, has introduced several important points that will be condensed into this section for easy referencing and quick understanding. The success of this research has allowed for conclusions involving both parts of the experiment to be made. Beginning with the wind tunnel characteristics discussed in Section 4.1, the presence of the ABL simulator within the test section played an essential role in providing results relevant to the civil engineering discipline. Without it, the turbulence levels within the tunnel would not have sufficed in mimicking the real-world terrain effects subjected to high-rise structures. Not only did the ABL simulator increase the turbulence intensity levels by an average of 20% in comparison to an empty test section, but it also increased the atmospheric boundary layer thickness from 10 cm (3.9 in.) to 40 cm (15.7 in.). This ensured that the test models, each with a height of 25.4 cm (10 in.), were completely within this boundary layer during the second part of testing. Despite the success of the ABL simulator in producing accurate velocity and turbulence intensity profiles, it was not successful in producing appropriate power spectrum plots. This is attributed to the small test section dimensions. While turbulence spectrums have negligible effects on the mean forces and pressures of this study, they are absolutely necessary for dynamic simulations. Therefore, if that is the research objective of a particular project, the experimental conditions discussed here will not be suitable for providing accurate results due to the lack of low frequency signals displayed in the power spectrum plots of turbulence. The specimen testing revealed that the V-grooved cylinder was the most effective surface pattern in reducing drag forces on a high-rise structure out of all the model types tested. The drag coefficients, C D, were consistently lower at all Reynolds numbers than those of the smooth and U-grooved cylinder, in the cases including the ABL simulator. Additionally, the contour plots for the V-grooved cylinder in the cases including the ABL simulator showed the highest, or least negative, pressure coefficients along all pressure tap heights between 70 and 110. This is followed by the U-grooved cylinder and, finally, the smooth cylinder. 50
Although the precise location of the separation point is difficult to obtain, based on the pressure coefficient vs. angle plots, the V-grooved cylinder appeared to have the earliest separation point, especially with the presence of the ABL simulator. This simply implies that the pressure drag subjected to the V-grooved cylinder could be reduced further if the separation is somehow delayed. In a case where the V-grooved cylinder were to have flow separation occur at the same point as the U-grooved cylinder, the pressure and drag coefficients would be expected to reduce even further. Another point worth mentioning is the effectiveness of the V-grooved cylinder under laminar flow. While it was not the poorest performing cylinder, the results presented in the cases without the ABL simulator were particularly worse for the V-grooved cylinder than for either of the other ones. This is likely attributed to the physical dimensions of the riblets and will be discussed in Section 5.1. 5.1 Future Work For researchers who would like to expand on the work completed here, the recommended adjustments and/or additions will be discussed in this section. The most significant, yet important, change would be to conduct testing in a larger wind tunnel facility. Several issues discussed in the results can be attributed to the limitations associated with a short test section. A larger facility will also be capable of producing faster wind velocities, which allows for data sets within a larger range of supercritical Reynolds numbers. Adjusting the dimensions of the V-grooved riblets will also improve results for data obtained in laminar flow. Although this is not directly relevant to structural civil engineering, it can make all the difference for several other engineering disciplines. 51
APPENDIX Figure A-1. Surface patterns on smooth (left), U-grooved (center), and V-grooved (right) cylinders 52
Figure A-2. Sketch of tap locations 53
Figure A-3. LabVIEW block diagram for specimen data acquisition Figure A-4. 20 m/s velocity profiles at specimen location 54
Figure A-5. 40 m/s velocity profiles at specimen location Figure A-6. 40 m/s velocity profiles 60 cm from beginning of test section 55
Figure A-7. 40 m/s velocity profiles 125 cm from beginning of test section Figure A-8. 20 m/s turbulence intensity profiles at specimen location 56
Figure A-9. 40 m/s velocity profiles at specimen location Figure A-10. 20 m/s power spectral density at 15 cm height without ABL simulator 57
Figure A-11. 20 m/s power spectral density at 30 cm height without ABL simulator Figure A-12. 20 m/s power spectral density at 15 cm height with ABL simulator 58
Figure A-13. 20 m/s power spectral density at 30 cm height with ABL simulator Figure A-14. 40 m/s power spectral density at 15 cm height without ABL simulator 59
Figure A-15. 40 m/s power spectral density at 30 cm height without ABL simulator Figure A-16. 40 m/s power spectral density at 30 cm height with ABL simulator 60
Figure A-17. 60 m/s power spectral density at 15 cm height without ABL simulator Figure A-18. 60 m/s power spectral density at 30 cm height without ABL simulator 61
Figure A-19. 20 m/s pressure coefficient variations without ABL simulator Figure A-20. 20 m/s pressure coefficient variations with ABL simulator 62
Figure A-21. 40 m/s pressure coefficient variations without ABL simulator Figure A-22. 40 m/s pressure coefficient variations with ABL simulator 63
Figure A-23. Adjusted 60 m/s pressure coefficient variations without ABL simulator 64
Figure A-24. 20 m/s smooth cylinder pressure contour with (left) and without (right) ABL simulator 65
Figure A-25. 20 m/s U-grooved cylinder pressure contour with (left) and without (right) ABL simulator 66
Figure A-26. 20 m/s V-grooved cylinder pressure contour with (left) and without (right) ABL simulator 67
Figure A-27. 40 m/s smooth cylinder pressure contour with (left) and without (right) ABL simulator 68
Figure A-28. 40 m/s U-grooved cylinder pressure contour with (left) and without (right) ABL simulator 69
Figure A-29. 40 m/s V-grooved cylinder pressure contour with (left) and without (right) ABL simulator 70