The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 212 Wind tunnel studies on the effects of porous elements on the aerodynamic behavior of civil structures M. Belloli a, D. Rocchi a, L. Rosa a, A. Zasso a a Politecnico di Milano, Dipartimento di Meccanica, via La Masa 1, 2156 Milano, Italy ABSTRACT: This paper deals with the role of the porosity on the aerodynamic behavior of two civil structures: the Spire, a tall slender tower 8.44m high, and the buttresses of a tall building. The Spire is an original architectonic complement. Its covering, made of perforated steel plates, forms and intricate three-dimensional spiral characterized by five different diameters. The buttresses connect the building core around one fourth of its height to viscous dampers at their bottom, in order to increase the overall damping value of the building. The geometry of the buttresses is characterized by a variable cross-section along the longitudinal axis, permeable to the air. Wind tunnel tests on a 1:5 scaled rigid and aeroelastic model of the Spire and on a 3:1 scaled sectional model of the buttress were carried out in order to investigate the aerodynamic behavior of the structures using different geometry of the porosity or replacing with not-porous surfaces. The analysis carried out highlights the great influence of the porosity in the dynamic response of both structures, mainly in relation to vortex shedding induced vibrations. KEYWORDS: Porous surface; Vortex Shedding; Wind Tunnel Tests; Wind Loads; Aeroelastic Model; High Slender Tower. 1 INTRODUCTION Nowadays porosity has a wide range of application in engineering and civil fields. In agricultural, porous screens have been largely used for many years in many functions: temperature regulating, shading, wind-breaking, anti-hail, anti-frost or for preventing the entrance of insects or birds. In some ventilation system, screens are used to control dust as filter function. As an alternative to these historical functions, the porosity finds new applications in civil structures. In fact modern buildings have required to reach higher energetic standards and this is achieved not only, using new construction materials, but also applying accessory elements to the structures. Porous structural elements are ones of the most common, since they have many functionalities: architectural design, temperature regulating function, shading function, wind-breaking effect and, last but not least, vortex shedding mitigation. In literature there are many experimental and numerical CFD studies about flow through porous surface, e.g. [1,2,3]. Most of them regard wind loads on structures such as greenhouses, panels or roof [4,5,6,7,8,9]; these results reveals that the wind loads on perforated structure are generally lower than those on same non-porous surfaces. More systematic studies regard flow around porous cylinders [1,11]: in particular they show how vortex shedding can be reduced or suppressed manipulating the flow around the cylinder through the application of suction or blowing. Not a great number of experimental data are on the contrary available about wind interaction with permeable accessory elements in civil structures. For instance some recent studies attest that the porosity of a surface could modifies the wake formation and reduces the intensity of the vortex shedding and the overall crosswind load [12,13]. This paper deals with the role of the porosity on the aerodynamic behavior considering as illustrative examples two civil structures: the Spire, a tall slender tower 8.44m high, and the buttresses of a tall building. Wind tunnel tests were carried out in order to investigate the aerody- 1132
namic behavior of the structures using different geometry of the porosity or replacing with notporous surfaces. This analysis is very interesting as the structures could be subjected to atmospheric icing which can make non-porous parts of the perforated panels and so change their aerodynamic behavior. 1.1 The Spire The Spire, which is erected on the top of a 139m new tall building, consists of a supporting lattice framework structure 8.44m high, covered with perforated steel plates. Considering the height of the building, the Spire maximum elevation from the ground is 22m (Figure 1). The external cover is not regular, but forms an intricate three-dimensional spiral characterized by five different diameters which decrease along the height of the structure. Changing the diameter along the height of the structure mitigates the vortex shedding phenomenon, but in the present case each section of the spire has an adequate length to introduce energy to the whole system and to induce oscillations of the spire to lead to fatigue damages [14,15]. Figure 1. The Spire at the top of the new tall building. Its maximum elevation from the ground is 22m. 1.2 The buttress The second structure studied in this paper are the long buttresses of a tall building, Figure 2(a). This building is an office tower 22m tall and it has a rectangular floor plan of 61.5m x 24m. The tower is characterized by two couples of external steel buttresses inclined with respect to the vertical axis and connected to the tower core around one fourth of its height along the front and the back facades. The buttresses are connected to their bottom ends to viscous dampers in order to increase the overall damping value of the building with respect to the minor side, providing beneficial effect to the base moment. The length of the buttress is 61.6m end connections included. Their geometry is characterized by a variable cross-section along the longitudinal axis made by 3 Circular Hollow Section (CHS) interconnected rigidly by steel plates (Figure 2(b)); between the transverse connections the area within the three longitudinal CHS is permeable to the air. At mid span the three CHS reach the maximum c/c distance of 1.2m (see section 3, Figure 2); whereas at the two ends the three sections converge into a single circular tube (see sections 1 and 2, Figure 2). 1133
The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 212 Section 2 Section 1 Section 3 Figure 2. The 22m high tall building. (a) Global view. (b) Buttress geometry and transverse typical sections. The 3D view represents the central part of the buttress (section 3). 2 EXPERIMENTAL SET-UP Wind tunnel tests were performed at the 1.5MW closed-circuit wind tunnel at the Politecnico di Milano, Italy. The large dimensions of the boundary layer test section (4m high, 14m wide and 36m long) allows a very large geometry scale to be used while maintaining lower blockage effects. The tests were performed in smooth flow condition (turbulence intensity I u < 2%) which is considered on the safe side with respect to instability and vortex shedding excitation. 2.1 The model of the Spire The experimental tests were performed using two models of the Spire: a rigid and an aeroelastic model. In this work we will focus on the results obtained on the aeroelastic model. A comparison between the results from the two models, to verify the presence of possible aeroelastic effects, can be found in [16], while a study about the local wind loads on the permeable panels by means of pressures measurements can be found in [17]. The large dimensions of the test section allowed to choose a geometry scale λ L = 1/5. The aeroelastic model was designed as a spine model, adopting the Froude similitude criteria for scale reduction: the elastic properties of the real structure were reproduced by means of an aluminum bar made up by rectangular section of four different dimensions. An external cover, constituted of 16 modules, was adopted to reproduce the wind interaction shape and to match the correctly scaled mass. A more detailed description about the aeroelastic model set-up can be found in [17], while Figure 3(a) shows the model in the test section. Great care was taken for scaling the external surface geometry and porosity: in order to guarantee the kinematic similitude the perforated panels must be scaled not geometrically but the same loss coefficient must be maintained [7]. The loss coefficient k is defined as: pu pd k = (1) ρu 2 / 2 1134
being p u and p d respectively the upstream and the downstream static pressure on either side of the grid, ρ is the density of the air and U is the mean wind speed. This coefficient is a indicator of the resistance to flow through a porous surface, including the effects of the open area β (defined as the ratio between the area open to through flow and the total area of the panel) as well as the shape of the perforations. The pressure loss coefficients were evaluate experimentally in a smaller wind tunnel, in which its entire round cross section (diameter = 4mm) was covered by the grid being tested. (a) (b) Figure 3. (a) The aeroelastic model in wind tunnel test section. (b) The perforated panels on the real structure. In the real structure the panels have round hole perforation (radius 7.5mm) with triangular pitch of 21mm, Figure 3(b). Their open area β is 46.3% and the loss coefficient k varies between 4.5 and 4.8 (experimentally evaluated). On the models round hole perforated plates with radius.4mm and triangular pitch of 1mm were used. These plates were electrical discharge machining formed, they had an open area β=58% and a loss coefficient k very close to the real structure (4<k<4.5). Global forces measurement is carried out by means of a dynamometric six-components force-balance linked at the base of the model. The exposure angle and the reference system is shown in Figure 4(a). (a) (b) (c) Figure 4. (a) Exposure angle and reference system. (b) Heights of the cobra probes (h) and the accelerometers (Acc). (c) The aeroelastic model made completely non-porous. 1135
The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 212 The model was instrumented with 6 accelerometers fixed on the structural bar in order to monitor the accelerations according to its bending modes (x-dir and y-dir). Vortex shedding excitation was investigated using four Cobra Probes, a multi-hole pressure probe developed by Turbulent Flow Instrumentation (TFI). They were fixed downwind the Spire at the heights shown in Figure 4(b); the same figure shows the positions of the accelerometers. In order to evaluate the effects of the porosity on the wind loads as well as the vortex shedding excitation, some tests were duplicated making impermeable the entire external surface of the aeroelastic model, Figure 4(c). Great care has been taken in order to keep the surface roughness unchanged adding the tape on the external surface. 2.2 The model of the buttress The tests were carried out on a sectional model of the buttress (3/1 geometric scaled, aspect ratio L/B=6) realized in carbon fiber. The model is rigid and it was elastically suspended to realize a one degree of freedom system able to vibrate orthogonally to the incoming mean wind direction (cross flow direction), Figure 5. Structural modal damping was kept as low as possible in order to highlight the VIV phenomenon and it was adjustable by adding damping to the model through eddy current dampers or viscous dampers. In this way it was possible to change the Scruton Number and to investigate the oscillation amplitudes as a function of this parameter. Figure 5. Overall view of the suspended model in the test section for dynamic tests. Tests were performed at 3 angles of attack α, in according with Figure 6(a) and on different layouts. Porous screens: the gap between the cylinders is closed by a porous screen, Figure 6(b). Solid screens: the gap between the cylinders is completely closed, Figure 6(c). Different model surface finishing were also tested. (a) (b) (c) Figure 6. Sectional model of the buttresses. (a) Cross-section and wind direction angle. (b) Open, porous screens. (c) Close, solid screens. 1136
3 RESULTS The analysis has demonstrated that the permeability has a great importance on the aerodynamic behavior of both structures. In the following the main results from the experimental wind tunnel tests are presented. 3.1 The Spire Figure 7 shows the overall wind load at the base of the aeroelastic model of the Spire. The results are expressed in terms of the drag coefficient C D (α) and C L (α), Equation (2), where F D is the drag force and F L is the lift (side) force function of the exposition angle α, q H is the mean wind pressure at the base of the Spire and A rif is the reference area. The peak factor method for Gaussian process has been used for the evaluation of the peak values. C D, L ( ) F ( α ) D, L α = (2) q A H rif The figure shows the along-wind component C D, Figure 7(a) and the cross-wind component C L, Figure 7(b), of the overall wind load measured at the base of the aeroelastic model of the Spire in the in-service configuration (perforated panels) and making not-porous all the panels (close configuration). Due to the three dimensional external shape of the covering both coefficients manifest a great variability changing the exposition angle α. Considering the drag coefficient C D, the non-porous configuration shows a moderate increment of the mean value and a higher increment of the peak values. In particular at α=-3deg and α=-6deg the mean value is almost the same, but the peak value is nearly doubled. The effect of the porosity on cross wind coefficient is more relevant. The non-porous Spire does not manifest a modification in mean value, which stays close to zero, on the contrary the peak values show a great increment: the maximum peak occurs about at a=-9 deg, where its value is about ten-times larger than the equivalent from porous panels. The analysis of the wind load coefficients reveals that making nonporous the panels slightly increases the along-wind force on the structures and generates a strong cross-wind dynamic load. The increment of the cross wind dynamic is strictly linked to a strong vortex shedding excitation, described in the next paragraph. Drag coefficient Cd (-) 2.5 2 1.5 1 Mean-Imperm. Peak-Imperm. Mean-Porous Peak-Porous Lift coefficient Cl (-) 12 1 8 6 4 2 Mean-Impermeable Peak-Impermeable Mean-Porous Peak-Porous.5-18 -12-6 6 12 18 Exposition angle α (deg) -2-18 -12-6 6 12 18 Exposition angle α (deg) Figure 7. Overall wind load at the base of the Spire, smooth flow. Comparison between porous and non-porous panels. (a) Drag coefficient, C D. (b) Lift coefficient, C L (Note the difference in the y-axis scale). 1137
The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 212 The vortex shedding investigation has been performed analyzing the cross-wind velocities downwind the Spire measured by the four cobra probes. In the discussion of the results the crosswind velocity v i refers the probe at height h i placed downwind a section of the Spire characterized by a diameter D i. Figure 8 shows the spectra of the cross-wind velocities vi in the wake of the Spire the porous panels. The test speed was selected in order to generate from the section D2 a vortex shedding resonant with the first vibration mode of the structure (f1=5.9hz). The peak at the frequency f1 in the spectra of the component v2, Figure 8(c), confirms this condition. D4 - v4.4.2 D3 - v3.4.2 f = 8.3Hz 5 1 15 2 25 3 Freq. (Hz) - (a).2 5 1 15 2 25 3 Freq. (Hz) - (b).2 D2 - v2.15.1.5 f = 5Hz D1 - v1.15.1.5 5 1 15 2 25 3 Freq. (Hz) - (c) 5 1 15 2 25 3 Freq. (Hz) - (d) Figure 8. Aeroelastic model with porous panels. U=2.18m/s, SF. Spectra of the cross-wind velocities v i downwind the Spire (Model scale data). The section D2 has a full length scale of about 2m, a quarter of the total height of the structure, making its length not negligible compared with the respective mean diameter of about 5m. For this reason, even if changing the diameter along the height of the structure is known that mitigates the vortex shedding phenomenon, this section could have an adequate length to introduce enough energy to whole structure and induce vibrations [14,18,19]. The previous assumption, generally valid for non-porous circular surfaces, is disproved in the present case inspecting the spectra of the cross-wind velocities at the other levels. The excitation of the whole structure due to vortex shedding phenomenon is excluded by the following considerations: the spectra of the component v3, Figure 8(b), shows a peak at the Strohual frequency of the section D3 (f=8.3 Hz) and not at the first natural frequency of the model. The peaks in the spectra have a broad bandwidth, indicating a vortex shedding not fully synchronized. Lastly the crosswind displacements time histories calculated from the integration of the accelerometers signals are very low at all the levels considered (the maximum full scale peak to peak displacement is 5mm) and no regular vibrations are visible. On the contrary, the aerodynamic behavior of the Spire completely non-porous is totally different. Figure 9 shows the spectra of the cross-wind velocities vi in the wake of the Spire fully non-porous. The test speed was selected in order to have a vortex shedding generated by the section D3 resonant with the first vibration mode of the structure (f1=4.5hz). The clear narrow bandwidth peak at the frequency f1 in the spectra of the component v3, Figure 9(b), confirms this condition. A narrow bandwidth peak at the same frequency is also present in the spectrum of the component v4, Figure 9(a), while downwind the sections at the lower levels, D1 and D2, a peak is also present but it is located at the Strohual frequency of the section and not at the first natural frequency of the model, Figure 9(c-d). This analysis reveal that vortex shedding induced vibration is now present, but due to the very low wind speed the energy introduced is not enough to induce an important vibration of the entire Spire. 1138
.4.4 D4 - v4.3.2.1 f = 4.5Hz D3 - v3.3.2.1 f = 4.5Hz 5 1 15 2 25 3 Freq. (Hz) - (a).1 5 1 15 2 25 3 Freq. (Hz) - (b).1 D2 - v2.5 f = 2.Hz D1 - v1.5 5 1 15 2 25 3 Freq. (Hz) - (c) 5 1 15 2 25 3 Freq. (Hz) - (d) Figure 9. Aeroelastic model with panels fully non-permeable. U=1.28m/s, SF. Spectra of the cross-wind velocities v i downwind the Spire (Model scale data). The top-spire cross-wind time history displacement calculated from the integration of the accelerometers shows an evident constant vibration of the Spire due the vortex shedding excitation. The vibration is small, 2mm model equal to 1mm full scale, nevertheless since the low but very frequent wind speed, it could be enough to lead to fatigue damages to the structure. The absence of a vortex induced vibration on the permeable structure is given by the porosity: a detailed analysis by means of pressure measurements conducted in [16] has shown that permeability reduces the wind load because the air flows through the holes of the permeable panel, tending to equalize the mean pressure and attenuate the peak across the panel. As a consequence the vortex shedding excitation is strongly reduced. 3.2 Isozaki tower buttresses Vortex induced vibration (VIV) tests were performed on the sectional model of the buttress. Aim of the tests was to compare the aerodynamic behavior of the buttresses using different screens layouts, different porosity in the area permeable to the air and different surface roughness. Aerodynamic stability of the buttress shape has been also investigated by studying the aerodynamic damping trend increasing the mean wind speed. The results are presented in function of the Scruton Number, defined as Eq.(3), where m is the mass per unit length of the body, h s critical damping ratio, ρ is the air density and B is the overall dimension of the cross section. 2π mhs Sc = (3) 2 ρb The solid screens layout (Figure 6(c)) was firstly investigated. This configuration highlighted significant problems related to the fluid-structure interaction. In particular, considering the angle of attack α=-3deg (Figure 6(a)), the model showed significant vortex induced vibrations with also the presence of instability phenomena that led to large uncontrolled flow-induced vibrations unsuitable at low Scruton number (Sc=.1, Figure 1(a)) and even at high value of Scruton number (u/b=4.5% at Sc=23, where u is the experimental displacement and B the model characteristic dimension, Figure 1(b)). Oscillations were still present even if the Scruton number was increased up to 5. 1139
The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 212 Non-dimensional oscillation amplitude u/b (%) 3 2 1-1 -2-3 Test stopped Test stopped 6 8 1 12 14 16 18 2 Time (s) -6 5 1 15 2 Time (s) (a) (b) Figure 1. (a) Solid screens layout, α=-3deg, instability and VIV. (a) Sc=.1. (b) Sc=25. The choice was to use porous screens as an alternative configuration to the "solid screens". These screens have round hole perforation (radius 5mm) with triangular pitch of 14mm and porosity β=46%. This layout was stable at the angles of attack α=deg and α=3deg, while vortex induced vibrations were still present at α=-3deg. Figure 11(b) shows the non-dimensional oscillation amplitude u/b (%) function of the Scruton Number Sc: it is possible to see that increasing the structural damping to provide a Scruton number higher than 12 the oscillations stopped. The stability of the structure was checked also changing the surface roughness of the model. Non-dimensional oscillation amplitude u/b (%) 2 1.5 1.5 >7.5% 5 1 15 2 25 3 Scruton number (-) Figure 11. Porous screens layout, α=3deg. VIV in function of Scruton number at the most critical angle of attack. Non-dimensional oscillation amplitude u/b (%) 6 4 2-2 -4 Porous screens -3 Porous screens -3 rough 4 CONCLUSIONS The analysis has demonstrated that the permeability has a great importance on the aerodynamic behavior of the structures considered in the present work. Tests showed that the porosity of the external surface gives advance on both the mean load reduction as well vortex shedding attenuation. This is due to the passage of the flow through the panels which modifies locally the pressure distribution and, as a result, the wind loads and the dynamic response of the whole 114
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