CIRCULATION CONTROLLED AIRFOIL ANALYSIS THROUGH 360 DEGREES ANGLE OF ATTACK

Proceedings of the ASME 2009 3rd International Conference of Proceedings Energy Sustainability of ES2009 Energy Sustainability ES2009 July July 19-23, 2009, 2009, San San Francisco, California, USA ES2009-90341 CIRCULATION CONTROLLED AIRFOIL ANALYSIS THROUGH 360 DEGREES ANGLE OF ATTACK Henry Z. Graham IV, Meagan Hubbell, Chad Panther, Jay Wilhelm, Gerald M. Angle II, James E. Smith Center for Industrial Research Application (CIRA) Mechanical and Aerospace Engineering Department West Virginia University, Morgantown, WV, 26506, USA ABSTRACT Wind turbines are a source of renewable energy with an endless supply. The most efficient types of wind turbines operate by utilizing the lift force of its blades to create a rotational force. The power capabilities of a wind turbine are tied to the blades ability to convert the aerodynamic forces into rotational energy. Vertical axis wind turbines (VAWT), unlike the more common horizontal axis (HAWT) type, do not need to be directed into the wind and can place the transmission and electrical power generation components at the bottom of the turbine shaft, near the ground. Currently VAWTs cannot feather or pitch the blades, in the same fashion as a HAWT, for a lift change to control power generation and/or rotational speed at different or changing wind speeds. A method of increasing the lift of a blade without physically moving the blade is to use circulation control (CC), via a blowing slot over a rounded trailing edge. The CC air flow entrains the air around the blade to create more lift. Adding an actuated valve for the blowing slot allows a CC-VAWT to control the amount of lift generated, as well as the location of the augmentation relative to the wind direction, resulting in augmented power generation. In order to study the performance capabilities of a CC- VAWT, a NACA0018 blade was modified to incorporate circulation control. This modified shape was analyzed using computational fluid dynamics at two Reynolds numbers and a wide range of angles of attack. The lift to drag ratio of the CC- VAWT blade shows benefits at low Reynolds numbers over a NACA0018 blade for post stall angles of attack, but there is a decrease in the lift to drag before stall due to a significant increase in drag of the circulation control models. Further CFD refinement and experimental investigations are recommended to validate the predicted effects circulation control will have on the performance of a VAWT. INTRODUCTION Demand for energy is ever increasing as the world population continues to grow. Wind turbines are capable of alleviating the increase in demand by providing a renewable source of power by capturing wind energy and converting it into electricity. Wind turbines can be classified by the force which drives the rotation of the turbine blades. Hence, there are both lift and drag types of wind turbines. Modern wind turbines use aerodynamic lift as the driving mechanism for their blades through the air [1]. Drag devices use the force of the wind striking the turbine blade to push the rotor about its axis, generating torque. Lift devices are nearly two times more efficient at capturing the energy in the wind relative to the turbine s frontal area [1]. Wind turbines can also be classified into two orientations, based on the axis with which the blades rotate. The most common orientation is the Horizontal Axis Wind Turbine (HAWT), which is similar to a propeller. The other type is the Vertical Axis Wind Turbine (VAWT), an example of which is shown in Figure 1. The vertical orientation of a lift driven turbine has two distinct advantages over a HAWT. First, unlike conventional HAWT s, which need to be reoriented as the wind direction varies, VAWT s are operational in winds from all directions. Second, the VAWT places the generator, gearing, and other power equipment near the ground, making service repairs convenient and safe. However, the same equipment on a HAWT is mounted atop a tower, often hundreds of feet in the air, causing maintenance to be difficult, dangerous, and thus costly. 1 Copyright 2009 by ASME

of the wing without physically altering the airfoil, resulting in increased lift [6]. This paper is focused on quantifying the improvement in lift of the airfoil through the use of circulation control. One of the common properties of jet flow is the blowing coefficient. This is the ratio of mass flow of the blowing jet to the free stream air and is defined by Equation (1) [6]. C tv 2 j j (1) 2 0.5 cv Figure 1: Vertical Axis Wind Turbine [2] The purpose of this research effort was to model a CC- VAWT blade in a computational fluid dynamics (CFD) environment. This blade has not been extensively analyzed for use in vertical axis wind turbines, suggesting a need to generate full angle of attack lift and drag information. In order to validate the results of the CFD simulations, a NACA0018 without circulation control will be used as a comparison to quantify the CC-VAWT blade s enhanced performance [3]. CIRCULATION CONTROL Previous research and development efforts have been devoted to the application of circulation control on the turbine blades of VAWTs [4,5], but were left unfinished. These efforts which investigated circulation control by removing the sharp trailing edge of a conventional airfoil and replacing it with a rounded surface and using pressurized air injected into the boundary layer around the airfoil to control the flow field. Blowing slots can be incorporated on both the upper and lower airfoil surfaces, at the apex of the rounded trailing edge. The pressurized air exiting the slots, entrains the free stream air which increases the momentum traveling over the airfoil. The jet naturally follows the rounded trailing edge via the Coanda effect [6]. The Coanda effect is the tendency of flow to stay attached to the boundary layer and follow the direction of a smoothly rounded edge. As a result, this form of circulation control acts in part as boundary layer control, delaying separation of flow and deflecting the flow across the blade s upper surface downward. As the flow is deflected downward, the virtual camber of the blade is increased, similar to the effect of a mechanical flap used on airplanes [2]. This enhancement in the airfoil s apparent camber provides a significant increase in lift with only a minimal drag penalty. The goal of applying circulation control to a VAWT s blades is to augment the wind speed operating range, ultimately improving the power generated. Circulation control is an attractive enhancement to turbine blades because it essentially enlarges the effective area The blowing coefficients were examined in the range of 0% to 10%. The blowing coefficient was limited to this range because higher amounts of augmentation would require excessive levels of power to achieve the desire results, thus not justifiable [7]. The computational model, shown in Figure 2a and 2b, was designed with a semicircular trailing edge, which allows for the creation of blowing slots which were created on the upper and lower surfaces. Figure 2a: Model of the CC-VAWT Blade Blowing Slot Figure 3b: Trailing Edge of Model with Upper and Lower Blowing Slots The CC-VAWT blade computational model was created with a trailing edge radius to chord length ratio of 0.0169 [8]. 2 Copyright 2009 by ASME

In addition the slot heights have a height to chord length ratio of 4.17x10-4. These non-dimensional values were shown to have the optimal performance after a comparison of differing dimensions on this same trailing edge radius ratio [9]. CFD CC-VAWT BLADE EVALUATION During rotational startup or low tip speed ratio, (λ < 1), a VAWT could encounter angles of attack ranging from negative 180 to positive 180 degrees [10]. As a VAWT reaches its rotational operating range, at a tip speed ratio between 4 and 6, the blade angle of attack drops significantly to within stall of the blade. Evaluation of the blade performance was initially evaluated at two Reynolds numbers, which will represent startup and normal operating speed, 40k and 360k respectively. The full range of angle of attack was examined for low speed scenario, while the linear and stall regions for normal operating speed. CFD SETTINGS The design and meshing of the model was performed in the Gambit 2.4.6 software package [11]. The model was designed with a unit chord length to allow for scaling for future prototypes. The meshed model of the CC-VAWT was created using an unstructured triangular mesh scheme with approximately 1,017,000 cells. To evaluate the computational model, Fluent 6.3.26 [12] was used with a Full Reynolds Stress turbulence scheme, an ideal gas density solver and 2 nd order solution controls. Three grid adaptations were applied to each mesh; a region, boundary and velocity gradient. These were applied to allow a greater concentration of nodes near critical areas around the airfoil, such as the area where the air is undergoing rapid changes such as the circulation control blowing jets along the trailing edge. Each of the cases were then solved to between 4 th and 5 th order convergence, which took approximately 30,000 iterations. A pressure inlet was used upstream of the airfoil and for both the upper and lower blowing slots. A velocity of 2.9 m/s was used at the inlet for the 40k Re case. The velocities produced by the slots at a Re of 40k are 10.4 m/s and 39 m/s for a C μ of 1% and 10% respectively. The free stream velocity for the Re 360k case was 26 m/s with velocities produced by the blowing slots of 111.6 m/s and 382.2 m/s for the 1% and 10% blowing coefficients respectively. Walls were used both above and below the airfoil but placed approximately 10 chord lengths from the airfoil as not to interfere with flow development. In addition, a pressure outlet was used downstream of the airfoil. Streamlines The effect of circulation control can most easily be identified by inspecting the streamlines of the flow around the airfoil. For the CC-VAWT blade with no blowing the streamlines are relatively uniform experiencing little circulation as shown in Figure 4, indicating a low lift coefficient. These streamlines are relatively similar to those that would be seen around an unmodified NACA0018. Figure 4: Streamlines of the CC-VAWT at C μ of 0% Figure 5 and Figure 6 show the streamlines of the flow around the CC-VAWT for a C μ of 10%. These figures visualize the increased circulation around the airfoil due to blowing, resulting in an augmented lift. The most noticeable difference is that the streamline near the blowing slot is attached to the rounded trailing edge, approximately 100 degrees from the exit plane of the jet. Figure 5: Streamlines of the CC-VAWT at C μ of 10% RESULTS The computational results of this investigation have been divided into a comparison of the streamlines, low speed and high speed analysis which correspond to a λ of approximately 1 and 5. These tests will be run at a Reynolds number of 40,000 and 360,000. 3 Copyright 2009 by ASME

Figure 6: Trailing Edge View of the Streamlines of CC-VAWT at C μ of 10% Low Speed The lift and drag coefficients for low speed simulation were predicted for every three degrees angle of attack from -21 to 21 degrees, and every six degrees from 21 to 339 degrees for a Reynolds number of 40,000, which simulates a tip speed ratio, λ < 1. The coefficients of lift and drag were compared to an unmodified, i.e. a traditionally pointed trailing edge, NACA0018 experimental airfoil data produced by Sandia National Laboratories [3]. The lift coefficients for the angles of attack from 0 to 360 degrees are shown by Figure 7. The lift curves are for blowing coefficients of 0%, 1% and 10%, and compared to the experimental data at a Reynolds number of 40,000. A close up of the lift curves are shown in Figure 8 for angles of attack from 0 to 21, which highlights the increase in lift production with increased blowing over the NACA0018 [3]. As seen in Figure 8, the non-blown case follows the experimental data at low angles of attack, less than 6 degrees. Above which the modified trailing edge influences the aerodynamic characteristics of the airfoil, without circulation control blowing. Figure 8: Lift Coefficient for Re of 40k 0 to 21 Degrees An increase in blowing provides a noticeable increase the lift coefficients. Figure 9 shows the drag coefficients for the full range of angles of attack at each blowing coefficient. As shown in Figure 9, the drag coefficient is greater than the NACA0018 for angles near 0 and 180 degrees. Above approximately 30 degrees angle of attack, the drag produced by the circulation controlled airfoils becomes less than the NACA0018. The drag coefficient which occurs from 180 to 360 degrees angle of attack was predicted to be lower than the 0 to 180 degree range due to the direction of blowing with respect to the wind speed. The blown jet interacts with the free stream air differently resulting in a reduced drag. Another cause of the discrepancy is the potential under prediction of separation, and consequent difficulty in predicting drag by the CFD model which has been shown to occur [13]. Figure 10 is a close up view of the drag coefficients from 0 to 21 degrees angle of attack and shows a jump in the drag coefficient values from the 1% blowing coefficient case to the 10% blowing coefficient at pre-stall locations. Figure 9: Drag Coefficient for Re of 40k - Full Range Figure 7: Lift Coefficients for Re of 40k - Full Range A comparison of lift to drag for the CC-VAWT is shown in Figure 11 from 0 to 21 degrees angle of attack and in Figure 12 for the full angle of attack range. The NACA0018 has a large 4 Copyright 2009 by ASME

peak in the lift to drag ratio between 0 degrees and approximately 12 degrees. Beyond which the circulation controlled airfoils have a greater lift to drag ratio than the NACA0018 airfoil. The circulation control airfoils currently have a higher drag up to 12 degrees than the NACA0018 [3] accounting for their lower lift to drag ratios. This increased drag coefficient could be addressed by utilizing both the upper and lower blowing slot to create a blown trailing edge. The lift to drag ratios for the circulation control airfoils, shown by Figure 12, follows that of the unmodified experimental data except for the regions near stall. Figure 11 and Figure 12 show that there is an improvement in the lift to drag ratio past stall, with a decrease in the lift-drag ratio between about 1 to 11 and 165 to 195 degrees. Figure 12: Lift to Drag Ratio for Re of 40k - Full Range Figure 10: Drag Coefficient for Re of 40k - 0 to 21 Degrees High Speed The lift and drag coefficients for Re of 360,000 simulations were also predicted every three degrees in angle of attack from 0 to 18 degrees. This Reynolds number simulates a tip-speedratio of the turbine of approximately five. Figure 13 plots the lift coefficients versus angles of attack for a Re of 360,000 for the various blowing conditions over 0 to 18 degrees angle of attack, which shows the anticipated increase in the lift coefficient that corresponds to an increase in the blowing coefficient. Figure 11: Lift to Drag Ratio for Re of 40k - 0 to 21 Degrees Figure 13: Lift Coefficient for Re of 360k - Full Range Figure 14 is a plot of the drag coefficient versus angle of attack for the 360,000 Reynolds number cases, similar to the low Re cases a significant increase in the drag coefficient corresponds to an increase in blowing coefficient. In looking at a force balance for the vertical axis turbine this drag increase has a negative effect on performance and therefore it is not desirable at operational tip speed ratios. 5 Copyright 2009 by ASME

altering the aerodynamic characteristics depending on the location of the turbine blade relative to the direction, and speed of the wind. Overall this work is a starting point for the verification that a CC-VAWT blade could provide more torque when used on a VAWT than a NACA0018 blade and possible improve the efficiency of the VAWT. An experimental investigation into circulation control is needed for validation, and improvement to the computational prediction techniques. The current computational model shows an increase in lift due to circulation control which can be used for estimation of potential improvement to lift data of a NACA0018. Figure 14: Lift Coefficient for Re of 360k - Full Range Figure 15 shows the lift to drag ratio at a Re of 360,000 versus an angle of attack from 0 to 18 degrees and reveals the detrimental effects of the increased drag on the performance of the airfoil in these conditions. Comparing the CC-VAWT blade to the NACA0018, the un-augmented airfoil has a significant advantage over the circulation control airfoils under constant blowing conditions. Figure 15: Lift to Drag Ratio for Re of 360k - 0 to 18 Degrees CONCLUSIONS AND RECOMMENDATIONS While it may not be effective to utilize circulation control for the entire range of λ for a VAWT [14], the use of circulation control could be beneficial at low rotational speeds, and assist start-up. Further analysis of CFD predicted lift and drag data at more tip speed ratios could provide improved performance predictions. The current CFD modeling shows that there is a decrease in the lift-drag ratio for the circulation controlled airfoils pre-stall. This signifies that constant circulation control may not be beneficial to the performance of a VAWT. Previous research signifies that further study of the circulation control application is needed to determine the optimal characteristics for the current application [7,8,15,16]. Furthermore, experimental validation of the computational data is also needed. Investigation into the use of a cyclic blowing scenario is also seen as a potential solution to the lift-to-drag ratio by REFERENCES [1]. Gipe, P., Wind Energy Comes of Age. John Wiler and Sons, 1995, ISBN 047110924X, 9780471109242. [2]. Wolfe, W., Analysis of Test Results for the WVU Straight Bladed Darrieus Wind Turbine, Thesis, West Virginia University, Morgantown, WV, 1978. [3]. Sheldahl, R. E., Kilmas, P. C., Characteristics of Seven Airfoil Sections Through 180 Degrees Angle of Attack for Use in Aerodynamic Analysis of Vertical Axis Wind Turbines, Alburquerque, NM: SAND80-2114, Sandia National Laboratories, 1981. [4]. Fanucci, J., Walters, R., Innovative Wind Machines: The Theoretical Performances of a Vertical Axis Wind Turbine, In Proc. Of the VAWT Technology Workshop, Sandia Lab. Report SAND 76-5586, 1976. [5]. Walters, R., Innovative Wind Machines, West Virginia University, 1975. [6]. Gibbs, E. H., Analysis of Circulation Controlled Airfoils, Ph.D Dissertation, Department of Aerospace Engineering, West Virginia University, Morgantown, WV, 1975. [7]. Englar, R. J., Two-Dimensional Subsonic Wind Tunnel Tests of Two 15-Percent Thick Circulation Control Airfoils, August 1971, Naval Research and Development Center Report 212-204, Technical Note AL-211. [8]. Angle, G., Aerodynamic Benefits of Near-Surface- Actuated Circulation Control Blowing Slots for Rotorcraft Use, Ph.D. Dissertation, West Virginia University, Morgantown, WV, 26506, 2008. [9]. Graham, H. Z., Panther, C., Meagan, H., Wilhelm, J., Angle, G. M., Smith, J. E., Airfoil Selection for a Straight Bladed Circulation Controlled Vertical Axis Wind Turbine, ASME Proceedings of ES2009 Conference, ES2009-09343, San Francisco, CA, July 19-23, 2009. [10]. Strickland, J., Webster, B., Nguyen, T., A Vortex Model of the Darrieus Turbine: An Analytical and Experimental Study, Sandia National Labs Report SAND79-7058. [11]. ANSYS, Inc. (2009). Gambit 2.4.6. Retrieved from http://www.ansys.com/products/default.as [12]. ANSYS, Inc. (2009). Fluent 6.3.26. Retrieved from http://www.ansys.com/products/default.as [13]. Smith, J. L., Graham, H. Z., Smith, J. E., The Validation of an Airfoil in the Ground Effect Regime Using 2-D 6 Copyright 2009 by ASME

CFD Analysis, 26 th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, Seattle, WA, 2008-4262, June, 23-26, 2008. [14]. Trevelyan, C., Application of Circulation Control Aerofoils to Wind Turbines, Ph.D. Dissertation, Loughborough University, Nov., 2002. [15]. Baker, W. J., Simulation of Steady Circulation Control for the General Aviation Circulation Control (GACC) Wing, Proceedings of the 2004 NASA/ONR Circulation Control Workshop, Hampton, VA, pp. 819-829, Mar. 16-17, 2005 [16]. Wilkerson, J. B., An Assessment of Circulation Control Airfoil Development, David Taylor Naval Ship Research and Development Center, Bethesda, MD, Report 77-0084. 7 Copyright 2009 by ASME