Flow Separation Control with a Truncated Ellipse Airfoil in Cycling Aerodynamics WONG HSI, J. J. MIAU Department of Aeronautics and Astronautics (DAA), National Cheng Kung University, Tainan, Taiwan (R.O.C) Research Center for Energy Technology and Strategy, National Cheng Kung University Tainan, Taiwan (R.O.C) INTRODUCTION. In competitive cycling, aerodynamic resistance is always an issue. In order to gain aerodynamic advantages and structural stiffness of the bike, the teardrop shape is widely used by bicycle manufacturers today. It is a combination of a circular cylinder and symmetric NACA airfoil. Although it does provide drag reduction, this design still suffers from early flow separation and drag penalty at certain yaw angle with Reynolds number in range of 3 x 10 4 to 6 x 10 4. In our previous study [1], we investigated the pressure distribution and flow development on a number of airfoils (Symmetry NACA and Teardrop airfoil) with 2D Computational Fluid Dynamics (CFD), then we obtained a new airfoil design called the Truncated Ellipse (TE). This design is based on base drag theory [2], by technically cutting off the tail of an ellipse to create an insulating boundary layer at the rear of the TE airfoil shows a much lower drag coefficient and flow separation delay compared with Symmetry NACA and Teardrop shape airfoil. In this study we continue our research with 3D CFD simulation and flow visualization experiment on flow pattern around TE and Teardrop shape airfoil. The experimental method includes water channel flow visualization and wind tunnel oil film method. Experimental model is a teardrop shape airfoil and a TE airfoil with the same chord length and width, both of the airfoil models were printed by a 3D printer with 350mm height. Using a dye-injection technique and the method of Particle Image Velocimetry (PIV) in a water channel and the oil-film method in a wind tunnel, we are able to visualize the flow pattern corresponding to the two airfoil models, without actual force measurement in the wind tunnel. Subsequently we compare our CFD simulations with the experimental results. The simulations and experimental results show that the TE has flow separation occurred near the base of the airfoil at a 0 degree yaw angle and delayed flow separation at low yaw angles (<15 degrees) due to the development of a separation bubble near the leading edge. The yaw angle denotes the angle of the wind relative to the bicycle, equivalent to the angle of attack to the airfoil model mentioned in the present paper, (the separation bubble increase the kinetic energy of the flow near the surface and push the separation point further downstream). This explains why the TE airfoil model has much better aerodynamic performance than the teardrop shape. Our research has implications for the future prospects of aerodynamic bicycle design.
Numerical Method. The airfoil discussed in this study is a Truncated Ellipse (TE) and a Teardrop airfoil, both have the same thickness T=20mm and chord length C=50mm. TE is evolved by cutting 29mm away from an ellipse with a chord length C=79mm to form a flat base. CFD simulations were conducted using the commercial finite volume solver FLUENT 14.0, in viewing that the flow velocities considered studied here is lower than (M<0.3), the pressured-based type is selected and allowed to implement the incompressible flow assumption. As our major concern is the drag coefficient, thus the turbulent model chosen is the K-Omega with SST (Shear Stress Transfer), Low-Re and Curvature Corrections options. Husson [3] conducted a 2D CFD regarding flows over different cross-sectional foils, in his work, he mentioned using the K-Omega SST turbulence model rendered a good precision close to the wall as well as in the wake far from the wall. It also has a good behavior under high adverse pressure gradient and separating flow. In these present study, we conducted 3D CFD simulations to visualize the static pressure gradient on the surface of both airfoil models in order to locate the separation bubble region. The simulations were performed using a hp Z620 workstation which contain 24 CPUs and 40GB RAM. Water Channel Flow Visualization. A closed loop water channel in DAA has a 0.6m 0.6m 2.5m test section with the maximum flow speed 0.4m/s and turbulence intensity less than 1%.Using the methods of dye-injection and Particle Image Velocimetry (PIV) to visualize the flow pattern around TE and Teardrop airfoil, we are able to study the separation region and wake behind them. The Reynolds number of the experiments is about 7200. Figure 1. TE airfoil dye injection method, 0 degree yaw angle.
Figure 2. Dye injection at TE s trailing edge with 0 degree yaw angle. Figure 3. TE airfoil PIV visualization at trailing edge with 0 degree yaw angle.
Figure 4. Dye injection method on teardrop-shape airfoil, 0 degree yaw angle. Figure 5. Teardrop-shape airfoil with PIV visualization at 0 degree yaw angle. WONG HSI, J. J. MIAU 2015
Wind Tunnel Oil Film Method. The open type wind tunnel in DAA has a test section with dimeter 0.5m and 1.5m long. By using oil film method (oil indicator is balck in color) we able to locate separation region and identifive the position of separation bubble. Reynolds number of the experiments performed is about 4.3 x 10 4. Figure 6. Oil film method on TE airfoil with 12.5m/s (45km/hr) flow velocity at 0 yaw angle. Figure 7. Comparison of CFD simulations (2D,3D) and experimental result on TE airfoil, 15 degree yaw.
Fig.8. Oil film method on Teardrop shape airfoil with 12.5m/s flow velocity at 15 yaw angle. Flow Figure 9. Comparison of CFD simulations (2D,3D) and experimental result on Teardrop airfoil, 0 degree yaw.
Separation region Along chord length Figure 10. Wall shear stress on the upstream of TE airfoil at 0 yaw angle. 1 st separation 2 nd separation Figure 11. Wall shear stress on the upstream of TE airfoil at 15 yaw angle.
RESULT AND DISCUSSION. We fully aware that the Reynolds number of water channel and wind tunnel cannot reached dynamic similarity. But according to Erickson Gary [4], even though the Reynolds number in water channel is lower, the fundamental structure of the flow bears the similarity, regardless of the difference of the Reynolds numbers of the two experimental facilities. With the dye-injection method in water channel we could observe that the TE model is characterized with separation region at the base of the airfoil (see Fig.2). The formation of insulating boundary layer at the rear of TE airfoil separates the other flow and flow behind TE create a dead space (or virtual tail) [2] as shown in Fig.3. This flow pattern in water is very similar to our CFD simulations in air. CFD simulations also shows the occurrence of the separation bubble at the leading edge when yaw angle reaches 10 to 15 degree, but in the water channel experiment the separation bubble doesn t occur and the flow pattern are different from our simulations. As Fig.4 and Fig.5 shows the early flow separation of Teardrop shape airfoil at 0 degree yaw angle, this phenomena is also due to the absence of separation bubble. The absent of separation bubble may be due to the differences of Reynolds number. With wind tunnel oil-film experiment, we found the flow separation of the TE airfoil at 0 yaw angle happened at the base of the airfoil (present at Fig.6), this result is consistent with CFD simulation and water channel experiment. Figure 10 shows the wall shear stress on the upstream of TE airfoil at 0 yaw angle, this indicate the separation occur at approximately 95% of the chord length of the leading edge. The separation bubble occurs between 10 and 15 degrees yaw angle, the position of the separation bubble matches the CFD results (see Fig.7 and Fig.11). The 3D CFD simulation in Fig.7 and 9 are the static pressure gradient which shows characteristic drop of pressure at the position of the separation bubble, 2D CFD are the velocity gradient which identify the appearance of separation bubble. Fig.9 shows the separation bubble of Teardrop shape airfoil appears at 0 degree yaw angle which delay flow separation, but when the yaw angle increases the early flow separation occurs (see Fig.8). CONCLUSION. In water channel the flow structure around the airfoils are similar only when there are no formation of separation bubbles. In the other hand wind tunnel oil film method can locate the flow separation region and separation bubble position. Even without actual force measurement, we can conclude that TE airfoil has better aerodynamic performance than Teardrop-shape airfoil through CFD simulation studies and oil film visualization verification. ACKNOWLEDGEMENT. Specially thanks to GIANT Bicycle Inc for technical discussions. REFERENCE [1] J. J. MIAU, WONG HSI, S. HUSSON, 2D CFD study of cross-section foils performances in cycling aerodynamics, 16 th international symposium on flow visualization, June 24 2014, Okinawa, Japan. [2] Dr. Sighard F. Hoerner, Aerodynamic Drag, Otterbein Press, Dayton Ohio, 1951, pg 3-18 to 3-22. [3] Sebastien Husson, 2D CFD study of cross-section foils performances in cycling aerodynamics, 2014, pg 40-42.
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