Effects of the Reynolds Number

To assess the effects of the Reynolds number on the flow field and the resulting aerodynamic loading, numerical results were obtained for the same kinematics by Kang et al. [324] for Re = 6 x 104, 3 x 104, and 1 x 104. In this Reynolds number range, the Reynolds number sensitivity to the qualitative flow structures is small. This is in contrast to the flows at lower Reynolds number regimes (i. e., O(102)), where the viscosity plays a more important role than at the current Reynolds number, or for the more conventional SD7003 airfoil with a larger radius of curvature at the leading edge [323].

For the deep stall case, Figure 3.35 shows the time history of force coefficients at Re = 1 x 104, 3 x 104, and 6 x 104. Both lift and drag coefficients are negligibly affected by the Reynolds number variation. For all Reynolds number cases, the lift coefficient reaches its maximum near t/T = 0.25, decreases, and starts to increase from t/T = 0.75.

At these moderately high Reynolds numbers, forces due to pressure that arise from large-scale vortical effects, such as LEVs, dominate over viscous forces. The dependence of the small Reynolds number on the u1/Um profiles shown in

Figure 3.36 and the qualitative similarity in the large-scale flow features (t/T = 0.50 in Fig. 3.36) suggest that the resulting aerodynamic forces are insensitive to the change in the Reynolds numbers. Figure 3.37 shows the time histories of the force coefficient from the numerical computations. At Re = 1 x 104 the maximum lift coefficient is around t/T = 0.25, which is slightly lower (CL max10K = 1.86) than for Re = 6 x

104 (CL, max, 60K = 1.92) and 3 x 104 (CL, max,30K = 1.90). Simllarly the time his­tories of the drag coefficient coincide during the downstroke, whereas for Re = 1 x 104 the drag is slightly greater between t/T = 0.50 to 1.00 than at Re = 3 x 104 and 6 x 104.