Separation control on a thick airfoil with multiple slots blowing at small speeds

Using multiple slots blowing at small speeds to efficiently control boundary-layer separation on a thick airfoil is studied. The method of numerically solving the Reynolds averaged Navier-Stokes equations was used for the study. The following results were obtained. For multiple slots blowing at small speeds, the wall jet from each slot does not decay very fast (unlike the single slot blowing at the same momentum rate, the wall jet of which has much larger speed) and the near-wall momentum is continuously reinforced by the blowing slots sequentially located on the airfoil upper surface; as a result, under the same momentum blowing rate, multiple slots blowing at small speeds control the boundary layer separation more effectively than single slot blowing at large speed. Moreover, the power expenditure of multiple slots blowing at small speeds is much smaller than that of single slot case. Therefore, using multiple slots blowing at small speeds, high lift and high efficiency (as measured by equivalent lift-drag ratio) can be achieved at the same time. For the multi-slotted thick airfoil (40 percent thickness, 10 slots) considered in this paper, with the blowing total pressure the same as the free-stream total pressure, high lift (C_L = 2.3) and large equivalent lift-drag ratio (almost 40) were realized; with a slightly larger blowing total pressure, even higher lift (C_L = 3.2) but slightly smaller equivalent lift-drag ratio (about 36) were obtained.