Rotating air scoop as airfoil boundary-layer control

Abstract

C CL = airfoil chord = lift coefficient = maximum coefficient of lift CP = pressure coefficient Re = Reynolds number U = freestream velocity Uc = cylinder surface velocity X = distance along chord a. = angle of attack Introduction T a recent study, Mokhtarian and Modi have assessed moving surface boundary-layer control effectiveness with reference to a symmetrical Joukowsky airfoil modified with a leading-edge rotating cylinder. Results of the test program and the numerical models suggest the following: 1) The numerical scheme, accounting for wall confinement and separated-flow effects, gives useful information concerning moving surface boundary-layer control. The predicted pressure distributions are in good agreement with experiment almost up to the point of complete separation from the airfoil surface except near the trailing edge where more accurate results of the flowfield would require the modeling of the separated-flow region using the full Navier-Stokes equations. 2) The concept of moving surface boundary-layer control appears to be quite promising. In general, the leading-edge rotating cylinder extends the lift curve without substantially affecting its slope, thus effectively increasing the maximum lift and delaying stall. This Note represents an extension of the preceding study and investigates the effect of the leading-edge cylinder geometry. A symmetrical Joukowsky model, of «15% maximum thickness-to-chord ratio, is considered with three different configurations of the leading-edge cylinder. These include a) a solid circular cylinder, b) a scooped cylinder, and c) a reversed scooped cylinder (see Fig. 1). The cylinders were designed for clockwise rotation to inject momentum into the upper-surface boundary layer. Configuration b) was designed as an air scoop to enhance the cylinder's effect in displacing the air. It would slow down the flow over the lower surface and redirect more flow over the upper surface. Configuration c, on the other hand, was designed as a vortex generator. The relatively large angles of attack used in the experiments result in a considerable blockage of the wind-tunnel test section, from 21% at a = 30 deg to 30% at a = 45 deg. The wall confinement leads to an increase in local wind speed, at the location of the model, thus resulting in an increase in aerodynamic forces. Several approximate correction procedures have been reported in the literature to account for this effect. However, these procedures are applicable mostly to streamlined bodies with attached flow. A satisfactory procedure applicable to bluff bodies with large blockage is still not available.