Control of flow around a low Reynolds number airfoil using longitudinal strips

Abstract

We suggest longitudinal strips attached to an airfoil surface as a new stall control device. Their effects on the stall characteristics and flow modifications are experimentally investigated. The airfoil considered is SD7003 and the Reynolds numbers are Re = 60 000 and 180 000 based on the chord length and freestream velocity. The drag and lift forces on the airfoil are measured by varying the angle of attack from alpha = 0 degrees to 16 degrees with and without strips. The optimal strip configuration is determined using a response surface method. Without strip, abrupt stalls occur at alpha = 11 degrees and 12.5 degrees for Re = 60 000 and 180 000, respectively, whereas broad stalls occur without much changing the stall angles by optimal strips. The lift coefficient and lift-to-drag ratio are significantly increased by the strips at post-stall angles of attack. A corner vortex is generated at each corner of strip near the leading edge. Clockwise and counterclockwise streamwise vortices are generated at the left- and right-facing corners, respectively, and they slowly move away from the corners while travelling downstream. These vortices provide additional momentum to the airfoil suction surface, resulting in fully attached flow above the strips and reattachment of flow above grooved surface. The longitudinal strips presented here are different from other devices such as the vortex generator, trip wire, burst control plate, and zigzag tapes used for the separation control of low Reynolds number airfoil, in that the strips are installed nearly on the whole airfoil surface but with their heights lower than the boundary layer thickness, and produce positive control effects at post-stall angles of attack but little affect the aerodynamic performance at prestall angles of attack.