BOUNDARY-LAYER-TRIPPING STUDIES OF COMPRESSIBLE DYNAMIC STALL FLOW

The challenging task of properly tripping the boundary layer of a lead ing-edge-stalling airfoil experiencing compressible dynamic stall at R eynolds numbers between 3.6 x 10(5) and 8.1 x 10(5) has been addressed . Real-time interferometry data of the flow over an oscillating airfoi l have been obtained at freestream Mach numbers of 0.3 and 0.45. The a irfoil was tripped by separately placing five different trips of varyi ng lengths near the leading edge. The trip heights ranged from 40 to 1 75 mu m. The resulting flow and airfoil performance were evaluated usi ng the criteria of elimination of the laminar separation bubble that o therwise forms, delay of dynamic stall onset to higher angles of attac k, and production of consistently higher suction peaks. Quantitative a nalysis of the interferograms showed that the laminar separation bubbl e was still present with the smallest trip and premature dynamic stall occurred with the largest trip. The right trip was determined to be a distributed roughness element extending from 0.5 to 3% chord. Its hei ght was found to compare reasonably with the airfoil boundary-layer th ickness at the dynamic stall vortex formation angle of attack, at a lo cation slightly upstream of the vortex origin in the adverse pressure gradient region.