Boundary layer tripping studies of compressible dynamic stall flow

Abstract

The challenging task of properly tripping the boundary layer of a leading-edge-stalling airfoil experiencing compressible dynamic stall at Reynolds numbers between 3.6 X 10 s and 8.1 X 10 s has been addressed. Real-time interferometry data of the flow over an oscillating airfoil have been obtained at freestream Mach numbers of 0.3 and 0.45. The airfoil was tripped by separately placing five different trips of varying lengths near the leading edge. The trip heights ranged from 40 to 175 y^m. The resulting flow and airfoil performance were evaluated using the criteria of elimination of the laminar separation bubble that otherwise forms, delay of dynamic stall onset to higher angles of attack, and production of consistently higher suction peaks. Quantitative analysis of the interferograms showed that the laminar separation bubble 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 height was found to compare reasonably with the airfoil boundary-layer thickness at the dynamic stall vortex formation angle of attack, at a location slightly upstream of the vortex origin in the adverse pressure gradient region.

Methods to control dynamic stall for wind turbine applications

“…For the smaller height of 1%c, the stall process is actually hastened for both pitch rates. This premature stall might be caused by a thickening effect of the boundary layer, similar to that observed for large trip heights [58]. In this case, it appears that the DSV is not formed at all and the airfoil seems to sustain a constant lift during the pitch-up process.…”

Dynamic- and post-stall characteristics of pitching airfoils at extreme conditions

“…For example, Lorber and Carta, 7 Dimmick, 8 Jumper and Dimmick 9 and Jumper and Schreck 10 investigated the changes in the pitch rate and pitch axis, where it was demonstrated that increasing the reduced frequency, κ=ωC/2U∞ (where ω is the angular velocity, C is the airfoil chord and U ∞ is the freestream velocity) and moving the pitch axis aft of the leading edge resulted in greater lift coefficients and increased angles of attack prior to deep-stall conditions. Likewise, other studies detailed the effect of performance parameters including Reynolds number, 11,12 Mach number 13–15 and airfoil geometry. 16,17 These primary studies all shed light on the characteristics of dynamic stall, with their focus predominantly based upon the mitigation of the dynamic-stall force production about rotor blades of helicopters where pitch angles are low and the pitch angle varies sinusoidally.…”