Passive Flow Control of Dynamic Stall via Surface-based Trapped Vortex Generators

A novel passive flow control concept - based on the local modification of an airfoil's surface - is proposed and examined via CFD for the mitigation of the negative effects of dynamic stall, i.e. for the reduction of peak negative pitching moment while not deteriorating significantly the original lift and drag characteristics. 2D CFD simulations of a NACA 0012 airfoil exposed to a freestream of Mach 0.3 and Re = 3.76×10⁶ and undergoing a 15°±10° pitch oscillation with a reduced frequency of 0.101 were conducted. The baseline airfoil simulations were carefully verified and validated, showing excellent agreement with wind tunnel data. Twenty-six different local geometry modifications were proposed and examined, all functioning as a trapped-vortex generator. The surface modifications were examined on both the upper and lower surfaces. In case of the upper surface modifications, the best geometries could reduce the peak negative pitching moment by as much as 37-63%, while sacrificing only 2-10% of peak lift and reducing drag by 14-38%. On the other hand, the lower surface modifications demonstrated the ability to increase lift by 4-16% with only minor penalty in pitching moment and drag.