Numerical study of lift augmentation in massively separated turbulent flows with forcing

At high angles of attack the flow over any lifting body experiences massive separation. The resulting shear layers naturally tend to roll up into large vortical structures. The presence and motion of these structures has a significant impact on the continuing flowfield development. By applying an artificial forcing to the shedding shear layer the rollup can be intensified or reduced, and the entire vorticity field may be altered. Control of these vortical structures by forcing to enhance lift has been the goal of numerous investigations, and some progress has been made in understanding how an unsteady forcing can be used to advantage.

This study describes work done in determining the effectiveness of using computational simulations to model massively separated, turbulent, unsteady flowfields subject to small-amplitude mechanical forcing. Algorithm development and validation highlight certain concerns that arise in the modeling of such unsteady flowfields using the Reynolds-Averaged Navier-Stokes equations, and questions of grid density, numerical diffusion, timestep size, and turbulence modeling are addressed with a suite of sample calculations.

Simulation of flowfields undergoing mechanical forcing by flap is accomplished using an overset grid methodology with the flow solver. Excellent agreement is demonstrated for flow over a NACA 63₃-018 airfoil with a small flap placed near the leading edge separation point. Computed values of the unforced lift and drag coefficients agree with given experimental values to less than 2%. Forcing frequencies for the experimental configuration were in the range of global shedding frequency, and the maximum variation of the aerodynamic coefficients was seen when the forcing frequency was equal to the shedding frequency. Excellent agreement with experiment is also found for the cases involving forcing, and the data is used to explain the effect of forcing in terms of a global receptivity mechanism.