Numerical Investigation of a Low Aspect Ratio Flat Plate.

摘要

This report presents the key findings resulting from an AFOSR grant. In this reseasrch we extended our previous results on the linear pitch-ramp- return of a two-dimensional (2D) flat plate to that of a three-dimensional (3D) flat plate of various aspect ratios, in an attempt to quantify 3D effects and the boundary effects on flapping wing aerodynamics. Specifically we compared the leading edge vortex (LEV) between the two-2D and the corresponding 3D cases, and the vortex structure and force histories between a small and a large computational domain. The computational approach solved the incompressible Navier-Stokes equations using a Poisson-like method on overlapping grids. In the computation, the impact of domain size and the presence of walls (as in the experiment) were investigated in particular detail, as earlier work on high- frequency pure-plunge cases has shown remarkable sensitivity of forces to domain size, as well as large differences between wall-bounded and unbounded flows. Comparisons were made with experiments performed at the Air Force Research Laboratory at Wright Patterson. Our study showed that a smaller domain leads to higher lift and drag coefficients on the flat plate. The sinusoidal motion has an earlier stall than the linear-ramp motion. Non-circulatory loads were found to be localized to phases of motion where acceleration is large. The low aspect ratio wing was seen to have slight difference in leading edge vortex evolution relative to that of the two-dimensional case.