Abstract
The effects of trailing edge shape on the vortex dynamics and surface pressure distributions on pitching foils are examined using direct numerical simulation. Results are presented for a Reynolds number of and a Strouhal number of 0.5, which corresponds to the Strouhal number for maximum efficiency as found by experiment [Van Buren et al., Phys. Rev. Fluids 2, 014702 (2017)]. For the trailing edge shapes studied here (square, convex, and concave), the maximum instantaneous thrust occurs at the moment when the vortex detaches from the trailing edge. At the same time, the surface pressure gradients across the panel surfaces increase, while viscous forces drop. The leading edge vortices are seen to merge with the side edge shear layers, and as they wrap around the trailing edge the thrust generation is depressed just prior to the instance of maximum thrust. This process, combined with momentum transfer associated with switching the direction of pitching, produces irregular variations in the thrust and side forces. Compared to the square panel, the wake of a concave trailing edge panel exhibits strong three-dimensional effects on its side edges leading to slower wake contraction, smoother vortex bending, and lower thrust. In contrast, the convex trailing edge panel allows detaching structures to grow longer prior to their detachment, resulting in bent structures formed closer to the trailing edge, which leads to faster wake contraction and higher thrust. Skeleton models of the wakes help explain their underlying structure.
11 More- Received 25 July 2018
DOI:https://doi.org/10.1103/PhysRevFluids.4.033101
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