The turbulence structure of the wake of a thin flat plate at post-stall angles of attack

Abstract

The influence of post-stall angles of attack, $\alpha$, on the turbulent flow characteristics behind a thin high aspect ratio flat plate was investigated experimentally. Time-resolved stereo particle image velocimetry was used in an open-section wind tunnel at a Reynolds number of 6600. The mean field was determined along with the wake topology, force coefficients, vortex shedding frequency, and the terms in the transport equation for the turbulent kinetic energy k. Coherent and incoherent contributions to the Reynolds stress and k-transport terms were estimated. Over the measured range of $20^\circ \le \alpha \le 90^\circ$, quasi-periodic vortex shedding is observed and it is shown that most of the fluctuation energy contribution in the wake arises from coherent fluctuations associated with vortex shedding. As the angle of attack is reduced from $90^\circ$, the length of the recirculation region and the drag decrease, while the shedding frequency increases monotonically. In contrast, mean lift and k are maximized at $\alpha \approx 40^\circ$, suggesting a relationship between the bound vortex circulation and the levels of k. Structural differences in the mean strain field, wake topology, relative contributions to the k-production terms, and significant differences in the incoherent field suggest changes in the wake dynamics for $\alpha > 40^{\circ }$ and $20^{\circ } \le \alpha \le 40^{\circ }$. For $\alpha > 40^\circ$, coherent contributions to the fluctuation field result in a large region close to the plate exhibiting small levels of negative mean production and generally low levels of advection, despite very high levels of production just downstream of the recirculation region.