Separated flow over blunt plates with different chord-to-thickness ratios: Unsteady behaviors and wall-pressure fluctuations

Abstract

This study presented an extensive analysis of the influence of chord-to-thickness ratios on unsteady behaviors of the separated flow and their coupling with wall-pressure fluctuations at the Reynolds number ReD=1.58×104. Three systems with different chord-to-thickness ratios were chosen for comparison: L/D=3.0, 6.0, and 9.0. The separated flow fields and wall-pressure fluctuations were measured using planar Particle Image Velocimetry (PIV) operating at 1Hz and a microphone array sampling at 1kHz, respectively. The distributions of the turbulent kinetic energy and the reverse-flow intermittency indicated that the shortest plate (L/D=3.0) was subjected to full coverage of the separated flow. In the moderate system (L/D=6.0), the time-averaged reattachment station was determined to be x/D=4.75, while in 7% of all realizations the fluid close to the trailing edge surface remained slightly reversed due to the unsteady reattachment of the separated shear layer to the trailing edge and to irregular trailing edge vortex shedding. However, the separated flow always reattached on the plate’s surface in the longest system L/D=9.0. The measured wall-pressure fluctuations demonstrated combinative influences of hydrodynamic pressure of the large-scale vortical structures above the plate and acoustic excitation propagated by the energetic unsteady wake. This resulted in the salient phase changeover characteristics of the correlative patterns near x/D=1.5 in the shortest system, L/D=3.0. Close to the leading edge, the wall-pressure fluctuations in the L/D=6.0 system were over-exposed to acoustic excitation propagated by the unsteady wake behind the trailing edge, while in the longest system, L/D=9.0, such influences were relatively attenuated. Proper Orthogonal Decomposition (POD) analysis was performed on the down-sampled wall-pressure fluctuations paired with the flow fields, resulting in a series of energetic flow structures and their corresponding wall-pressure fluctuation profiles. Subsequently, the conditional averaging technique was applied to determine the relationship between the spatial characteristics of the large-scale vortical structures and profiles of the wall-pressure fluctuations.