Unsteady behaviors of separated flow over a finite blunt plate at different inclination angles

Abstract

This study comprises an extensive analysis of unsteady behaviors of a separated flow over a finite blunt plate at three different inclination angles, θ = 0°, 3°, and 6°. It was found that these three distinctly different flow patterns resulted from increasing inclination angles: reattachment, intermittent reattachment, and non-reattachment. The separated flow fields and wall-pressure fluctuations were experimentally measured with planar particle image velocimetry (PIV) operating at 1 Hz and a microphone array sampling at 5 kHz, respectively. Flow patterns were discussed in terms of the time-averaged flow fields and distributions of the statistical quantities (i.e., the reverse-flow intermittency, or velocity fluctuation intensity). A slender separation bubble formed in the front area of the plate (0 < x/D < 4.6) in the system where θ = 0° and then it enlarged to the whole surface of the plate in the system where θ = 3°. In contrast, in the system where θ = 6°, the plate surface was entirely engulfed by a large recirculation zone extending to the near-wake region. In the wall-pressure fluctuation analysis, two characteristic frequencies, St = 0.036 and 0.107, could be readily identified in all three systems; these corresponded to the flapping of separation bubble and the shedding of large-scale vortical structures, respectively. In addition, in the system where θ = 0°, wall-pressure fluctuations of the Karman vortex were detected at St = 0.154 but were suppressed in the systems where θ = 3° and 6° due to the extensive interaction between the shedding of large-scale vortical structures and the unsteady wake. Subsequently, a field-programmable gate array taking full advantage of dynamic mode decomposition (DMD) on the wall-pressure fluctuations was constructed, and a real-time conditional signal corresponding to individual unsteady behavior was generated to fire the phase-locking PIV measurement. High-resolution spatiotemporal evolutions of dominant flow behaviors (i.e., enlargement-and-shrinkage motion of the separation bubble and shedding motion of the large-scale vortical structures) were determined. In the system where θ = 6°, a separation bubble enlarged and shrank dramatically together with the shedding of large-scale vortical structures, leading to a large recirculation zone over the blunt plate, distinct from the behavior in the systems where θ = 0° and 3°. Finally, a joint dominant mode analysis of flow structures and wall-pressure fluctuations was further evaluated, which delineated the complex unsteady processes in separated flow clearly and provided more information and references for other studies on wind engineering, fluid-induced structure vibrations, and acoustic emissions.