On the wake with and without vortex shedding suppression behind a two-dimensional square cylinder in proximity to a plane wall

Abstract

A comparative study of the wake characteristics behind a two-dimensional square cylinder in proximity to a plane wall was made on two systems, i.e. G/ D=0.25 and 0.5, which corresponds to the wakes with and without suppression of the vortex shedding, respectively. Here, G is the gap distance and D is the width of the square cylinder. Synchronized measurements of wall-pressure fluctuations and velocity fluctuations were made using a microphone array and a split-fiber film, respectively. For the system G/ D=0.5, a regular Karman-like vortex shedding at fD/ U 0=0.13 was found by inspection of the streamwise velocity spectra. When the gap width is reduced to G/ D=0.25, a low-speed separation bubble was formed in the region 4.25< x/ D<9.25, while the intermittent existence of large-scale vortices at the reduced frequency fD/ U 0=0.116 was identified in the upper shear layer. The convective vortex dynamics were analyzed in terms of the cross-correlation of the wall-pressure fluctuation, coherence and cross-correlation of wall-pressure fluctuations with velocity fluctuations at various positions located at the lower edge of the cylinder ( y/ D= G/ D) and the outer edge of the upper shear layer. The convection velocities of the Karman-like vortices in the system G/ D=0.5 was determined to be U c / U 0=0.64. However, in the system G/ D=0.25 the convection velocities before and behind x/ D=3.25 were determined to be U c / U 0=0.32 and 0.64, respectively.