The mode B structure of streamwise vortices in the wake of a two-dimensional blunt trailing edge

Abstract

The structure of streamwise vortices that arise due to secondary instabilities in the wake of a two-dimensional blunt body with a chord-to-thickness ratio of 12.5 was investigated using high-speed stereoscopic particle image velocimetry. Reynolds numbers spanning an order of magnitude from to 25 800 were considered, where is the height of the blunt trailing edge. A modified two-dimensional -criterion ( ) was applied to identify the streamwise vortices. The wavelength of the streamwise vortices, defined as the spanwise distance between adjacent streamwise vortex pairs in the wake, was investigated by applying an autocorrelation algorithm to snapshots of . The most probable wavelength was found to range from to with increasing , and the mean wavelengths increased from to . These wavelength values appeared to increase asymptotically. Visual inspection and cross-correlation analyses based on showed that the streamwise vortices maintain their directions of rotation during primary shedding cycles. The latter analysis was carried out at low because of a large amount of wake distortion and the absence of time-resolved data at high . The characteristics of the streamwise vortex structure found here match those of mode B, which, at similar , dominates the wakes of circular and square cylinders and has also recently been shown to exist in the wake of an elongated blunt body with a larger chord-to-thickness ratio of 46.5 (Gibeau et al., J. Fluid Mech., vol. 846, 2018, pp. 578–604).