Analysis of the wake flow behind concave curved cylinders with velocity measurements by particle image velocimetry and modal decomposition

Abstract

The properties of the wake flow behind concave curved cylinders have been investigated analyzing simultaneously the mean velocity profiles extracted along concentric arcs parallel to the cylinder axis and the dynamically most relevant coherent structures obtained by spectral proper orthogonal decomposition. These analyzes have allowed us to determine the position, the extension, and the evolution of the different wake regimes. The velocity measurements have been obtained through the stereo particle image velocimetry (stereo-PIV) technique. The study considers various Reynolds numbers, based on cylinder diameter (240<Re<840), and curvatures. The quarter-of-ring cylinder is first analyzed. Near the cylinder root, the flow exhibits a topology dominated by an oblique vortex shedding, contrary to what is observed in the literature for similar geometric configurations. We attribute this disagreement to differences in the treatment of the free-end conditions that play a role in triggering the shedding regime. We find that the local shedding inclination is driven by the axial velocity, whereas its extension and wavelength depend mainly on the Reynolds number. The near free-end region presents, instead, two counter-rotating standing vortices that induce a cross-wise velocity directed toward the cylinder root and a tip vortex that expands evolving downstream. As the Reynolds number increases, the wake presents irregularities and the shedding spreads becoming nearly normal to the incoming flow. At smaller curvatures, the cylinder free-end becoming more inclined with respect to the incoming flow, and the free-end effects are enhanced. The interaction with the vortex sheets of the standing vortices that develop at the leeward side weakens. As a consequence, these vortices stretch in the stream-wise direction giving rise to the trailing vortices that stem from the cylinder surface and affect the cross-wise velocity distributions.