Volumetric PIV Measurement in the Wake of Wavy Cylinder

Abstract

The three-dimensional laminar flow over a straight cylinder with sinusoidal variation of cross-section was investigated using a volumetric Particle Image Velocimetry at Reynolds numbers of 15,000 and 20,000 and compared with the wake of a straight cylinder. It was observed that the boundary layer separation line was also wavy since the flow separated earlier with saddle-type attachment and later for the case of nodal attachment. Consequently, the wake was narrower behind the nodes and wider behind the saddles. The results were compared with a straight cylinder wake. Spatial characteristics of the flow suggest the presence of streamwise vortices that form midway between the saddle and the node and extend downstream. Interactions of these vortices with the von Karman vortices result in significant distortion of the coherent structures, increase the vortex formation length and suppress the large scale spanwise vortex formation. The Proper Orthogonal Distribution of the PIV data in several planes indicated that the overall energy contents of the wavelength λ/D = 1.6 was lower than the straight circular cylinder.