Investigation of 3-D Coherent Structures in the Turbulent Cylinder Wake using

Abstract The wake of a circular cylinder at Re = 2700 is investigated by means of Tomographic PIV (Tomo-PIV), a recently developed three-dimensional velocimetry technique. The paper presents the principles of Tomo-PIV and discusses in details the implementation of the technique to the experimental conditions. With a measurement volume of approximately 40x40x10 mm2 two configurations are chosen in order to describe both the 2D wake features and the span wise organization of secondary rollers dominating the 3D flow organization. The present data allow to directly evaluate the measurement error imposing mass conservation inside the volume (viz. solenoidal velocity field). The visualization of the wake instantaneous structure is obtained by means of vorticity vector magnitude iso-surfaces, or decomposing the vorticity in the span wise and stream-wise(normal)/bi-normal component, which return the separate contribution of the Kármán rollers and the interconnecting structures between them respectively. The properties of the secondary rollers are given in terms of span wise wavelength, peak vorticity, vorticity stretching and their size and shape. A value of the span wise wavelength λz/D = 1.2 is recurring and the rollers organization into pairs is found to be similar to the instability Mode B occurring in lower Reynolds number wakes investigated with previous studies. 1