Tomographic Particle Image Velocimetry Measurements in a Bluff-Body Wake Flow

The wake flow of a bluff-based cylindrical body, representing a generic model of a rocket configuration, is investigated experimentally for subsonic inflow conditions. This study concentrates on turbulent structures in the shear layer between recirculation bubble and external flow. Tomographic particle image velocimetry was applied to determine the instantaneous velocity distribution of three-dimensional measurement volumes. The results are compared to standard planar particle image velocimetry measurements considering mean velocity and turbulence levels. The availability of volumetric data enables the application of vortex identification schemes based on the velocity gradient tensor del(v), in this case Q- and lambda(Ci)-criterion. The selection of an appropriate onset threshold to separate vortical structures from measurement noise is discussed. A statistical analysis was carried out for different regions of interest, located between 0.2 and 1.6 model diameters downstream of the base plane. The decomposition into axial, radial, and tangential vortex components reveals a distinct spatial evolution of the eddy structure. It can be shown that tangential (or toroidal) eddies, initially induced by the shear-layer instability and, thus, having a preferred sense of rotation, gradually break up into three-dimensional structures without favored rotational direction. The current results agree with recent numerical studies that predict comparable features.