Investigating Step Cylinder Wake Topology with Tomographic PIV

Step cylinders have numerous applications, for instance, radio communication towers, airport control-towers, antenna members, etc. Numerous applications motivate researchers to investigate this bluff body both numerically and experimentally. The single step cylinder model is comprised of a small-diameter cylinder (d) attached coaxially to a large-diameter cylinder (D). In this study vortex shedding from such a bluff body is investigated experimentally using Tomographic Particle Image Velocimetry (TOMO PIV). Experiments are conducted in the low-speed w-tunnel of the aerospace engineering department of TU Delft. The cylinders' diameter ratio D/d is ranging from 1.14 to 2.67, and the Reynolds numbers based on the large cylinder are ranging from 2000 to 5100. The TOMO PIV system consists of six CCD cameras subtending an arc and a Nd:YAG laser. LaVision Davis 8 is used for image acquisition and processing. The use of TOMO PIV enables quantitative visualization and analysis of the intricate vortex topology and vortex dynamics. The aim of the study is the exploration of the near wake of the step cylinder, focusing on the region of stepwise discontinuity. Considering history of the investigations for this geometry, there is no study available for this range of Reynolds number, and this range of the diameter ratios. For the range of the parameters investigated, turbulent vortex shedding occurs in the single-step cylinder wake. Thus, the investigations will put more insight into the flow development in the wake of this bluff body. The results showed that distinct spanwise vortex cells form in the step cylinder wake: a single vortex shedding cell in the wake of the small cylinder, and one vortex shedding cell in the wake of the large cylinder. However, in the wake of the large cylinder wake an interaction region is also introduced. The difference in the diameters leads to a variation in the vortex shedding frequency, which introduces intricate three-dimensional vortex interactions in the wake of the step cylinder. Once in a while, the vortex shedding becomes out of phase, which introduces a dislocation at the boundary of two cells. The dislocation is exhibited by a half-loop connection, in which two vortices within the same cell connect to each other. Results showed that there is no significant Reynolds number effect on the average spanwise extent of the vortex cells and the transition region between neighboring cells. However, it was found that in such a high Reynolds number streamwise vortices are introduced into the wake which complicates the vortex dynamic, and vortex interactions.Solid and Fluid MechanicsMechanical EngineeringMechanical, Maritime and Materials Engineerin