An investigation of the near wake of a circular cylinder using a video-based digital cross-correlation particle image velocimetry technique

A cross-correlation particle image velocimetry (PIV) technique has been developed to measure the spatiotemporal in-plane velocity vector field evolution of time-dependent flows. A novel iterative two-stage cross-correlation scheme of two sequential images of flow tracers has been incorporated in the image analysis. The implementation in hardware and software of this complete recording and analysis system are described. The expected accuracy of the velocity measurements was investigated and is discussed. The technique has been applied to study the near wake behind a circular cylinder at low Reynolds numbers (Re-d). The measurements presented pertain to cylinders with d = 12.5 and 25 mm (l/d = 19.5 and 9.8, respectively). The respective Reynolds numbers Re-d are 875 and 769. Two planes of this flow are considered in this study: (1) plane normal to the cylinder axis (xy plane) and (2) a plane containing the cylinder axis and the stream direction (xz plane). Instantaneous in-plane velocity vector fields and out-of-plane vorticity fields are presented for both planes. The effect of spatial resolution on peak vorticity is discussed using velocity vector field measurements in the near wake of the cylinder that were conducted using different spatial resolutions. The three-dimensional nature of the near wake of circular cylinders at low Re-d is demonstrated using quantitative in-plane velocity vector field and out-of-plane vorticity measurements. An upstream influx of relatively high velocity fluid into the stagnant near-wake region in the xy plane and the subsequent deflection of the fluid normal to this plane as it approaches the stagnation region at the cylinder are shown to be responsible for the generation of three-dimensional flow in the near wake of a circular cylinder.