POD Analysis in the Wake of Wall-Mounted Cylinders

摘要

The flow patterns over a finite square cylinder of aspect ratio H/d = 3 were analyzed experimentally. The measurements were carried out in a subsonic wind tunnel using the technique of Time Resolved Particle Image Velocimetry (TR-PIV). The near wake flow structures and vortex shedding characteristics were investigated using instant velocity maps and proper orthogonal decomposition (POD). Tests were performed at a speed of 20m/s, resulting in Re - 65300, with the cylinder facing the flow at 90°. For the wind tunnel tests, the cylinders were fixed on flat plate, creating a boundary layer which interacted with the cylinder wake. The 2D PIV measurements were conducted at three horizontal planes (z/H = 0.3, z/H = 0.5, z/H = 1) and the symmetry x-z plane. Due to the complexity of the phenomena, the flow was characterized both in terms of average behavior and time-resolved velocity fields. Both symmetrical and anti-symmetrical vortices structures occur in the cylinder wake, which can be identified based on the coefficients of the first four POD modes. The results indicated that the alternating Karman vortex structures are dominant, described by the two first POD modes. However, this structure is sometimes suppressed, leading to periods of symmetrical vortex shedding.