POD analysis of the wake behind a circular cylinder coated with porous media

The wake of a circular cylinder coated with porous media was experimentally investigated by hot-wire anemometry, particle image velocimetry (PIV) and smoke-wire visualization. Mean velocity and Reynolds stresses in the streamwise and lateral are eliminated obviously for porous layer coating, especially in the centerline. The vortex formation length is elongated, and the width of the wake region is expanded by coating porous media. The Strouhal number of vortex shedding is 0.162 for the porous cylinder, which is smaller than 0.182 for the bare one due to the widened wake. Additionally, porous layer significantly regulates the flow field, which can be detected distinctly by smoke-wire visualization and transient PIV data. The proper orthogonal decomposition (POD) was further used to analyze PIV data with a view to extract dominant coherent structures in the wake. Moreover, a phase-averaged low-order model was applied to provide a comprehensive description of the dynamics of the first four modes. The first two POD modes, showing the signature of asymmetrical vortex shedding and predominating in the wake of the bare cylinder, are weakened dramatically for porous layer coating and present abrupt degradation in the midline. Meanwhile, mode 3 and mode 4, representing the symmetrical vortex shedding, occupy higher energy for porous cylinder. The study reveals that the porous layer coating delays the roll-up of shear layer and weakens the interaction between them, and thus, the vortex region is shifted further downstream.