Experimental study on the development of wake vortices behind screen cylinders

This work examined the development of the large-scale vortex structures in the wake of two screen cylinders with porosities β = 48% and 61% in the intermediate region at a Reynolds number of 7000 using two X-type hot-wire probes. The screen cylinders were made of stainless steel screen meshes rolled into a cylindrical shape. The results were compared with those of a solid cylinder (β = 0%) under the same flow conditions. It was shown that the formation of the large-scale vortices in the screen cylinder wakes involves different mechanisms. For the screen cylinder with β = 48%, they were generated due to wake instability (Kármán vortex), which closely resembled that of a solid cylinder. In contrast, for the screen cylinder with β = 61%, the vortices were formed owing to the shear-layer (Kelvin–Helmholtz) convective instability. These findings were justified by the streamwise evolution of the vortex shedding frequency, a stability analysis in the near wake using the mean streamwise velocity profiles, and the coherent and incoherent wake structures obtained by the phase-averaged analysis at different downstream locations.