Coherent structures in a screen cylinder wake

Experiments were conducted in a wind tunnel to examine the development of coherent structures in a screen cylinder wake over a streamwise range of 10≤x/d≤60 (where d is the diameter of the cylinder) at a Reynolds number Re of 7000. The screen cylinder was made of a stainless-steel wire mesh with a porosity (open area ratio) of 67%. Results showed that the flow can be classified into two distinct regimes, characterized by the averaged contributions from the coherent motion to the Reynolds stresses where vortices changed from growing to decaying. The first regime exhibited the growth of small-scale Kelvin-Helmholtz vortices in the shear layer through vortex merging. These small-scale vortices matured quite a distance downstream to form the "large-scale" coherent structures, marked by the change of the root-mean-square transverse velocity fluctuations from a twin-peak to a single peak profile, significantly extending the vortex formation length. The second regime was manifested by the slow decay of the fully formed large-scale vortices, designated by the weak vorticity exchange across the wake centerline. Conceptual models of the vortex formation and decay processes as well as the momentum transport in the screen cylinder wake were proposed and illustrated, and comparisons were made with that of a solid cylinder wake.