Experimental study on wake structures of a circular cylinder enclosed in a screen shroud using PIV

The wake vortices were manipulated by enclosing a bare circular cylinder with a screen shroud of porosities β = 37%, 48%, 61%, and 67%, where β was defined as the ratio of the open area to the total area. The diameter ratio between the screen shroud and the bare cylinder was fixed at 2. Experiments were conducted in a water flume using particle image velocimetry at a Reynolds number, Re_d, based on the bare cylinder diameter, d, of 1600. Depending on the porosity of the screen shrouds, the wake vortices were formed through two kinds of processes. The first one was found for β = 37% and 48% and was attributed to wake mode instability, or absolute instability, of the shear layers generated from the edge of the screen shrouds. The bleeding flow from the gap between the screen shroud and the bare cylinder delayed the interactions between the shear layers and elongated the vortex formation length. The second one was found for β = 61% and 67% and was associated to the merging of the shear layer vortices originated from the edge of the screen shroud with the vortices shed from the bare cylinder due to the gap flow. The latter was due to absolute instability caused by the reverse flow immediately downstream of the bare cylinder. The vortices in the combined shear layers grew in size through merging and engulfing processes and were finally shed in the wake due to convective instability. It was also found that the existence of the screen shroud attenuated the strength of the vortices and weakened the Reynolds stresses in the wake.