Experimental Investigation of Starting Flow in the Wake of a Perforated Plate*

The flow over a perforated or porous plate is commonly encountered in civil engineering, and its practical importance is growing. Since the seminal work of Castro in 1971, it has been recognized that the steady state mean flow produced by a perforated plate contains an attached vortex dipole. The dipole moves further from the plate and its strength decreases (as does vortex shedding) for plates with larger open area fractions until it disappears entirely. Despite renewed interest in the steady perforated plate problem, almost no attention has been paid to a perforated plate’s starting flow. This leaves many questions unanswered. Does the disappearance of attached vortices in the steady case have a correspondence to behavior in the startup flow? How is the vortex formation process effected by perforation?

A combination of PIV, dye flow visualization, and force measurement are used to study the flow physics of a perforated plate. The plate is set in motion impulsively using a gantry placed above a tank of water, and the startup process is observed. Investigations focus on the vortex formation process from nascent vortex generation to pinch-off, and the effect of the plate’s perforation level on these processes.