An Experimental Investigation of Wake Edge

Abstract

The unsteady Velocity and sound pressure fields in the vicinity of two flat plates aligned with the direction of flow were studied experimentally. The plates were colinear so that the wake of the upstream plate impinged on the downstream plate. These tests were performed at mean velocities ranging from the critical velocity required to achieve unsteady flow in the region between the plates up to velocities high enough to produce audible sound. The objective of these experiments was to resolve apparently conflicting reports in the literature concerning the role of the feedback of unsteady disturbances from a downstream edge in determining the properties of the unsteady flow in an upstream wake. The results of the observations reported here lead to the conclusion that feedback influences the character of the flow only when the unsteadiness arises from the instability of a laminar wake. It does not influence the shedding of a separated wake. Regular variations observed in the far-field sound pressure level with changes in spacing between blunt plates are attributed to the interaction of multiple sound sources rather than to feedback. Nomenclature L = plate length P = sound pressure level Re = Reynolds number based on plate thickness, Ut/v S = plate spacing t = plate thickness U = mean velocity u' =rms level of the fluctuating component of streamwise velocity