Structure of random velocity fluctuations in a two-dimensional vortex-street wake.

Abstract

This paper describes experimentall the structure of random components of the velocity fluctuations in the vortex-street wake of a thin normal plate at a Reynolds number of 23 OOO. Measurements were made by a phase-averaging technique at positions eight heights downstream of the plate. The random components were defined as the corresponding instantaneous velocities subtracted by their phase-averaged values, viz. the periodic components. The random components were decomposed into several narrow-band frequency components, whose contributions to the Reynolds shearing stress are found to generally attain a maximum at saddles associated with the periodic components. Moreover the Reynolds shearing stress is dominated by the low-frequency components whose longitudinal length scale is greater than that of the periodic components. These facts raise a doubt as to the well·documented idea that most of Reynolds shearing stress is produced by the vortex stretching near the saddles. The low-frequency random components may possibly be produced by a modulation of the shape, strength and position of each realization of the periodic components.