Oscillation of the Wake behind a Flat Plate parallel to the Flow

Oscillation of the pure laminar wake behind a flat plate placed in a uniform stream at zero incidence angle was investigated experimentally at Reynolds numbers between 10 2 and 10 5 . (The Reynolds number R is U l /ν, where U is the model speed, l is the plate length and ν is the kinematic viscosity.) For a sufficiently small R the corresponding wake was perfectly laminar and vortex filament was seen nowhere. But when R of about 700 was reached, the wake began to oscillate sinusoidally some distance downstream. As R was increased further the oscillation became more and more conspicuous and discrete vortex filaments appeared in the loop of the sinusoidal wave. Frequency N was measured by the hot-wire technique and it was found that the Strouhal number N l / U is nearly proportional to 1/2 power of R . On the other hand, wave length λ was determined from the photographs taken by aluminium dust method, and the relation between l /λ and R was obtained. Spacing ratio and velocity of the vortex filaments were also estimated. These experimental results were compared with those of the wakes behind various bluff bodies, and some knowledges about the origin of oscillation of the wake were obtained. The value of wake Strouhal number was nearly constant for all shapes of body, while the wave velocity changed remarkably with the shape of the body.