An experimental investigation is conducted to determine the effect of mean flow unsteadiness on the vortex street behind a plane wake. Complex demodulation and bispectral analysis techniques are used to obtain quantitative measurements of the modulation and nonlinear couplings generated by the mean flow unsteadiness. The results show that low frequency mean flow unsteadiness increases the amplitude and phase modulation levels of the frequency component that corresponds to the vortex street. This increase in modulation can be seen as an outcome of an increase in the level of nonlinear couplings between the mode at the unsteadiness frequency, the vortex street frequency component and its harmonics. A simple numerical example is presented to show that modulation effects can be seen as variations in the spacings between the successive elements of the vortex street.
The spectral energy distribution of slender body wakes is an important factor in determining the magnitude of self-induced vibrations and drag and lift forces acting on the body that generates the wake and on other members placed downstream of that body. The basic dynamic~ of steady wakes and in particular the characteristics of the How separation and of the downstream vortex street under steady conditions are now fairly understood. The vortex street is advected downstream at a frequency that is determined by the length scale of the wake and the free stream velocity. Under steady conditions, the vortex street is ordered and the vertical and horizontal separations between the vortices is fairly constant. In offshore applications, non steady wave phenomenon is a natural characteristic of the flow field and its effects on the vortex street have not been studied. The objective of this paper is to examine the influence of low frequency mean flow unsteadiness on an established vortex street.
