The effect of nonharmonic forcing on bluff-body aerodynamics at a low Reynolds number

The aerodynamic response of a circular cylinder to nonharmonic forcing of the inflow velocity is studied by numerically solving the equations of two-dimensional fluid motion on an orthogonal curvilinear mesh. The effect of varying the inflow velocity waveform while maintaining other forcing parameters constant at a Reynolds number of 180 is considered in this study. The forcing frequency is 84% of the natural vortex shedding frequency in the unforced wake while the peak-to-peak amplitude of velocity oscillation is 65% of the reference velocity. Results are reported for the drag and lift coefficients and the flow field in terms of streamline patterns and vorticity distributions. It is shown that the wake is locked-on to the forcing frequency for all cases tested but the aerodynamic response is systematically modified by the imposed changes in the velocity waveform. The magnitude and the phase of the fluctuating drag and lift forces and the mean drag force are affected. These effects are associated with changes in the mechanism of vortex formation and shedding in the wake of the cylinder; it is found that the rolling up of the individual shear layers on both sides can be manipulated to promote shedding of single vortices or vortex pairs.