FLOW ABOUT A CIRCULAR-CYLINDER WITH A SINGLE LARGE-SCALE SURFACE PERTURBATION

An experimental study of the flow around a cylinder with a single straight perturbation was conducted in a wind tunnel. With this bluff body, positioned in a uniform crossflow, the vortex shedding frequency and other flow characteristics could be manipulated. The Strouhal number has been shown to be a function of the perturbation angular position, theta(p), as well as the perturbation size and Reynolds number. As much as a 50% change in Strouhal number could be achieved, simply by changing theta(p) by 1 degrees. The perturbation size compared to the local boundary layer thickness, delta, was varied from approximately 1 delta to about 20 delta. The Reynolds number was varied from 10,000 to 40,000. For perturbation sizes approximately 5 delta to 20 delta and Reynolds numbers of 20,000 to 40,000, a consistent Strouhal number variation with theta(p) was observed. A detailed investigation of the characteristic Strouhal number variation has shown that varying theta(p) had a significant influence on the boundary layer separation and transition to turbulence. These significant changes occurring in the boundary layer have been shown to cause variations in the spacing between the shear layers, base pressure, drag, lift, and the longitudinal spacing between the vortices in the vortex street.