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, _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 _p by 1°. The perturbation size compared to the local boundary layer thickness, , was varied from approximately 1 to about 20 . The Reynolds number was varied from 10,000 to 40,000. For perturbation sizes approximately 5 to 20 and Reynolds numbers of 20,000 to 40,000, a consistent Strouhal number variation with _p was observed.A detailed investigation of the characteristic Strouhal number variation has shown that varying _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.List of Symbols a longitudinal spacing of vortices in the vortex street - C _d drag coefficient - C _dc drag coefficient corrected for blockage effect - C _l lift coefficient - C _p pressure coefficient, p _i –p /q - C _pb base pressure coefficient - C _pbc base pressure coefficient corrected for blockage effect - d perturbation diameter - d ^* spacing between the shear layers; defined as conditionally averaged spacing between points in the shear layers corresponding to 0.99u _max/U - D cylinder diameter; diameter of the circumscribing circle for a cable - f _v vortex shedding frequency - H wind tunnel test section cross-sectional width - L spanwise length of the cylinder - p _i tap pressure - p free stream static pressure - q free stream dynamic pressure - Re Reynolds number based on cylinder diameter - rms root-mean-square - S Strouhal number, f _v D/U - S _max maximum value of S - S _min minimum value of S - t time - u _c vortex convection velocity - u _max maximum velocity in the shear layer - U free stream velocity - U _c free stream velocity, corrected for blockage effect - x streamwise dimension referenced from the back of the cylinder - z lateral wake dimension, i.e., perpendicular to the free stream velocity vector and cylinder axis, referenced from the cylinder axis - x spacing between two hot wire probes aligned streamwise - phase difference between two hot wire probes aligned streamwise - boundary layer thickness - angle from stagnation point in degrees - _p perturbation angular position - _{b p} where S drops back to about the S of a cylinder - _c critical angle, angular position where S drops steeply with 1° change in - _{m p} where S was minimum - _{r p} after S recovers from drop in value - _{t p} where S starts to increase from about the S of a cylinder