Secondary vorticity pattern

Figure 3.46 shows the instantaneous stream-wise vorticity contours above a circular cylinder obtained in the present work using PIV. The axis of the vorticity component is along the flow direction. The PIV results of Brede et al (1996) are shown for comparison. The Reynolds number of the present work is 330, while that of the reference is 290. At these Reynolds numbers, secondary (streamwise) vortices are generated along with the spanwise (Karman) vortices. The secondary vortices are arranged along a line parallel to the cylinder axis. The sense of rotation alternates along the spanwise direction. The secondary vortices correspond to mode B of Williamson (1997). The spanwise wavelength of the secondary vortices is around one cylinder diameter, the spacing between the vorticity peaks being, in general, a constant. These trends are realized in both sets of experiments. Overall, the favorable comparison of vorticity patterns with the published literature.

Note: The supplementary material accomparying Module 3 contains addtional PIV images for flow past a cylinder of square cross section.

Figure 3.44: Variation of drag coefficient with incidence angle. The data of the present study in the top gure is for mid-plane of the cylinder. The numerical simulation of Sohankar et al. (1998) reports an average over the cylinder length. The drag coefficient data averaged over 3-planes has been shown in the bottom figure.