Three-dimensional effects in turbulent bluff body wakes

Abstract

Recent investigations have shown that is is possible to control three-dimensional patterns in a cylinder wake at low Reynolds numbers (where the vortex shedding is laminar) by altering the end boundary conditions. However, very little work has been done to understand three-dimensional phenomena at higher Reynolds numbers. In the present study, we demonstrate the effect of end conditions on the cylinder wake at moderately high Reynolds numbers (200 < Re < 10,000). By suitably manipulating the end conditions, it is possible to induced oblique and parallel vortex shedding patterns across large spans (80 cylinder diameters) over aa large Re range. Measured parameters in the wake display marked differences between oblique and parallel shedding. The practical significance of such a study is that the total spanwise-integrated unsteady fluid forces on the body can be dramatically reduced to a value close to zero, by inducing oblique vortex shedding or indeed other three-dimensional phenomena. We have found that the instability of the separated shear layer is also affected by the end conditions: with parallel shedding, the instability first manifests itself at Re = 1200; but, with oblique shedding, the instability is inhibited until a significantly higher Reynolds number of about 2600. We show that the variation of normalized shear-layer frequency with Reynolds number is not accurately represented by a Re 0.5 power law, which has hitherto been used extensively in the literature. A power law that closely models not only our data, but all the data from earlier studies, is of the form, f SL f K =0.0235xRe 0.67 Physical reasons why one should naturally experct an exponent larger than 0.5 are included.