Three-dimensional effects in turbulent bluff-body wakes

Abstract

There has recently been a surge in activity concerning the development of three-dimensionality in the wakes of nominally two-dimensional bluff bodies, yielding the realization that end effects can influence the wake vortex shedding pattern over long spanlengths. Much of this work has been focused on low Reynolds numbers (Re), but virtually no studies have investigated to what extent it is possible to control shedding patterns at higher Reynolds numbers, through the use of end manipulation. In the present paper, we demonstrate that it is possible to induce parallel shedding, oblique shedding and vortex dislocations, by manipulation of the end conditions, over a large range of Reynolds number. Such patterns affect the frequency of primary wake instability and its amplitude of fluctuation, as they do at low Reynolds number, although distinct differences are found at the higher Reynolds numbers.

We find that imposition of oblique shedding conditions at high Reynolds number leads to a spatial variation of both the oblique shedding angle and shedding frequency across the span, and to sparse dislocations which are not restricted to the spanwise end regions, as they are at low Reynolds numbers (under similar geometrical conditions). In the wake transition regime (Re=190–250), it is confirmed that the spontaneous appearance of vortex dislocations in mode-A shedding precludes the control of shedding patterns using end manipulation. However, it has proven possible to extend the regime of Reynolds number where dislocations ‘naturally’ exist to Re>250, by introducing them artificially through end control, where they would otherwise not occur. The possibility of introducing dislocations and of inducing oblique vortex shedding at higher Reynolds numbers has practical significance, if one can deliberately decorrelate the vortex shedding, and hence reduce the spanwise-integrated unsteady fluid forces on the body.

We confirm the existence of a transition in the mode of shedding at Re≈5000 (originally found by Norberg 1987) under conditions where parallel shedding is attempted. This mode transition displays similarities to an inverse of the mode A→mode B transition that is found in the wake transition regime. It is clear that vortex dislocations occur beyond Re=5000, although it is not clear why the flow is unstable to such a mode. Furthermore, there appears to be some support for the suggestion that vortex dislocations may be a feature of the flow for Re at least up to 30×10³, as evidenced by the work of Norberg (1994).