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).
Feedback control of unstable flows: a direct modelling approach using the Eigensystem Realisation Algorithm
