Spatial perturbation of a wing-tip vortex using pulsed span-wise jets
A. L. Heyes, D. A. R. Smith,
Abstract The separation distance required between
transport aircraft to avoid wake vortices remains a limiting
factor on airport capacity. The dissipation of the wake can
be accelerated by perturbing co-operative instabilities
between multiple pairs of vortices. This paper presents the
results of a preliminary experimental investigation into the
use of pulsed span-wise air jets in the wing tip to perturb a
single tip vortex in the very near ﬁeld. Velocity measure-
ments were made using PIV and hot-wire anemometry.
The results demonstrate that the vortex position can be
modulated at frequencies up to 50 Hz and, as such, the
method shows promise for forcing instability in multiple
The trailing vortex wake from aircraft dictates the minimum
spacing that must be maintained on landing at airports.
Since the late 1960s, there has been a continued effort to ﬁnd
means to diminish the hazard associated with the wake,
enabling a reduction in aircraft spacing and a consequential
increase in airport capacity. Despite this attention, and
although there now exists a greater understanding of vortex
wakes, there is still no successful alleviation method
implemented in service. With the imminent introduction of
new, larger aircraft and the continuing growth in air travel
this problem becomes ever more critical.
Trailing vortices are formed from streamwise vorticity
generated in the ﬂow around discontinuities in the wing
surface, such as ﬂap side edges and wing tips, and the roll
up of the sheet of vorticity shed from the trailing edge.
Vortices are also formed at the wing-fuselage junction and
from the tailplane. Therefore the trailing vortex ‘‘system’’
from an aircraft consists of a number of pairs of vortices
symmetrically orientated on either side of the aircraft (e.g.
deBruin et al. 1996; Fabre et al. 2001). Down stream of the
aircraft, vortices of the same sign will typically merge and
ﬁnally a single counter-rotating vortex pair results and
persists, depending on atmospheric conditions, for some
considerable time after the passage of the aircraft.
It has long been recognised that such vortex systems
can be destroyed by instabilities. Crow (1970) described
the mechanism by which a counter-rotating vortex pair
develops a sinusoidal instability, which ampliﬁes under
mutual inductance of the pair and leads to linking on the
vortices and formation of vortex rings. The vortex rings
then dissipate at a later time.
More recently it has been recognised that similar
instabilities occur in the near ﬁeld where more than one
pair of vortices are present. Crouch (1997) studied two
pairs of co-rotating vortices representing tip and outboard
ﬂap side edge vortices and discovered shorter wavelength
instabilities with higher growth rates than the Crow
Similarly Fabre et al. (2002) investigated instability in
two pairs of counter rotating vortices. They calculated the
optimal wavelength of perturbation that produced the
highest growth rate of instability for a wide range of vortex
strengths and spacings. Whilst very large growth rates
could be achieved for some cases, these mainly affected the
inboard pair of vortices, leaving the stronger, outboard
pair less affected. For the most advantageous effect, they
proposed a long-wave perturbation on the inner vortices.
They suggest that whilst this will have a lower growth rate
than shorter wavelength perturbations, it will have a larger
effect on the outer vortices, and is therefore likely to be
more beneﬁcial for wake alleviation.
Ortega et al. (2003) carried out an experimental study of
instability in a wake of similar conﬁguration to that of
Fabre et al. They used a rectangular plan-form wing, the
outboard pair of vortices was produced at the wing-tips
and the inboard pair of vortices produced by triangular
ﬂap extensions to the trailing edge. A rapidly growing
instability developed within 20 spans downstream of the
wing and converted the coherent two-dimensional ﬂow to
a three dimensional one. The authors argue that this
results in rapid reduction in the 2D rotational kinetic
energy and therefore the hazard posed by the wake. In this
experiment the instability developed naturally, with
perturbations to the vortices caused by background
turbulence in the towing tank.
The potential for forcing instability was recognised
early on. Crow and Bate (1976) suggested oscillating the
aircraft’s control surfaces in order to bring about the
Experiments in Fluids 37 (2004) 120–127
Received: 24 July 2002 / Accepted: 5 October 2003
Published online: 27 March 2004
Ó Springer-Verlag 2004
A. L. Heyes, D. A. R. Smith (&)
Department of Mechanical Engineering,
Imperial College London, Exhibition Road,
London, SW7 2BX, UK
The authors gratefully acknowledge the ﬁnancial support of EP-
SRC, DERA Bedford. We are also grateful to Professors J.H.
Whitelaw and P. Bearman for their advice and support
throughout the work.