EFFECT OF AMBIENT TURBULENCE ON VORTEX INSTABILITY
Dr. (Pete) H.-T. Liu
Physical modeling of the effect
of ambient turbulence on wing-tip vortices was conducted in a towing tank,
which measured 18 m long, 1.2 m wide and 0.9 m deep. The towing system
was capable of towing two oil-bearing carriages independently or simultaneously.
The carriages rode on thin oil films released from four pads supporting
the carriages on two tracks, one round and one flat. The tracks were aligned
such that the vertical displacement of the instrumentation carriage was
within 2 mm in the 12-m working section . Trailing vortices were generated
by towing a rectangular wing with a span 5.1 cm and a chord of 2.5 cm
at several negative angles of attack . Ambient turbulence was generated
by towing upstream of the wing 3 grids with different mesh sizes .
In the absence of ambient turbulence (left column), sinusoidal linking
is the only form of instability, as predicted by Crow .In weak turbulence,
both sinusoidal and bursting instabilities are observed (middle column);
the dominant wavelength of the sinusoidal instability deceases with increasing
turbulence intensity. In strong turbulence, the vortex instability is
predominantly of the bursting type (right column).
References: Liu, H.-T. (1992)
"Effects of Ambient Turbulence on the Decay of a Trailing Vortex Wake,"
AIAA J. of Aircraft, 29, 255-263.
 Liu, H.-T. (1995) "Energetics of Grid
Turbulence in a Stably Stratified Fluid," J. Fluid Mech. 296, 127-157.
 Crow, S. C. (1970) "Stability Theory for a Pair of Trailing Vortices,"
AIAA Paper 70-53.