An aircraft in flight will generate a pair of counter-rotating wake
vortices. A large aircraft can generate a wake vortex (or vortex pair) of
considerable strength, which can pose a hazard to any craft which may
encounter this wake vortex. The study of the mechanisms by which these
vortices decay and breakdown is, therefore, of great practical
importance. The evolution of the vortex pair is affected by its Reynolds
number (which is a measure of the strength of the vortices) and the
strength of stratification in the atmosphere.
Two flow instabilities, long wave and short wave,
play an important role in the evolution of a counter rotating vortex pair.
In certain situations it is thought that the short wave instability might
play the most significant role in the evolution of the vortex pair.
When stratification of the fluid is increased the growth rate of the
short wave instability is greatly enhanced. This enhancement in growth
rate is not fully understood.
The purpose of this work is to investigate the effect of varying levels
of ambient stable stratification on the growth of the short wave
instability. Direct Numerical Simulations are used to investigate the
flow for a range of stratification levels (1< Fr < infinity)
with a constant value of Reynolds number.
The effect of stratification on the short wave instability is
quantified by measuring wavelengths and growth rates.
An earlier onset and higher growth rate of the instability is
found for a flow with stratification. With low levels of
stratification (Fr > 1.5) an almost threefold increase in the
growth rate of the instability is observed. In this case the
wavelength of the instability is found to be the same as in the
unstratified case. As the level of stratification is further
increased (Fr < 1.5) the form of the instability becomes more
complex (with the amplification of two wavelengths) with a slight
reduction in the growth rate when compared with results for weak
stratification.