Jw. Naughton et al., AN EXPERIMENTAL-STUDY OF COMPRESSIBLE TURBULENT MIXING ENHANCEMENT INSWIRLING JETS, Journal of Fluid Mechanics, 330, 1997, pp. 271-305
Compressible jets with various amounts of swirl and compressibility ar
e investigated experimentally. The mixing-layer growth rate is obtaine
d from time-averaged images of the mixing layer using the planar laser
scattering (PLS) technique, and the swirl is quantified with laser Do
ppler velocimetry and intrusive probes. The results conclusively demon
strate that the addition of swirl to the jet increases entrainment by
up to 60% compared to a corresponding non-swirling case. Instantaneous
PLS images reveal modified turbulent structure in the mixing layer of
the swirling-jet cases. In particular, analysis of these images indic
ates that both the spatial extent and amplitude of the largest turbule
nt fluctuations are increased when swirl is added. Based upon these re
sults, a parameter beta that correlates the observed growth-rate enhan
cement is proposed. This parameter is derived assuming that the stream
wise vorticity, generated in the mixing layer by the addition of small
amounts of swirl, causes additional turbulent mixing that increases t
he growth rate. When the available growth-rate data for swirling jets
are plotted against this parameter, they collapse to a single curve wi
th increased enhancement for higher values of beta. This result implie
s that the degree of enhancement actually increases with compressibili
ty, although the dimensional growth rates for the present compressible
swirling-jet cases are still less than those of their incompressible
counterparts.