5% - eTheses Repository - University of Birmingham

Roshko (1974), observed that a turbulent mixing layer also
contains large-scale spanwise vortex motions which persist in the
presence of smaller scale turbulence. Winant & Browand (1974)
went further in reporting that vortex pairing was the main
mechanism behind shear layer growth at low Reynolds numbers.
Brown and Roshko found the same process to be equally important
at high Reynolds number. Dimotakis & Brown (1976) found this
behaviour to be a dominant feature even at Reynolds numbers of
10 7 . As detailed above, Hernan & Jiminez (1982) found that fluid
is engulfed during pairing and is entrained by the vortices
between pairing, typically with most of the growth occurring by
entrainment rather than engulfment. A comprehensive study of the
entrainment phenomenon by Panides & Chevray (1990), identified
key elements of the mechanisms from simultaneous flow
visualisation and one point two-component laser doppler
anemometry. Ho & Huerre (1984) indicated that not only pairing
interactions may occur, but also three-fold interactions or
higher order events, especially for locations close to the
initial instability.
b) Spreading angle and three dimensional instability- Muller
& Gyr (1986) used an analysis by Jiminez (1980) to calculate the
spreading angle of the shear layer. Jiminez found (figure 9 ;
from Muller & Gyr, 1986) that the ratio of the area P^ of
vortices of neighbouring generations was about 4 giving a ratio
of 2 for the linear scale since
=2 x 1.75 x 1.17 = 4.1
pairing entrainment engulfment
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