A model for predicting transition in flows with separation is presented in
this study. The two-layer model of turbulence is employed, along with a sui
tably defined intermittency function, which takes appropriate values in the
laminar-, transitional- and turbulent-flow regions. Correlations derived f
rom measurements are used for this purpose. Two test cases were selected; t
he flow over a long horizontal body with semi-circular leading edge and the
flow over the backward-facing step of small height (expansion ratio of 1:1
.01). In the former, oncoming flows with a free-stream turbulence level enc
ountered in practical applications was considered (0.2-5.6 percent) whereas
in the latter the corresponding level was much lower. The Reynolds numbers
, based on the diameter in the first case and on the step height in the sec
ond, lie in the range of 1600-6600, in which limited numerical investigatio
ns were previously available and where bubbles with laminar separation and
turbulent reattachment are expected. The predictions were found to compare
well with the corresponding measurements, both in terms of the lengths of t
he separation and the transition regions and of velocity and turbulence int
ensity profiles at various streamwise locations. The results show that, for
the transition criterion chosen, in all cases transition is completed down
stream of the reattachment point and that the rate at which it is completed
increases with the free-stream turbulence level.