Computation of separated-flow transition using a two-layer model of turbulence

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 suitably defined intermittency function, which takes appropriate values in the laminar-, transitional- and turbulent-flow regions. Correlations derived from measurements are used for this purpose. Two test cases were selected; the 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 encountered 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 second, lie in the range of 1600-6600, in which limited numerical investigations 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 the separation and the transition regions and of velocity and turbulence intensity profiles at various streamwise locations. The results show that, for the transition criterion chosen, in all cases transition is completed downstream of the reattachment point and that the rate at which it is completed increases with the free-stream turbulence level.