The semi-deterministic approach as way to study coherent structures. Case of a turbulent flow behind a backward-facing step

Abstract

The chapter has two main goals: using a conditional method to deduce an experimental coherent motion and separately developing the semideterministic modeling that directly provides the numerical coherent motion in the same configuration of the backward facing step flow, comparing precisely numerical and experimental results. Concerning the differences with a plane mixing layer, despite an ambient turbulent level much higher, it is checked that similar vortical coherent structures can be identified and that the higher turbulence level in the separated flow does not alter their coherence. The semideterministic approach is a rather rough approach based on a simple generalization of k-є from steady to unsteady case, but it correctly predicts the coherent motion in the flow. Characteristic frequencies and morphologies of coherent structures are in agreement with experimental ones. In contrary, two differences exist about the evolution of the size of coherent events when they go toward the reattachment area and the magnitudes of coherent vorticity and velocities. The comparison between numerical and experimental coherent motions is not made, and it shows that the semideterministic approach is able to predict the coherent motion, even though some modeling improvements are necessary to improve the introduction of the characteristics of incoherent turbulence.