Semi-Confined Flows

Underlying Flow Regime 3-34

Best Practice Advice

Key Physics

Key physical features of the UFR in question are: separation of the turbulent boundary layer from a smooth surface driven by adverse pressure gradient, a rapid development of three-dimensional turbulent structures in the separated shear layer, its reattachment to the plane wall, and further relaxation of the reattached turbulent boundary layer to the “normal” state.

Physical Modelling Issues

Turbulence Modelling

Based on the conclusions formulated above the following advice for the computations of the considered UFR may be given.

Use turbulence-resolving approaches (hybrid RANS-LES or global WMLES and WRLES), since none of the currently available RANS turbulence models, either linear eddy-viscosity or RSM, ensures capturing of the challenging physical features of the UFR indicated above. However a success of the scale-resolving approaches is also not guaranteed. In order to reach it:

Impose as realistic as possible turbulent content at the inflow of LES zone within zonal RANS-LES approaches.
Ensure a rapid development of 3D turbulent structures in the separated shear layer within nonzonal hybrid approaches (this demands employing of special grey-area mitigation tools).
Use sufficiently wide domains for simulation of nominally 2D geometries with periodic boundary conditions in the uniform direction.

Whether all the above recommendations are really implemented in a simulation should be checked by:

Visualizing the unsteady solutions (see, e.g., Figs. 13, 14) and getting a visual impression on the length of the RANS-to-LES transition in the zonal RANS LES simulations, on 3-dimensionality of the early region of the separated shear layer in the non-zonal hybrid simulations, and on sufficiency of the domain width for representation of large-scale structures showing up in a flow field.

Contributed by: E. Guseva, M. Strelets — Peter the Great St. Petersburg Polytechnic University (SPbPU)