Flow over a backward-facing step

This was a major project for a class in CFD. The aim of the project was to determine the reattachment length () by developing two- and three-dimensional models of the BFS flow using ANSYS Fluent, and validate the results by referring to prior research.

The general geometry & nomenclature

The model was based on the 1985 experiment by Driver & Seegmiller, which used a wind tunnel with = 36,800. The reattachment length is strongly dependent on the Reynolds number in the laminar and transitional regimes ( < 3400) but in turbulent flow, settles to ~ 7.

Results from the refined 2D model, clearly showing the primary recirculation zone

The model is notoriously bad at predicting , often underestimating by a factor of 2. In this model I compared the RNG and RSM models for the 2d case, and standard and realisable for the 3d case. The most realistic results came from the RNG model in 2D, giving a value = 5.33. This is still an underestimation, and I suspect is due to the mesh: I left some high aspect ratio elements in the region behind the step, which would have interfered with calculations of the primary and secondary vortices.

The full report can be found here.