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 (x_R) by developing two- and three-dimensional models of the BFS flow using ANSYS Fluent, and validate the results by referring to prior research.

fig1

The general geometry & nomenclature

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

 

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

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

The k-\epsilon model is notoriously bad at predicting x_R, often underestimating by a factor of 2. In this model I compared the k-\epsilon RNG and RSM models for the 2d case, and k-\epsilon standard and realisable for the 3d case. The most realistic results came from the k-\epsilon RNG model in 2D, giving a value x_R/H = 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.
 

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