A Computational Approach to Simulating the Effects of Realistic Surface Roughness on Boundary Layer Transition.

A surface roughness model extending the Langtry-Menter transition model has been implemented in a RANS framework. The model, originally proposed by Dassler, Kozulovic, and Fiala, introduces an additional scalar field roughness amplification quantity. This value is explicitly set at rough wall boundaries using surface roughness parameters and local flow quantities. This additional transport equation allows non-local effects of surface roughness to be accounted for downstream of rough sections. This roughness amplification variable is coupled with the Langtry-Menter model and used to modify the criteria for transition. Results from flat plate test cases show good agreement with experimental transition behavior on the flow over varying sand grain roughness heights. Additional validation studies were performed on a NACA 0012 airfoil with leading edge roughness. The computationally predicted boundary layer development demonstrates good agreement with the experimental results. New experimental tests using multiple roughness configurations were conducted to further validate and calibrate the model. Finally modifications are discussed to potentially improve the behavior of the Langtry-Menter transition model at high Reynolds numbers and angles of attack.

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