Edge Behavior of the

We examine the outer-edge behavior in a turbulent boundary layer (TBL) predicted by the k-ε model with reference to a mathematical analysis of Cazalbou–Spalart–Bradshaw (CSB) (Physics of Fluids, Vol. 6, No. 5, 1994, pp. 1797–1804). Particular attention is given to one-dimensional analyses at the edge of the turbulent region. An inverted parabola analysis demonstrates that the correct edge behaviors of k, ε and νt are reproduced for both high- and low-Re models. For a ramp analysis following CSB (involving the velocity field), a set of diffusion coefficients for low-Re k-ε that violates the CSB constraint, however, yields an unrealistic velocity profile at the interface between the turbulent and nonturbulent regions due to the excessively small turbulent diffusion. We therefore propose improved diffusion coefficients by considering both the CSB constraint and requirements in wall turbulence, and calculate a range of two-dimensional aeronautical flows with k-ε to clarify if the one-dimensional analyses support the predictions in realistic flows, fully coupled with the velocity field. Indeed, the corrected diffusion coefficients provide reasonable predictions for both the channel and TBL but also TBLs involving turbulent separation when Re becomes large. Over an airfoil, the correction improves the prediction for CD by a few counts.