A New Model for Free-Stream Turbulence Effects on Boundary Layers

A model has been developed to incorporate more of the physics of free-stream turbulence effects into boundary layer calculations. The transport in the boundary layer is modeled using three terms: 1) the molecular viscosity, ν; 2) the turbulent eddy viscosity, ε_T, as used in existing turbulence models; and 3) a new free-stream induced eddy viscosity, ε_f. The three terms are added to give an effective total viscosity. The free-stream induced viscosity is modeled algebraically with guidance from experimental data. It scales on the rms fluctuating velocity in the free-stream, the distance from the wall, and the boundary layer thickness. The model assumes a direct tie between boundary layer and free-stream fluctuations, and a distinctly different mechanism than the diffusion of turbulence from the free-stream to the boundary layer assumed in existing higher order turbulence models. The new model can be used in combination with any existing turbulence model. It is tested here in conjunction with a simple mixing length model and a parabolic boundary layer solver. Comparisons to experimental data are presented for flows with free-stream turbulence intensities ranging from 1 to 8% and for both zero and non-zero streamwise pressure gradient cases. Comparisons are good. Enhanced heat transfer in higher turbulence cases is correctly predicted. The effect of the free-stream turbulence on mean velocity and temperature profiles is also well predicted. In turbulent flow, the log region in the inner part of the boundary layer is preserved, while the wake is suppressed. The new model provides a simple and effective improvement for boundary layer prediction.