Evaluation of turbulence models for large-eddy simulations of flow over Askervein hill

The evaluation of turbulence closure models for large-eddy simulation has primarily been performed over flat terrain, where comparisons to theory and observations are simplified. We have previously developed improved closure models using explicit filtering and reconstruction, together with a dynamic eddy viscosity model and a near-wall stress term. This dynamic reconstruction model gave improved results over standard eddy-viscosity models for neutral boundary layer flow over flat but rough terrain, yielding the expected logarithmic velocity profiles near the wall. We now extend the results from the studies over flat terrain to flow over full-scale topography. We consider Askervein hill, an isolated hill in western Scotland, where a field campaign was conducted in 1983 under neutral stratification and steady wind conditions. This widely-studied flow provides a more challenging test case for the new turbulence models because of the sloping terrain, and separation in the lee of the hill. To provide a realistic turbulent inflow, a separate neutral boundary layer simulation with periodic boundary conditions is performed and data are extracted from a slice in the domain at every time step. This turbulent dataset is then used to specify the time-dependent inflow velocity at the western entrance of the Askervein domain. Results indicate that reconstruction and dynamic eddy viscosity models, used together with the near-wall stress model, improve the predictions of flow speed-up and turbulent kinetic energy over the hill. High resolution is needed, particularly in the vertical direction. This is the first time, to our knowledge, that reconstruction (scale-similarity) or dynamic turbulence models have been applied to full-scale simulations of the atmospheric boundary layer over terrain. Simulations with the lowest level of reconstruction are straightfoward. Increased levels of reconstruction present difficulties, however, and require modification of the closure model near the ground, pointing to the need for further study on the behavior of closure models in this region of the flow.