Dependence of Boundary Layer Separation on Surface Friction and Different Mountain Flow Regimes: An Analysis Based on Large-Eddy-Simulations

Boundary layer separation (BLS) may occur when a strong adverse pressure gradient force is imposed on boundary layer flow, leading to strong deceleration and detachment of streamlines from the surface. This process commonly occurs at the salient edge of very sharp obstacles. In stably stratified flows, pressure perturbations strong enough to cause BLS can also be induced by internal gravity waves.

A well-known phenomenon related to wave-induced BLS is that of leeside atmospheric rotors, i.e., boundary-layer zones characterized by strong turbulence, surface wind direction reversals, large values of spanwise vorticity and near-neutral stability. Due to the high intensity of turbulence, atmospheric rotors are known to pose a hazard to general aviation and road traffic and can significantly impact the energy yield of wind parks in mountainous terrain. Hence, the onset of BLS and formation of rotors have been extensively investigated in recent years. However, only a few systematic studies of the processes involved in the formation of rotors are available in the literature.

In this study, the CM1 model is used to explore the impact of different mountain flow regimes and of the surface exchange coefficient for momentum on the size and strength of rotors. Furthermore, a possible feedback mechanism of BLS onto the larger-scale flow is investigated. The results show that the strength and size of atmospheric rotors mostly depends on the flow features upstream of the mountain, whereas friction mainly influences the rotor's interior structure.

A set of two-dimensional simulations shows that the most intense rotors, as measured by the strength of the surface reversed flow, tend to occur in strongly non-linear and non-hydrostatic flows. The largest rotors, instead, are unexpectedly found in moderately non-linear flows. A possible explanation is related to low-level mountain-wave breaking caused by strongly nonlinear flows. In this case, the reflection of wave energy at a self-induced critical level favours the formation of a lee wave train and an attendant sequence of smaller rotors underneath the lee wave crests. Flow over this train of rotors excites propagating gravity waves, which are superimposed on the wave field of the larger-scale flow above.