Airflow patterns over and around a large three-dimensional hill

Abstract

Surface wind patterns and air flows within the planetary boundary layer over a large three-dimensional hill of moderate slope are grouped according to Froude number classes. An evolution of flow patterns is shown to occur as the Froude number increases. Separation of the surface flow begins at the base of the lee side of the mountain near the centerline, moving upward on the lee slope as the Froude number increases. Recirculating eddies follow the separation of the lee flow. Eventually the separation line moves forward to the windward side as the Froude number becomes very large. The recirculating eddy becomes unsteady, with indication of an intermittent counterrolating eddy near the lee surface in neutral flow. The lee-side turbulence is enhanced with respect to the windward side due to the large eddies in high Froude number regimes. The concept of a critical height for the approach flow is generally supported. The integral form of the Froude number does not appear to be superior to a bulk Froude calculation in representing a particular airflow pattern.