Turbulent diffusion from a point source in stratified and neutral flows around a three-dimensional hill—II. Laboratory measurements of surface concentrations

Abstract

Towing-tank and wind-tunnel measurements of the concentration distributions on the surface of a hill when a plume impinges from an upwind source are presented. The stability is varied between very stable and neutral. The results are compared with the theories developed in Part I. When the source is below the dividing-streamline height H D , the plumes impact on the front surface of the hill, yielding surface concentrations nearly the same as would be observed at the plume centerline in the absence of the hill. However, eddying in the wake can cause oscillations in the plume upwind so as to increase the area of impingement and decrease the average concentration. When the source is above H D , the plume surmounts the hill top, but if it is only slightly above H D , maximum surface concentrations can again essentially equal those that would be observed at the plume centerline in the absence of the hill. The maximum surface concentration decreases very rapidly with further increases in source height. The location and value of the maximum surface concentrations are found to be extremely sensitive to slight displacements of the source from the stagnation streamline when the source is below H D . The general assumptions of the potential flow models developed in Part I to provide estimates of surface concentrations on three-dimensional hills are useful.