An experimental investigation of flow and diffusion in the disturbed boundary layer over a ridge—II. Diffusion from a continuous point source

Abstract

The influence of a low but steep ridge on diffusion of material in a disturbed boundary layer is investigated through experiments conducted in a large wind tunnel. A two-dimensional triangular ridge with side slopes of 2 to 1 and height ( h) about 1 10 of the boundary-layer thickness was placed normal to the flow. In addition to the various flow measurements described in Part I (Arya and Shipman, 1980), observations were made of the vertical and horizontal concentration profiles downwind of a point source whose height and position relative to the ridge were varied. These were compared with diffusion measurements in the undisturbed boundary layer without the ridge. For a source located upwind of the ridge, the effect of the ridge is generally to reduce ground level concentrations (g.l.c.) on the lee side of the ridge. For the source located on the ridge top, concentrations in the cavity region are likely to be significantly increased if any part of the plume impinges on the separation streamline and is entrained into the cavity. The highest ground level concentrations result when the source is located within the cavity region. The magnitude and location of the maximum g.l.c. are considerably influenced by the ridge, especially when the source is located within a distance of 15 h downwind of the ridge crest. For an elevated source located in the lee of the ridge, but outside the cavity region, the maximum g.l.c. decreases rapidly with increasing distance of the source away from the ridge, while the location of the maximum g.l.c. shifts farther away from the source. Comparisons of vertical concentration profiles at various distances from a point source with and without the ridge bring out several interesting features of dispersion in the lee of a ridge. The concentration distributions within the cavity due to sources located upwind of or at the ridge top are nearly uniform (independent of height). For a source located on the lee side near the separation streamline, concentration profiles show the effect of plume downwash with resulting high g.l.c. where the streamline reattaches to the surface.