An experimental investigation of flow and diffusion in the disturbed boundary layer over a ridge—I. Mean flow and turbulence structure

Abstract

The influence of a low ridge on the mean flow and turbulence structure of a boundary layer is investigated through experiments conducted in a large meteorological wind tunnel. A two-dimensional triangular ridge, having a slope of 2 to 1 and height ( h) about 1 10 of the boundary layer thickness, was placed normal to the air flow. Observations were made of mean and turbulent flow in the boundary layer at various positions relative to the ridge and compared with measurements in flow without the ridge. The biggest effect of the ridge is to cause a separation bubble or cavity region of much reduced but recirculating mean flow and very intense turbulence; this region extends to a distance of 13 h downstream from the ridge top and to a maximum height of 2.5 h. In addition, there results an extended wake region in which mean velocity is reduced and turbulence is enhanced considerably to large distances ( x) behind the ridge. Observations indicate that the thickness of the wake increases roughly in proportion to ( (x h ) 1 2 , while the perturbations in mean velocity, Reynolds stress and variances of velocity fluctuations decay as ( x h ) −1 . Other observations of wake flows behind square blocks and two-dimensional gentle hills also show the obstacle-caused perturbations to decay in proportion to ( x h ) −1 , although the proportionality constants are found to be strongly dependent on the obstacle shape. There is no satisfactory theory at present to explain all these experimental results.