THE EFFECT OF ROUGHNESS BARS ON TRANSVERSE MIXING IN HYDRAULIC MODELS

Abstract

Similarity of horizontal diffusion in hydraulic models is generally difficult to achieve because the diffusion law is often not known and small scale ratio is often used. It is generally accepted that use of roughness elements is necessary to generate turbulence in the small-scale hydraulic models. Quantitative assessment of such roughness elements, e. g. roughness bars or strips, with regard to the horizontal diffusion, however, has not been studied extensively. In this paper, a flume study is carried out to obtain the velocity defect law in the presence of roughness bars. Based on the obtained velocity defect law, the horizontal turbulent diffusivity is derived using the Prandtl's concept of mixing length and the Reynolds' analogy. Experimentally verified law of horizontal diffusivity thus is expressed in terms of flow velocity, radius of the bar elements and spacings of roughness bars in two horizontal directions.