Author(s): Rachel Batt
Linked Author(s):
Keywords: Bed roughness; Density currents; Lock - release; CFD
Abstract: Results are presented in which the effects of the spacing of bed roughness elements on the front speed of horizontal, turbulent lock - release density currents are modelled numerically and validated using a new dataset from laboratory experiments. It has been observed (Kneller et al., 1999) that the effects of some bed roughnesses on density currents are substantial enough to change the fundamental fluid dynamics of the current with consequent implications for the fluid bodies affected by these complex flows. For example, changes to the propagation of density currents of pollutants, such as waste effluent, in rivers or reservoirs could have severe consequences. The experiments and simulations presented herein were designed to study the effects of changing the element spacing of one type of roughness on the front speed of a saline density current with a 5% density excess. The chosen roughness is a 6mm high beam type roughness with four spacings, 6, 12, 24 and 36mm, of which a 6mm spacing is categorised as a ‘d - type’ roughness while the other three are so - called ‘k - type’ roughnesses (Perry et al., 1969). The numerical model is shown to suffer from known limitations due to its two - dimensionality which are exaggerated in the rough case, however, the effects of element spacing on the flow can be deduced. The results clearly show an overall decrease in front speed with increased element spacing and transitions in speed inconsistent with the typical phases associated with density current flow over hydraulically smooth surfaces (Rottman and Simpson, 1983).
Year: 2007