Abstract
ABSTRACTResults from large-eddy simulations and complementary flume experiments of turbulent open channel flows over bed-mounted square bars at intermediate submergence are presented. Scenarios with two bar spacings, corresponding to transitional and k-type roughness, and three flow rates, are investigated. Good agreement is observed between the simulations and the experiments in terms of mean free surface elevations and mean streamwise velocities. Contours of simulated time-averaged streamwise, streamfunction and turbulent kinetic energy are presented and these reveal the effect of the roughness geometry on the water surface response. The analysis of the vertical distribution of the streamwise velocity shows that in the lowest submergence cases no logarithmic layer is present, whereas in the higher submergence cases some evidence of such a layer is observed. For several of the flows moderate to significant water surface deformations are observed, including weak and/or undular hydraulic jumps which affect significantly to the overall streamwise momentum balance. Reynolds shear stress, form-induced stress and form drag are analysed with reference to the momentum balance to assess their contributions to the total hydraulic resistance of these flows. The results show that form-induced stresses are dominant at the water surface and can contribute significantly to the overall drag, but the total resistance in all cases is dominated by form drag due to the presence of the bars.
Highlights
Important advances have been made in recent decades, a comprehensive understanding of the turbulence structure and the flow resistance in shallow rough-bed flows remains elusive
Note that the Bathurst data correspond to gravel bed roughness and were originally presented in terms of H /D84, where D84 corresponds to the 84th percentile used to represent the coarse gravel fraction (i.e. 84% of the gravel elements are smaller than D84)
Results from large-eddy simulations and complementary flume experiments of turbulent open channel flows over bed-mounted square bars at intermediate submergence have been presented
Summary
Important advances have been made in recent decades, a comprehensive understanding of the turbulence structure and the flow resistance in shallow rough-bed flows remains elusive. Bathurst (1985) summarized that most standard resistance equations are applicable only to rivers with gentle slopes, and those that have been developed for steep stretches are either empirical or valid only for deep flows. In many shallow flows of practical interest the mean flow depth, H, is of the same order of magnitude as the roughness height, k, and the well-known concept of the logarithmic boundary layer (LBL) may no longer be applicable (Jiminez, 2004; Raupach, Antonia, & Rajagopalan, 1991). Several early attempts at modifying the LBL for application to gravel bed rivers with steep slopes and low submergences resulted in a number of empirical or semi-empirical formulations for the well known Darcy-Weisbach friction factor, f, which is defined for flow in an open channel as follows:
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