Numerical and Experimental Studies of Flows in Open-Channels with Gravel and Vegetation Roughnesses.

摘要

This study aims to quantify the gravel and vegetation induced roughness effects on flows and mixings in open-channels using experimental and numerical methods. Firstly, a Double Averaged Navier-Stokes equation (DANS) model has been developed for depth-limited open-channel flows over gravels. Noting that the turbulence length scale within the gravel layer is governed by the gravel size, the DANS model incorporating the drag force method (DFM) and a modified Spalart-Allmars (S-A) turbulence closure is proposed. The turbulence length scale parameter in the S-A model is modified to address the change in the turbulence structure within the gravel layer. The computed velocity profiles agree well with the corresponding measured profiles in all cases. Particularly, the model reproduces the S-shape velocity profile for sparsely distributed large size roughness elements. Secondly, laboratory measurements of the velocity profiles and flow resistances of depth-limited open-channel flows over fixed gravel patches (GPs) under different bed slopes and flow rates were carried out. Two GPs with identical individual element size and different lengths were tested. The measured double-averaged (DA) velocity profiles were found to fit well with the log law and defect law with a non-universal Karman constant κ. Under relatively small submergence, the κ-value decreases to 0.22 for the fitting the velocity profiles by the logarithmic flow resistance law. Finally, the hydrodynamics of flows over a finite length flexible vegetation patch (VP) was investigated in the laboratory. The VP, which retarded the flow within the canopy and accelerated the overlying flow, appeared to be swaying under different flow rates. Comparing to the case with rigid VP, the high-level turbulence region within the adjustment region occurred at a farther downstream distance from the leading edge for the case with flexible VP. The existence of the Kelvin-Helmholtz (K-H) vortices within the shear layer is confirmed by both the flow visualization and the quadrant analysis. The flow evolution within the VP was successfully replicated by a three-dimensional Reynolds Averaged Navier-Stokes (RANS) model incorporating the DFM and the S-A turbulence closure. In summary, the present research contributes to the knowledge and understanding of open-channel flows with gravel and vegetation roughnesses.