Abstract
The modelling of complex free surface flows over weirs and in the vicinity of bridge piers is presented in a numerical model emulating open channel flow based on the Reynolds Averaged Navier-Stokes (RANS) equations. The importance of handling the turbulence at the free surface in the case of different flow regimes using an immiscible two-phase RANS Computational Fluid Dynamics (CFD) model is demonstrated. The free surface restricts the length scales of turbulence and this is generally not accounted for in standard two-equation turbulence modelling approaches. With the two-phase flow approach, large-velocity gradients across the free surface due to the large difference in the density of the fluids can lead to over-production of turbulence. In this paper, turbulence at the free surface is restricted with an additional boundary condition for the turbulent dissipation. The resulting difference in the free surface features and the consequences for the solution of the flow problem is discussed for different flow conditions. The numerical results for the free surface and stream-wise velocity gradients are compared to experimental data to show that turbulence damping at the free surface provides a better representation of the flow features in all the flow regimes and especially in cases with rapidly varying flow conditions.
Highlights
The interface between air and water or the free water surface is one of the most significant features of open channel flow
From the results presented for sub-critical flow throughout the channel with an embankment weir and a pair of bridge piers, it is seen that the representation of the free surface is similar in simulations with and without free surface turbulence damping (FSTD) treatment
From the results for the transitional flow regime with a plunging jet flow on an embankment weir, it is seen that the difference in the calculation of the free surface and the recirculation pattern is significantly influenced by the over-production of turbulence around the free surface in the absence of the FSTD algorithm
Summary
The interface between air and water or the free water surface is one of the most significant features of open channel flow. The presence of the free surface results in the elongation of the turbulent eddies parallel to the flow and dissipation of the turbulent energy in the direction normal to the free surface This turbulence dissipation at the free surface must be included using additional boundary conditions in RANS turbulence models. Numerical simulations are carried out for controlled and free stream outflow conditions to obtain different flow conditions such as sub-critical flow throughout the domain, change from sub-critical to super-critical flow regime, transitional flow, and change from sub-critical to super-critical and back to sub-critical These changing flow conditions are investigated for free surface flow over hydraulic structures such an embankment weir, a broad-crested weir and in the vicinity of bridge piers with focus on the boundary condition for the turbulence dissipation at the free surface. The change in the distribution of the eddy viscosity around the free surface and the stream-wise velocity profiles in the presence and absence of the free surface turbulence boundary condition is analyzed to investigate the influence and importance of turbulence damping at the free surface in the different cases
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