Experimental Investigation of Free Surface Gradients in a 90° Angled Asymmetrical Open Channel Confluence

Abstract

State of the art numerical models have come up with a number of possibilities to treat the free surface (e.g. rigid-lid approach, interface-tracking methods, interface-capturing methods). Depending on the case at hand, the complexity of the free surface treatment can be changed. To make a well-informed choice, the modeller should be able to test the model performance in a selection of cases with different relevant processes. In this paper, an experimental set-up is exploited to provide a selection of cases with high-resolution data of the free surface levels in an open channel confluence. This will allow to evaluate the relative importance of the surface gradients and presents data for numerical modellers to assess the capabilities of their chosen free surface treatment for the adopted flow conditions. The selected case is a 90° angled asymmetrical confluence, with subcritical flow, but with a downstream Froude number high enough to have important contributions of the free surface gradients to the overall momentum balances in the measurement domain. Complementary, Large Eddy Simulations with a horizontal rigid-lid treatment of the free surface are performed. This will allow to evaluate the performance of this numerical methodology in the selected case of the confluence flow. In general, the free surface levels are found to be of major importance to the overall momentum balances in the Confluence Hydrodynamics Zone, and thus correct modelling of the effects of the free surface proves to be a prerequisite for correct simulation of the flow. With the data presented in this paper, the performance of the numerical models can be further tested with respect to the treatment of the free surface.