Unsteady Friction Approaches Applied to Mixed Flow Modeling

Abstract Submission: For many years, various closed-pipe models were developed to represent highly dynamic unsteady flows, in which unsteady friction plays an important role in energy dissipation. On the other hand, various urban water systems experience highly dynamic flow conditions associated with mixed flow, i.e., when pressurized and free surface flow exist. The Saint-Venant equations, originally formulated for open-channel flows, can also be applied to represent mixed flow conditions through conceptual models such as the Preissmann Slot or the Two-Component Pressure Approach (TPA). Yet, unsteady friction formulations have never been applied within the context of Saint-Venant equations to address pressurized flows and mixed flow conditions, which consists in an important knowledge gap that needs to be addressed. This study adapts methods originally designed for unsteady pressurized flows for the Saint-Venant equations to improve the energy dissipation. Two unsteady friction methods were selected for this study. In the first method, the friction term depends on the average velocity, with a weighting function calculated based on the changes in velocity from previous time steps. The second method accounts for the damping caused by unsteady friction as a result of both local and convective acceleration. The numerical results are validated against experimental data available in the literature, showing potential improvements in Saint-Venant equation predictions.

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