Validation of the multiphase-flow model for freely overfalling flows
Overview: A free overfall is a situation in which the flow faces a sudden drop in the channel base, causing its separation from the solid boundary to form a free nappe. In open channels, free overfall offers the possibility of being used as a measuring device of discharge. Free overfall flows can be observed at nick point or head-cut of river channels. The water flow can evolve from subcritical to supercritical near the free fall region, and departure from hydrostatic distribution of pressure is thus expected.
The challenge: The engineering challenge here is to predict the pressure and velocity distribution of water stream which can turn from subcritical to supercritical regime. Of interests to the designers is the time evolution of surface profile, bed pressure, bed shear stress, for zero, adverse and favorable slopes. Other parameters are also important to determine, including the depth of flow (measured normal to bed) at the end and the
brink depth or end depth, and its dependence on the relative slope (defined as the ratio of bed slope to critical slope).
TransAT Solution: In contrast to analytical modelling where a large number of assumptions are made regarding the pressure and velocity distribution at the end section, the 2D simulation shown here is based on solving the multiphase flow equations where water and air are separated by a resolved interface, using the so called Level-Set approach. The simulation was performed for two grid resolutions (1mm and 4mm per cell). The obtained numerical results have been validated against experimental data as shown below (Normalized pressure vs relative height and axial velocity vs relative height), and showed a very good agreement for both mesh resolutions.
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