Investigating scale effects of a hydraulic physical model with 3D CFD

In the present study, the three-dimensional (3D) Computational Fluid Dynamics (CFD) Volume of Fluid (VOF) method is validated to reproduce hydraulic free surface flows over a labyrinth weir and a spillway for several flow rates using the open source toolbox OpenFOAM 3.0.1 and the commercial CFD package ANSYS Fluent 17.2. The CFD solvers are employed to simulate the 1:25 scale Froude number similarity physical model of the scheme, with validation conducted using experimental observations and measurements. It is found that both solvers are capable of accurately reproducing the velocities and depths measured in the physical model and are also able to capture complex flow features. The models are applied to simulate the prototype hydraulic flows so that scale effects from the physical model can be quantified. Results show the overall decrease in water depth and increase in velocity in the prototype can be up to 15% and 10%, respectively, for the lower flow rates, with scale effects reducing for larger flow rates. The prototype scale simulations also exhibit some variation in the labyrinth weir rating curve when compared to the scaled case; showing lower heads upstream of the crest for the same discharge. As theory would suggest, discrepancies in the rating curve at the two scales are more pronounced for low flow rates.