2017 – CFD Simulation of Pressure Fluctuations in Plunge Pools: In Search of a New Method

Shayan Maleki, James Apostolidis, Tom Ewing, Virgilio Fiorotto

The stability analysis of dam spillways and stilling basin chutes requires the knowledge of the spatially fluctuating pressure at the bottom of the structure with reference to the large vortex system with dimensions comparable with the structure characteristic length of the order O (0.1 –1 m). In this context only the small frequency pressure fluctuations (smaller than 1 –10 hz in prototype) must be analyzed in Large Eddy Simulation (LES)context; while the higher frequency pressure fluctuations could be filtered given their negligible importance in relation to stability computations with reference to the spatial Taylor macroscale and fluctuating pressure variance evaluation. These two quantities allow us to define the variance of the force acting on the structure, and as a consequence via statistical analysis, the design force on the structure. This procedure is historically performed via.physical hydraulic modelling (PHM)where these quantities are measured in a laboratory setup. Considering the limits of.current industry approach to Computational Fluid Dynamics (CFD), the use of Detached Eddy Simulation (DES) could become a valid low cost solution and could potentially be a valid method to perform preliminary studies in order to refine the design while avoiding expensive physical model modifications. In this paper, the pressure field at the base of a rectangular impinging jet is measured in laboratory flume setup and is compared with the numerical results obtained via equivalent DES simulations conducted in CFD.Maximum values and the structure of spatial correlation of the anisotropic field of fluctuating pressures are described in view of their relevance to the structural design of the lining of spillway stilling basins and other dissipations structures,as well as in view of their relevance to rock stability analysis. The comparison of the laboratory study with DES simulations presented in this paper shows a good agreement indicating.that this approach may eventually provide a lower.cost substitute for physical model studies in the design of stilling basins and plunge pools.However,it is acknowledged that virtually all stilling basins and plunge pools present a three-dimensional hydraulics complexity, and numerous.further studies need to be done.

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