Modelling tidal channels preliminary results in turbulence

This paper presents preliminary results from turbulence modelling in channels. Identifying key characteristics of turbulence within tidal flows and providing a basis for further study of turbine specific turbulence behaviour. Numerical simulations have been performed using Code_Saturne [1], EDFs open source finite-volume code. Comparisons of Large Eddy Simulation and unsteady-averaged Navier Stokes methods for capturing the turbulence signatures present in empty channel flow are presented. Paying particular attention to resolved power spectral density distributions of turbulence. As well as vorticity isoplane analysis. Synthetic eddy inlet conditions [2] were used to model fully turbulent flow at the inlet. Results of numerical simulations of a test flume are presented using three turbulence modelling techniques - LES, Rij - ε SSG and k - ε. It is found that the k - ε model performed most poorly at resolving the turbulent eddies. Whilst the Rij - ε SSG model is proven to be a possible alternative to the more computationally expensive LES model when looking at large to mid scale turbulence. Analysis of viscosity isoplanes indicates that both the Rij - ε SSG and k - ε turbulence models dampened out large scale turbulence at a greater rate than the LES model - though a reasonably similar distribution of turbulent eddies still propagated downstream with the Rij - ε SSG model. Whilst the results from all three models could benefit from further mesh refinement the turbulent forces acting across a hypothetical turbine region is show to be anti-symmetric. This anti-symmetry across the rotor plane requires further study and inclusion in wake-mixing models used in tidal farm layout tools.