LANS-α turbulence modeling for coastal sea: an application to Alderney Race

Abstract The Lagrangian-Averaged Navier-Stokes-α (LANS-α) turbulence model was implemented for the first time in a coastal hydrodynamic model. We present in this paper the details of the implementation, as well as the difficulties encountered. To overcome the difficulties, a convolution filter was used instead of the Helmoltz operator, and incompressibility was imposed in both rough and smooth velocities. The results of the second numerical implementation were tested against the results of simulations without LANS-α for a realistic application showing the tidal dynamic in Alderney Race, Normandy, France, which has the strongest currents in western Europe. The behavior of LANS-α is consistent with the conclusions of former studies, which supports our results. The findings are: i) LANS-α re-energizes the flow recovering higher-resolution turbulence statistics in lower-resolution simulations, leading to 30% savings in computing time, ii) LANS-α produces the two kinds of inertial range for barotropic turbulence, with turbulent energy decays in k − 5 / 3 and in k − 3 (k being the wave number) and iii) LANS-α strongly characterizes the turbulence induced by deformation. In the future, these results need to be compared to measurements and to other turbulence modeling approaches, including dissipative LES or DNS, to evaluate their relevance for the community.