Effects of time dependence in unstratified tidal boundary layers: results from large eddy simulations

A three-dimensional Large Eddy Simulation (LES) model is used to simulate oscillating tidal boundary layers and test previous results obtained from one-dimensional boundary layer models and turbulence measurements in tidal channels. The LES model produces low-order turbulence statistics in agreement with the semi-analytic theory and observations. It shows a logarithmic layer in the mean velocity profile and a linear distribution of Reynolds stress with water depth. However, the eddy viscosity profile predicted by the LES model is not parabolic but better matches a parabolic profile modified by wake effect observed in the outer part of depth-limited steady boundary layers. Low-order turbulence statistics can be scaled by the instantaneous friction velocity at the bottom boundary. Although turbulence intensities in three directions fluctuate over a tidal cycle, their normalized values are in good agreement with those determined from laboratory experiments of steady open-channel flows. The LES model confirms that tidal turbulence is in quasi-equilibrium. However, it also demonstrates the importance of flow acceleration/deceleration term in the depth-integrated momentum balance for the mean flow. Phase differences are found between flows at different heights above the bottom boundary.