Wave influence on turbulence length scales in free surface channel flows

In order to predict sediment movements in coastal environments, the interaction between these particles and turbulence should be better understood. Although previous studies have particularly shown the importance of the turbulence length scales on sediment transport for current flows, few measurements have been made on wave/current flows. The purpose of our experiments is to get a better knowledge on wave action on these characteristic length scales. For this study, in the context of a grid-generated turbulence, we aimed to describe evolution of turbulence macro and micro scales in two kinds of free surface flow. Indeed, current and wave/current flows are studied. Two data analysis techniques are used to estimate these characteristic length scales depending on flow conditions. Whereas a well-known energetic method is used for current flow, a specific analysis based on correlation measurements is lead to describe temporal evolution of turbulence length scale over the wave period. As a main result, we show that the free surface causes a vortex stretching for current flow and that turbulence length scales follow a periodic evolution with a frequency which is twice as the swell period. The turbulence length scales also depend on wave period and amplitude.