Effect of externally generated turbulence on wave boundary layer

This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel with a smooth bottom.

Turbulence was generated ´externally´ as the flow in the oscillator was passed through a series of grids, that extended from the cover of the water tunnel to about mid-depth. Two different types of grid porosities were used. Direct measurements of the bed shear stress and velocity measurements were carried out.

For the velocity measurements, mean and turbulence properties were measured in both the streamwise direction and in the direction perpendicular to the bed. A supplementary measurement for the undisturbed (without grids) case was also carried out, for comparison with the grid results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids.

It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore the friction coefficient. Other features related to the bed shear stress, such as transition, the friction factor and phase lead are discussed.

The range of the Reynolds number studied is 10.000 - 2.000.000