Recent experiments, supported by direct numerical simulation, indicate that turbulence structure near boundaries is governed by a non-dimensional shear rate which is the ratio of the turbulent energy production rate to the dissipation rate. For high values of the shear rate, organized low speed/high speed "streaky" structures are observed that periodically break down into "bursts." The qualitative features of the streaks and bursts are similar, no matter what the boundary condition, e.g. they are qualitatively alike at walls and at gas-liquid interfaces at the same shear rates.
At low shear rates, the structures observed are "patchy" and appear to arise from turbulence generated in regions removed from the boundary. For example, turbulent patches are formed at the free surface from bursts generated at the wall for open channel flows where there is no shear at the surface. Patches at fluid-fluid interfaces exhibit a damping of the velocity fluctuation component normal to the interface with redistribution of kinetic energy, through the pressure-strain correlation term, into the components parallel to the interface. As a consequence the intensity of fluctuations parallel to the interface is enhanced.
From the viewpoint of scalar transport at interfaces, these considerations lead to different dominant mechanisms depending on the shear rate. For shear rates high enough to form streaks and bursts in the interface region, excellent predictions of transport rates are obtained considering that the interfacial bursts govern the process. For shear rates that lead to patches, transport rates are related to parameters associated with these patches.
As the burst periods are seen to scale best with inner variables (local shear velocity and viscosity), transport rates are also predicted to scale with interfacial shear velocity for high shear rates. On the other hand, patch area and patch residence time−the two important parameters governing transfer rates at low shear rates, scale with mixed wall and outer flow variables, e.g. wall shear velocity and mean flow velocity. Consequently, the transfer coefficients also are predicted to scale with a mix of these variables.
For situations where the turbulence structure far from the interface is homogeneous and isotropic, e.g. grid generated, the modifications to structure close to a shear-free surface are well predicted by superposing the effect of image eddies to cancel the normal velocity components at the surface. Transfer rates based on these calculations indicate that the large eddies dominate when the turbulent Reynolds number, based on integral scales, is 0(100), but the small eddies (in the dissipation range) dominate when the turbulent Reynolds number is 0(1000). These results hold for non-wavy surfaces. Wave-turbulence interactions and are still under investigation.