Laboratory characterization of free surface turbulence: towards strong regimes

Free surface flows can span across a wide range of turbulent conditions, from quiescent rivers and lakes to very turbulent ship wakes. While differences between the turbulence regimes manifest clearly in the free surface characteristics, they are also present in the subsurface turbulence structure. For example, numerical studies under very strong turbulence support the vanishing of the energy redistribution mechanism near the surface, with direct application to bubble entrainment modelling. Experimental observations today mainly focus on weak turbulence regimes, leaving such findings unexplored. In this study we extend experimental investigations to stronger turbulent conditions than previously achieved, reaching bubble entraining regimes. Forcing and geometrical symmetry typically found in enclosed facilities are adapted here to an open configuration to generate homogeneous and isotropic turbulence at a distance below the surface. Alternative flow actuators enable higher turbulence levels without sacrificing flow quality; flow quality metrics (e.g. homogeneity coefficient) are on par with previous studies. In the vicinity of the surface, we observe the development of a zone where flow characteristics are influenced by the interaction between the air‑water interface and the underlying turbulent field–promoting turbulence redistribution, energy exchange and modulation of the integral length scales, similar to the one observed for the associated fluctuating velocities.