Study of a free surface in open-channel water flows in the regime from “weak” to “strong” turbulence

A liquid–air interface in an inclined open-channel water flows was studied experimentally as the flow changes from “weak” to “strong” turbulence. In this regime, the interface is highly agitated by bulk eddies and waves, but not broken. The surface deformation statistics were obtained under a variety of conditions, including different inclination angles and flow rates. The parameter space is described in terms of Reynolds, Froude, and Weber numbers. The surface-normal displacements were obtained via the time series of the fluctuating flow depth with an ultrasound transducer. Independently, the in-plane changes in surface structures were acquired with a high-speed camera. These structures are seen as surface cells. By applying a newly developed image processing technique, the cell celerity was found to agree well with the mean flow velocity. This suggests that the cells appear when a turbulent surface-renewal eddy interacts with the interface. As the flow changes to strong turbulence, the turbulence–interface interactions become dominant over the wave phenomena, and the turbulent structures at the surface become more 3D (similar to those in the bulk flow), compared to quasi-2D structures in the weak turbulence.