Turbulence measurements in a round jet beneath a free surface

Abstract

The results of an experimental investigation of a turbulent jet flow issuing from a circular nozzle beneath and parallel to a free surface are presented. Measurements of the mean velocity vector and all components of the Reynolds stress tensor were made using a three-component, underwater laser-Doppler velocimeter (LDV). Visualizations of the flow field using both fluorescent dye and free-surface shadowgraphs were made in support of the measurements. The jet is observed to form a shallow surface current, the lateral extent of which is significantly greater than that of the primary jet flow that produces it. Flow visualization reveals the surface current to consist largely of fluid structures ejected from the jet. These structures remain coherent within the current, apparently as a consequence of reduced turbulent mixing just beneath the surface. LDV measurements of the turbulence within the surface current reveal that near the jet centreline, where the interaction between the jet and the free surface is most intense, the velocity fluctuations normal to the free surface are diminished in approaching the surface, while the fluctuations parallel to the surface are enhanced. Away from the jet centreline, toward the edges of the surface current, the vertical and cross-stream fluctuations become approximately equal in magnitude, whereas the streamwise fluctuations become diminished. This is attributed, in part, to orbital motions beneath surface waves generated by the interaction of large-scale jet structures with the surface. When oleyl alcohol, an insoluble surface-active agent, was added to the free surface, the surface current was not observed. Comparisons between a jet issuing beneath both clean and surfactant-contaminated free surfaces, and a jet issuing beneath a solid wall are made to identify the role of streamwise vorticity and of secondary vorticity generated at boundaries on the development of these flows.