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.

The turbulent wake of a flat plate aligned with a uniform water flow and extending through the free surface was investigated experimentally. Laser-Doppler velocimetry (LDV) measurements show good agreement with published data for a two-dimensional wake, except in a shallow layer near the free surface. In this surface layer, the wake width is observed to double while the wake centerline velocity remains essentially unchanged from its value at depth, resulting in a wake momentum deficit that is twice that at depth. Instantaneous, full-field measurements of the velocity were made using digital particle image velocimetry (DPIV) to elucidate the role of vortical structures in the development of the surface layer. DPIV measurements reveal that in the deep wake, vortex structures predominantly of opposite sign exist on opposite sides of the wake centerline and contribute to a velocity and vorticity field that is two-dimensional in the mean. However, near the surface, vortex structures tend to become either surface-normal or surface-parallel and contribute to a velocity and vorticity field that is highly three-dimensional in the mean. The resulting surface layer is characterized by surface–parallel structures interacting with their images above the surface to retard and widen the surface flow to a depth comparable to the size of the vortex structures. Histograms taken from many independent DPIV realizations of the flow characterize the distribution of vorticity in the wake and verify a mean flow consistent with the LDV measurements. The existence of these structures in the surface layer is further confirmed by flow visualization using laser-induced fluorescence.

The effect of a free surface on the structure of a submerged turbulent jet is investigated experimentally. Three-component LDV measurements beneath a clean free surface show that the mean flow spreads laterally outward in a shallow surface layer much wider than the mean flow well below the surface. As the free surface is approached, velocity fluctuations normal to the surface are diminished while those parallel to the surface are enhanced. Laser-induced fluorescence is used to show that the surface layer contains fluid ejected from the jet. With the addition of surface-active agents, the surface layer is suppressed.

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A new tool for flow field measurement is in use at the David Taylor Model Basin (DTMB) The Digital Particle, Image Velocimetry (DPIV) technique developed by Willert and Gharib (1) has been used to measure the flow fields of a 2D standing wave and of a laminar vortex pair interacting with 2D traveling waves at a free surface. Application of the DPIV technique to free surface flows introduces new measurement challenges a􀍤 the position of the non-stationary surface is not known a priori, but must be determined from the video images themselves. The initial application of the DPIV technique at DTMB has shown that it is a useful tool for flow field measurement and visualization with a high throughput rate. It is possible to obtain flow field measurements in almost rea1 time, and the results are immediately available for use in directing experimental efforts. It is envisioned that a myriad of applications will be found for the DPIV technique at DTMB, including the mapping of flow fields around hull forms, appendages, and propulsors.