The influence of a free surface on the development of turbulence in a submerged jet.

Abstract

Radar images of the ocean surface made when surface ships are moving within the imaged area reveal distinctive surface signatures attributed to the interaction of the turbulent wake of the ship with the free surface. In order to study the behavior of turbulence near a free surface, the flow in a round, turbulent jet issuing beneath and parallel to a clean free surface was investigated experimentally. A three-component Laser Doppler Velocimeter (LDV) was used to make detailed measurements of the mean flow velocity and Reynolds stress tensor throughout the flowfield. Surface shadowgraphs and Laser Induced Fluorescence (LIF) were used to visualize features of the free surface deformations and the subsurface flowfield. The jet Reynolds number, $U\sb{e}d/v\approx$ 12,700, and Froude number, $U\sb{e}/(gh)\sp{1/2}\approx$ 5.66, were comparable to those of the jet flow investigation of Bernal and Madnia, 1988. Large-scale turbulent structures within the jet generated surface waves that were observed to propagate nearly perpendicular to the jet axis. Measurements of the wavelength and wave speed from shadowgraph images showed these waves to be gravity-capillary waves. The LDV measurements revealed that near the jet centerline, the RMS velocity fluctuations become anisotropic as the free surface is approached: The fluctuations normal to the surface are diminished, while those parallel to the surface are enhanced. Measurements made near the surface on either side of the jet revealed the existence of a shallow surface current much wider than the primary jet flow. Within this current, the magnitudes of the cross-stream and vertical RMS velocity fluctuations are approximately the same, but are greater than that of the streamwise fluctuations. This is attributed to motions arising from surface waves propagating perpendicular to the jet axis and superposed on the surface current. LIF images of the surface current show it to consist largely of fluid emitted from the jet. Beneath a clean free surface, these emissions propagate to considerable distance under the influence of their images above the surface. When a surfactant is placed onto the water surface, vortical fluid ejected from the jet interacts with secondary vorticity generated beneath the surfactant covered surface, and the surface current is suppressed.