Experiment, Theory and Application

: An experimental study of the interaction of an underwater turbulent round jet with the free surface was conducted. Flow visualization, surface curvature measurements and hot film velocity measurements were used to study this flow. It is shown that surface waves are generated by the large scale vortical structures in the jet flow as they approach the free surface. These waves propagate at an angle with respect to the flow direction. The propagation angle increases as the strength of the interaction is increased by increasing the momentum flux of the jet or reducing the distance of the jet to the free surface or both. Propagation of these waves in the flow direction is suppressed by the surface current produced by the jet. Far downstream the surface motions are caused by the large scale vortical structures interacting directly with the surface. The fundamental scaling parameters of the free-surface jet have been determined. The velocity scale is the velocity obtained from the combination of jet momentum, density and depth of the jet and the length scale is the distance of the et to the free surface. It is shown that the centerline velocity decay when scaled with these parameters collapses to a universal curve for different depths of the jet.

Interaction of a turbulent round jet with the free surface

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

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112092002891

Turbulence measurements in a round jet beneath a free surface

Results of an experimental study of the interaction of a turbulent jet with a free surface when the jet issues parallel to the free surface are presented. Three different jets, with different exit velocities and jet-exit diameters, all located two jet-exit diameters below the free surface were studied. At this depth the jet flow, in each case, is fully turbulent before significant interaction with the free surface occurs. The effects of the Froude number (Fr) and the Reynolds number (Re) were investigated by varying the jet-exit velocity and jet-exit diameter. Froude-number effects were identified by increasing the Froude number from Fr = 1 to 8 at Re = 12700. Reynolds-number effects were identified by increasing the Reynolds number from Re = 12700 to 102000 at Fr = 1. Qualitative features of the subsurface flow and free-surface disturbances were examined using flow visualization. Measurements of all six Reynolds stresses and the three mean velocity components were obtained in two planes 16 and 32 jet diameters downstream using a three-component laser velocimeter. For all the jets, the interaction of vorticity tangential to the surface with its ‘image’ above the surface contributes to an outward flow near the free surface. This interaction is also shown to be directly related to the observed decrease in the surface-normal velocity fluctuations and the corresponding increase in the tangential velocity fluctuations near the free surface. At high Froude number, the larger surface disturbances diminish the interaction of the tangential vorticity with its image, resulting in a smaller outward flow and less energy transfer from the surface-normal to tangential velocity fluctuations near the surface. Energy is transferred instead to free-surface disturbances (waves) with the result that the turbulence kinetic energy is 20 YO lower and the Reynolds stresses are reduced. At high Reynolds number, the rate of evolution of the interaction of the jet with the free surface was reduced as shown by comparison of the rate of change with distance downstream of the local Reynolds and Froude numbers. In addition, the decay of tangential vorticity near the surface is slower than for low Reynolds number so that vortex filaments have time to undergo multiple reconnections to the free surface before they eventually decay.

Turbulent structure in free-surface jet flows

This paper documents measurements of the mean velocity field and turbulence statistics of an isothermal, round jet entering a shallow layer of water The lower boundary of the jet was a solid wall and the upper boundary a free surface The jet axis was midway between the solid wall and the free surface in all cases Experiments were performed at a Reynolds number of 22,500 for water layer depths 15, 10, and 5 times the jet exit diameter (9 mm) Particle image velocimetry measurements were made on vertical and horizontal planes—both containing the axis of the jet The measurements were taken from 10 to 80 jet diameters downstream Results showed that, for the highly confined cases at downstream locations, the axial velocity was quite uniform over the depth, with a mild peak below the jet axis In the horizontal plane, the velocity profiles were slightly narrower than the free jet profile, but in the vertical plane, they were wider The mean vertical velocity profiles showed that entrainment was suppressed 

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Confinement Effects in Shallow-Water Jets

The decay of a jet discharging from a circular nozzle parallel to and displaced from a solid surface is investigated under conditions where the transitional process from circular-jet flow to oblate wall-jet flow begins in the initial, transition or self-preserving regions of the original jet. The influence of displacement of the nozzle from the plane on the developed three-dimensional wall jet downstream is demonstrated and it is found that the transitional interaction with the plane is more extended when the plane interacts first in the initial zone of the circular jet. Measurements of turbulence and Reynolds stress show the transverse mixing parallel to the plane to exceed that perpendicular to the plane, and are generally consistent with the spreading rates in these two directions, the ratio of which approaches 8·5 at large distances from the nozzle. It is shown that the interaction between the plane and jet involves a relatively large-scale coherent motion in which components of velocity directed towards or away from the surface are associated with outflow or inflow along the surface. This motion is more extended in the direction parallel to the surface and provides a mechanism for the increases in mixing rate in the direction parallel to the plane.

H. Winarto

Papers

Jet diffusion from a circular nozzle above a solid plane