TURBULENT STRUCTURE IN FREE-SURFACE JET FLOWS

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 an d jet-exit diameters, all located two jet-exit diameters below the fre e surface were studied. At this depth the jet flow, in each case, is f ully turbulent before significant interaction with the free surface oc curs. The effects of the Froude number (Fr) and the Reynolds number (R e) were investigated by varying the jet-exit velocity and jet-exit dia meter. Froude-number effects were identified by increasing the Froude number from Fr = 1 to 8 at Re = 12700. Reynolds-number effects were id entified by increasing the Reynolds number from Re = 12700 to 102000 a t 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 thre e-component laser velocimeter. For all the jets, the interaction of vo rticity tangential to the surface with its 'image' above the surface c ontributes to an outward how near the free surface. This interaction i s also shown to be directly related to the observed decrease in the su rface-normal velocity fluctuations and the corresponding increase in t he tangential velocity fluctuations near the free surface. At high Fro ude number, the larger surface disturbances diminish the interaction o f the tangential vorticity with its image, resulting in a smaller outw ard how 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 turbule nce kinetic energy is 20 % 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 t he rate of change with distance downstream of the local Reynolds and F roude numbers. In addition, the decay of tangential vorticity near the surface is slower than for low Reynolds number so that vortex filamen ts have time to undergo multiple reconnections to the free surface bef ore they eventually decay.