The Influence of Reynolds and Froude Number on the Structure of Turbulence in Free Surface Jets

In this study, circular jets issuing beneath and parallel to a free surface were examined and the interaction between the subsurface turbulence and the free surface was investigated. The effects of the main parameters, the Froude number and the Reynolds number, and qualitative features of the interaction of the subsurface flow and free-surface were identified using flow visualization, three-component laser velocimetry and particle image velocimetry. For all the cases, vorticity that is tangential to the surface interacts with its 'image' above the surface. This results in large tangential velocity fluctuations at the surface and contributes to an outward flow near the free surface. It is also shown to be 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 free surface. Energy is transferred instead to free-surface disturbances (waves). This results in turbulence kinetic energy levels that are lower by as much as 20% for this case. The loss of turbulence kinetic energy also results in lower Reynolds shear stresses at high Froude number. At high Reynolds number, the jet evolved more slowly in the presence of the free surface, based on examination of the local Reynolds and Froude numbers for the jet. When the surface deformations caused by surface -parallel vorticity are large, secondary vorticity is generated in the stagnation regions near the free surface. Topological signatures at the free surface were identified. These include an attached vortex tube being stretched by the mean shear, an unstable focus signature at the free surface, and a tangential vortex tube, oriented in the mean shear direction, interacting with the free surface.