Characteristics of a horizontal square jet interacting with the free surface

Abstract

The characteristics of a horizontal submerged square jet interacting with the free surface are investigated experimentally. A particle image velocimetry system is used to measure the mean flow and turbulent characteristics in the vertical symmetry plane of the jet at a Reynolds number of approximately 5500. It is shown that the effect of the free surface is to cancel the turbulent-nonturbulent interface and reduce the mean spanwise vorticity, Reynolds shear stress, and jet spreading rate in the upper shear layer in comparison to those measured in the lower shear layer. The results indicate that lowering of the jet offset height ratio increases the confinement effect of the free surface. Stronger confinement reduces the maximum mean streamwise velocity decay rate and mean surface velocity defect in the interaction region. The results also indicate a dramatic reduction in surface-normal turbulence intensity in the interaction region due to the damping effect of the free surface on the surface-normal velocity fluctuations. The decay of the surface-normal turbulence intensity near the free surface produces commensurate reductions in the surface-normal velocity fluctuation autocorrelation in the upper shear layer while enhancing the streamwise velocity fluctuation autocorrelation. The damping effect of the free surface on the surface-normal velocity fluctuations is found to be connected to the redistribution of the turbulent kinetic energy from the surface-normal velocity fluctuations to the streamwise velocity fluctuations.