Free surface effects on the statistical properties of a submerged rectangular jet

Abstract

The results of an experimental investigation of turbulent rectangular jets offset from a free surface are presented. Two rectangular jets of aspect ratio 2 and 4 were examined and the results were compared to a square jet at the same offset height ratio of h/De ≈ 2.7, where De is the circle-equivalent diameter of the nozzle. A particle image velocimetry (PIV) was used to measure the mean flow and turbulent characteristics at a Reynolds number of 7900 and a Froude number of 1.29. The results indicate significant enhancements in the jet spreading rate and mean streamwise velocity decay rate for the larger aspect ratio nozzles. The results reveal that the jet-free surface interaction had a greater impact on the mean surface-normal velocity than the mean streamwise velocity. The values of the turbulence intensities, Reynolds shear stress and structure parameter were found to be nearly independent of the nozzle aspect ratio. Surface mean velocity and turbulence intensities were also measured to characterize the influence of the shear layer on the free surface. The results indicate a damping of the surface-normal turbulence intensities compared to the streamwise turbulence intensity. The influence of the free surface was felt as an enhancement in streamwise fluctuating velocity two-point correlation and a suppression of the surface-normal fluctuating velocity two-point correlation. The free surface also increased the structure inclination angle in the upper shear layer compared to that in the lower shear layer.