Effect of Shallowness on the Flow Field of a Horizontal Turbulent Water Jet

Abstract

Particle image velocimetry (PIV) measurements are reported for an isothermal round jet entering a shallow layer of water. The lower boundary is a solid wall and the upper boundary is a free surface. Confinement effects in the horizontal plane are negligible but there is substantial confinement in the vertical direction. Measurements were made with a jet exit Reynolds number of 9000. Results are reported for receiving water depths of 5 and 15 times the jet exit diameter (9 mm). The velocity field was measured on both vertical and horizontal planes, where the jet axis was included in all cases. Spatial resolutions in the range 1-2 mm were achieved. This paper reports features of the mean velocity field and several large-scale turbulence structures identified by performing a Galilean transformation on the low-pass filtered instantaneous velocity field. The velocity field measured in the axial locations (58 – 72 times the jet exit diameter) showed that the axial velocity profile is almost uniform throughout the shallow water depth. In the horizontal plane, the velocity profiles at this axial location were only slightly different than for an unconfined jet for both depths considered. The Galilean decomposition revealed the presence of quite large vortices in the shallow layer which spanned about 30-50% of the liquid depth. These vortices are found to be responsible for the formation of the mild peak of the axial velocity located in the lower side of the shallow water layer. In addition, there is an evidence of the occurrence of the pairing process in the flow.