Effects of Nozzle Geometry on Turbulent Characteristics and Structure of Surface Attaching Jets

The effects of nozzle exit geometry on the characteristics of a surface attaching jet were investigated experimentally. Three different types of nozzle geometries were studied, including a circular, square and rectangular nozzle with the same exit area. The jets were discharged with an offset height of two nozzle width at a Reynolds number of 5500. A particle image velocimetry system was used to measure the flow. Instantaneous visualizations revealed that the largest enhancements in the near field mixing and entrainment occur in the minor plane of the rectangular nozzle compared to the square and circular nozzles resulting in more rapid expansion of the shear layer. This also caused a faster deflection of the jet towards the free surface, maximum velocity decay and spread rates of the rectangular jet with minor axis orientation. The jet-surface interaction was examined using surface velocity, vorticity thickness and surface turbulence intensities. It was found that the damping of the surface-normal velocity fluctuations at the free surface is more severe in the minor plane of the rectangular jet than the square and circular nozzles. The influence of the free surface was also felt in the profiles of mean velocity and Reynolds stresses. The attenuation of Reynolds shear stress due to confinement was more dramatic in the minor plane of the rectangular nozzle than the other geometries. To quantify the influence of nozzle geometry on the coherent structures in the interaction region, two-point correlations of the velocity fluctuations and swirling strength; and proper orthogonal decomposition were performed.