STUDY ON TURBULENT CIRCULAR JET IN THE VISCINITY OF A FREE SURFACE

Abstract

When a circular jet emerging from a orifice with a uniform velocity into a large stagnant mass of the fluid. The size of the jet increases steadily as it travels away from the orifice. Upto certain point from the orifice, there is a core of flow with undiminished velocity. After this point, turbulence generated on the boundaries penetrates to the axis and the mean velocity on the axis begins to decay along axial direction. Jets encountered in nature and in technological applications are usually confined due to the presence of rigid boundaries, free surfaces, or strong stratification. Among the most important of these is the free surface confined jet which is a frequently encountered problem in engineering practice and consists of a jet that enters a body of water, where the proximity of the top free surface is expected to influence the flow. The turbulent flow below a gas-liquid interface plays an important role in diverse areas ranging from environmental flows and industrial mixing processes to the remote sensing of ship wakes. Rajaratnam and Humphries (1984) studied the mean flow characteristics of a circular jet discharging parallel to a free-surface when the free surface is located at the edge of the nozzle, i.e. the centerline of the jet is at 0.5 nozzle diameters below the free surface. Swean et al. (1989) reported measurements of mean velocities and turbulent fluctuations in a two-dimensional turbulent jet issuing at a free surface. Liepmann and Gharib (1992) showed that the free surface can strongly influence the instabilities in the near-field region of a circular jet. Anthony and Willmarth (1992) studied the development of the jet below free surface and identified surface currents near the free surface comprising mainly of vortical structures ejected from the main jet. Grinstein et al. (1995) showed that non circular jets can have enhanced entrainment properties relative to comparable circular jets_ due to axis rotation of the jet cross section resulting from the deformation of vortex rings with non-uniform curvature. Sankar et al. (2009) studied the characteristics of a three dimensional square jet in the vicinity of a free surface. Shinneeb et al. (2011) studied characteristics of shallow water jets. The characteristics of jets in confined situations have been studied to a lesser extent even though a number of practical situations, for instance, cleaning jets and paint removal reflect such conditions. Several researchers have studied the characteristics of jets considering them to be free jets neglecting the fact that they were partially or fully confined. In the present study, effects of surface confinement on the characteristics of jet were considered. The effects of surface confinement on a neutrally-buoyant turbulent circular jet discharging from a circular orifice into water tank were investigated. Experiments were performed in long glass walled tank of interior dimensions of 2.5 m in length, 1.25 m in width and 1.25 m in height. Jet was produced in the water tank by allowing water through a orifice of 5cm diameter. The confinement resulted from the proximity of an upper free surface. The jet exit Reynolds numbers 212,500, 350,000 and 425,000 were investigated. The depth of the water above jet axis and jet exit velocity was the principle parameters. The axial and lateral confinements were negligible. Three different degrees of surface confinements were investigated. Measurements were taken on two orthogonal axes along the jet axis: one parallel and one perpendicular to free surface. For each case, measurements were taken at five locations downstream of jet exit. In the present study hydraulic behavior of circular turbulent jets were examined through experimentations. The deviations of centerline velocity decay of surface confined jets from centerline velocity decay of free jets were also studied. Charts were developed to calculate velocity profiles at any section downstream of the jet exit. Variation of flux entrainment and kinetic energy along axial distance were investigated under nine different sets of surface confinement and jet exit velocity. The results showed that surface confinement causes deviation of centerline velocity decay from centerline velocity decay of a free jet. As the confinement increases, deviation of centerline velocity from the free jet centerline velocity increases. Results also showed that as the jet exit velocity decreases, deviation of centerline velocity from centerline velocity of free jet increases. Investigation also showed that the surface confinement results narrowing of velocity profile on horizontal axis and spreads it drastically on the vertical axis. The mixing efficiency of the fluid in the vertical axis is significantly inhibited by the surface confinement while there is a slight effect in the horizontal axis. The velocity profiles at any location downstream of jet exit, for different jet exit velocity are similar in shape. Investigation also showed that entrainment flux of turbulent circular jet increases along axial distance and kinetic energy decreases along the axial distance.