A comparative study of circular and rectangular bended plunging jets

In the present work we study the topology, mixing properties, turbulence quantities, dependence on the outlet geometry of a sharp-edged orifice plunging jet which first issues horizontally in air and then plunges in a water pool. The investigated orifices shapes are circular and rectangular. Data are acquired at different Reynolds numbers in the range 11000-25000, based on the orifice diameter (equal to 2 cm) and on the average exit velocity, as derived from flow rate measurements. Velocity fields in vertical and horizontal planes are measured using planar time-resolved Particle Image Velocimetry. Results show a clear asymmetry of the cross-velocity profiles both in circular and rectangular cases, with the latter that revealed a shape which is Reynolds num-ber dependent. Axial velocity decays, potential core lengths and spreading rates highlight an opposite trend between the two jet geometries, thus suggesting a higher mixing for the lowest Reynolds number circular jet and the highest rectangular one. Plunging angle shows a dependency on Reynolds number. Moreover, it seems to play a role in the evolution of the upper and lower side of the jet due to the onset of a co-flow. Ambient mass entrainment points out the different interactions of the two plunging jets with the ambient flow: in circular case, it entrains fluid from the surroundings, from horizontal to vertical planes in streamwise direction, while in rectangular one it ejects flow from vertical to horizontal planes. Finally, Strouhal numbers are derived for main vortices frequencies along jet centerline, other than upper and lower sides.