PIV Measurements in a Confined Jet

This paper reports PIV measurements made at three locations in an axisymmetric, confined jet that is approaching a free surface from below. The apparatus consists of a tank 40.5 cm × 40.5 cm at its base and 61 cm high. A 9 mm diameter nozzle is centered in the base of the tank and directs a jet of water upwards. The jet produced has a top-hat velocity profile with a maximum deviation of 0.32% of the mean and an axial relative turbulence intensity of 0.60%. The water is removed from the tank by an overflow around the perimeter of the tank. The PIV measurements achieved a spatial resolution of between 0.425–1.08 mm. The measurements show details of the velocity field in three regions of the flow; at the jet exit, near the surface on the centerline of the jet, and near the top corner of the tank. The centerline velocity remains at the exit velocity until ≈5D from the exit. The axial confinement of the jet begins to significantly influence the centerline velocity at ≈13D from the free surface. All entrained fluid is deflected downward from the horizontal surface flow as it approaches the overflow around the perimeter of the tank. This creates a large recirculation region in the upper region of the tank driven by the downward flow along the wall and the upward flow of the jet itself at the center of the tank.