Mixing and re-entrainment in a negatively buoyant jet

Abstract

An experimental study of inclined negatively buoyant jets released from a sharp-edged orifice was carried out to better understand the re-entrainment phenomenon, i.e. the mixing of jet fluid with itself instead with external fluid, and how it biases the jet behaviour. Experiments reproducing an orifice in the lateral wall of an outfall pipe on the sea bottom were conducted using light-induced fluorescence. The mean and variance of the concentration field have been determined to study the hydrodynamic features along the jet axis, including turbulent mixing and entrainment as the densimetric Froude number varies. The re-entrainment phenomenon tends to appear as the angle exceeds 75° with respect to the horizontal, and its onset occurs for lower angles as the Froude number increases. The re-entrainment biases the behaviour of the jet trajectory, making it bend on itself, and causing a reduction of both the maximum height and distance at the location where entrainment of external fluid reaches the jet axis.

Keywords:

• Brine discharge
• inclined negatively buoyant jet
• light-induced fluorescence
• mixing
• ocean outfall
• re-entrainment