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The near-field structure of strongly buoyant jet issuing from a square nozzle at low Froude and Reynolds numbers is studied by using LIF flow visualization and time-resolved scanning PIV. These experimental techniques allow the visualization of unsteady three-dimensional flow phenomenon occurring in the near-field of strongly buoyant jet. It is found that the buoyant jet is unstable to the positive buoyancy forces, which promote the inflow motion near the nozzle exit. The surrounding low temperature fluid moves into the nozzle inside along the nozzle corner and mixes with the high temperature fluid deep into the nozzle. Then, the flow pattern inside the nozzle becomes highly complex to promote the laminar to turbulent transition of the jet. The statistical flow characteristics of the strongly buoyant jet are evaluated from the scanning PIV measurement, and the result indicates the presence of axisymmetric distributions of mean flow and velocity fluctuations in the circle of diameter equal to the square side of the nozzle.

In the present paper, the onset of inflow in the near field of a vertical buoyant jet issuing from a square duct is studied by experimental flow visualization and numerical simulation. The experimental critical condition for the onset of inflow is obtained from the scanning LIF visualization in the near field of the buoyant jet at various combinations of Froude numbers and Reynolds numbers. The experimental result shows that the critical Froude number increases with an increase in Reynolds number of the buoyant jet. The critical condition is also examined by numerical simulation based on the Navier-Stokes equation and energy conservation equation, under the assumption that the flow separation occurs at the duct exit. The main feature of the inflow observed by experiment is well reproduced in the numerical results.

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