Global Behaviors of a Round Buoyant Jet in a Counterflow

Spreading of a buoyant jet into a counterflowing ambient flow stream is investigated in the laboratory with the laser-induced fluorescence (LIF) technique. The jet behaviors are found to depend on two flow parameters. The densimetric Froude number F characterizes the ratio between the initial buoyant force on the jet effluent and the initial jet momentum. The jet-tocurrent velocity ratio (R) compares the initial jet exit velocity to the speed of the counterflow. Experiments are carried out at Froude numbers F=3–11 and velocity ratios R between 2.5 and 18. Time-averaged patterns of the jet are revealed by the mean LIF concentration field of jet effluent. Global features of the jet including its trajectory, spreading, and dilution are analyzed from the mean LIF images. The buoyant jet penetrates forward into the counterflow for some distance before it is turned backwards by the counterflow. Buoyancy in the effluent results in some vertical rise of the jet in this first region of forward flow, especially at a high velocity ratio. The flow then transits into the second region of backward flow. Buoyant jet effluent is advected backwards by the counterflow and at the same time continuing to rise in a plume-like manner. Radial profiles of mean effluent concentration at successive stations along the jet centerline are found to follow reasonably well the Gaussian distribution. The predictions of the Lagrangian model JETLAG are compared with the experimental data for this complex situation of buoyant jet in counterflow. The jet trajectory data are also interpreted using a length scale analysis.