Author(s): G. Querzoli; A. Cenedese

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Abstract: The flow generated by the injection of negatively buoyant fluid in water at rest was investigated by means of both the laser induced fluorescence (LIF) visualization and the particle tracking velocimetry. Statistics on series of LIF images acquired during the same experiment have been evaluated to give quantitative information about the localization of the structures. Such a phenomenon is controlled by two non-dimensional groups, the Reynolds and the Richardson number. Series of experiments have been carried out changing the Richardson number, whereas the Reynolds number was kept constant at a low value such that the initial flow was laminar. Visualizations permitted the identification of the structures that appear as a consequence of the instability of the flow. On the upper boundary Kelvin-Helmoltz instabilities occur that are very similar to the ones observed in a simple jet. The scenario on the lower boundary is completely different due to the unstable stratification. A descending plume develops as the distance from the outlet increases until it experiences transverse instability. It is very similar to the one generated by a cold cylinder in calm water. The presence of the plume generates perturbations that destabilize the upper boundary so that even a small amount of buoyancy is sufficient to considerably anticipate the apparition of Kelvin-Helmoltz instability and then the transition to turbulence of the flow. The velocity measurement permitted to quantitatively clarify that the buoyant jet is not axisymmetric. Its asymmetry decreases with the distance from the outlet. Evaluating the transverse velocity profiles it is possible to clearly see that the buoyant jet consists of two elements: a core characterized by high velocities and a surrounding zone, where the fluid moves slowly, that generates the downward plume.

DOI: https://doi.org/10.1080/00221680509500112

Year: 2005