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Abstract
A negatively buoyant jet (NBJ) corresponds to the physical phenomenon that develops when a fluid is discharged upwards into a lighter environment or downwards into a heavier receptor fluid. In a NBJ the flow is initially driven mostly by the momentum, so it basically behaves as a simple jet released with the same angle, while far from the outlet the buoyancy prevails, bending the jet axis down and making it similar to a plume. The coexistence in the same phenomenon of both the characteristics of simple jets and plumes makes the NBJs a phenomenon still not entirely explained but, considering also the numerous practical applications, very interesting to study. Here some of the experimental results are presented. The laboratory experiment were obtained on a model simulating a typical sea discharge of brine from desalination plants: a pipe laid down on the sea bottom, with orifices on its lateral wall, releasing brine (heavier than the sea water) with a certain angle to the horizontal, in order to increase the jet path before sinking to the seafloor. A non-intrusive image analysis technique, namely Feature Tracking Velocimetry, is applied to measure velocity fields, with the aim at understanding the influence of some non-dimensional parameters driving the phenomenon (e.g. Reynolds number, release angle) on the structure of the NBJ and of the turbulence.
Highlights
In contrast with the previous quoted authors, that discharged the jet at the end of a pipe, Ferrari and Querzoli (2004 [26], 2010 [1]) studied negatively buoyant jet (NBJ) emitted from a sharp edged orifice, as this kind of release allows a larger entrainment (Mi et al 2001 [27], 2007 [28]); the inclination was from 45° to 90°, and these works explain the non-axisymmetry of NBJs, due to the different stratifications in the upper and lower boundary of a NBJ
A paper based on Particles Image Velocimetry (PIV) measures for inclined NBJs, released at the end of a pipe, is the one by Lai and Lee (2012) [31]: the release angles were between 15° and 60° and, the authors compared their results with two previous work of Shao and Law (2010 [32], 2011 [33]), with inclination of 30° and 45°
Comparing the two plots in the figure 4 and 5, it is important to notice that the jet axis reaches almost the same height, this is due to the re-entrainment phenomenon, i.e. the mixing of the jet fluid with itself instead of being fed by the external fluid (Ferrari et al 2010 [1]), this condition occurs for inclination exceeding 75°
Summary
In contrast with the previous quoted authors, that discharged the jet at the end of a pipe, Ferrari and Querzoli (2004 [26], 2010 [1]) studied NBJs emitted from a sharp edged orifice, as this kind of release allows a larger entrainment (Mi et al 2001 [27], 2007 [28]); the inclination was from 45° to 90°, and these works explain the non-axisymmetry of NBJs, due to the different stratifications in the upper and lower boundary of a NBJ (see Ferrari and Querzoli, ISSF 2011 [29]) They showed that the maximum height of the jet axis increases from 45° to approximately 80°, in agreement with the results of Bloomfield and Kerr (2002) [21], to decrease, because of the re-entrainment phenomenon. The main target of this work is to investigate the influence of the Reynolds number on some turbulent quantities that govern the mixing processes, via a novel Feature Tracking Velocimetry (FTV) technique
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