The effect of background turbulence on jet entrainment: an experimental study of a plane jet in a shallow coflow

Abstract

Many effluents are released into water bodies or into the atmosphere, and dilution is relied on to minimize the toxic effects of the pollutants on the environment. Dilution occurs due to entrainment and subsequent mixing of the "clean" (or cleaner) ambient fluid and the effluent stream. In the near field, dilution occurs due to momentum or buoyancy driven entrainment into the effluent stream, which is proportional to a characteristic velocity of the jet or plume.Whereas in the far field, dilution occurs due to turbulent diffusion at a rate dependent on the level of turbulence in the receiving fluid. In order to predict the expected dilution of an effluent stream, it is necessary to answer the question, at what point does the dilution mechanism change and how does it affect dilution rates. An experimental study examining the velocity and concentration decay of plane jets released into shallow coflows with increasing levels of external turbulence, indicated that levels of external turbulence just large enough to disrupt the jet structure reduced dilution rates significantly. This indicates that models, in which the jet dilution and turbulent diffusion are superimposed, will not always give a conservative estimate of effluent dilution.

Effect of background turbulence on an axisymmetric turbulent jet

“…This hypothesis was confirmed experimentally by Gaskin et al (2004) that the plume can be destroyed when the convective velocity of the plume is of the same order as the RMS velocity of the background turbulence. At the breakup location an increase in the spreading rate was observed.…”

“…The decrease of mass flow rate is in contrast to the theory of superposition of jet dilution and turbulent diffusion which predicts an increase in the entrainment into the jet in the presence of external turbulence (Wright, 1994). On the other hand, our results confirm i) the hypothesis of Hunt (1994) that the tendency of the jet to break up into distinct eddies by the external turbulence will decrease the entrainment into the jet, and ii) the more qualitative experimental study of Gaskin et al (2004) who detected a decreased entrainment in a plane jet issuing into a shallow co-flow.…”

“…The dilution then occurs mainly due to turbulent diffusion at a rate dependent on the ambient turbulence levels. This was verified experimentally by Gaskin et al (2004) for a jet in a shallow co-flow. They reject the theory of superposition of the dilution resulting from the jet driven entrainment and the background-turbulence-drivenentrainment, and confirm a reduction in the rate of dilution in the near-field once the jet self-preserving structure is disrupted.…”

“…On the other hand, Hunt (1994) hypothesized that background turbulence could lower dilution rates by breaking up the jet structure. Experiments have provided qualitative confirmation of this hypothesis (Gaskin et al, 2004). Consequently, it is clear that a thorough and systematic study of jets in background turbulence is required to further confirm or refute the above hypotheses.…”

Momentum and scalar transport at the turbulent/non-turbulent interface of a jet

“…Engulfment appears to be dominant in the case of a free plane shear layer or when jets, plumes or wakes are strongly perturbed by internal or external fluctuations. In that case the basic structure changes, and engulfment or detrainment dominates the entrainment process (Bhat & Narasimha 1996;Gaskin, McKernan & Xue 2004;Hunt et al 2006;Khorsandi, Gaskin & Mydlarski 2007). The present quantitative experimental approach may shed light on the entrainment process in other flows and for other flow conditions.…”