3 results
Turbulent transfer and entrainment in a low-density jet
- P. Salizzoni, S. Vaux, M. Creyssels, M. Amielh, L. Pietri, F. Anselmet
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- Journal:
- Journal of Fluid Mechanics / Volume 968 / 10 August 2023
- Published online by Cambridge University Press:
- 08 August 2023, A27
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We investigate the dynamics of a low-density round jet, with a focus on the mechanisms governing the turbulent momentum and mass transfers as well as on the entrainment of ambient fluid. To that purpose, we combine a theoretical analysis, laboratory experiments and numerical simulations. The theoretical analysis relies on a general formulation of the entrainment decomposition for the case of large density differences, revealing the role of the processes contributing to the entrainment: turbulent kinetic energy production and variation in the shape of the mean velocity radial profiles. The spatial evolution of these terms has been evaluated by means of challenging experiments, providing a unique data set of combined velocity and density statistics of a low-density jet and an air jet. The same flows are investigated by means of large-eddy simulation (LES). Other than for providing complementary information on flow statistics, LES is here used to investigate the role of varying conditions imposed at the source, notably concerning the shape of the inlet velocity profile and the presence of a bottom wall surrounding the source. Experimental and numerical results provide clear insight on how a reduced density within the jet enhances the turbulent kinetic energy production (compared to an iso-density jet) and modifies the shape of the mean velocity profiles. Despite its clear influence on the flow statistics, the reduced density has overall little influence on the entrainment rate, which also shows little sensitivity to varying source conditions.
Control of light gas releases in ventilated tunnels
- L. Jiang, M. Creyssels, G. R. Hunt, P. Salizzoni
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- Journal:
- Journal of Fluid Mechanics / Volume 872 / 10 August 2019
- Published online by Cambridge University Press:
- 10 June 2019, pp. 515-531
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The release of buoyant harmful gases within enclosed spaces, such as tunnels and corridors, may engender specific health, industrial and transportation risks. For safety, a simple ventilation strategy for these spaces is to impose a flow along the tunnel, whose velocity is defined as ‘critical’, that confines the front of harmful buoyant gases immediately downstream of the source of emission. Determining the critical velocity as a function of the geometrical and dynamical conditions at the source is a fundamental fluid mechanics problem which has yet to be elucidated; this problem concerns the dynamics of non-Boussinesq releases relating to large differences between the densities of the buoyant and the ambient fluids. We have investigated this problem theoretically, by means of a simplified model of a top-hat plume in a cross-flow, and in complementary experiments by means of tests in a reduced-scale ventilated tunnel, examining releases from circular sources. Experimental results reveal: (i) the existence of two flow regimes depending on the plume Richardson number at the source $\unicode[STIX]{x1D6E4}_{i}$, one for momentum-dominated releases, $\unicode[STIX]{x1D6E4}_{i}\ll 1$, and a second for buoyancy-dominated releases, $\unicode[STIX]{x1D6E4}_{i}\gg 1$, with a smooth transition between the two; and (ii) the presence of relevant non-Boussinesq effects only for momentum-dominated releases. All these features can be conveniently predicted by the plume-based model, whose validity is, strictly speaking, limited to releases issuing from ‘small’ sources in ‘weak’ ventilation flows. Analytical solutions of the model are generally in good agreement with the experimental data, even for values of the governing parameters that are beyond the range of validity for the model. The solutions aid to clarify the effect of the source radius, and reveal interesting behaviours in the limits $\unicode[STIX]{x1D6E4}_{i}\rightarrow 0$ and $\unicode[STIX]{x1D6E4}_{i}\rightarrow \infty$. These findings support the adoption of simplified models to simulate light gas releases in confined ventilated spaces.
Dynamical variability of axisymmetric buoyant plumes
- A. Ezzamel, P. Salizzoni, G. R. Hunt
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- Journal:
- Journal of Fluid Mechanics / Volume 765 / 25 February 2015
- Published online by Cambridge University Press:
- 26 January 2015, pp. 576-611
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We present experimental measurements conducted on freely propagating, turbulent, steady thermal air plumes. Three plumes are studied with differing source conditions, ranging from jet-like, momentum flux dominated releases, to pure plume releases, characterised by a balance between the momentum, volume and buoyancy fluxes at the source. Velocity measurements from near the source to a height of tens of source diameters were made using particle image velocimetry (PIV), providing a high spatial resolution. Temperatures were measured with thermocouples. From these measurements, we investigate the vertical development of the plume fluxes and radial profiles of the mean velocity and temperature. These allow us to analyse the local self-preserving characteristics of the mean flow and to estimate the dependence with height of the plume Richardson number ${\it\Gamma}$. In addition, we analyse the similarity of one-point and two-point second-order velocity statistics, and we discuss the role of ${\it\Gamma}$ on the vertical development of the bulk dynamical parameters of the plume, namely, the turbulent viscosity, the turbulent Prandtl number and the entrainment coefficient ${\it\alpha}_{G}$. Comparison with previous experimental results and with estimates of the entrainment coefficient based on the mean kinetic energy budget allow us to conclude on the influence of ${\it\Gamma}$ on the entrainment process and to explain possible physical reasons for the high scatter in estimates of ${\it\alpha}_{G}$ in the literature.