Measurements and image analysis of the turbulent field in an axisymmetric jet subject to a sudden velocity decrease

Borée, Jacques; Atassi, N.; Charnay, G.; Taubert, Lutz

doi:10.1016/s0894-1777(96)00100-8

Cited by 27 publications

(25 citation statements)

References 10 publications

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“…On the other hand, in the region in figure 9(a) in which ∂ z M m > 0, the behaviour of α disp1 > 0 indicates an enhanced entrainment resulting from the acceleration of the jet in the longitudinal direction. When ∂ z M m > 0, both the spacing of the streamlines evident in figure 9(a) and the observation of an enhanced radial inflow reported by Borée et al (1997), support the qualitative predictions that can be obtained from the framework that was outlined in § 2.4. Indeed, we will demonstrate in Part 2 An advantage of examining entrainment via balances of mean momentum and energy in an unsteady jet is that it is not necessary to develop a storage model for the volume of the jet independently.…”

Section: The Unsteady Jetsupporting

confidence: 77%

“…Bremhorst & Hollis 1990, and references therein). Similarly, Borée et al (1997) observed a relatively large radial inflow in the vicinity of a rapid reduction in the velocity of an unsteady jet.…”

mentioning

confidence: 77%

“…In particular, the behaviour of the turbulence production is an area that would benefit from further attention (see e.g. Borée et al 1997) and could be analysed from an integral perspective using the framework that we described in § 2. In Part 2 of this work we will produce a simple closure for unsteady jets based on Taylor are satisfied, which is consistent with the skew-symmetry one finds in the advection and gradient operators on a continuum, and in turn means that the total energy on a periodic domain is changed only as a result of external forcing and/or viscous dissipation.…”

Section: Discussionmentioning

confidence: 99%

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Energy dispersion in turbulent jets. Part 1. Direct simulation of steady and unsteady jets

Craske

Reeuwijk

2014

J. Fluid Mech.

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We study the physics of unsteady turbulent jets using direct numerical simulation (DNS) by introducing an instantaneous step change (both up and down) in the source momentum flux. Our focus is on the propagation speed and rate of spread of the resulting front. We show that accurate prediction of the propagation speed requires information about the energy flux in addition to the momentum flux in the jet. Our observations suggest that the evolution of a front in a jet is a self-similar process that accords with the classical dispersive scaling z ∼ √ t. In the analysis of the problem we demonstrate that the use of a momentum-energy framework of the kind used by Priestley & Ball (Q. J. R. Meteorol. Soc., vol. 81, 1955, pp. 144-157) has several advantages over the classical mass-momentum formulation. In this regard we generalise the approach of Kaminski et al. (J. Fluid Mech., vol. 526, 2005, pp. 361-376) to unsteady problems, neglecting only viscous effects and relatively small boundary terms in the governing equations. Our results show that dispersion originating from the radial dependence of longitudinal velocity plays a fundamental role in longitudinal transport. Indeed, one is able to find dispersion in the steady state, although it has received little attention because its effects can then be absorbed into the entrainment coefficient. Specifically, we identify two types of dispersion. Type I dispersion exists in a steady state and determines the rate at which energy is transported relative to the rate at which momentum is transported. In unsteady jets type I dispersion is responsible for the separation of characteristic curves and thus the hyperbolic, rather than parabolic, nature of the governing equations, in the absence of longitudinal mixing. Type II dispersion is equivalent to Taylor dispersion and results in the longitudinal mixing of the front. This mixing is achieved by a deformation of the self-similar profiles that one finds in steady jets. Using a comparison with the local eddy viscosity, and by examining dimensionless fluxes in the vicinity of the front, we show that type II dispersion provides a dominant source of longitudinal mixing.

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Section: The Unsteady Jetsupporting

confidence: 77%

mentioning

confidence: 77%

Section: Discussionmentioning

confidence: 99%

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Energy dispersion in turbulent jets. Part 1. Direct simulation of steady and unsteady jets

Craske

Reeuwijk

2014

J. Fluid Mech.

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“…On the contrary, the measurements by Borée et al (1996) indicate that the radial profiles of phase-averaged axial velocity and other velocity moments deviate from the self-similar profiles of a statistically steady jet as the jet decelerates, and that the profiles vary axially through the confined deceleration region (Borée et al 1996(Borée et al , 1997. Here, we hypothesise that the flow field in a decelerating jet asymptotes towards a new temporally stationary self-similar state in which the radial profiles of normalised velocity moments are axially uniform.…”

Section: Introductionmentioning

confidence: 66%

Self-similar properties of decelerating turbulent jets

2017

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The flow in a decelerating turbulent round jet is investigated using direct numerical simulation. The simulations are initialised with a flow field from a statistically stationary turbulent jet. Upon stopping the inflow, a deceleration wave passes through the jet, behind which the velocity field evolves towards a new statistically unsteady self-similar state. Assumption of unsteady self-similar behaviour leads to analytical relations concerning the evolution of the centreline mean axial velocity and the shapes of the radial profiles of the velocity statistics. Consistency between these predictions and the simulation data supports the use of the assumption of self-similarity. The mean radial velocity is predicted to reverse in direction near to the jet centreline as the deceleration wave passes, contributing to an approximately threefold increase in the normalised mass entrainment rate. The shape of the mean axial velocity profile undergoes a relatively small change across the deceleration transient, and this observation provides direct evidence in support of previous models that have assumed that the mean axial velocity profile, and in some cases also the jet spreading angle, remain approximately constant within unsteady jets.

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“…This rapid reduction in equivalence ratio near the injector is driven by advection and enhanced mixing that arises from the reduction of flow through the nozzle approaching the end of the injection. Changes in mixing, or entrainment, during rapid velocity variations at the nozzle have been measured in turbulent gas jets by several authors [16][17][18][19] . For diesel jets, Musculus and coworkers [20][21][22] showed how charge temperature and density as well as the profile of end-of-injection transient affect the evolution and propagation of large-scale structures downstream in the jet to increase entrainment.…”

Section: Introductionmentioning

confidence: 99%

Effect of post injections on mixture preparation and unburned hydrocarbon emissions in a heavy-duty diesel engine

O’Connor

Musculus

Pickett

2016

Combustion and Flame

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a b s t r a c tThis work explores the mechanisms by which a post injection can reduce unburned hydrocarbon (UHC) emissions in heavy-duty diesel engines operating at low-temperature combustion conditions. Post injections, small, close-coupled injections of fuel after the main injection, have been shown to reduce UHC in the authors' previous work. In this work, we analyze optical data from laser-induced fluorescence of both CH 2 O and OH and use chemical reactor modeling to better understand the mechanism by which post injections reduce UHC emissions. The results indicate that post-injection efficacy, or the extent to which a post injection reduces UHC emissions, is a strong function of the cylinder pressure variation during the post injection. However, the data and analysis indicate that the pressure and temperature rise from the post injection combustion cannot solely explain the UHC reduction measured by both engine-out and optical diagnostics. The fluid-mechanic, thermal, and chemical interaction of the post injection with the main-injection mixture is a key part of UHC reduction; the starting action of the post jet and the subsequent entrainment of surrounding gases are likely both important processes in reducing UHC with a post injection.

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Measurements and image analysis of the turbulent field in an axisymmetric jet subject to a sudden velocity decrease

Cited by 27 publications

References 10 publications

Energy dispersion in turbulent jets. Part 1. Direct simulation of steady and unsteady jets

Energy dispersion in turbulent jets. Part 1. Direct simulation of steady and unsteady jets

Self-similar properties of decelerating turbulent jets

Effect of post injections on mixture preparation and unburned hydrocarbon emissions in a heavy-duty diesel engine

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