Plane Jet Research Articles

Examination of the effect of differential molecular diffusion in DNS of turbulent non-premixed flames

The effect of differential molecular diffusion (DMD) in turbulent non-premixed flames is studied by examining two previously reported DNS of temporally evolving planar jet flames, one with CO/H2 as the fuel and the other with C2H4 as the fuel. The effect of DMD in the CO/H2 DNS flames in which H2 is part of fuel is found to behave similar to laminar flamelet, while in the C2H4 DNS flames in which H2 is not present in the fuel it is similar to laminar flamelet in early stages but becomes different from laminar flamelet later. The scaling of the effect of DMD with respect to the Reynolds number Re is investigated in the CO/H2 DNS flames, and an evident power law scaling (∼Re−a with a a positive constant) is observed. The scaling of the effect of DMD with respect to the Damköhler number Da is explored in both laminar counter-flow jet C2H4 diffusion flames and the C2H4 DNS flames. A power law scaling (∼Daa with a a positive constant) is clearly demonstrated for C2H4 nonpremixed flames.

An analytical model for simulating air concentration in drop and deflector jets

In the present study, a quasi 3D analytical model of drop and deflector jet is developed for calculating air concentration distribution in the flow. Continuity and momentum equations were solved in the whole flow domain of this model, in which air content in the flow is also considered. A numerical simulation is also performed to verify the results of the analytical model in parallel with the available experimental and another analytical data. Analytical method has been done both for throwing and plane jets and equations have been obtained, separately, and were compared with experimental data. Results showed good agreement between compared models.

Use of dual planar jets for the reduction of flow-induced noise

The planar jet has been proposed to reduce the flow-induced noise. However, the self-noise emission from a single planar jet is a limiting factor on the performance based characteristics of this technology. Therefore, this research paper reports an alternative dual planar jets geometry in an attempt to minimize the self-noise of the single planar jet, thus, further enhancing total noise reduction in the application. Tandem cylinders are considered as the generic bluff body, which represents the source of flow-induced noise. Tests are conducted to capture noise reduction effects when using a single planar jet geometry. Subsequently, two different configurations of the dual planar jets geometry were tested in comparison. It is observed that the dual jets geometry achieves more noise reduction when compared to the single jet. Moreover, conclusions are made on the most efficient configuration in the dual jets geometry.

Entropy generation concept for a turbulent plane jet with variable density

This study presents a numerical investigation of the entropy generation of a turbulent plane jet with variable density. Turbulent simulations are carried out using the first order k-ε turbulence model. The Finite Volume Method (FVM) is used to discretize governing equations. The effect of the jet temperature at the inlet nozzle on the flow and mainly on the entropy generation has been studied. So far, a comparison of numerical and experimental results gives a satisfactory agreement. The present investigation suggests that high value of entropy generation correspond to the higher inlet hot jet temperature. Moreover, the viscous dissipation is found to be important near the nozzle exit and it becomes negligible in the affinity region of the jet. Furthermore, the entropy generation rate as well as the Merit number grows progressively to reach an asymptotic value along the flow direction as the inlet jet temperature increases.

Direct numerical simulation of planar turbulent jet with chemical reaction

Direct numerical simulation of planar turbulent jet with chemical reaction

二次元液相噴流中の乱流･非乱流界面近傍における化学反応場に及ぼす量論混合比の影響

二次元液相噴流中の乱流･非乱流界面近傍における化学反応場に及ぼす量論混合比の影響

二次元乱流噴流における速度勾配と変動静圧の同時計測

二次元乱流噴流における速度勾配と変動静圧の同時計測

Modulating flow and aerodynamic characteristics of a square cylinder in crossflow using a rear jet injection

The temporally evolved flow behaviors around a square cylinder subject to modulation of a planar jet issued from the cylinder’s downstream surface into the wake were studied using the laser-assisted smoke flow visualization method and synchronized hot-wire anemometers. The drag force asserted on the square cylinder was obtained by measuring the surface pressures. Four characteristic flow modes (wake-dominated, transitional, critical, and jet-dominated) were observed in different regimes of freestream Reynolds number and jet injection ratio. In the wake-dominated mode, the jet swung periodically back and forth on the downstream surface due to the wake vortex shedding. In the transitional mode, the vortex shedding in the wake vanished so that the flow around the cylinder presented no periodic oscillations. In the critical mode, the wake width became smaller and therefore made the vortex shedding frequency larger than that observed in the wake dominated mode. In the jet-dominated mode, the jet had a large momentum that entrained wake fluids and therefore stabilized the instabilities of the wake, separated boundary layers on lateral surfaces, and stagnation point on the upstream surface. Two standing vortices appeared in the near wake beside the high-momentum jet. The width of the wake was decreased substantially by jet entrainment. The drag coefficient decreased with an increase in the jet injection ratio. The downstream surface jet injection caused the pressure coefficients to decrease at the upstream surface and to increase at the downstream surface. Therefore, the drag coefficients were decreased significantly by 26%, 33%, and 38% at the injection ratios of 0.5, 1.5, and 2.5, respectively.

Prediction on Turbulent Flow of Two Parallel Plane Jets Using κ-ε Model

In the present study, computational details of two parallel plane jets with nozzle separation ratio = 4.25 and Re = 11,000 are presented. A study of the turbulence has also been done. The computational domain size, grid resolution, and different discretization schemes on the predictions are discussed. The existence of a recirculation flow region, a merging region and a combined region in the two parallel plane jets configuration has been predicted qualitatively by model. Power law scheme is used for discretizing the convective terms. Calculations were made using a Power law scheme. A code is used to solve a laminar, two dimensional viscous fluid flow and heat transfer. At the last it is shown that this code is ready to use for modification for mean flow solution of the turbulent two parallel plane jets. The numerical results are compared with the experimental results. Computational details of various mean flow and turbulent parameters are presented and described with separation ratio of 4.25 and Re = 11,000. The results are compared with Nasr and Lai and are found to be in good agreement with it. The two parallel plane jets develop like a single free jet in the combined region. The outer shear layer spreads faster than the inner shear layer in the near field.

A Comprehensive Presentation of the Turbulent Plane Jet Theory with Passive Scalar

We present a unified vision of the existing theoretical models for the turbulent plane jet, leading to new analytical profiles for scalar concentration and turbulent quantities, including a complete turbulent kinetic energy budget. Integrals of the budget terms are also computed. The present model is split into two variants. Both compare fairly well with referenced experimental data.

Flow Characteristics of a Tabbed Plane Jet

Flow Characteristics of a Tabbed Plane Jet

Numerical study on rarefied unsteady jet flow expanding into vacuum using the Gas-Kinetic Unified Algorithm

The unsteady process of rarefied jet flows expanding into a vacuum is numerically solved by the Gas-Kinetic Unified Algorithm (GKUA) based on the Boltzmann model equations. The discrete velocity ordinate method (DVOM) is adopted to discretize the velocity space of the molecular velocity distribution function, while the corresponding numerical integral technique is developed to evaluate macroscopic flow variables, including number density, flow velocity, temperature and so on. The time-explicit finite difference scheme is constructed to capture the unsteady evolution of the discrete velocity distribution functions at each DVO point by using the unsteady time-splitting method. The multi-block docking grid technique is designed for different regions of flow field in physical space with self-adaptive adjustment. Then, the supersonic planar jet flows with the wide range of Knudsen numbers, from 100 to 0.1, are solved numerically and discussed, including startup to steady state and shutting down. The GKUA results of free-molecule jet flows are analyzed and compared with the Maxwellian analytical solutions for collisionless plume flows, in which good agreement shows the validity and accuracy of the present numerical method. When taking the collision effect into account, the unsteady jet flows with different Knudsen numbers are computed by the present GKUA method. It is shown that the collision term of the Boltzmann model equation plays an important role in this rarefied gas diffusion into the back flow when Kn ≤ 10. However, the colliding relaxation term have a little influence on the kernel region in the startup process for Kn ≥ 0.1. In general, the convective transport term of the Boltzmann model equation dominates the kernel region of the jet flow during the unsteady process of gas expanding into a vacuum. The numerical experience indicates that the present GKUA can provide a vital tool for solving unsteady flow problems covering various flow regimes by directly tracing the time evolution of the Boltzmann-type velocity distribution function.

LES study of the combined effects of groups of vortices generated by a pulsating turbulent plane jet impinging on a semi-cylinder

The present work is a numerical study of a submerged pulsating plane turbulent jet impinging on a convex semi-cylinder placed at a distance equal to twice the nozzle-exit width, using the Large Eddy Simulation approach. The temperature of the jet is higher than that of the surrounding air and the impingement wall. The Reynolds number, based on the nozzle-exit width and the time-averaged velocity is equal to 5600. The inflow is forced at a frequency of 600Hz by imposing a sinusoidal velocity profile at the nozzle exit. The study focuses on the effects of the organized structures, shed from the free-shear layer, on the boundary layer developing along the curved surface and the corresponding heat transfer. In particular, the present contribution is an investigation, in an unexplored research field, namely, the simultaneous time-dependent responses of the Nusselt number, the friction coefficient and the pressure profiles to the passage of the coherent structures along the curved wall.The passage of the primary coherent vortices induces secondary vortices within the boundary layer and entrains both the hot air, from the jet, and the cold air, from the surroundings, which is undesirable in heating applications. The primary vortices, the induced vortices and their pairing have a major effect on the heat and fluid flow. Thus, air is entrained towards the wall immediately downstream of the induced vortices (downwash) and ejected away from the wall just upstream (upwash). It is shown that the dynamic field can be uncorrelated with the thermal field and may not be sufficient for a good prediction of the expected heat transfer.

Incident shock strength evolution in overexpanded jet flow out of rocket nozzle

The evolution of the incident shock in the plane overexpanded jet flow or in the axisymmetric one is analyzed theoretically and compared at the whole range of governing flow parameters. Analytical results can be applied to avoid jet flow instability and self-oscillation effects at rocket launch, to improve launch safety and to suppress shock-wave induced noise harmful to environment and personnel.The mathematical model of “differential conditions of dynamic compatibility” was applied to the curved shock in non-uniform plane or axisymmetrical flow. It allowed us to study such features of the curved incident shock and flow downstream it as shock geometrical curvature, jet boundary curvature, local increase or decrease of the shock strength, flow vorticity rate (local pressure gradient) in the vicinity of the nozzle lip, static pressure gradient in the compressed layer downstream the shock, and many others. All these quantities sufficiently depend on the flow parameters (flow Mach number, jet overexpansion rate, nozzle throat angle, and ration of gas specific heats). These dependencies are sometimes unusual, especially at small Mach numbers. It was also surprising that there is no great difference among all these flowfield features in the plane jet and in the axisymmetrical jet flow out of a nozzle with large throat angle, but all these parameters behave in a quite different way in an axisymmetrical jet at small and moderate nozzle throat angles.

Two-dimensional impinging jets in hydrodynamic rotational flows

The free streamline theory in hydrodynamics is an important and difficult issue not only in fluid mechanics but also in mathematics. The major purpose in this paper is to establish the well-posedness of the impinging jets in steady incompressible, rotational, plane flows. More precisely, given a mass flux and a vorticity of the incoming flows in the inlet of the nozzle, there exists a unique smooth impinging plane jet. Moreover, there exists a smooth free streamline, which goes to infinity and initiates at the endpoint of the nozzle smoothly. In addition, asymptotic behavior in upstream and downstream, uniform direction and other properties of the impinging jet are also obtained. The main ingredients of the mathematic analysis in this paper are based on the modified variational method developed by H. W. Alt, L. A. Caffarelli and A. Friedman in the elegant works [1,17], which has been shown to be powerful to deal with the steady irrotational flows with free streamlines.

Numerical Study of Two Dimensional Laminar Jet Flow Past a Sudden Expansion

A computational study of laminar plane jet flow past a plane symmetric sudden expansion configuration is presented. The dependence of asymmetric flow instability on the velocity profile at the expansion region has been studied. This variation in velocity profile at the expansion region is brought about by varying the length of the inlet duct. The attachment lengths of the jet for a range of Reynolds number (Re) values and aspect ratios (ratio of duct width to jet width) have been presented. An in-house two dimensional incompressible flow code has been developed for solving the problem. The Critical Reynolds Number (Recr) at which jet develops asymmetry with respect to mid-plane of the channel for a certain aspect ratio, increases as the inlet duct length is decreased. Hence, this indicates that the stability of the jet increases with the decrease in the inlet duct length. The shorter attachment length (SAL) remains nearly constant with increase in Re but longer attachment length (LAL) increases with increase in Re for various aspect ratios.

Impinging planar jet flow on a horizontal surface with slip

The flow of a planar jet (sheet) impinging onto a solid flat plate with slip is examined theoretically. The jet is assumed to spread out in a thin layer bounded by a hydraulic jump, and draining at the edge of the plate. In contrast to an adhering jet, a slipping jet does not admit a similarity solution. Taking advantage of the different scaling in each region, series expansions are used in the developing and fully viscous layers, which are matched at the transition point. We show that a slipping film exhibits a singularity in the normal stress at the leading edge of the boundary layer, as opposed to the singularity in velocity and shear stress for an adhering film. The boundary-layer and film heights are both found to decrease with slip relative to a smooth substrate, roughly like $\sqrt{30x/Re}-2S$, whereas the slip velocity intensifies like $S\sqrt{Re/30x}$ with slip. Here, $x$ is the distance along the plate, $S$ is the slip length and $Re$ is the Reynolds number (in units of the jet width). The transition is delayed by slip. Guided by the measurements of Duchesne et al. (Europhys. Lett., vol. 107, 2014, p. 54002) for a circular adhering jet, the hydraulic-jump height and location are determined for a planar jet, and are found to increase with the Froude number (flow rate) like $Fr^{1/4}$ and $Fr^{5/8}$ respectively, essentially independently of slip length.

Critical slot size for deflagration initiation by hot products discharge into hydrogen–air atmospheres

This computational study addresses deflagration initiation of lean, stoichiometric, and moderately rich hydrogen–air mixtures by the sudden discharge of a hot planar jet of its adiabatic combustion products. The objective is to determine the minimum slot size required for ignition, a relevant quantity of interest for safety and technological applications concerning the accidental ignition of hydrogen. For sufficiently small jet velocities, the numerical solution of the problem requires integration of the two-dimensional Navier–Stokes equations for chemically reacting ideal-gas mixtures, supplemented by standard descriptions of the molecular transport terms and a reduced chemical–kinetic mechanism suitable for hydrogen–air combustion. The computations provide the variation of the critical slot size for hot-jet ignition with both the jet Reynolds number and the equivalence ratio of the mixture. In particular, it is seen that, while the Reynolds number exerts only a relatively weak effect on the ignition process, the influence of the equivalence ratio is much more pronounced, with the smallest slot widths found for stoichiometric or slightly-rich conditions. The numerical results show three different ignition modes, with the flame developing from a clearly identified ignition kernel located either at the core of the leading vortex pair (mode 1), at the symmetry plane near the leading edge of the starting jet (mode 2), or at the jet stem connecting the jet exit with the starting vortex (mode 3).

Investigation of tone generation in ideally expanded supersonic planar impinging jets using large-eddy simulation

The generation of tones in a supersonic planar jet impinging on a flat plate normally has been investigated by performing compressible large-eddy simulations using low-dissipation and low-dispersion finite differences. At the exit of a straight nozzle of height$h$, the jet is ideally expanded, and has a Mach number of 1.28 and a Reynolds number of$5\times 10^{4}$. Four distances between the nozzle and the plate between$3.94h$and$9.1h$have been considered. Flow snapshots and mean velocity fields are first presented. The variations of turbulence intensities and of the convection velocity in the jet shear layers are then examined. The properties of the jet near fields are subsequently described, in particular by applying Fourier decomposition to the pressure fields. Several coexisting tones appear to be generated by aeroacoustic feedback loops establishing between the nozzle lip and the flat plate, which also lead to the presence of hydrodynamic–acoustic standing waves. The tone frequencies are consistent with those given by the aeroacoustic feedback model and with measurements for high-aspect-ratio rectangular jets. The jet oscillation modes at these frequencies are characterized, and found to agree with experimental data. Their symmetric or antisymmetric natures are shown to be well predicted by a wave analysis carried out using a vortex sheet model of the jet, providing the allowable frequency ranges for the upstream-propagating acoustic waves. Thus, it is possible, for an ideally expanded impinging planar jet to predict both the frequencies of the tones and the symmetric or antisymmetric nature of the corresponding oscillation modes by combining the aeroacoustic feedback model and the wave analysis.

On the sensitivity of planar jets

Planar jets are widely researched flows. Many aspects of their instability have been well-known for a long time. One of them is the observation that the jet is more sensitive to disturbances near the orifice than elsewhere. This paper aims at giving an explanation to this hitherto unexplained phenomenon. To this end, linear stability investigations on various velocity profiles were carried out using the Orr–Sommerfeld (OS) equation. The velocity profiles were provided by analytical approximations and numerical computational fluid dynamics (CFD) simulations. A special method called compound matrix method (CMM) was used for solving the OS equation to obtain sufficiently accurate results. The adaptation of the method for symmetric jets was briefly derived. The stability of various velocity profiles was compared based on the local spatial growth rate. The results of the comparison clearly show that velocity profiles near the orifice are more unstable than the downstream ones. The outcomes also show that the velocity profile close to the orifice and the wall thickness of the nozzle have impact on the stability properties of the flow there.