Analysis of the possibilities of the methods for calculating turbulent jet noise

Two methods for calculating the noise of turbulent exhaust jets of civil aircraft nozzles are considered. The first method is chiefly intended for engineering mass-volume calculations and is based on the solution of the averaged Navier-Stokes equations closed by a two-equation turbulence model. The second method uses direct numerical simulation of large eddies in a turbulent jet and the Kirchhoff surface for calculating noise spectra and radiation patterns in the far field. The possibilities and certain important restrictions of these methods are analyzed. The results obtained using these methods are compared with experimental data.     

Soot distribution in turbulent nonpremixed chloromethane — Air flames

The results of an experimental study on soot and temperature distribution in turbulent, nonpremixed chloromethane-air jet flames are presented. Transient measurements of soot volume fraction and temperature are made using a three-line optical pyrometry method. This method enables measurement of the “total” (i.e., absorption-related), and “hot” (i.e., emission-related) soot. Significant amounts of cold soot with hot soot is observed to coexist for all of the measurements made in these flames. Images of soot presence using a gated camera provide information about the mixing phenomena. The effects due to differential probe lengths are also discussed.     

高速喷流下的柔性旋转火箭发射系统瞬态响应

对于柔性旋转火箭发射系统，考虑高速燃气喷流作用下的系统耦合振动．建立发射系统动力学模型，数值计算发射管流场结构，确定系统瞬态响应和燃气流冲击对火箭发射姿态的影响，更加真实地模拟柔性旋转火箭的发射动力学环境．     

Numerical modeling of the formation of aerosol particles in jet engine plumes

The formation of liquid sulfate aerosols in the isobaric axisymmetric plume of a subsonic aircraft is modeled numerically. The specific features of the appearance and evolution of sulfate aerosols attributable to 2D effects, such as the parameter nonuniformity in the initial section (at the nozzle outlet), mixing of the hot engine jet with the cold air stream, and the transverse turbulent diffusion of aerosol particles and gas mixture components. The equations of gas dynamics for a turbulent axisymmetric jet, the equations of chemical kinetics, the equations for the liquid fractions (water and sulfuric acid), and the relations for the binary nucleation, condensation growth and coagulation of aerosol particles are used. The distributions of the parameters determining the formation of the aerosol phase in the exhaust plume of a B-747 aircraft are obtained and the geometry of the nucleation zone in this plume is determined.     

Differential Diffusion Modelling in LES with RCCE-Reduced Chemistry

The present work shows results obtained from the incorporation of a soot model into a combined Large Eddy Simulation and Conditional Moment Closure approach to modelling turbulent non-premixed flames. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, where acetylene is employed as the incipient species responsible for soot nucleation. The concentrations of the gaseous species are calculated using a Rate-Controlled Constrain Equilibrium approach to reduce the number of species to solve from a detailed gas-phase kinetic scheme involving 63 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for atmospheric methane flames, Overall, the study demonstrates that the model, when used in conjunction with a representation of differential diffusion effects, is capable of predicting soot formation at a fundamental level in the turbulent non- premixed flames considered.     

Computations for a jet impinging obliquely on a flat surface

A SIMPLE-C algorithm and Jones-Launder k-ε two-equation turbulence model are used to simulate a two-dimensional jet impinging obliquely on a flat surface. Both the continuity and momentum equations for the unsteady state are cast into suitable finite difference equations. The pressure, velocity, turbulent kinetic energy and turbulent energy dissipation rate distributions are solved and show good agreement with various experimental data. The calculations show that the flow field structure of the jet impinging obliquely on a flat surface is strongly affected by the oblique impingement angle. The maximum pressure zone of the obliquely impinging jet flow field moves towards the left as the oblique impingement angle is decreased.     

Kinetic droplet evaporation in a turbulent air jet

Droplet evaporation in a turbulent air jet is considered for conditions such that the evaporation rate is determined by the evaporation kinetics for the individual droplets rather than by the diffusion rate of the air in the jet. Numerical solution of the equations by computer has shown that the mean air speed in the jet has little effect on the droplet evaporation in the range covered. A simplified solution is presented for the dispersal of an evaporating impurity in a jet. Experiments confirm that this solution is correct, and they show that the turbulent pulsations play a large part, with the result that an initially monodisperse system becomes more and more polydisperse away from the jet.     

Numerical Simulation of Diesel Particulate Filter Regeneration Considering Ash Deposit

Non-combustible ash can be deposited on channel walls through the full length of a diesel particulate filter (DPF). This type of ash can affect the exhaust condition and heat transfer process during the periodical soot regeneration of DPF. A model of soot regeneration is established in this paper to describe the effects of ash deposits on exhaust condition and heat transfer. Mass, momentum, and energy balances are considered for the multiphase system of gas, soot, and wall. The good agreement with experimental data confirms that the model is able to describe the processes. The axial patterns of wall temperature and soot regeneration rate during DPF regeneration are investigated by numerical simulation. Results indicate that the deposited ash layer reduces the exhaust flow rate and increases the heat conduction resistance during DPF regeneration, thus leading to high wall temperature and soot oxidation rate. When 5 g/L of ash is deposited, the complete oxidation of soot can be achieved 90 s faster with a 60 ^°C increase in wall temperature. The gain on soot oxidation rate increases with increasing amounts of deposited ash and reaches a plateau when the deposited ash approaches 15 g/L. Owing to the sintering and melting of ash when the temperature reaches 900 ^°C and the consequent uncontrolled regeneration, the soot carrying capacity should be identified based on the amount of deposited ash within DPF.     

Unsteady phenomena of an oscillating turbulent jet flow inside a cavity: Effect of aspect ratio

Self-sustained oscillatory phenomena in confined flow may occur when a turbulent plane jet is discharging into a rectangular cavity. An experimental set-up was developed and the flow analysis has been made using mainly hot-wire measurements, which were complemented by visualisation data. Previous studies confirmed that periodic oscillations may occur, depending on the location of the jet exit nozzle inside the cavity, and also the distance between the side-walls. The present study deals with the symmetrical interaction between a turbulent plane jet and a rectangular cavity and the influence of the geometrical characteristics of the cavity on the oscillatory motion. The size and aspect ratio of the cavity were varied together with the jet width compared to that of the cavity. The study is carried out both numerically and experimentally. The numerical method solves the unsteady Reynolds averaged Navier–Stokes equations (URANS) together with the continuity equation for an incompressible fluid. The closure of the flow equations system is achieved using a two-scale energy-flux model at high Reynolds number in the core flow coupled with a wall function treatment in the vicinity of the wall boundaries. The fundamental frequency of the oscillatory flow was found to be practically independent of the cavity length. Moreover, the oscillations are attenuated as the cavity width increases, until they disappear for a critical value of the cavity width. Contour maps of the instantaneous flow field are drawn to show the flow pattern evolution at the main phases of oscillation. They are given for several aspect ratios of the cavity, keeping constant values for the cavity width and the jet thickness. The proposed approach may help to investigate further the oscillation mechanisms and the entrainment process occurring in pressure driven jet–cavity interactions.     

Buoyancy effects on turbulent swirling flows

A three-parameter model of turbulence applicable to free boundary layers has been developed and applied for the prediction of axisymmetric turbulent swirling flows in uniform and stagnant surroundings under the action of buoyancy forces. The turbulent momentum and heat fluxes appearing in the time-averaged equations for the mean motion have been determined from algebraic expressions, derived by neglecting the convection and diffusion terms in the differential transport equations for these quantities, which relate the turbulent fluxes to the kinetic energy of turbulence, k, the dissipation length scale of turbulence, L, and the temperature covariance, T ′². Differential transport equations have been used to determine these latter quantities. The governing equations have been solved using fully implicit finite difference schemes. The turbulence model is capable of reproducing the gross features of pure jet flows, buoyant flows and swirling flows for weak and moderate swirl. The behaviour of a turbulent buoyant swirling jet has been found to depend solely on exit swirl and Froude numbers. The predicted results indicate that the incorporation of buoyancy can cause significant changes in the behaviour of a swirling jet, particularly when the buoyancy strength is high. The jet exhibits similarity behaviour in the initial region for weak swirl and weak buoyancy strengths only, and the asymptotic case of a swirling jet under the action of buoyancy forces is a pure plume in the far field. The predicted results have been found to be in satisfactory agreement with the available experimental data and in good qualitative agreement with other predicted results.     

Large eddy simulation of vertical turbulent jets under JONSWAP waves

The effect of random waves on vertical plane turbulent jets is studied numerically and the mechanism behind the interaction of the jet and waves is analyzed. The large eddy simulation method is used and the σ-coordinate system is adopted. Turbulence is modeled by a dynamic coherent eddy model. The σ-coordinate transformation is introduced to map the irregular physical domain with a wavy free surface and an uneven bottom onto a regular computational domain. The fractional step method is used to solve the fil...     

A Length-Scale Equation

We derive an equation for the average length-scale in a turbulent flow from a simple physical model. This is a tensorial length-scale. We use as a model the evolution of a blob of turbulent kinetic energy under the influence of production, dissipation, and transport, as well as distortion by the mean motion. A single length-scale is defined which is biased toward the smallest of the scales in the various directions. Constants are estimated by consideration of homogeneous decay. Preliminary computations are carried out in a mixing layer and a two-dimensional jet, using the new length-scale equation and the equation for the turbulent kinetic energy. The results are compared with data and with the predictions of the classical k-epsilon equations; the new results are quite satisfactory. In particular, the plane jet/round jet anomaly is approximately resolved. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the analysis of an impinging jet on ground effects

Laser Doppler measurements and flow visualization are presented for a turbulent circular jet emerging into a low-velocity cross-stream and, then, impinging on a flat surface perpendicular to the jet-nozzle axis. The experiments were performed for a Reynolds number based on the jet-exit conditions of 6 × 10⁴, a jet-to-crossflow velocity ratio of 30 and for an impingement height of 5 jet diameters and include mean and turbulent velocity characteristics along the two normal directions contained in planes parallel to the nozzle axis. The results, which have relevance to flows found beneath VSTOL aircraft in ground effect, show the presence of a complex 3-D scarf vortex formed around the impinging jet. In zones where measurement data are not available, the flow details are numerically-visualized using a solution of the finite difference form of the fully threedimensional Reynolds-averaged Navier-Stokes equations, incorporating the turbulence viscosity concept. The turbulent structure of the flow is affected by flow distortion at the impinging zone, which results in an unconventional behaviour of the dimensionless structure parameters that determine the empirical constants in engineering models of turbulence. The relative magnitude of the terms involved in the transport equations for the turbulent stresses is quantified from the experimental data in order to assess the importance of these effects and show the extent to which the turbulent structure of the impingement zone is affected by extra rates of strain.     

Numerical Investigation of Multi-Jet Channel Flows

The results of a numerical simulation of the three-dimensional outflow of a system of circular supersonic turbulent jets into a cocurrent supersonic (or subsonic) air flow in a partially bounded region are given. Solutions are obtained by the splitting method using a matrix sweep of the parabolized Navier-Stokes equations. Assuming that the flow is nonseparated in the boundary layer, features of the three-dimensional structure of the jet system are investigated as functions of the pressure ratio number and the jet and cocurrent flow Mach numbers.     

Numerical modeling of a two-dimensional vertical turbulent two-phase jet

The results of numerically modeling two-dimensional two-phase flow of the “gas-solid particles” type in a vertical turbulent jet are presented for three cases of its configuration, namely, descending, ascending, and without account of gravity. Both flow phases are modeled on the basis of the Navier-Stokes equations averaged within the framework of the Reynolds approximation and closed by an extended k-? turbulence model. The averaged two-phase flow parameters (particle and gas velocities, particle concentration, turbulent kinetic energy, and its dissipation) are described using the model of mutually-penetrating continua. The model developed allows for both the direct effect of turbulence on the motion of disperse-phase particles and the inverse effect of the particles on turbulence leading to either an increase or a decrease in the turbulent kinetic energy of the gas. The model takes account for gravity, viscous drag, and the Saffman lift. The system of equations is solved using a difference method. The calculated results are in good agreement with the corresponding experimental data which confirms the effect of solid particles on the mean and turbulent characteristics of gas jets.     

Use of the turbulence energy equation in the theory of jet flows

In this study some of the assumptions introduced in [1] in developing a closed system of equations for a turbulent boundary layer will be simplified. With the aid of the system of equations of [1], a theoretical solution is found for the problem of a jet in an accompanying flow, it being assumed that the structure of the jet turbulence depends solely on local conditions. Experiment has shown that the turbulence in such a jet does depend also on the prehistory of the flow. At large distances from the source, the theoretical characteristics of the jet agree well with the experimentally determined characteristics of the wake beyond a body. Also examined is the problem of the boundary layer between two homogeneous flows, flowing with different velocities.Translated from Izvestiya Akademii Nauk SSSR. Mekhanika Zhidkosti i Gaza, No. 2, pp. 75–81, March–April, 1973.     

Nichtisotherme ebene Freistrahlen unter Schwerkrafteinfluß

Higher-order boundary layer theory is used to study the behaviour of nonisothermal laminar and turbulent free jet flows. In addition to the Prandtl boundary layer equations, an equation is used to describe the equilibrium of forces normal to the flow direction. This equilibrium exists between the buoyancy forces caused by gravity and the centrifugal forces resulting from the curvature in the flow. The proper selection of reference values permits the characteristics of the jet flow to be expressed as universal functions in which only the initial jet orientation and the Prandtl number in the case of laminar flow are input parameters. When the volume flow is given in addition to the momentum and thermal energy, the characteristic parameter are the Archimedes number for turbulent flow and the modified Archimedes number for laminar flow. The jet flow is calculated using an integral method in which the eddy viscosity and the turbulent Prandtl number are given as functions of the local Archimedes number. Comparison of experimental data from the literature and from our laboratory on nonisothermal free jets with the theoretical results, show satisfactory agreement. The universal diagrams given in the paper are valid forall plane laminar (Pr=0.7) and turbulent nonisothermal jets.     

Some problems in the theory of plane jets of conducting fluid

Using the boundary-layer equations as a basis, the author considers the propagation of plane jets of conducting fluid in a transverse magnetic field (noninductive approximation).The propagation of plane jets of conducting fluid is considered in several studies [1–12]. In the first few studies jet flow in a nonuniform magnetic field is considered; here the field strength distribution along the jet axis was chosen in order to obtain self-similar solutions. The solution to such a problem given a constant conductivity of the medium is given in [1–3] for a free jet and in [4] for a semibounded jet; reference [5] contains a solution to the problem of a free jet allowing for the dependence of conductivity on temperature. References [6–8] attempt an exact solution to the problem of jet propagation in any magnetic field. An approximate solution to problems of this type can be obtained by using the integral method. References [9–10] contain the solution obtained by this method for a free jet propagating in a uniform magnetic field.The last study [10] also gives a comparison of the exact solution obtained in [3] with the solution obtained by the integral method using as an example the propagation of a jet in a nonuniform magnetic field. It is shown that for scale values of the jet velocity and thickness the integral method yields almost-exact values. In this study [10], the propagation of a free jet is considered allowing for conduction anisotropy. The solution to the problem of a free jet within the asymptotic boundary layer is obtained in [1] by applying the expansion method to the small magnetic-interaction parameter. With this method, the problem of a turbulent jet is considered in terms of the Prandtl scheme. The Boussinesq formula for the turbulent-viscosity coefficient is used in [12].This study considers the dynamic and thermal problems involved with a laminar free and semibounded jet within the asymptotic boundary layer, propagating in a magnetic field with any distribution. A system of ordinary differential equations and the integral condition are obtained from the initial partial differential equations. The solution of the derived equations is illustrated by the example of jet propagation in a uniform magnetic field. A similar solution is obtained for a turbulent free jet with the turbulent-exchange coefficient defined by the Prandtl scheme.     

The mathematical simulation of various condensation regimes in turbulent isobaric jets

Mathematical models are considered and calculations made for flows in turbulent isobaric steam—air jets in the presence of condensation of the water vapor they contain. The models consist of gasdynamic equations for a turbulent jet, equations for a differential two-parameter model of turbulence, thermodynamic relations, and kinetic equations. A study is made of steam—air jets in a regime of condensation in equilibrium, when the flow region is broken down into zones of frozen flow and flow in equilibrium, described by the equations for a turbulent jet with the use of the traditional thermodynamic relations and of the thermodynamic relations for condensation in equilibrium. An analysis is made of the influence of pulsating motion on the kinetic parameters: rate of nucleation, the critical size of the nuclei, and rate of growth of the drops. It is shown that the rate of nucleation, determined from a quasilaminar averaging model, is several orders of magnitude less than the mean value obtained by averaging using the density distribution of the passive admixture concentration probability. A numerical study is made of the heterogeneous condensation in turbulent jets on extraneous particles entering from the nozzle. Kinetic equations are written down for the case when the rate of growth of the drops does not depend on their radius. A study is made of the dynamics of the transition of heterogeneous condensation from disequilibrium to equilibriumTranslated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 59–67, January–February, 1985.     

Concentration fluctuations in a turbulent jet

Using Spalding's model of turbulence in a turbulent shear flow, we have calculated the root-mean-square value of the concentration fluctuations inside a turbulent jet. Although we used the same equations and the same solution technique as Spalding, we have not been able to find precisely his numerical results derived for a jet issuing into a fluid at rest with the same density as the jet. The differences between our numerical results, Spalding's numerical results and the experimental data of Becker, Hottel and Williams are fairly small only if the initial values of the turbulence energy and the mixing length inside the jet and the turbulence in the ambient fluid are taken into account in the model. For a turbulent jet issuing into a turbulently flowing surrounding stream of different density, we found that the relative concentration fluctuations can increase considerably. This brings out the importance of taking into account property variables in analysing turbulent mixing processes.