Showing papers on "Supersonic speed published in 2010"

Dielectric Barrier Discharge Plasma Actuators for Flow Control

Abstract: The term plasma actuator has now been a part of the fluid dynamics flow-control vernacular for more than a decade. A particular type of plasma actuator that has gained wide use is based on a single–dielectric barrier discharge (SDBD) mechanism that has desirable features for use in air at atmospheric pressures. For these actuators, the mechanism of flow control is through a generated body-force vector field that couples with the momentum in the external flow. The body force can be derived from first principles, and the effect of plasma actuators can be easily incorporated into flow solvers so that their placement and operation can be optimized. They have been used in a wide range of internal and external flow applications. Although initially considered useful only at low speeds, plasma actuators are effective in a number of applications at high subsonic, transonic, and supersonic Mach numbers, owing largely to more optimized actuator designs that were developed through better understanding and modeling of...

Large-Eddy Simulation of Jet Mixing in Supersonic Crossflows

Abstract: Large-eddy simulation of an underexpanded sonic jet injection into supersonic crossflows is performed to obtain insights into key physics of the jet mixing. A high-order compact differencing scheme with a recently developed localized artificial diffusivity scheme for discontinuity-capturing is used. Progressive mesh refinement study is conducted to quantify the broadband range of scales of turbulence that are resolved in the simulations. The simulations aim to reproduce the flow conditions reported in the experiments of Santiago and Dutton [Santiago, J. G., and Dutton, J. C., "Velocity Measurements of a Jet Injected into a Supersonic Crossflow," Journal of Propulsion and Power, Vol.132,1997, pp. 264―273] and elucidate the physics of the jet mixing. A detailed comparison with these data is shown. Statistics obtained by the large-eddy simulation with turbulent crossflow show good agreement with the experiment, and a series of mesh refinement studies shows reasonable grid convergence in the predicted mean and turbulent flow quantities. The present large-eddy simulation reproduces the large-scale dynamics of the flow and jet fluid entrainment into the boundary-layer separation regions upstream and downstream of the jet injection reported in previous experiments, but the richness of data provided by the large-eddy simulation allows a much deeper exploration of the flow physics. Key physics of the jet mixing in supersonic crossflows are highlighted by exploring the underlying unsteady phenomena. The effect of the approaching turbulent boundary layer on the jet mixing is investigated by comparing the results of jet injection into supersonic crossflows with turbulent and laminar crossflows.

Simulation of Turbulent Mixing Behind a Strut Injector in Supersonic Flow

Abstract: The flowfield downstream of a strut-based injection system in a supersonic combustion ramjet is investigated using large-eddy simulation with a new localized dynamic subgrid closure for compressible turbulent mixing. Recirculations are formed at the base of the strut in the nonreacting flow and trap some of the injected fluid. The high levels of turbulence along the underexpanded hydrogen jets and in the shear layer lead to a high level of mixing of fuel and freestream fluids. Furthermore, the shear layer unsteadiness permits efficient large-scale mixing of freestream and injected fluids. In the reacting flowfield, the flame anchoring mechanism is, however, found to depend more on a recirculation region located downstream of the injectors than on their sides. A region of reverse flow is formed that traps hot products and radicals. Intermittent convection of hot fluid toward the injector occurs and preheats the reactants.

Two-dimensional supersonic plasma acceleration in a magnetic nozzle

Abstract: A two-dimensional model of the expansion of a collisionless, electron-magnetized, low-beta, current-free plasma in a divergent magnetic nozzle is presented. The plasma response is investigated in terms of the nozzle/plasma divergence rate, the magnetic strength on ions, and the Hall current at the nozzle throat. Axial acceleration profiles agree well with those estimated from simple one-dimensional models. A strong radial nonuniformity develops downstream. There is a separation between ion and electron/magnetic streamtubes which leads to the formation of, first, a longitudinal electric current density, which indicates that current ambipolarity is not fulfilled, and, second, a small ion azimuthal current that competes negatively with the electron azimuthal (Hall) current. The analysis of the mechanisms driving thrust, ion momentum, and ion energy unveils the dual electrothermal/electromagnetic character of the magnetic nozzle. In general, the thrust includes the contributions of volumetric and surface Hall currents, this last one formed at the plasma-vacuum interface. Plume efficiency, based on radial expansion losses, is computed. Plasma detachment and the transonic matching with the upstream plasma are not addressed.

Finite Rate Chemistry Large-Eddy Simulation of Self-Ignition in a Supersonic Combustion Ramjet

Abstract: In this study, large-eddy simulation is used to analyze supersonic flow, mixing, and combustion in a supersonic combustor equipped with a two-stage fuel injector strut. The present study focuses on mixing, ignition, and flame stabilization and the degree of detail required by the reaction mechanism in the large-eddy simulation model framework. An explicit large-eddy simulation model, using a mixed subgrid model and a partially stirred reactor turbulence-chemistry interaction model, is used in an unstructured finite volume setting. The model, and its components, has been carefully validated in a large number of other studies. To bestow further validation and to provide supplementary information about the physics of mixing and supersonic combustion, experimental data from the National Aerospace Laboratory of Japan's supersonic combustor, equipped with the two-stage strut injector and connected to ONERA's vitiation air heater, are employed. The large-eddy simulation predictions are compared with the experimental centerline wall pressure distribution and the planar laser-induced fluorescence imaging of hydroxide-ion radicals distributions in several cross sections of the combustor, showing excellent qualitative and quantitative agreements. The large-eddy simulation results are furthermore used to elucidate the complicated flow, mixing, and combustion physics imposed by the multi-injector two-stage injector strut. The importance of the combustion chemistry appears weaker than expected but with the one-step mechanism resulting in a too early ignition (caused by local shock wave heating) and a more stable flame, as compared with the more detailed two- and seven-step mechanisms.

Dynamics of sonic jet injection into supersonic crossflow

Abstract: The interaction between a sonic air jet and a supersonic air crossflow is simulated using large-eddy simulation (LES). A hybrid numerical methodology is used here to capture shock waves locally with minimal dissipation of the turbulent structures. The dynamic subgrid closure model employed for the LES permits a fully localized evaluation of the closure coefficients, such that there are no ad hoc adjustable parameters. Simulation of the experimental study of Santiago and Dutton (J. Propul. Power, vol. 13, 1997, pp. 264–273), where detailed measurements of the mean velocity and turbulent fluctuations have been acquired, is reported. The LES results show fairly good agreement with the experimental data for the mean and statistical fluctuations of the velocity field. The numerical study is then extended to two other jets in crossflow conditions to study the impact of the free-stream Mach number and of the jet to free-stream momentum ratio on the structure of the jet and on the dynamics of the interaction. The...

Physics and Regimes of Supersonic Combustion

Abstract: Understanding the physics of supersonic combustion is the key to design a performing engine for scramjet-powered vehicles. Despite studies on supersonic combustion dating back to the 1950s, there are still numerous uncertainties and misunderstandings on this topic. The following questions need to be answered: How does compressibility affect mixing, flame anchoring, and combustion efficiency? How long must a combustor be to ensure complete mixing and combustion while avoiding prohibitive performance losses? How can reacting turbulent and compressible flows be modeled? Experimental results in the past have shown that supersonic combustion of hydrogen and air is feasible and takes place in a reasonable distance, which is a necessary requirement in actual hypersonic vehicles powered by supersonic combustion ramjets. These results are explained based on a theoretical analysis of the physical mechanisms driving mixing and combustion in supersonic airstreams, where they are found to be different from those in the incompressible regime. In particular, the classic Kolmogorov scaling is shown to be no longer strictly valid, and the flame regime is predicted to be significantly affected by compressibility and different from that of subsonic flames. This analysis is also supported by the results of the numerical simulations presented, showing that by generating sufficiently intense turbulence, a supersonic combustion flame is short and can indeed anchor within a small distance from fuel injectors, with the flame typically burning in the so-called flamelets-in-eddies regime.

Numerical investigation of transverse jets through multiport injector arrays in supersonic crossflow

Abstract: A three-dimensional numerical study has been performed of the effects of sonic gaseous hydrogen injection through multiple transverse injectors subjected to a supersonic crossflow. Solutions were obtained for a series of injection configurations in a Mach 4.0 crossflow, with a global equivalence ratio of o = 0:5. Results indicate a different flow structure than for a typical single jet, with the development of two clearly defined wake vortices, including a stagnation point and reversed flow region immediately behind each downstream jet. While the overall penetration was reduced under the investigated conditions, significant improvements were observed when nondimensionalizing against the equivalent jet diameter for each modeled injector row. This was found to be the result of increased jet-to-freestream momentum ratio due to the subsonic flow regions between each injector. Further enhancements were also observed in terms of mixing performance for the multijet cases. Improvements of up to 5% in the overall mixing efficiency were experienced by using multiple jets due to increased mixant interface area and intermediate stirring through wake vortices between each injector. No improvement in far-field mixing was observed. Overall, it has been demonstrated that there are benefits to be gained through the injection of gaseous hydrogen from many small injectors rather than fewer large injectors. Copyright © 2010 by A. S. Pudsey and R. R. Boyce. Published by the American Institute of Aeronautics and Astronautics, Inc.

Wave-Packet Models for Large-Scale Mixing Noise:

Abstract: A wave-packet Ansatz is used to model jet noise generation by large-scale turbulence. In this approach, an equivalent source is defined based on the two-point space-time correlation of hydrodynamic pressure on a conical surface surrounding the jet plume. The surface is sufficiently near the turbulent flow region to be dominated by non-propagating hydrodynamic signatures of large-scale turbulent structures, yet sufficiently far that linear behavior can be assumed in extending the near-field pressure to the acoustic field. In the present study, a 78-microphone array was used to measure hydrodynamic pressure on the conical surface in order to identify parameters for the model and to validate the approach. Six microphones were distributed in the azimuthal direction at each of 13 axial locations spanning the first 8 jet diameters, allowing decomposition of azimuthal modes m = 0 and m = 1. We show that a source model based on a Gaussian correlation function provides a consistently good representation of the noise source attributed to large-scale structures. The results provide evidence that large-scale wave-like structures, known to dominate aft radiation at supersonic phase speeds, are also relevant at subsonic speeds.

The density variance -- Mach number relation in supersonic, isothermal turbulence

Abstract: We examine the relation between the density variance and the mean-square Mach number in supersonic, isothermal turbulence, assumed in several recent analytic models of the star formation process. From a series of calculations of supersonic, hydrodynamic turbulence driven using purely solenoidal Fourier modes, we find that the `standard' relationship between the variance in the log of density and the Mach number squared, i.e., sigma^2_(ln rho/rhobar)=ln (1+b^2 M^2), with b = 1/3 is a good fit to the numerical results in the supersonic regime up to at least Mach 20, similar to previous determinations at lower Mach numbers. While direct measurements of the variance in linear density are found to be severely underestimated by finite resolution effects, it is possible to infer the linear density variance via the assumption of log-normality in the Probability Distribution Function. The inferred relationship with Mach number, consistent with sigma_(rho/rhobar) ~ b M with b=1/3, is, however, significantly shallower than observational determinations of the relationship in the Taurus Molecular Cloud and IC5146 (both consistent with b~ 0.5), implying that additional physics such as gravity is important in these clouds and/or that turbulent driving in the ISM contains a significant compressive component. Magnetic fields are not found to change this picture significantly, in general reducing the measured variances and thus worsening the discrepancy with observations.

A numerical study of compressible turbulent boundary layers

Abstract: Compressible turbulent boundary layers with free-stream Mach number ranging from 2.5 up to 20 are analyzed by means of direct numerical simulation of the Navier–Stokes equations. The fluid is assumed to be an ideal gas with constant specific heats. The simulation generates its inflow condition using the rescaling-recycling method. The main objective is to study the effect of Mach number on turbulence statistics and near-wall turbulence structures. The present study shows that supersonic/hypersonic boundary layers at zero pressure gradient exhibit close similarities to incompressible boundary layers and that the main turbulence statistics can be correctly described as variable-density extensions of incompressible results. The study also shows that the spanwise streak’s spacing of 100 wall units in the inner region y + 15 still holds for the considered high Mach numbers. The probability density function of the velocity dilatation shows significant variations as the Mach number is increased, but it can also be normalized by accounting for the variable-density effect. The compressible boundary layer also shows an additional similarity to the incompressible boundary layer in the sense that without the linear coupling term, near-wall turbulence cannot be sustained. © 2011 American Institute of Physics. doi:10.1063/1.3541841

Large Eddy Simulation of Stable Supersonic Jet Impinging on Flat Plate

Abstract: This paper describes a numerical study based on large eddy simulation of the flow induced by a supersonic jet impinging on a flat plate in a stable regime. This flow involves very high velocities, shocks, and intense shear layers. Performing large eddy simulation on such flows remains a challenge because of the shock discontinuities. Here, large eddy simulation is performed with an explicit third-order compressible solver using a unstructured mesh, a centered scheme, and the Smagorinsky model. Three levels of mesh refinement (from 7 to 22 million cells) are compared in terms of instantaneous and averaged flowfields (shock and recirculation zone positions), averaged flow velocity and pressure fields, wall pressure, root mean square pressure fields, and spectral content using one and two-point analyses. The effects of numerical dissipation and turbulent viscosity are compared on the three grids and shown to be well controlled. The comparison of large eddy simulation with experimental data shows that the finest grid (a 22 million cell mesh) ensures grid-independent results not only for the mean and rms fields but also for higher statistics such as single and two-point correlation functions.

Computational Fluid Dynamics Simulation of Supersonic Oxygen Jet Behavior at Steelmaking Temperature

Abstract: Supersonic oxygen jets are used in steelmaking and other different metal refining processes, and therefore, the behavior of supersonic jets inside a high temperature field is important for understanding these processes. In this study, a computational fluid dynamics (CFD) model was developed to investigate the effect of a high ambient temperature field on supersonic oxygen jet behavior. The results were compared with available experimental data by Sumi et al. and with a jet model proposed by Ito and Muchi. At high ambient temperatures, the density of the ambient fluid is low. Therefore, the mass addition to the jet from the surrounding medium is low, which reduces the growth rate of the turbulent mixing region. As a result, the velocity decreases more slowly, and the potential core length of the jet increases at high ambient temperatures. But CFD simulation of the supersonic jet using the k−e turbulence model, including compressibility terms, was found to underpredict the potential flow core length at higher ambient temperatures. A modified k-e turbulence model is presented that modifies the turbulent viscosity in order to reduce the growth rate of turbulent mixing at high ambient temperatures. The results obtained by using the modified turbulence model were found to be in good agreement with the experimental data. The CFD simulation showed that the potential flow core length at steelmaking temperatures (1800 K) is 2.5 times as long as that at room temperature. The simulation results then were used to investigate the effect of ambient temperature on the droplet generation rate using a dimensionless blowing number.

Numerical Investigation of Transverse Hydrogen Jet into Supersonic Crossflow Using Detached-Eddy Simulation

Abstract: A three-dimensional unsteady reacting flowfield that is generated by transverse hydrogen injection into a supersonic mainstream is numerically investigated using detached-eddy simulation and a finite-rate chemistry model. Grid refinement with the grid-convergence-index concept is applied to the instantaneous flowfield for assessing the grid resolution and solution convergence.Validation is performed for the jet penetration height, and the predicted result is in good agreement with experimental trends. The results indicate that jet vortical structures are generated as the interacting counter-rotating vortices become alternately detached in the upstream recirculation region. Although the numerical OH distribution reproduces the experimental OH–planar-laser-induced fluorescence well, there are some disparities in the ignition delay times due to the restricted availability of experimental and numerical data. The effects of the turbulence model on combustion are identified by a comparative analysis of the Reynolds-averaged Navier–Stokes and detached-eddy simulation approaches. Their effects are quantified by the production ofH2O, which is the primary species of hydrogen combustion.

Supersonic Air Flow due to Solid-Liquid Impact

Abstract: A solid object impacting on liquid creates a liquid jet due to the collapse of the impact cavity. Using visualization experiments with smoke particles and multiscale simulations, we show that in addition, a high-speed air jet is pushed out of the cavity. Despite an impact velocity of only 1 m/s, this air jet attains supersonic speeds already when the cavity is slightly larger than 1 mm in diameter. The structure of the air flow closely resembles that of compressible flow through a nozzle—with the key difference that here the “nozzle” is a liquid cavity shrinking rapidly in time

Reduced-Order Modeling of Two-Dimensional Supersonic Flows with Applications to Scramjet Inlets

Abstract: Control-oriented models of hypersonic vehicle propulsion systems require a reduced-order model of the scramjet inlet that is accurate to within 10% but requires less than a few seconds of computational time. To achieve this goal, a reduced-order model is presented, which predicts the solution of a steady two-dimensional supersonic flow through an inlet or around any other two-dimensional geometry. The model assumes that the flow is supersonic everywhere except in boundary layers and the regions near blunted leading edges. Expansion fans are modeled as a sequence of discrete waves instead of a continuous pressure change. Of critical importance to the model is the ability to predict the results of multiple wave interactions rapidly. The rounded detached shock near a blunt leading edge is discretized and replaced with three linear shocks. Boundary layers are approximated by displacing the flow by an empirical estimate of the displacement thickness. A scramjet inlet is considered as an example application. The predicted results are compared to two-dimensional CFD solutions and experimental results. Nomenclature a = local soundspeed [m/s] c = specific heat [J/kg K] h = specific enthalpy [J/kg] H = length normal to flow [m] M = Mach number n = number of a given quantity L = length tangent to flow [m] p = pressure [Pa] Pr = Prandtl number r = radius [m] R = normalized gas constant [J/kg K] R = 8314.47 J/kmol K T = temperature [K] u = velocity magnitude [m/s] W = molecular weight [kg/kmol] x = forward body-frame coordinate [m] Y = mass fraction z = vertical body-frame coordinate [m] = shock angle = ratio of specific heats = thickness of layer [m] " = ratio = flowpath angle = dynamic viscosity [kg/m s] = ln p0=p = += 2

Circulation and dissipation on hot jupiters

Abstract: Many global circulation models predict supersonic zonal winds and large vertical shears in the atmospheres of short-period Jovian exoplanets. Using linear analysis and nonlinear local simulations, we investigate hydrodynamic dissipation mechanisms to balance the thermal acceleration of these winds. The adiabatic Richardson criterion remains a good guide to linear stability, although thermal diffusion allows some modes to violate it at very long wavelengths and very low growth rates. Nonlinearly, wind speeds saturate at Mach numbers {approx}2 and Richardson numbers {approx}<1/4 for a broad range of plausible diffusivities and forcing strengths. Turbulence and vertical mixing, though accompanied by weak shocks, dominate the dissipation, which appears to be the outcome of a recurrent Kelvin-Helmholtz instability. An explicit shear viscosity, as well as thermal diffusivity, is added to ZEUS to capture dissipation outside of shocks. The wind speed is neither monotonic nor single valued for a range of shear viscosities larger than about 10{sup -3} of the sound speed times the pressure scale height. Coarsening the numerical resolution can also increase the speed. Hence global simulations that are incapable of representing vertical turbulence and shocks, either because of reduced physics or because of limited resolution, may overestimate wind speeds. We recommend that suchmore » simulations include artificial dissipation terms to control the Mach and Richardson numbers and to capture mechanical dissipation as heat.« less

Mixing in Supersonic Turbulence

Abstract: In many astrophysical environments, mixing of heavy elements occurs in the presence of a supersonic turbulent velocity field. Here, we carry out the first systematic numerical study of such passive scalar mixing in isothermal supersonic turbulence. Our simulations show that the ratio of the scalar mixing timescale, ?c, to the flow dynamical time, ?dyn (defined as the flow driving scale divided by the rms velocity), increases with the Mach number, M, for M 3, and becomes essentially constant for M 3. This trend suggests that compressible modes are less efficient in enhancing mixing than solenoidal modes. However, since the majority of kinetic energy is contained in solenoidal modes at all Mach numbers, the overall change in ?c/?dyn is less than 20% over the range 1 M 6. At all Mach numbers, if pollutants are injected at around the flow driving scale, ?c is close to ?dyn. This suggests that scalar mixing is driven by a cascade process similar to that of the velocity field. The dependence of ?c on the length scale at which pollutants are injected into flow is also consistent with this cascade picture. Similar behavior is found for the variance decay timescales for scalars without continuing sources. Extension of the scalar cascade picture to the supersonic regime predicts a relation between the scaling exponents of the velocity and the scalar structure functions, with the scalar structure function becoming flatter as the velocity scaling steepens with Mach number. Our measurements of the volume-weighted velocity and scalar structure functions confirm this relation for M 2, but show discrepancies at M 3, which arise probably because strong expansions and compressions tend to make scalar structure functions steeper.

An MDOE Investigation of Chevrons for Supersonic Jet Noise Reduction

Abstract: The impact of chevron design on the noise radiated from heated, overexpanded, supersonic jets is presented. The experiments used faceted bi-conic convergent-divergent nozzles with design Mach numbers equal to 1.51 and 1.65. The purpose of the facets was to simulate divergent seals on a military style nozzle . The nozzle throat diameter was equal to 4.5 inches. Modern Design of Experiment (MDOE) techniques were used to investigate the impact of chevron penetration, length, and width on the resulting acoustic radiation. All chevron configurations used 12 chevrons to match the number of facets in the nozzle. Most chevron designs resulted in increased broadband shock noise relative to the baseline nozzle. In the peak jet noise direction, the optimum chevro n design reduced peak sound pressure levels by 4 dB relative to the baseline nozzle. Th e penetration was the parameter having the greatest impact on radiated noise at all observatio n angles. While increasing chevron penetration decreased acoustic radiation in the pea k jet noise direction, broadband shock noise was adversely impacted. Decreasing chevron length increased noise at most observation angles. The impact of chevron width on radiated noise depended on frequency and observation angle. I. Introduction He application of chevrons (serrations applied to a nozzle trailing edge that protrude into the exhaus ting flow) to military aircraft is particularly attractive becaus e existing engines can be retrofitted rather than r edesigned to incorporate these devices. At takeoff, high perfor mance tactical aircraft typically have overexpanded , supersonic jet-exhausts that contain noise sources not present in the subsonic exhausts of commercial aircraft en gines. As a result, chevrons that have been optimized for noise reduction in commercial aircraft may not perform a dequately on tactical aircraft. While a reasonably large number of investigations have studied the impact of chevr ons on subsonic jets, similar studies for supersonic flows are limi ted. The present investigation uses a Modern Desig n of Experiments (MDOE) approach to explore the impact of chevron design on the acoustic radiation of overe xpanded supersonic jets. An overexpanded jet resulting from operating a convergent-divergent nozzle at a stagnation pressu re below that corresponding to the nozzle design Mach number contains a quasi-periodic shock cell structure that can persist for several diameters downstream of the nozzle exit. T he constructive interference of sound waves produce d by the interaction of large-scale jet disturbances with th e shock waves within the shock cell structure resul ts in broadband shock noise 1,2,3 . Shock noise can dominate the acoustic spectra at upstream and broadside observation angles relative to the nozzle exit. Additionally, mixing noise sources are present and are associated with l arge scale jet disturbances (radiating in the downstream direction ) that become very effective noise sources when the ir phase speeds (relative to the ambient speed of sound) bec ome supersonic 4 , and with fine scale turbulence 5 (radiating in the upstream direction). Mixing noise sources are also present in subsonic jets but the large-scale distu rbances typically have subsonic phase speeds. In subsonic jets, properly designed chevron no zzles produce lower overall acoustic radiation leve ls than those of a corresponding round nozzle 6,7,8,9 . Experiments have shown that increasing chevron p enetration decreases low frequency noise and often increases high frequency noise (sometimes referred to as high frequency cros sover). The number of chevrons also impacts the acoustic radiat ion but not as significantly as the penetration. J et shear velocity (the velocity difference between the inner and oute r jet streams) impacts chevron acoustic performance with increases in shear velocity increasing low frequenc y noise reduction but sometimes increasing high fre quency noise * Researcher, Acoustics Branch, MS 54-3, 21000 Brookpark Rd., Cleveland, OH 44135. † Reseacher, Acoustics Branch, MS 54-3, 21000 Brookpark Rd., Cleveland, OH 44135, Associate Fellow.

Raman Scattering Measurements of Gaseous Ethylene Jets in a Mach 2 Supersonic Crossflow

Abstract: The structures of sonic ethylene jets delivered from orifices of three different diameters and two injection angles (30 and 90deg) into aMach 2 supersonic crossflowwere studied experimentally. The ratio of the cross-sectional areas of the largest and smallest injectors is 25:1. Time-averaged spontaneous vibrational Raman scattering was used to quantify injectant concentrations by constructing two-dimensional spanwise concentration images from the onedimensional linewise Raman scattering images. Based on the present data set, new penetration height correlations were developed to treat cases with injection angles of both 30 and 90 deg. Excluding the influence of wall boundary layer, the present measurements show that the properties of fuel plume structures, such as shape, size, and concentration profiles, are scalable with the injector size. Themeasured ethylene concentrations were also compared with predictions from the revised jet penetration code, which was calibrated primarily with hydrogen and helium. Discrepancieswere observedbetween themeasurements and the jet penetration code predictions for the structures of ethylene fuel plumes. The experimental data generated from the present study can be used to validate the numerical simulations.

Computational Fluid Dynamics Modeling of Supersonic Coherent Jets for Electric Arc Furnace Steelmaking Process

Abstract: Supersonic coherent gas jets are now used widely in electric arc furnace steelmaking and many other industrial applications to increase the gas–liquid mixing, reaction rates, and energy efficiency of the process. However, there has been limited research on the basic physics of supersonic coherent jets. In the present study, computational fluid dynamics (CFD) simulation of the supersonic jet with and without a shrouding flame at room ambient temperature was carried out and validated against experimental data. The numerical results show that the potential core length of the supersonic oxygen and nitrogen jet with shrouding flame is more than four times and three times longer, respectively, than that without flame shrouding, which is in good agreement with the experimental data. The spreading rate of the supersonic jet decreased dramatically with the use of the shrouding flame compared with a conventional supersonic jet. The present CFD model was used to investigate the characteristics of the supersonic coherent oxygen jet at steelmaking conditions of around 1700 K (1427 °C). The potential core length of the supersonic coherent oxygen jet at steelmaking conditions was 1.4 times longer than that at room ambient temperature.

Ignition, Flame Structure and Near-Wall Burning in Transverse Hydrogen Jets in Supersonic Crossflow

Abstract: We have investigated the properties of transverse sonic hydrogen jets in high-temperature supersonic crossflow at jet-to-crossflow momentum flux ratios $J$ between 0.3 and 5.0. The crossflow was held fixed at a Mach number of 2.4, 1400 K and 40 kPa. Schlieren and $\\text{OH}^{\\ast }$ chemiluminescence imaging were used to investigate the global flame structure, penetration and ignition points; $\\text{OH}$ planar laser-induced fluorescence imaging over several planes was used to investigate the instantaneous reaction zone. It is found that $J$ indirectly controls many of the combustion processes. Two regimes for low ( ${<}1$ ) and high ( ${>}3$ ) $J$ are identified. At low $J$ , the flame is lifted and stabilizes in the wake close to the wall possibly by autoignition after some partial premixing occurs; most of the heat release occurs at the wall in regions where $\\text{OH}$ occurs over broad regions. At high $J$ , the flame is anchored at the upstream recirculation region and remains attached to the wall within the boundary layer where $\\text{OH}$ remains distributed over broad regions; a strong reacting shear layer exists where the flame is organized in thin layers. Stabilization occurs in the upstream recirculation region that forms as a consequence of the strong interaction between the bow shock, the jet and the boundary layer. In general, this interaction – which indirectly depends on $J$ because it controls the jet penetration – dominates the fluid dynamic processes and thus stabilization. As a result, the flow field may be characterized by a flame structure characteristic of multiple interacting combustion regimes, from (non-premixed) flamelets to (partially premixed) distributed reaction zones, thus requiring a description based on a multi-regime combustion formulation.

Improved Phased Array Imaging of a Model Jet

Abstract: An advanced phased array system, OptiNav Array 48, and a new deconvolution algorithm, TIDY, have been used to make octave band images of supersonic and subsonic jet noise produced by the NASA Glenn Small Hot Jet Acoustic Rig (SHJAR). The results are much more detailed than previous jet noise images. Shock cell structures and the production of screech in an underexpanded supersonic jet are observed directly. Some trends are similar to observations using spherical and elliptic mirrors that partially informed the two-source model of jet noise, but the radial distribution of high frequency noise near the nozzle appears to differ from expectations of this model. The beamforming approach has been validated by agreement between the integrated image results and the conventional microphone data.

Molecular Weight and Shock-Wave Effects on Transverse Injection in Supersonic Flow

Abstract: An experimental study of the effects of injectant molecular weight on transverse-jet mixing in a supersonic crossflow is reported. In addition, the effects of an oblique shock impinging near the injection station were investigated. The examined gaseous injector is circular in geometry and angled downstream at 30 deg to the horizontal. Test conditions involved sonic injection of helium, methane, and air at a jet-to-freestream momentum flux ratio of 2.1 into a nominal Mach 4 air cross stream with average Reynolds number 5.77e + 7 per meter to provide a range of injectant molecular weights from 4―29. Sampling probe measurements were used to determine the local helium and methane concentration. A miniature five-hole pressure probe, pitot and cone-static pressure probes, and a diffuser-thermocouple probe were employed to document the flow. The goals of this effort are twofold. The first goal is to broaden and enrich the database for transverse injection in high-speed flows. Second, these data will aid greatly in the development of advanced turbulence models with a wide range of applicability for high-speed mixing flows. The main experimental results showed that an impinging shock reduces penetration and increases mixing for injectants of all molecular weights. Higher molecular weight injectant seems to increase penetration, but the effect is weak. The effects of shock impingement were strongest when this occurred downstream of, and closest to, the injection port.

Characteristics of the Shock Noise Component of Jet Noise

Abstract: The characteristics of the flow and the noise of shock-containing jets have been studied for nearly three decades. It is now established that broadband shock-associated noise is generated by the interaction of the downstream-convecting coherent structures of the jet flow with the shock cells in the jet plume. Past analyses of far-field data have been carried out with the total measured noise, which contains both the turbulent mixing noise and shock noise. In this study, these two components are first separated and extracted from the total spectra. Both convergent and convergent-divergent nozzles are considered. The decomposition is made possible by a recently developed scaling methodology for turbulent mixing noise, which provides excellent collapse of the mixing noise spectra from jets at all velocities but at a fixed temperature ratio. The characteristics of the shock component alone are investigated. A surprising effect of jet temperature on shock noise is established for the first time: the levels increase as the jet is first heated; however, the levels do not increase with further increase in jet temperature. The physical phenomenon responsible for this saturation of levels is not known at this time. The intensity for shock noise in the forward quadrant does not scale as the fourth power (shock exponent) of √|M 2 j -M 2 D | but spans a range from 2.9 to 6.17, depending on the radiation angle and the jet temperature ratio. It is not straightforward to collapse the shock spectra. It is also established for the first time that nonlinear propagation effects are manifested at lower radiation angles, in which the shock component is dominant. The physical phenomenon that triggers the onset of nonlinear propagation for the shock noise could not be identified. The characteristics of the correlation functions at the lower inlet angles for subsonic and supersonic jets are different, attesting to the different noise generation mechanisms.

The procedure for investigation of the efficiency of purification of natural gases in a supersonic separator

Abstract: A procedure is suggested for determining the field of application of supersonic separators. The formulation of the problem is given as regards the choice of optimal values of Mach number. The compositions of gas-liquid mixtures are determined as a function of the initial parameters (composition of gas, temperature, pressure, and Mach number). The results are given in the form of temperature, pressure, and Mach number dependences of the composition of the liquid phase of gas-liquid mixture. Primary estimation is made of the efficiency of purification of natural gases depending on their initial parameters.

Formation of collapsing cores in subcritical magnetic clouds: three-dimensional MHD simulations with ambipolar diffusion

Abstract: We employ the three-dimensional magnetohydrodynamic simulation including ambipolar diffusion to study the gravitationally-driven fragmentation of subcritical molecular clouds, in which the gravitational fragmentation is stabilized as long as magnetic flux-freezing applies. The simulations show that the cores in an initially subcritical cloud generally develop gradually over an ambipolar diffusion time, which is about a few � 10 7 years in a typical molecular cloud. On the other hand, the formation of collapsing cores in subcritical clouds is accelerated by supersonic nonlinear flows. Our parameter study demonstrates that core formation occurs faster as the strength of the initial flow speed in the cloud increases. We found that the core formation time is roughly proportional to the inverse of the square root of the enhanced density created by the supersonic nonlinear flows. The density dependence is similar to that derived in quasistatically contracting magnetically supported clouds, although the core formation conditions are created by the nonlinear flows in our simulations. We have also found that the accelerated formation time is not strongly dependent on the initial strength of the magnetic field if the cloud is highly subcritical. Our simulation shows that the core formation time in our model subcritical clouds is several � 10 6 years, due to the presence of large-scale supersonic flows (� 3 times sound speed). Once a collapsing core forms, the density, velocity, and magnetic field structure of the core does not strongly depend on the initial strength of the velocity fluctuation. The infall velocities of the cores are subsonic and the magnetic field lines show weak hourglass shapes.