Mach Disk Research Articles

Effects of the jet-to-crossflow momentum ratio on a sonic jet into a supersonic crossflow

Numerical investigation of a transverse sonic jet injected into a supersonic crossflow was carried out using large-eddy simulation for a free-stream Mach number M=1.6 and a Reynolds number Re = 1.38×105 based on the jet diameter. Effects of the jet-to-crossflow momentum ratio on various fundamental mechanisms dictating the intricate flow phenomena, including flow structures, turbulent characters and frequency behaviors, have been studied. The complex flow structures and the relevant flow features are discussed to exhibit the evolution of shock structures, vortical structures and jet shear layers. The strength of the bow shock increases and the sizes of the barrel shock and Mach disk also increase with increasing momentum ratio. Turbulent characters are clarified to be closely related to the flow structures. The jet penetration increases with the increase of the momentum ratio. Moreover, the dominant frequencies of the flow structures are obtained using spectral analysis. The results obtained in this letter provide physical insight in understanding the mechanisms relevant to this complex flow.

Simulation of thermophysical flow in axisymmetric nozzle with expansion chamber

AbstractAxisymmetric nozzle flows with a free‐jet expansion are simulated considering several substances and several flow conditions, and the thermophysical properties in the nozzle and the free‐jet region are predicted. The present numerical method is based on the preconditioning method developed by Yamamoto and the mathematical models of thermophysical properties of the substances. As numerical examples show, gas flows of carbon dioxide, water, and nitrogen under a subcritical pressure condition are first calculated. Calculated distances to the Mach disk are compared with the experimental results, and also the density distributions are compared among these three substances. Second, carbon dioxide flows while changing the pressure from subcritical to supercritical values are calculated and the effect of pressure on the flow field is investigated. Third, flows of water vapor with and without nonequilibrium condensation are calculated and the effect of condensation on the flow field is investigated. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Hysteretic phenomenon of shock wave in a supersonic nozzle

In recent years, hysteretic phenomena in fluid flow systems drew attention for their great variety of industrial and engineering applications. When the high-pressure gas is exhausted to atmosphere from the nozzle exit, the expanded supersonic jet with the Mach disk is formed at a specific condition. In two-dimensional expanded supersonic jet, the hysteresis phenomenon for the reflection type of shock wave is occurred under the quasi-steady flow and the transitional pressure ratio between the regular reflection and Mach reflection is affected by this phenomenon. However, so far, there are very few researches for the hysteretic phenomenon of shock wave in a supersonic internal flow and the phenomenon has not been investigated satisfactorily. The present study was concerned with the experimental and numerical investigations of hysteretic phenomena of shock wave in a supersonic nozzle, and discussed the relationship between hysteresis phenomenon and rate of the change of pressure ratio with time.

Numerical simulation of supercritical carbon dioxide flows across critical point

Numerical study of supercritical carbon-dioxide flows across the critical point is presented. The present numerical method is based on the preconditioning method developed by Yamamoto and mathematical models of thermophysical properties for carbon dioxide programmed in the program package for thermophysical properties of fluids, developed by Kyushu University. First, the two-dimensional natural convection of carbon dioxide between two parallel plates is calculated while changing the bulk pressure. The calculated thermophysical properties of the carbon-dioxide flow under supercritical pressure are compared with those in a gas condition. Next, the natural convection of carbon dioxide in an O-shaped cyclic channel is calculated, and the effect of the density difference induced by the phase change to the flow is investigated. For application to high-speed flows, supercritical carbon dioxide flows through a nozzle with free-jet expansion (known as the process of rapid expansion of supercritical solutions) process are calculated. The calculated shock distance to the Mach disk generated in the free jet is compared with experiments and the density variations in the nozzle while changing the inlet temperature are numerically predicted.

Shock structure deformation in plasma jet caused by strong magnetic field

The shock structure deformation in a plasma jet caused by a strong magnetic field was studied using an optical method. The plasma jet images in the enclosing vacuum chamber were taken with a digital single-lens reflex camera through a viewing window. Since the relative values of the measured light intensity are known to have a strong correlation with the number density of excited atoms, the vertical distribution of light intensity can be converted to a radial distribution of excited atom density using an Abel-inversion. The results of this conversion with a magnetic field were compared in the present experiments with those without the presence of a field. It is possible to obtain a smooth radial distribution of estimated number density using an approximate line. The density along the centerline increases obviously with the magnetic field. The minimal density point around the Mach disk was moved slightly upstream with the application of the field. The density distribution inside of the jet boundary was changed considerably by the application of the strong field. The jet boundary itself was not found to be affected significantly by the field although the boundary was found to be slightly contracted to the center axis with application of the field. It has been confirmed that the density distributions are mainly affected by the magnetic field at the corresponding location.

Energy Deposition Applied to a Transverse Jet in a Supersonic Crossflow

An experimental investigation of a sonic underexpanded jet injected normally into a Mach 2.25 crossflow is reported. The jet exit geometry was circular and was operated at a jet-to-crossflow momentum flux ratio of 1.7. The unperturbed flowfield was analyzed with schlieren imaging, particle image velocimetry velocity data, surface oil flow visualizations, and pressure-sensitive paint measurements. As a means of excitation to the flowfield, an energy pulse from a Q-switched neodymium-doped-yttrium-aluminum-garnet laser was focused in the center of the jet exit at three different vertical locations. The focused laser pulse resulted in the induced optical breakdown of air, creating a plasma to perturb the flowfield. The perturbed resulting flowfield was analyzed with schlieren photography and particle image velocimetry. Analysis of phase-averaged schlieren images suggested that the resulting blast wave from the laser pulse disrupted the structure of the barrel shock and Mach disk. The two-component velocity field data revealed that the excitation pulse also caused a perturbation to the jet-shear layer and induced the formation of vortices that convect downstream.

Exhaust of Underexpanded Jets from Finite Reservoirs

We examine the response of an underexpanded jet to a depleting, nite reservoir with experiments and simulations. An open-ended shock tube facility with variable reservoir length is used to obtain images of nitrogen and helium jet structures at successive instances during the blowdown from initial pressure ratios of up to 250. The reservoir and ambient pressures are simultaneously measured to obtain the instantaneous pressure ratio. We estimate the time-scales for jet formation and reservoir depletion as a function of the specic heat ratio of the gas and the initial pressure ratio. The jet structure formation time-scale is found to become approximately independent of pressure ratio for ratios greater than 50. In the present work, no evidence of time-dependence in the Mach disk shock location is observed for rates of pressure decrease associated with isentropic blowdown of a nite reservoir while the pressure ratio is greater than 15. The shock location in the nitereservoir jet can be calculated from an existing empirical t to innite-reserv oir jet data evaluated at the instantaneous reservoir pressure. For pressure ratios below 15, however, the present data deviate from a compilation of data for innite-reserv oir jets. A new t is obtained to data in the lower pressure regime. The self-similarity of the jet structure is quantied and departure from similarity is noted to begin at pressure ratios lower than about 15, approximately the same ratio which limits existing empirical ts.

Unsteady high‐pressure flow experiments with applications to explosive volcanic eruptions

Motivated by the hypothesis that volcanic blasts can have supersonic regions, we investigate the role of unsteady flow in jets from a high‐pressure finite reservoir. We examine the processes for formation of far‐field features, such as Mach disk shocks, by using a shock tube facility and numerical experiments to investigate phenomena to previously unobtained pressure ratios of 250:1. The Mach disk shock initially forms at the edges of the vent and moves toward the centerline. The shock is established within a few vent diameters and propagates downstream toward the equilibrium location as the jet develops. The start‐up process is characterized by two different timescales: the duration of supersonic flow at the nozzle exit and the formation time of the Mach disk shock. The termination process also is characterized by two different timescales: the travel time required for the Mach disk shock to reach its equilibrium position and the time at which the Mach disk shock begins significantly to collapse away from its equilibrium position. The critical comparisons for the formation of steady state supersonic regions are between the two start‐up timescales and the termination timescales. We conclude that for typical vulcanian eruptions and the Mount St. Helens directed blast, the Mach disk shock could have formed near the vent, and that there was time for it to propagate a distance comparable to its equilibrium location. These experiments provide a framework for analysis of short‐lived volcanic eruptions and data for benchmarking simulations of jet structures in explosive volcanic blasts.

Shock wave configurations and flow structures in non-axisymmetric underexpanded sonic jets

An experimental and numerical study of underexpanded free sonic jet flows issuing from rectangular, elliptical and slot nozzles has been undertaken. Aspect ratios (AR) of 1, 2, and 4 are described at pressure ratios (exit plane pressure to ambient pressure), of 2 and 3. There is good qualitative agreement between the experimental observations and the numerical predictions. In the case of rectangular jets, a complex system of shock waves forming the incident shock system is identified. This shock wave system originates at the corners of the nozzle exits, and proceeds downstream. Mach reflections are found to occur on the incident shock wave surface as well as the presence of a Mach disk terminating the first jet cell. This Mach disk has the shape of a square, a hexagon, or an octagon depending on the nozzle shape. For slot and elliptical jets, the formation of the incident shock wave was not observed along the minor axis plane of the nozzle for AR > 2. The incident shock wave was observed to originate downstream of the nozzle exit in the major axis plane. This wave system undergoes a transition to Mach reflection as it propagates downstream of the nozzle exit. In all cases tested, the shape of the jet boundary is significantly distorted. In rectangular jets, the narrowing of the jet boundary along the diagonal axis of the nozzle exit is observed, and in the case of the elliptical and slot jets axis switching is noted.

ナトリウム-水反応および圧縮性混相流数値解析コードSERAPHIMの高度化 : 高度不足膨張噴流に対する解析手法の適用性検証(<特集>第14回動力・エネルギー技術シンポジウム)

SERAPHIM is a computer program for the simulation of the compressible multiphase flow involving the sodium-water chemical reaction under a tube failure accident in a steam generator of sodium cooled fast reactors. In this study, the numerical analysis of the highly underexpanded air jets into the air or into the water was performed as a part of validation of the SERAPHIM program. The multi-fluid model, the second-order TVD scheme and the HSMAC method considering a compressibility were used in this analysis. Combining these numerical methods makes it possible to calculate the multiphase flow including supersonic gaseous jets. In the case of the air jet into the air, the calculated pressure, the shape of the jet and the location of a Mach disk agreed with the existing experimental results. The effect of the difference scheme and the mesh resolution on the prediction accuracy was clarified through these analyses. The behavior of the air jet into the water was also reproduced successfully by the proposed numerical method.

Transient Shock Formation of Pulsed Electrothermal Plasma Discharge Confined in an Extended Bore

An electrothermal gun possesses a great potential to be an efficient source of pulsed plasma discharge for nanomaterials production or thermal plasma spray coatings. A plasma discharge by intense pulsed power is numerically studied utilizing time-dependent gas dynamics equations which are solved by FCT (flux-corrected transport) algorithm in two-dimensional domain of the interior capillary bore region and the external region of extended bore. Plasma conditions at the bore exit, mass ablation of polycarbonate bore wall, and degree of ionization are determined at different levels of transient arc current profile. As a way to controlling the plasma discharge, the extended bore at the capillary exit is considered and the flow pattern of pulsed plasma discharge in the extended bore exhibit complex shock structure from slightly to highly underexpanded jet depending on the level of arc current profiles. Flow instability of oscillating Mach disk is found at higher level of arc current profile cases.

Supersonic flow imaging via nanoparticles

Due to influence of compressibility, shock wave, instabilities, and turbulence on supersonic flows, current flow visualization and imaging techniques encounter some problems in high spatiotemporal resolution and high signal-to-noise ratio (SNR) measurements. Therefore, nanoparticle based planar laser scattering method (NPLS) is developed here. The nanoparticles are used as tracer, and pulse planar laser is used as light source in NPLS; by recording images of particles in flow field with CCD, high spatiotemporal resolution supersonic flow imaging is realized. The flow-following ability of nanoparticles in supersonic flows is studied according to multiphase flow theory and calibrating experiment of oblique shock wave. The laser scattering characteristics of nanoparticles are analyzed with light scattering theory. The results of theoretical and experimental studies show that the dynamic behavior and light scattering characteristics of nanoparticles highly enhance the spatiotemporal resolution and SNR of NPLS, with which the flow field involving shock wave, expansion, Mach disk, boundary layer, sliding-line, and mixing layer can be imaged clearly at high spatiotemporal resolution.

Numerical simulation of the XZ Tauri supersonic astrophysical jet

Computational gas dynamical simulations using the WENO-LF method are applied to modeling the high Mach number astrophysical jet XZ Tauri, including the effects of radiative cooling. Mach 55 simulations of the pulsed proto-jet are presented and analyzed in terms of interacting nonlinear waves: terminal Mach disks, bow shocks, and Meshkov-Richtmyer instabilities of the leading jet contact boundary.

Transient Three-Dimensional Side-Load Analysis of a Film-Cooled Nozzle

Transient t hree -dimensional numerical investigations on the side load physics for a n engine en comp assing a film cooled nozzle extension and a regeneratively cooled thrust chamber , were performed. The objectives of this study are t o identify the side load physics and to compute the associated aerodynamic side load using an anchored computational meth odology. The computational methodology is based on an unstructured -grid, pressure based computational fluid dynamics and heat transfer formulation, and a transient inlet history based on an engine system simulation. Computations simulating engine startup a t ambient pressures corresponding to sea level and three high altitudes were performed . In addition, computations for both engine startup and shutdown transients for a stub nozzle operating at sea level were also performed. For engine startup s with the no zzle extension attached , computational result s show the dominant side load physics are the turbine exhaust gas film coolant induced asymmetric Mach disk flow and the subsequent jump of the shock foot or the jump of the separation line which generated the p eak side load. That peak side load decreases rapidly and progressively as the ambient pressure decreases with the increasing altitudes . For the stub nozzle operating at sea level, t he peak side load reduces drastically and comes from the simultaneous free -shock separation to partial restricted shock transition and the shock breathing a t the nozzle lip during startup, whereas the magnitude of the side load during shut down is almost negligible . The computed side load characteristics for the sea level cases agree re asonably well with those of available data from the tests of a similar engine.

Simulation of gas flow through tubes of finite length over the whole range of rarefaction for various pressure drop ratios

The rarefied gas flow through circular tubes of finite length has been investigated computationally by the direct simulation Monte Carlo method. The reduced flow rate and the flow field have been calculated as functions of the gas rarefaction, the length-to-radius ratio, and the pressure ratio along the tube. The gas rarefaction, which is inversely proportional to the Knudsen number, is varied from 0 to 2000, i.e., the free-molecular, transitional, and hydrodynamic regimes are embraced. A wide range of the length-to-radius ratio, namely, from 0 corresponding to the orifice flow up to 10 representing a sufficiently long tube, has been considered. Several values of the pressure ratio between 0 and 0.7 have been regarded. This pressure range covers both gas flow into vacuum and into a background gas. It has been found that the rarefaction parameter has the most significant effect on the flowfield characteristics and patterns, followed by the pressure ratio drop, while the length-to-radius ratio has a rather modest impact. Several interesting findings have been reported including the behavior of the flow rate and other macroscopic quantities in terms of these three parameters. In addition, the effect of gas rarefaction on the choked flow and on the Mach disks at large pressure drops is discussed. Comparison of some of the present numerical results with available experimental data has shown a good agreement.

LABORATORY EXPERIMENTS, NUMERICAL SIMULATIONS, AND ASTRONOMICAL OBSERVATIONS OF DEFLECTED SUPERSONIC JETS: APPLICATION TO HH 110

Collimated supersonic flows in laboratory experiments behave in a similar manner to astrophysical jets provided that radiation, viscosity, and thermal conductivity are unimportant in the laboratory jets, and that the experimental and astrophysical jets share similar dimensionless parameters such as the Mach number and the ratio of the density between the jet and the ambient medium. Laboratory jets can be studied for a variety of initial conditions, arbitrary viewing angles, and different times, attributes especially helpful for interpreting astronomical images where the viewing angle and initial conditions are fixed and the time domain is limited. Experiments are also a powerful way to test numerical fluid codes in a parameter range where the codes must perform well. In this paper we combine images from a series of laboratory experiments of deflected supersonic jets with numerical simulations and new spectral observations of an astrophysical example, the young stellar jet HH 110. The experiments provide key insights into how deflected jets evolve in 3-D, particularly within working surfaces where multiple subsonic shells and filaments form, and along the interface where shocked jet material penetrates into and destroys the obstacle along its path. The experiments also underscore the importance of the viewing angle in determining what an observer will see. The simulations match the experiments so well that we can use the simulated velocity maps to compare the dynamics in the experiment with those implied by the astronomical spectra. The experiments support a model where the observed shock structures in HH 110 form as a result of a pulsed driving source rather than from weak shocks that may arise in the supersonic shear layer between the Mach disk and bow shock of the jet's working surface.

Effect of atomization gas pressure variation on gas flow field in supersonic gas atomization

In this paper, a computational fluid flow model was adopted to investigate the effect of varying atomization gas pressure ( P 0) on the gas flow field in supersonic gas atomization. The influence of P 0 on static pressure and velocity magnitude of the central axis of the flow field was also examined. The numerical results indicate that the maximum gas velocity within the gas field increases with increasing P 0. The aspiration pressure (Δ P ) is found to decrease as P 0 increases at a lower atomization gas pressure. However, at a higher atomization gas pressure increasing P 0 causes the opposite: the higher atomization gas pressure, the higher aspiration pressure. The alternation of Δ P is caused by the variations of stagnation point pressure and location of Mach disk, while hardly by the location of stagnation point. A radical pressure gradient is formed along the tip of the delivery tube and increases as P 0 increases.

Unsteady correlation between pressure and temperature field on impinging plate for dual underexpanded jets

In this paper, dual underexpanded impinging jets are experimentally studied by measuring unsteady wall static pressure and temperature fields using high response semiconductor pressure sensors and infrared imaging camera(100 captures per sec) with dual converging sonic nozzles. The flow field is also visualized by means of high speed schlieren system (up to 70000 frame per second) to clarify the unsteady behavior of the jet structures such as barrel shock wave, and Mach disk. Experiments were performed with a blow down tunnel changing the parameters such as the nozzle to plate distance, the distance between the two nozzles, and wind tunnel pressure ratio. The nozzle to plate distance L and the distance between the two nozzles H are non-dimensionalized by the nozzle diameter D covering H/D=1.5, 2.0, and 3, L/D= 2, 3, and 4, and po/pb=1.0 − 5.0, where po and pb are stagnation pressure and back pressure respectively. Although the response of the infrared camera is quite different from that of the high speed camera, the unsteady correlation between the pressure and temperature fields is confirmed under some conditions. Also the high speed schlieren pictures and the wall static pressure fluctuations suggest that the generation of the Mach disk promote the unsteady behavior of the jets. After the confirmation of the correlation, a simple way to find the severe fluctuating region can be provided according to the two dimensional unsteady temperature images without a lot of unsteady pressure measurements.

Deagglomeration of nanoparticle aggregates via rapid expansion of supercritical or high‐pressure suspensions

AbstractDeagglomeration of suspensions of alumina and titania nanopowders (i.e., nanoparticle aggregates) via rapid expansion of supercritical suspensions (RESS) or high‐pressure suspensions (REHPS) was studied. The size distribution of fragmented nanopowders was characterized by online Scanning Mobility Particle Spectrometer (SMPS) and Aerodynamic Particle Sizer (APS), and by offline Scanning Electron Microscopy (SEM). SMPS and SEM measurements indicate that the average agglomerate sizes were well below 1 μm, consistent with the length scales observed in our complementary RESS/REHPS mixing experiments using alumina and silica nanopowders. The APS measurements, on the other hand, were affected by reagglomeration during sampling and yielded an agglomerate size range of 1 to 3 μm. Analysis of the RESS/REHPS process through compressible flow models revealed that both the shear stress in the nozzle and the subsequent impact of the agglomerates with the Mach disc in the free expansion region can lead to micron or sub‐micron level deagglomeration. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Large-Eddy Simulations of a Supersonic Jet and Its Near-Field Acoustic Properties

Large-eddy simulations of imperfectly expanded jet flows from a convergent-divergent nozzle with a sharp contraction at the nozzle throat have been carried out. The flowfield and near-field acoustics for various total pressure ratios from overexpanded to underexpanded jet flow conditions have been investigated. The location and spacing of the shock cells are in good agreement with experimental data and previous theoretical results. The velocity profiles are also in good agreement with data from experimental measurements. A Mach disk is observed immediately downstream of the nozzle exit for overexpanded jet conditions with nozzle pressure ratios much lower than the fully expanded value. It is found that this type of nozzle with a sharp turning throat does not have a shock-free condition for supersonic jet flows. The near-field intensities of pressure fluctuations show wavy structures for cases in which screech tones are observed. The large-eddy simulations predictions of the near-field noise intensities show good agreement with those obtained from experimental measurements. This good agreement shows that large-eddy simulations and measurements can play complementary roles in the investigation of the noise generation from supersonic jet flows.