Transonic Velocities Research Articles

Formation of Galactic Disks. II. The Physical Drivers of Disk Spin-up

Using a representative sample of Milky Way (MW)–like galaxies from the TNG50 cosmological simulation, we investigate physical processes driving the formation of galactic disks. A disk forms as a result of the interplay between inflow and outflow carrying angular momentum in and out of the galaxy. Interestingly, the inflow and outflow have remarkably similar distributions of angular momentum, suggesting an exchange of angular momentum and/or outflow recycling, leading to continuous feeding of prealigned material from the corotating circumgalactic medium. We show that the disk formation in TNG50 is correlated with stellar bulge formation, in qualitative agreement with a recent theoretical model of disk formation facilitated by steep gravitational potentials. Disk formation is also correlated with the formation of a hot circumgalactic halo with around half of the inflow occurring at subsonic and transonic velocities corresponding to Mach numbers of ≲2. In the context of recent theoretical works connecting disk settling and hot halo formation, our results imply that the subsonic part of the inflow may settle into a disk while the remaining supersonic inflow will perturb this disk via the chaotic cold accretion. We find that disks tend to form when the host halos become more massive than ∼(1–2) × 1011 M ⊙, consistent with previous theoretical findings and observational estimates of the predisk protogalaxy remnant in the MW. Our results do not prove that either corotating outflow recycling, gravitational potential steepening, or hot halo formation cause disk formation, but they show that all these processes occur concurrently and may play an important role in disk growth.

Massive clumps in W43-main: Structure formation in an extensively shocked molecular cloud

Aims. W43-main is a massive molecular complex undergoing starburst activities, located at the interaction of the Scutum arm and the Galactic bar. We aim to investigate the gas dynamics, in particular, the prevailing shock signatures from cloud to clump scales. We also look to assess the impact of shocks on the formation of dense gas and early-stage cores in OB cluster formation processes. Methods. We carried out NOEMA and IRAM-30 m observations at 3 mm towards five molecular gas clumps in W43 main located within large-scale interacting gas components. We used CH3CCH and H2CS lines to trace the extended gas temperature and CH3OH lines to probe the volume density of the dense gas components (≳105 cm−3). We adopted multiple tracers that are sensitive to different gas density regimes to reflect the global gas motions. The density enhancements constrained by CH3OH and a population of NH2D cores are correlated (in the spatial and velocity domains) with SiO emission, which is a prominent indicator of shock processing in molecular clouds. Results. The emission of SiO (2–1) is extensive across the region (~4 pc) and it is contained within a low-velocity regime, hinting at a large-scale origin for the shocks. Position-velocity maps of multiple tracers show systematic spatio-kinematic offsets supporting the cloud-cloud collision-merging scenario. We identified an additional extended velocity component in the CCH emission, which coincides with one of the velocity components of the larger scale 13CO (2−1) emission, likely representing an outer, less-dense gas layer in the cloud merging process. We find that the ‘V-shaped’, asymmetric SiO wings are tightly correlated with localised gas density enhancements, which is direct evidence of dense gas formation and accumulation in shocks. The dense gas that is formed in this way may facilitate the accretion of the embedded, massive pre-stellar and protostellar cores. We resolved two categories of NH2D cores: those exhibiting only subsonic to transonic velocity dispersions and those with an additional supersonic velocity dispersion. The centroid velocities of the latter cores are correlated with the shock front seen via SiO. The kinematics of the ~0.1 pc NH2D cores are heavily imprinted by shock activities and may represent a population of early-stage cores forming around the shock interface.

Effects of vacancy concentration on the edge dislocation motion in copper by atomic simulations

Nonequilibirum vacancy concentration widely appears in crystals under many extreme loading conditions, but receives relatively few attentions. In this work, we systematically explore the influence of a serial of different vacancy concentrations on the edge dislocation motion in copper using molecular dynamics (MD) simulations. Our result shows that the vacancy would hinder the dislocation motion, but the mechanism depends on the detailed dislocation motion regions. In thermally activated region, its influence is mainly reflected by modifying the dynamic and static threshold stresses required for edge dislocation initiation and continuous motion. In the linear region, the hindering mechanism is gradually transformed from phonon damping to vacancy pinning with the increasing vacancy concentration. In contrasts, the dislocation structure is almost unchanged under different vacancy concentrations in the non-linear region. Under high applied stress, high vacancy concentration will cause the dislocation velocity to jump back and forth between transonic and subsonic velocities more frequently. It has been attributed to the reactions between the dislocation and vacancies. The latter may result in dislocation local constriction and climbing. Moreover, a mobility equation suitable for describing edge dislocations at different non-equilibrium vacancy concentrations is proposed, which fits the MD results well. Finally, the roles of the nonequilibirum vacancy concentration on the edge dislocation motion is interpreted using the degrading elastic property and stacking fault energy.

Aerodynamic performance of a characteristic airfoil at low-Reynolds number and transonic flow under Mars sand-containing environment

The propeller tip of an unmanned aerial vehicle on Mars operates at a low-Reynolds number (Re = 1–5 × 104) and transonic velocity range (Ma = 0.7–1.2). Generally, this implies flow separation and shock waves in the flow field. Furthermore, the impact of Martian atmospheric sand particles significantly affects the aerodynamic performance, and numerical simulations of this issue have received increasing attention recently. Aimed at a characteristic airfoil, the study of a low-Reynolds number transonic flow and the influence of sand-containing flow on airfoil aerodynamic performance were analyzed in this study. The discrete phase model was adopted to simulate the two-phase flow considering Re = 8100–56 800 and Ma = 0.2–1.4. The results indicate that, compared with the atmospheric environment on Earth, the low-pressure atmosphere on Mars can delay the generation of the surface shock wave of the airfoil as well as alter the shock wave structure, significantly increasing the lift-to-drag ratio at high subsonic velocities (Ma = 0.6–0.8). Moreover, due to the weak compressive wave generated at the separation position, the low-pressure atmosphere weakens the strength of the trailing-edge oblique shock wave and reduces the drag when Ma = 0.9–1.4. Compared with a sand-free environment, sand-containing flow can affect the separation and transition positions of laminar separation bubbles, in addition to alter the shock wave structure. At different Mach numbers, the lift and drag of the airfoil first increased and then decreased as the sand particles flowed in the horizontal direction during the entire process of particles entering and exiting the airfoil flow field.

Quantitative Investigation of Ballistics Flow Fields by Background Oriented Schlieren Technique

The ballistics field is known by the presence of several complex phenomena such as muzzles and flying projectiles flow fields. Consequently, numerical simulations are commonly used to model these complicated flows. However, the validation process of these codes has proven to be problematic due to the lack of experimental quantitative data. In this context, the present paper describes the application of the Background Oriented Schlieren technique (BOS) as a quantitative investigation tool in the ballistics field. We illustrate that BOS can accurately capture the main characteristics of the studied configurations: Firstly, we discuss the visualization and the density field reconstruction around a Bullet Simulated Projectile BSP flying at supersonic velocities and a sniper projectile flying at supersonic and transonic velocities. We demonstrate that these fields are in satisfactory agreement with the results of Taylor and Maccoll’s equation and numerical simulation. Then, the findings of the BOS visualization of the precursors and the propellant flow fields are presented. To this end, the salient features accurately captured by the BOS technique such as vortex rings, shock bottles, Mach, and blast wave are described both qualitatively and in terms of density profiles. Two improved approaches that are essential to the aforementioned analysis are proposed: the first is related to density field reconstruction based on Abel inversion and the second approach is a phase separation procedure.

Mathematical model of external ballistics of projectiles

The paper presents mathematical model of external ballistics of projectile, fired from cannon. The drag force has a decisive influence on the dynamics of the projectile. Direct calculation of the functional dependence of the drag force on the set of parameters is quite controversial and has not been utilized so far. Therefore, the author calculus the functional relativity, approximately, solving the inverse problem of mechanics. The flight range of a projectile at certain aiming angles, noted in the firing tables, determine the drag force according to the effect on a projectile of its weight and Coriolis force. Its functional dependence is described separately at the stages of projectile’s motion with supersonic, transonic and subsonic velocities. Due to the functional dependence of the drag force of a projectile, the influence of a charge projectile, initial velocity, atmospheric pressure, air temperatures and projectile’s mass effect on kinematic parameters of its motion can be calculated. Knowledge of this dependence of the drag force of the projectile’s movement allows to automate, using appropriate software, the determination of the elevation angle depending on the location of the target and the values of deterministic and nondeterministic factors.

Study of Plasma-Based Vortex Generator in Supersonic Turbulent Boundary Layer

The problem of flow control under conditions of a turbulent boundary layer at transonic and supersonic free-stream velocities is considered. Such flows are integral components of the flight process and exert significant effects on the flow around both the aerodynamic object as a whole and its individual elements. The present paper describes investigations of a combined control device (“plasma wedge”), which is a wedge mounted along the flow with the energy supply at one side of the wedge owing to a spark discharge. The strategy of flow control by this device is based on increasing the momentum in the boundary layer, which enhances its resistance to the adverse pressure gradient and, as a consequence, its resistance to flow separation further downstream. The study includes experimental and computational aspects. The examined flow evolves on a rectangular flat plate with a sharp leading edge at the free-stream Mach number M = 1.45 and unit Reynolds numbers Re1 = 11.5·106 1/m. The experiments are performed to study the velocity fields and the pressure distribution in the wake behind the actuator. The results show that a streamwise vortex is formed in the wake behind the actuator when the discharge is initiated. Reasonable agreement of the experimental data with numerical simulations allows one to conclude that the Reynolds-averaged Navier–Stokes equations are suitable tools for solving the problem considered.

Ion kinetic effects and instabilities in the plasma flow in the magnetic mirror

Kinetic effects in plasma flow due to a finite ion temperature and ion reflections in a converging–diverging magnetic nozzle are investigated with collisionless quasineutral hybrid simulations with kinetic ions and isothermal Boltzmann electrons. It is shown that in the cold ions limit, the velocity profile of the particles agrees well with the analytical theory, predicting the formation of the global accelerating potential due to the magnetic mirror with the maximum of the magnetic field and resulting in the transonic ion velocity profile. The global transonic ion velocity profile is also obtained for warm ions with isotropic and anisotropic distributions. Partial ion reflections are observed due to a combined effect of the magnetic mirror and time-dependent fluctuations of the potential as a result of the wave breaking and instabilities in the regions when the fluid solutions become multi-valued. Despite partial reflections, the flow of the passing ions still follows the global accelerating profile defined by the magnetic field profile. In simulations with reflecting boundary condition imitating the plasma source and allowing the transitions between trapped and passing ions, the global nature of the transonic accelerating solution is revealed as a constrain on the plasma exhaust velocity that ultimately defines plasma density in the source region.

Gas dynamics features of physico-chemical processes in the SRM large-sized charges at open firing

The problem of utilization of solid rocket motors (SRM) with large-sized charges is a relevant objective. This is due to the decommissioning of SRM that failed or were rejected during production and operation, as well as with the implementation of strategic arms reduction treaties. Comparative analysis of SRM with large-sized charge utilization methods showed that the simplest, most productive, and relatively safe technique is open firing. At the same time, the information on the dynamics of harmful substances emission into the environment is needed to conduct quantitative assessment of the concentrations of environmentally hazardous combustion products formed during open firing. The calculation of intraballistic parameters in SRM makes it possible to estimate the consumption parameters of harmful gaseous and condensed combustion products entering the atmosphere. Also, it allows analyze the spatial and temporal course of the concentrations of these substances and optimize ways to reduce environmental hazards. The mathematical model and results of gas dynamics parameters calculation at the open firing of large-sized SRM with the demounted nozzle unit and forward bottom are submitted. The key features of realized flow modes are marked: significant pressure differentials along the length of the SRM in the initial stage of its activity; transonic velocities of intrachannel flow; erosion effects on the initial stages of burning rate.

Pressure- and Size-Dependent Aerodynamic Drag Effects on Mach 0.3–2.2 Microspheres for High-Precision Micro-Ballistic Characterization

The acceleration of microparticles to supersonic velocities is required for microscopic ballistic testing, a method for understanding material characteristics under extreme dynamic conditions, and for projectile gene and drug delivery, a needle-free administration technique. However, precise aerodynamic effects upon supersonic microsphere motion at sub-300 Reynolds numbers have not been quantified. We derive drag coefficients for microspheres traveling in air at subsonic, transonic, and supersonic velocities from the measured trajectories of microspheres launched by laser-induced projectile acceleration. Moreover, the observed drag effects on microspheres in atmospheric (760 Torr) and reduced pressure (76 Torr) are compared with existing empirical data and drag coefficient models. We find that the existing models adequately predict the drag coefficient for subsonic microspheres, while rarefaction effects cause a discrepancy between the model and empirical data in the supersonic regime. These results will improve microsphere flight modeling for high-precision microscopic ballistic testing and projectile gene and drug delivery.

Compressible Gas Flow past a Plate with Upstream-Moving Surface

The numerical solution of the problem of compressible gas flow past a plate with upstream-moving surface is obtained. Unstable time-periodic and stable steady-state solutions of this problem are discussed. A new type of unsteady periodic flow in which the varying characteristics turn out to be integrally asymmetric at transonic velocities is obtained.

Improved Estimation of Air-to-Air Missile Aerodynamic Characteristics with Using CFD Method

The paper describes a solving procedure of the special Finite Volume Method (FVM) problem dealing with calculation of a flow field around air-to-air missile at subsonic, transonic and supersonic velocity range in order to obtain a coefficient of drag (cD). Some emphasis is put on numerical solution and fundamentals of FVM and use of a k-ω turbulence model for simulation of subsonic and supersonic flow fieldsaround air-to-air missile AIM-9 Sidewinder version M. Obtained results of missile aerodynamics characteristics are verified using semi-empirical software and by analysis of flow fields.

On three-dimensional boundary layer stability analysis on curved aerodynamic surfaces

Numerical simulation of three-dimensional flow past a prolate spheroid located at an angle of attack of 10° accompanied by the laminar-turbulent transition was performed for subsonic (M∞ = 0.136) and transonic (M∞ = 0.75) velocities. The general-purpose gas-dynamic package ANSYS Fluent in integration with a laminar-turbulent transition module based on software bundle LOTRAN 3 was used. The principles of numerical simulation of the main flow are discussed: requirements for the near-wall region allocation and independence of the results from various methods of presetting the position of forced turbulence. It was shown that two instability mechanisms take place in the boundary layer of the spheroid: the Tollmien-Schlichting instability and the cross-flow instability with a predominance of the last on side surfaces. A comparison of predicted position of the transition for different flow regimes was performed.

Chandra Monitoring of the J1809–1917 Pulsar Wind Nebula and Its Field

Abstract PSR J1809–1917 is a young (τ = 51 kyr) and energetic ( erg s−1) radio pulsar powering an X-ray pulsar wind nebula (PWN) that exhibits morphological variability. We report on the results of a new monitoring campaign by the Chandra X-ray Observatory (Chandra), carried out across six epochs with a ∼7 week cadence. The compact nebula can be interpreted as a jet-dominated outflow along the pulsar’s spin axis. Its variability can be the result of Doppler boosting in the kinked jet, whose shape changes with time (akin to the Vela pulsar jet). The deep X-ray image, composed of 405 ks of new and 131 ks of archival Chandra data, reveals an arcminute-scale extended nebula (EN) whose axis of symmetry aligns with both the axis of the compact nebula and the direction toward the peak of the nearby TeV source HESS J1809–193. The EN’s morphology and extent suggest that the pulsar is likely moving through the ambient medium at a transonic velocity. We also resolved a faint 7′ long nonthermal collimated structure protruding from the PWN. It is possibly another instance of a “misaligned outflow” (also known as a “kinetic jet”) produced by high-energy particles escaping the PWN’s confinement and tracing the interstellar magnetic field lines. Finally, taking advantage of the 536 ks exposure, we analyzed the point sources in the J1809 field and classified them using multiwavelength data. None of the classified sources in the field can reasonably be expected to produce the extended TeV flux in the region, suggesting that PSR J1809–1917 is indeed the counterpart to HESS/eHWC J1809–193.

Study on energy distribution characteristics of cyclone in Laval nozzle

Supersonic separator is a device for dehydrating and separating heavy hydrocarbons in natural gas. Laval nozzle and cyclone in the separator have a great influence on the separation efficiency. At present, there are few studies on the energy distribution characteristics of the cyclone, and the law of the influence of cyclone on subsonic and transonic flow fields is not clear. Based on numerical simulation, the paper studies the influence of gas-liquid density on the separation performance of the separator, studies the energy distribution characteristics of methane flow in Laval nozzle under subsonic and transonic velocities, and deduces the energy equation containing axial kinetic energy term and radial kinetic energy term in the nozzle. The results show that the gas-liquid density has some effects on the separation efficiency of the separator. The distribution of tangential velocity and tangential kinetic energy in the nozzle is similar before the cyclone along the axis, but after the cyclone, the distribution of tangential velocity and tangential kinetic energy is significantly different. The proposal of tangential kinetic energy has important guiding significance to the structure design of supersonic separator.

Formation of a transonic region in a variable-section channel for different stagnation temperatures of the flow

Initiation of operation of a ramjet engine with distributed fuel injection along the combustion chamber is studied numerically. A principally important issue is the presence of a compressed air jet producing a throttling effect and preliminary deceleration of the flow to transonic velocities. The Reynolds-averaged Navier-Stokes equations closed by the SST k-ω turbulence model are solved. Hydrogen is used as a gaseous fuel. Transverse injection of the fuel is considered. A pulsed transonic flow regime is obtained. It is demonstrated that the characteristic scale of vortex regions increases with an increase in the stagnation temperature approximately to 1700 K; consequently, the degree of air-hydrogen mixing is significantly enhanced.

Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H Iself absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ~ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the13CO emission. The peak velocities of the HISA and13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 1020to 1021cm−2. The column density of molecular hydrogen, traced by13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H Iemission (the warm and cold neutral medium), and13CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.

An Integrally Asymmetric Flow Past a Plate with an Upstream-Moving Surface

This paper considers the flow of a viscous ideal gas past a plate with an upstream-moving surface. A new type of nonstationary periodic flow with integral asymmetry of aerodynamic characteristics is discovered at transonic free-stream velocities.

Calculations of shock-wave flow structure in axisymmetric channel with near-wall ethylene burning with throttle air jet

The CFD modelling of the ramjet start with low losses of the stagnation pressure was carried out. The basic feature of this start technology was the jet of compressed air creating effect of a throttle which aids in preliminary decelerating of the flow up to transonic velocities. The comparison with experiment was performed for the hydrogen burning. We studied the axial and near-wall supply of gas fuel. The physical mechanisms stimulating stable presence of transonic mode in the part with constant section were investigated within the frameworks URANS approach. It was shown that fuel supply blockage is a way to control near-wall supplying of gaseous fuel at considerably low pressure falls. And blocking the ignition processes is a way to control the axial supply at quite high pressure falls.

Bow shock stand-off distance for subsonic decelerating bodies

Projectile accelerations above $$500~\hbox {ms}^{-2}$$ are commonly encountered in aerodynamic applications, but suitable validation data are rare in this regime. Experimental transonic velocity range data for a sphere decelerating under its own drag have been used to validate a numerical model for decelerating objects. The validated model is then used to explore the flow field ahead of objects decelerating from supersonic to subsonic Mach numbers. To model the non-inertial frame of the projectile, source terms were included in the momentum and energy equations in a computational fluid dynamics model. In decelerating cases, the bow shock formed in supersonic flight persists in the subsonic regime. The differences in the flow field between the steady and unsteady cases are explained using the concept of flow history. In the experiment, a tubular insert was present near the observation window in the ballistic range. The insert was numerically modelled, and it is shown that the resulting bow shock behaviour can be explained in terms of the Kantrowitz criterion, in conjunction with flow history. The RAE2822 aerofoil was used to explore the effects of shock overtaking and propagation during deceleration from supersonic to low subsonic Mach numbers. In this case, the bow shock wave persists from the initial supersonic speed to projectile Mach numbers lower than 0.4. The expansion wave and tail shock are shown to overtake the decelerating projectile and propagate forward, behind the bow shock.