Simulations Of Jets Research Articles

An Integrated Approach of Simulation, Modeling and Computer-aided Design of Hot Abrasive Jet Machining Setup

In the past few decades, sophisticated machining industries have rapidly improved in order to achieve the required shape of a part within a specific time while also not affecting material properties. Accuracy of machined components in all industries is important. In the case of subassemblies of components, geometrical accuracy of hole is vital. The researchers updated their machinery from time to time from this perspective. In the machining process, environment and time are the vital parameters that are mainly affected, so in this case, the design of experiments that are very useful in the machining region needs to be considered to overcome these challenges. This study aims to analyze the fabrication of hot abrasive jet machining (HAJMing) with different abrasive temperatures using fluidized bed system as well as accomplishment of cutting performance in hot-abrasive jet machining concerning target surface erosion. Additionally, this research study accomplishes the experimental study and computational fluid dynamics (CFD) technique for erosive footprint prediction extent in HAJMing constraints on target surface for intricately shaped tapered holes generation. The use of hot abrasives in the HAJM process has demonstrated an interest in improving cutting performance for material erosion. Moreover, this proposed experimental work contains the manufacturing, design and fabrication of hot abrasive jet machine (HAJM) using commercially available hardware and software. So, the components are manufactured indigenously as per the designed parameters for the purpose of improving the machining performance. Simulation for material erosion mechanism of HAJMing is used to obtain the nature of produced workpiece profile. Additionally, FB-HAJMing also deals with the sustainability assessment under environmental-friendly hot abrasive-assisted machining conditions.

On the opportunity of Laser Plasma simulation of Plasma Jets formation in moderate magnetic fields ∼ kGs

Today a number of various experiments with Laser-Produced Plasmas (LPP), done at very-high Magnetic Fields (up to B0 > MG) are using to model the physics of Astrophysical and Space Jets, a various processes of their formation and possible long-range propagation at various angles S to magnetic fields. We discuss the opportunity and present the first results of new-type experiments on simulation of Jets with LPP at KI-1 facility of ILP, at moderate magnetic field ∼ kGs, oriented quasi-transverse (S ≍ 600) of LPP-blob expansion (with velocity V 0) relative to B 0. They were done on the base of all our preliminary studies, both at large-scale, high-vacuum chamber (0120 cm of KI-1) and others devices with LPP (oriented earlier at V0 transverse to B0, with $ = 900).

Espresso and Stochastic Acceleration of Ultra-high-energy Cosmic Rays in Relativistic Jets

Abstract In the espresso scenario, ultra-high-energy (UHE) cosmic rays (CRs) are produced via a one-shot reacceleration of galactic-like CRs in the relativistic jets of active galactic nuclei, independently of the scattering rate dictated by magnetic fluctuations. In Mbarek & Caprioli (2019) we traced test-particle CRs in high-resolution magnetohyrodynamic (MHD) jet simulations and found that the associated spectral slope, chemical composition, and anisotropy are consistent with UHECR phenomenology. In this work, we extend such an analysis by including subgrid pitch-angle scattering to model small-scale magnetic turbulence that cannot be resolved by MHD simulations. We find that a large scattering rate unlocks stochastic acceleration and fosters the energization of lower-energy CRs, which eventually leads to harder UHECR spectra. Yet, the particles that achieve the highest energies (up to the Hillas limit) are invariably produced by espresso acceleration and their spectrum is independent of the assumed subgrid scattering rate.

Numerical Modeling and Physical Interplay of Stochastic Turbulent Acceleration for Nonthermal Emission Processes

Abstract Particle acceleration is a ubiquitous phenomenon in astrophysical and space plasma. Diffusive shock acceleration (DSA) and stochastic turbulent acceleration (STA) are known to be the possible mechanisms for producing very highly energetic particles, particularly in weakly magnetized regions. An interplay of different acceleration processes along with various radiation losses is typically observed in astrophysical sources. While DSA is a systematic acceleration process that energizes particles in the vicinity of shocks, STA is a random energizing process, where the interaction between cosmic ray particles and electromagnetic fluctuations results in particle acceleration. This process is usually interpreted as a biased random walk in energy space, modeled through a Fokker–Planck equation. In the present work, we describe a novel Eulerian algorithm, adopted to incorporate turbulent acceleration in the presence of DSA and radiative processes like synchrotron and inverse Compton emission. The developed framework extends the hybrid Eulerian−Lagrangian module in a full-fledged relativistic Magneto-hydrodynamic (RMHD) code PLUTO. From our validation tests and case studies, we showcase the competing and complementary nature of both acceleration processes. Axisymmetric simulations of an RMHD jet with this extended hybrid framework clearly demonstrate that emission due to shocks is localized, while that due to turbulent acceleration originates in the backflow and is more diffuse, particularly in the high-energy X-ray band.

The impact of young radio jets traced by cold molecular gas

AbstractRanging from a few pc to hundreds of kpc in size, radio jets have, during their evolution, an impact on their gaseous environment on a large range of scales. While their effect on larger scales is well established, it is now becoming clear that they can also strongly affect the interstellar medium (ISM) inside the host galaxy. Particularly important is the initial phase ( years) of the evolution of the radio jet, when they expand into the inner few kpc of the host galaxy. Here we report on results obtained for a representative group of young radio galaxies using the cold molecular gas as a tracer of jet‐ISM interactions. The sensitivity and high spatial resolution of ALMA and NOEMA are ideal to study the details of this process. In many objects, we find massive molecular outflows driven by the plasma jet, even in low‐power radio sources. However, the observed outflows are limited to the circumnuclear regions and only a small fraction of the ISM is leaving the galaxy. Beyond this region, the impact of the jet seems to change. Fast outflows are replaced by a milder expansion driven by the expanding cocoon created by the jet‐ISM interaction, resulting in dispersing and heating the ISM. These findings are in line with predictions from simulations of jets interacting with a clumpy medium and suggest a more complex view of the impact of AGN than presently implemented in cosmological simulations.

Addressing the Impact of Non-intrinsic Uncertainty Sources in the Stability Analysis of a High-Speed Jet

It is acknowledged by mechanical and aeronautical engineers that many important industrial fluid mechanics simulation-based decisions are made by analysing Reynolds-Averaged Navier-Stokes (RANS) simulations. However, despite that the use of RANS simulations has become a necessity, still unrealistic conditions such as not considering experimental variability or blindly trust a turbulence model happens in practice. This is an important concern in simulation of high-speed jet flows, as the reliability and the accuracy of RANS is still an issue. In this work non-intrusive Uncertainty Quantification (UQ) has been applied to the linear stability analysis of 3D RANS simulations of an under-expanded supersonic jet under uncertainty. The Parabolized Stability Equations (PSE) are utilised for the first time in the literature in a probabilistic approach for the propagation of uncertainty upon CFD simulations. As PSE does not include turbulence nor stagnation pressure as parameters in their mathematical formulation (named non-intrinsic parameters in this work), this framework enables a recommended practice to quantify the impact of uncertainty from earlier stages on the latter jet instability analysis. For this objective, an artificial stochastic base-flow is generated from UQ on RANS to develop the final quantification of uncertainties on jet stability characteristics with PSE. Spatial uncertainty quantification results show that the considered stagnation pressure uncertainty (equivalent to mass-flow rate uncertainty) affects mainly to the shock-cells area, which is in agreement with the jet flow mechanics. The tested turbulent viscosity ratio uncertainty from the Spalart-Allmaras turbulence model has been observed more influential on the shear layer region in general, and with a special role on the growth rate of the perturbations. Frequency (in terms of Strouhal number), has been found influential on the spatial localisation of the turbulent wave packets. This procedure is proposed as an standard approach in the field. The framework may help to understand how instability features are being affected by expected or unexpected uncertainty sources and can guide to future robust developments in the industry, for instance in jet noise reduction.

Scale resolving simulations of a high-temperature turbulent jet in a cold crossflow: Comparison of two approaches

A high-temperature turbulent jet in a cold crossflow is investigated with the help of two scale-resolving simulation approaches. This work aims at improving the methodologies used to predict the thermal footprint of exhaust gases issuing from helicopter engines onto the fuselage. Specific attention is brought to the capability of scale resolving simulations to correctly reproduce flow dynamics and turbulent mixing. Mean flow features, turbulent quantities and temperature fields are compared and validated against wind tunnel test measurements. In addition, the present work highlights the importance of synthetic turbulence injection at pipe inlet to obtain a fair prediction of both flow dynamics and temperature field.

Severe Accidents in Sodium-Cooled Fast Reactors: Computational Fluid Dynamics Sensitivity Studies on the Thermal Ablation of Core Catcher

Abstract The present work aims at testing the computational fluid dynamics capabilities to simulate erosion of materials which interact with a corium mass. The main foreseen application is the design of external/internal core catchers or in-vessel retentions devices used to mitigate severe accidents for sodium-cooled fast reactors. 2D axisymmetric simulations of a corium jet impinging a sacrificial solid material show evidence of a pool-effect, previously observed in experiments, which contributes to limit the ablation process. Complementary sensitivity calculations assess the influence of jet diameter, temperature, and velocity.

Impact of relativistic jets on the star formation rate: a turbulence-regulated framework

ABSTRACT We apply a turbulence-regulated model of star formation to calculate the star formation rate (SFR) of dense star-forming clouds in simulations of jet–interstellar medium (ISM) interactions. The method isolates individual clumps and accounts for the impact of virial parameter and Mach number of the clumps on the star formation activity. This improves upon other estimates of the SFR in simulations of jet–ISM interactions, which are often solely based on local gas density, neglecting the impact of turbulence. We apply this framework to the results of a suite of jet–ISM interaction simulations to study how the jet regulates the SFR both globally and on the scale of individual star-forming clouds. We find that the jet strongly affects the multiphase ISM in the galaxy, inducing turbulence and increasing the velocity dispersion within the clouds. This causes a global reduction in the SFR compared to a simulation without a jet. The shocks driven into clouds by the jet also compress the gas to higher densities, resulting in local enhancements of the SFR. However, the velocity dispersion in such clouds is also comparably high, which results in a lower SFR than would be observed in galaxies with similar gas mass surface densities and without powerful radio jets. We thus show that both local negative and positive jet feedback can occur in a single system during a single jet event, and that the SFR in the ISM varies in a complicated manner that depends on the strength of the jet–ISM coupling and the jet break-out time-scale.

Coherent structures in a compressible turbulent plane jet

Coherent structures represented by proper orthogonal decomposition (POD) modes from large-eddy simulation of a compressible plane jet are analyzed. The leading POD modes of the fluctuating velocity components u′, v′, and w′, and the pressure fluctuation p′ are considered. Spatial and temporal features of these modes are analyzed by the modes and the spectra of the temporal coefficients, respectively. Interactions between leading modes of u′, v′, and w′ and the relationship between the modes of u′, v′, and w′ and of the p′ are discussed. The results show that the leading mode of u′ is in the shape of longitudinal stripes, and the mode is dominated by the jet column frequency Stc. For spanwise fluctuation w′, the leading modes are a pair of modes in the shape of a train of ridges along streamwise, and they are dominated by the fundamental frequency St0. The leading modes for v′ are a pair of modes in a row of ribs. For the dominant frequencies of the v′ modes, suppression of subharmonic frequency at St0/2 and formation of sideband frequencies around (St0 ± Stc)/2 are observed. These spatiotemporal features suggest the mode of v′ is modulated by the modes of u′ and w′. Both the leading modes of p′ and of v′ are in the shape of puffs outside the averaged shear layer. These puff modes are related as an oscillation system, which is confirmed by the spectral-POD modes.

Application of spectral proper orthogonal decomposition and resolvent analysis to periodically forced supersonic jets

The mechanisms by which near-nozzle forcing alters the turbulence structure and far-field sound of turbulent jets are not well understood. We perform large-eddy simulations of subsonic and supersonic axisymmetric isothermal turbulent jets subjected to an axisymmetric periodic forcing. The triple decomposition framework and spectral proper orthogonal decomposition are used to study the effect of the forcing on the underlying turbulence spectrum and determine how the periodic forcing alters coherent non-phase-locked structures. For subsonic jets, high-amplitude (1% of the jet velocity) low-frequency, Stf = 0.3, forcing is required to achieve a small change to the underlying turbulence spectrum and most energetic modes of the subsonic jet, despite producing a very energetic, phase-locked (tonal) response. The changes in the spectrum and the phase-locked structures are predicted well via resolvent analysis and linearized Navier-Stokes computations performed on the new turbulent mean, respectively. This suggests that there is little nonlinear interaction between the phase-locked structures and natural turbulence. High-frequency forcing, Stf = 1.5, shows similarly linear behavior except for around St≈0.75, which appears to be associated with vortex pairings triggered by the natural turbulence. Results of a similar analysis performed on a Mach 1.5 ideally expanded isothermal jet will also be presented.

A Simulation Study of Ultra-relativistic Jets–I. A New Code for Relativistic Hydrodynamics

Abstract In an attempt to investigate the structures of ultra-relativistic jets injected into the intracluster medium (ICM) and the associated flow dynamics, such as shocks, velocity shear, and turbulence, we have developed a new special relativistic hydrodynamic (RHD) code in the Cartesian coordinates, based on the weighted essentially non-oscillatory (WENO) scheme. It is a finite difference scheme of high spatial accuracy, which has been widely employed for solving hyperbolic systems of conservation equations. The code is equipped with different WENO versions, such as the fifth-order accurate WENO-JS, WENO-Z, and WENO-ZA, and different time-integration methods, such as the fourth-order accurate Runge–Kutta (RK4) and strong stability preserving RK (SSPRK), as well as the implementation of the equations of state (EOSs) that closely approximate the EOS of the single-component perfect gas in relativistic regimes. In addition, it incorporates a high-order accurate averaging of fluxes along the transverse directions to enhance the accuracy of multidimensional problems, and a modification of eigenvalues for the acoustic modes to effectively control the carbuncle instability. Through extensive numerical tests, we assess the accuracy and robustness of the code, and choose WENO-Z, SSPRK, and the EOS suggested in Ryu et al. as the fiducial setup for simulations of ultra-relativistic jets. The results of our study of ultra-relativistic jets using the code is reported in an accompanying paper.

All-regime two-phase flow modeling using a novel four-field large interface simulation approach

Two-phase flows may occur in many regimes, ranging from dispersed flow, slug or plug flow to stratified flow. To capture all those regimes with a monolithic approach, a so-called all-regime two-phase model should be developed. This paper presents such an all-regime two-phase modeling approach called Four-Field Large Interface Simulation (FF-LIS), in which we consistently consider two phases (gas and liquid) which can both be either resolved or sub-grid, resulting in four fields. The efficacy of this approach hinges on the accuracy with which the inter-scale mass, momentum and energy transfer terms between dispersed and resolved regimes are captured. In this paper, we introduce three phenomenological mechanisms to control the inter-scale transfer: absorption, break-up and coalescence. The separation of multiphase scales allows for the explicit modeling of transfer between them, whereas other methods often rely on an algebraic regime map in which transfer between scales is accounted for implicitly. We apply FF-LIS to the simulation of both bubbly and fully resolved single bubble flow, as a way of validating FF-LIS in those two two-phase limits. Next, the transfer terms are illustrated by simulation of bubbles rising in a column. As the bubble concentration increases, it is shown that a transfer from sub-grid to resolved occurs. Finally, we apply FF-LIS to the simulation of two-phase horizontal pipe flow in which the several flow regimes, depending on the superficial velocities, are recovered, and to the simulation of a plunging jet in which FF-LIS is shown to produce an entrained bubble plume. FF-LIS presents a computational platform for faithful two-phase resolved and sub-grid simulation of relevant real-world two-phase problems, and allows two-phase modelers for the explicit control on what can be resolved, and what should be sub-grid.

Large deformation effects on the combustion structure of a hydrogen/air rich premixed flame

The effects of large flame deformation on the premixed flame combustion structure were studied by means of a 2-D numerical experiment with detailed chemical kinetics and accurate transport properties, in which a high-speed unburnt gas volume was injected into a stationary hydrogen-air premixed flame of equivalence ratio 4.5. It was found that an extraordinarily high fuel consumption rate appears in the course of the gas volume penetration into the premixed flame. Full use of the numerical data was made to understand the cause of this phenomenon. It was found that enthalpy accumulation that takes place when the flame merges plays a key role: an increase in high-temperature unburnt O2 produced by the accumulated enthalpy accelerates the productions of OH and O, leading to the acceleration of H2-consuming reactions. A similarly extraordinarily high fuel consumption rate in a turbulent rich premixed flame has previously been observed in the 3-D simulation of a hydrogen jet lifted flame, and therefore the mechanism found in this study may be one of the key mechanisms to understand in the behavior of turbulent premixed flames.

From electrons to Janskys: Full stokes polarized radiative transfer in 3D relativistic particle-in-cell jet simulations

Context.Despite decades of dedicated observation and study, the underlying plasma composition of relativistic extragalactic jets remains largely unknown.Aims.Relativistic magnetohydrodynamic (RMHD) models are able to reproduce many of the observed macroscopic features of these outflows (e.g., recollimation shocks, jet sheaths and spines, bow shocks, and enshrouding jet cocoons). The nonthermal synchrotron emission detected by very long baseline interferometric arrays, however, is a by-product of the kinetic-scale physics occurring within the jet, physics that is not modeled directly in most RMHD codes. This paper attempts to discern the radiative differences between distinct plasma compositions within relativistic jets using small-scale 3D relativistic particle-in-cell (PIC) simulations.Methods.We made use of a polarized radiative transfer scheme to generate full Stokes imaging of two PIC jet simulations, one in which the jet is composed of an electron-proton (e− − p+) plasma (i.e., a normal plasma jet), and the other in which the jet is composed of an electron-positron (e− − e+) plasma (i.e., a pair plasma jet). We examined the differences in the morphology and intensity of the linear polarization and circular polarization (CP) emanating from these two jet simulations.Results.Our PIC simulations, when scaled into physical units, are ∼150 cubic kilometers in size. We find that the fractional level of CP (measured relative to integrated total intensity) emanating from thee− − p+plasma jet is orders of magnitude larger than the level emanating from ane− − e+plasma jet of a similar speed and magnetic field strength. In addition, we find that the morphology of both the linearly and circularly polarized synchrotron emission is distinct between the two jet compositions. These results highlight the following: (i) the potential of high-resolution full-Stokes polarimetric imaging to discern between normal plasma and pair plasma jet emission in larger scale systems and (ii) the challenges faced by kinetic simulations in modeling this emission self-consistently. We also demonstrate the importance of slow-light interpolation and we highlight the effect that a finite light-crossing time has on the resultant polarization when ray-tracing through relativistic plasma. Placing a firm constraint on the plasma content of relativistic extragalactic jets will help to advance our understanding of jet feedback.

Effect of Impact Cavity Shape Induced by Supersonic Oxygen Jet on the Dynamic Characteristics of Molten Bath in Converter

The cover image illustrates the numerical simulation of a four-hole supersonic jet impinging on the molten steel bath during the top-blown converter steelmaking process. The modified cavity shape index is put forward to quantitatively describe the scattered cavities profile. Furthermore, the relationship between the cavity shape and dynamic characteristics of the molten bath is also analyzed. Further details can be found in the article, number 2100179, by Qing Liu, and co-workers.

High-fidelity simulations of electrolyte jets in an electric field

High-fidelity simulations of electrolyte jets in an electric field

Numerical simulations of cryogenic jets at supercritical pressure conditions.

Numerical simulations of cryogenic jets at supercritical pressure conditions.

The effect of an inline blockage on the formation of a turbulent free jet

In many equilibrium-based models for droplet size prediction, the fundamental parameters of turbulence are removed in favour of bulk quantities like velocity, and diameter. In this work we show that the upstream conditions of the jet are of critical importance and cannot be neglected, as they influence the turbulence field in jets in ways that are not accounted for in models based entirely on bulk parameters. This work presents the results of a single-phase investigation into the effect of upstream disturbances on a turbulent free jet.Reynolds Averaged Navier-Stokes Simulations of turbulent free jets with disturbances (in the form of orifice plates with a β ratio of 0.5) located at varying locations upstream of the pipe exit have been undertaken. Validation against experimental data for an orifice plate in a pipe found the Standard k-ε model to be the most accurate of the RANS models tested for this geometry.The additional turbulence generated from the upstream disturbances was found to be advected downstream and did not dissipate prior to the jet exit. The increased turbulence levels and altered form of the turbulence profile at exit was found to affect the evolution and observed turbulence levels of the free jet.Two distinct ‘modes’ were observed for the influence of the disturbances on the free jet. When the orifice plate is near the exit the free jet is dominated by the behaviour of semi-confined jet forming from the blockage. For the deeper set disturbances, the predicted form of the free jet was similar to that of the free jet from a straight pipe, though the turbulence levels are still elevated at the exit, affecting spreading rates and turbulence levels in the jet.Measures based on averages of TKE and TDR over hemispheres of varying radii are proposed for the development of models that do take into account the turbulent state of the exiting jet.

Two Steps Forward and One Step Sideways: The Propagation of Relativistic Jets in Realistic Binary Neutron Star Merger Ejecta

Abstract The association of GRB170817A with GW170817 has confirmed the long-standing hypothesis that binary neutron star (BNS) mergers are the progenitors of at least some short gamma-ray bursts (SGRBs). This connection has ushered in an era in which broadband observations of SGRBs, together with measurements of the time delay between the gravitational waves and the electromagnetic radiation, allow for probing the properties of the emitting outflow and its engine to an unprecedented detail. Because the structure of the radiating outflow is molded by the interaction of a relativistic jet with the binary ejecta, it is of paramount importance to study the system in a realistic setting. Here we present a three-dimensional hydrodynamic simulation of a relativistic jet propagating in the ejecta of a BNS merger, which were computed with a general relativistic magnetohydrodynamic simulation. We find that the jet’s centroid oscillates around the axis of the system, due to inhomogeneities encountered in the propagation. These oscillations allow the jet to find the path of least resistance and travel faster than an identical jet in smooth ejecta. In our setup the breakout time is ∼0.6 s, which is comparable to the expected central engine duration in SGRBs and possibly a non-negligible fraction of the total delay between the gravitational and gamma-ray signals. Our simulation also shows that energy is carried in roughly equal amounts by the jet and by the cocoon, and that about 20% of the injected energy is transferred to the ejecta via mechanical work.