Low-density Jet Research Articles

Chaos via type-II intermittency in a forced globally unstable jet

We explore the transition to chaos in a prototypical hydrodynamic oscillator, namely a globally unstable low-density jet subjected to external time-periodic forcing. As the forcing strengthens at an off-resonant frequency, we find that the jet exhibits a sequence of nonlinear states: period-1 limit cycle $\rightarrow $ quasiperiodicity $\rightarrow$ intermittency $\rightarrow$ low-dimensional chaos. We show that the intermittency obeys type-II Pomeau–Manneville dynamics by analysing the first return map and the scaling properties of the quasiperiodic lifetimes between successive chaotic epochs. By providing experimental evidence of the type-II intermittency route to chaos in a globally unstable jet, this study reinforces the idea that strange attractors emerge via universal mechanisms in open self-excited flows, facilitating the development of instability control strategies based on chaos theory.

Coupling dynamics of capsule interior defects and its impact on hydrodynamic instabilities at ablation fronts for inertial confinement fusion implosions

The micrometer-scale internal defect in the capsule is one of the most important factors that limit implosion performance in inertial confinement fusion (ICF) experiments, which creates instability seeds as shocks propagate through the capsule shell. Here, we report the generation mechanism of vortex pairs resulting from the interaction of shock waves with multiple bubbles, as well as the origin of more intricate perturbation waves than those observed in the case of single defects. Based on the subsequent evolution of hydrodynamic instability, it is evident that the vortex pairs induce the emergence of low-density (light-bubble case) or high-density (referred to as heavy-bubble case) jets on the ablative front. The presence of multiple side-by-side defects can rapidly amplify the dimensions of the jet. These jets could be responsible for the “meteor shower” observed in implosion experiments. Converging disturbed waves between vertically aligned defects lead to a more complex nonlinear flow field evolution compared to the scenario with a single defect. A systematic study of localized perturbation growth as a function of defect placement is presented. We investigate the dependence of circulation in the flow field on the locations of the defects. The scanning results of defect scenes with different sizes revealed the reason why the depth of fluid penetration is affected by the position and size, and found that the effects of the position and size on the perturbation expansion width can be equivalent to a certain extent. The extension of the perturbation width when the defect is off-axis limits the degree of penetration of the perturbation depth. The results contribute to a more comprehensive understanding of physical processes, such as the seeding mechanism, shell integrity, and mass injection into the central region, which may be applied to inform the development of more effective strategies to mitigate implosion degradation in ICF implosion experiments.

Neoclassical transport of tungsten ion bundles in total-f neoclassical gyrokinetic simulations of a whole-volume JET-like plasma

The application of a bundling technique to model the diverse charge states of tungsten impurity species in total-f gyrokinetic simulations is demonstrated. The gyrokinetic bundling method strategically groups tungsten ions of similar charge, optimizing computational efficiency. The initial radial configuration of these bundles and their respective charges are derived from a coronal approximation and the quasi-neutrality of the plasma. A low-density JET H-mode like plasma is simulated using the neoclassical version of XGC across the entire plasma volume, spanning from the magnetic axis to the divertor. An accumulation of tungsten is observed at the pedestal top, as a result of low-Z tungsten ions moving inward from the scrape-off-layer into the core region and high-Z tungsten ions moving outward from the core into the pedestal. This organization of the fluxes cannot be captured by a single tungsten-ion simulation. Large up-down poloidal asymmetries of tungsten form in the pedestal and strongly influence the direction of neoclassical fluxes. The temperature screening effect and its correlation with asymmetries are analyzed.

A semi-supervised framework for analyzing the potential core of a low-density jet

The potential core is a region of crucial hydrodynamic importance in a low-density jet. In this study, we propose a semi-supervised data-driven method for detecting the potential core of an axisymmetric low-density jet and computing its length. We exploit a neural network to classify the Schlieren images of a low-density helium jet and infer the nozzle exit velocity as well as the length of the potential core region. We also employ the explainable artificial intelligence approach and verify the reliability of the presented framework. The results show that the information about the low-density jet and its potential core length can reasonably be extracted from the jet image, significantly reducing the experimental effort of the conventional sensor-based framework.

Study on the Effect of PTFE/Cu Composite Material Preparation Process on Penetration Performance.

The jet formed by the traditional metal liner has a slender shape. The diameter of the jet head is consistent with that of the tail, and the ductility is good. When it is used to penetrate the target, it has a good damage effect. The low-density jet formed by the PTFE/Cu liner, according to the different preparation processes and densities, has different degrees of radial expansion. This phenomenon may lead to the expansion of the jet head during the penetration process, resulting in a damage effect, which is different from the previous jet on the target. In this paper, the numerical simulation of PTFE/Cu liners with different preparation processes penetrating steel targets is carried out, and the effects of different preparation processes and liner density on the penetration characteristics of jets penetrating steel targets are compared and analyzed. The PTFE/Cu shaped charge liner was processed according to different preparation processes, and the jet penetration steel target experiment was carried out, so as to verify and analyze the numerical simulation results.

Turbulent transfer and entrainment in a low-density jet

We investigate the dynamics of a low-density round jet, with a focus on the mechanisms governing the turbulent momentum and mass transfers as well as on the entrainment of ambient fluid. To that purpose, we combine a theoretical analysis, laboratory experiments and numerical simulations. The theoretical analysis relies on a general formulation of the entrainment decomposition for the case of large density differences, revealing the role of the processes contributing to the entrainment: turbulent kinetic energy production and variation in the shape of the mean velocity radial profiles. The spatial evolution of these terms has been evaluated by means of challenging experiments, providing a unique data set of combined velocity and density statistics of a low-density jet and an air jet. The same flows are investigated by means of large-eddy simulation (LES). Other than for providing complementary information on flow statistics, LES is here used to investigate the role of varying conditions imposed at the source, notably concerning the shape of the inlet velocity profile and the presence of a bottom wall surrounding the source. Experimental and numerical results provide clear insight on how a reduced density within the jet enhances the turbulent kinetic energy production (compared to an iso-density jet) and modifies the shape of the mean velocity profiles. Despite its clear influence on the flow statistics, the reduced density has overall little influence on the entrainment rate, which also shows little sensitivity to varying source conditions.

An Experimental Investigation of Supercritical Methane Injection Characteristics in a CO2 Environment

Abstract Clean energy generation is gaining significant attention from industries, academia, and governments across the globe. The Allam cycle is one such technology that has been under focus due to its efficiency, environmental friendliness, and economics. This is a direct-fired cycle operating at supercritical conditions using carbon dioxide as a working fluid. Fuel or oxidizer jet mixing with CO2 is a vital phenomenon that governs combustion efficiency, and it is not well understood for the Allam cycle conditions. This paper experimentally investigated the jet characteristics of a methane jet injected into a subcritical to supercritical carbon dioxide environment. A wide range of injection pressures and temperatures were targeted between subcritical to supercritical conditions. Unlike previous studies, the current work focused on injecting lower-density (methane) jets into higher-density (carbon dioxide) environments. Schlieren imaging and methane absorption measurements were simultaneously performed with a CMOS high-speed camera and a 3.39 μm infrared laser. Specifically, we looked at the classical injection parameter of jet spreading angle, which was classically established to be mainly a density ratio function. Here, the jet cone angle was obtained from the postprocessed schlieren imaging. The jet cone angle is a critical characteristic parameter that describes the entrainment rate in a jet; thus, it is a crucial parameter in understanding the nature of the jet. The laser measurements were only used as an additional check to confirm the entry time of methane into the chamber filled with carbon dioxide. Notably, this paper makes a detailed comparison between the jet cone angles of jets with a density ratio. The result showed that the classical correlations, such as Abramovich's theory applied to submerged turbulent gas jets developed for low-density ratio jets, were unsuitable for higher-density ratio jets. It was also observed that the divergence angles were dependent not only on density ratio but also on other parameters such as pressure ratios and reduced pressures.

Self-regulated AGN feedback of light jets in cool-core galaxy clusters

ABSTRACT Heating from active galactic nuclei (AGNs) is thought to stabilize cool-core clusters, limiting star formation and cooling flows. We employ radiative magnetohydrodynamic (MHD) simulations to model light AGN jet feedback with different accretion modes (Bondi–Hoyle–Lyttleton and cold accretion) in an idealized Perseus-like cluster. Independent of the probed accretion model, accretion efficiency, jet density and resolution, the cluster self-regulates with central entropies and cooling times consistent with observed cool-core clusters in this non-cosmological setting. We find that increased jet efficiencies lead to more intermittent jet powers and enhanced star formation rates. Our fiducial low-density jets can easily be deflected by orbiting cold gaseous filaments, which redistributes angular momentum and leads to more extended cold gas distributions and isotropic bubble distributions. In comparison to our fiducial low momentum-density jets, high momentum-density jets heat less efficiently and enable the formation of a persistent cold gas disc perpendicular to the jets that is centrally confined. Cavity luminosities measured from our simulations generally reflect the cooling luminosities of the intracluster medium and correspond to averaged jet powers that are relatively insensitive to short periods of low-luminosity jet injection. Cold gas structures in our MHD simulations with low momentum-density jets generally show a variety of morphologies ranging from discy to very extended filamentary structures. In particular, magnetic fields are crucial to inhibit the formation of unrealistically massive cold gas discs by redistributing angular momentum between the hot and cold phases and by fostering the formation of elongated cold filaments that are supported by magnetic pressure.

Multi-GeV Electron Bunches from an All-Optical Laser Wakefield Accelerator

We present the first demonstration of multi-GeV laser wakefield acceleration in a fully optically formed plasma waveguide, with an acceleration gradient as high as 25 GeV/m. The guide was formed via self-waveguiding of <15 J, 45 fs (<∼ 300 TW) pulses over 20 cm in a low-density hydrogen gas jet, with accelerated electron bunches driven up to 5 GeV in quasimonoenergetic peaks of relative energy width as narrow as ∼15%, with divergence down to ∼1 mrad and charge up to tens of picocoulombs. Energy gain is inversely correlated with on-axis waveguide density in the range Ne0=(1.3–3.2)×1017 cm−3. We find that shot-to-shot stability of bunch spectra and charge are strongly dependent on the pointing of the injected laser pulse and gas jet uniformity. We also observe evidence of pump depletion-induced dephasing, a consequence of the long optical guiding distance.7 MoreReceived 2 December 2021Revised 4 April 2022Accepted 1 August 2022Corrected 21 October 2022DOI:https://doi.org/10.1103/PhysRevX.12.031038Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLaser driven electron accelerationLaser wakefield accelerationAccelerators & BeamsPlasma Physics

Experimental study of the Hopf bifurcation in rectangular low-density jets

Experiments are performed to understand the nature of Hopf bifurcations in rectangular low-density jets. Results showed that the rectangular low-density jet could undergo a transition from a globally stable state to a globally unstable state through the subcritical Hopf bifurcation in addition to the supercritical Hopf bifurcation. There is a critical aspect ratio of rectangular low-density jets above which Hopf bifurcations are supercritical. Below the critical aspect ratio, the rectangular low-density jets undergo subcritical Hopf bifurcations for a range of jet-to-ambient density ratios, and outside this range, the Hopf bifurcations are supercritical.

Global oscillations in low-density round jets with parabolic velocity profiles

Experiments and spatiotemporal stability analysis are carried out to study the global oscillations in laminar low-density round jets with parabolic velocity profiles. The experimental results of laminar low-density jets with parabolic velocity profiles exhibit global axisymmetric oscillations. The spatiotemporal stability results based on base profiles from numerical simulations are consistent with the present experimental results. These results differ from the prediction of stability study by Coenen et al. (Phys. Fluids, vol. 20, 2008, p. 074104). They reported that the low-density jets with near parabolic velocity profiles show global helical oscillations. It is observed that the method used by Coenen et al. is not able to predict the nature of global oscillations observed in experiments for low-density jets with near parabolic profiles. The present spatiotemporal stability results demonstrate that the base flows from Navier–Stokes equations are required to predict the critical conditions observed in experiments for low-density jets with near parabolic velocity profiles. The breakdown distance of globally unstable low-density jets scales with $(Re-Re_c)^{-1/2}$ ( $Re_c$ is the critical Reynolds number), consistent with the scaling law obtained from the nonlinear global mode of the Ginzburg–Landau equation.

Study of Mixing Enhancement in Helium Jet/Shock Interaction by Laser Pulses

The mechanism of low-density jet mixing enhancement by a pulsed laser plasma is investigated through a combined numerical and experimental method. A helium jet is injected in parallel to a Mach 2.4 supersonic airstream, and interacts with an oblique shock generated by a compression ramp of 20 deg. The helium jet Mach numbers are set to be 1.01 and 1.80 for the sonic and supersonic fuel jets in the scramjet combustor, respectively. For jet mixing enhancement control, the laser pulses at frequencies ranging from 5 to 25 kHz are introduced inside the jet. The experimental results show that laser pulses significantly improve the jet mixing with jet width increased by 30% seen from the averaged schlieren images. Three-dimensional unsteady Reynolds-averaged Navier–Stokes equations are solved, and the numerical results further reveal that the jet mixing enhancement is strongly associated with large-scale vortex rings stimulated by the laser pulses interacting with shock waves. These large-scale vortices prompt the jet cross-sectional area and mixing efficiency two times larger. However, the total pressure recovery coefficient is reduced by 0.16% downstream. Meanwhile, the turbulent kinetic energy of jet flow is significantly increased and favorable for the jet mixing.

Asynchronous and synchronous quenching of a globally unstable jet via axisymmetry breaking

We explore experimentally whether axisymmetry breaking can be exploited for the open-loop control of a prototypical hydrodynamic oscillator, namely a low-density inertial jet exhibiting global self-excited axisymmetric oscillations. We find that when forced transversely or axially at a low amplitude, the jet always transitions first from a period-1 limit cycle to $\mathbb {T}^2$ quasiperiodicity via a Neimark–Sacker bifurcation. However, we find that the subsequent transition, from $\mathbb {T}^2$ quasiperiodicity to $1:1$ lock-in, depends on the spatial symmetry of the applied perturbations: axial forcing induces a saddle-node bifurcation at small detuning but an inverse Neimark–Sacker bifurcation at large detuning, whereas transverse forcing always induces an inverse Neimark–Sacker bifurcation irrespective of the detuning. Crucially, we find that only transverse forcing can enable both asynchronous and synchronous quenching of the natural mode to occur without resonant or non-resonant amplification of the forced mode, resulting in substantially lower values of the overall response amplitude across all detuning values. From this, we conclude that breaking the jet axisymmetry via transverse forcing is a more effective control strategy than preserving the jet axisymmetry via axial forcing. Finally, we show that the observed synchronization phenomena can be modelled qualitatively with just two forced coupled Van der Pol oscillators. The success of such a simple low-dimensional model in capturing the complex synchronization dynamics of a multi-modal hydrodynamic system opens up new opportunities for axisymmetry breaking to be exploited for the open-loop control of other globally unstable flows.

Ionization induced plasma grating and its applications in strong-field ionization measurements

An ionization induced plasma grating can be formed by spatially selective ionization of gases by the interference of two intersecting ultra-short laser pulses. The density modulation of a plasma grating can approach unity since the plasma is produced only where the two pulses constructively interfere and ionization does not occur in destructive interference regions. Such a large density modulation leads to efficient Thomson scattering of a second ultra-short probe pulse once the Bragg condition is satisfied. By measuring the scattering efficiency, it is possible to determine the absolute electron density in the plasma grating and thereby deduce the ionization degree for a given neutral gas density. In this paper, we demonstrate the usefulness of this concept by showing two applications: ionization degree measurement of strong-field ionization of atoms and molecules and characterization of extremely low-density gas jets. The former application is of particular interest for ionization physics studies in dense gases where the collision of the ionized electron with neighboring neutrals may become important-sometimes referred to as many-body ionization; and the latter is useful for plasma-based acceleration that requires extremely low-density plasmas.

Multi-pass probing for high-sensitivity tomographic interferometry

Optical probing is an indispensable tool in research and development. In fact, it has always been the most natural way for humankind to explore nature. However, objects consisting of transparent materials with a refractive index close to unity, such as low-density gas jets, are a typical example of samples that often reach the sensitivity limits of optical probing techniques. We introduce an advanced optical probing method employing multiple passes of the probe through the object to increase phase sensitivity, and relay-imaging of the object between individual passes to preserve spatial resolution. An interferometer with four-passes was set up and the concept was validated by tomographic characterization of low-density supersonic gas jets. The results show an evident increase of sensitivity, which allows for the accurate quantitation of fine features such as a shock formed by an obstacle or a barrel shock on the jet boundary in low ambient gas pressures. Despite its limitations in temporal resolution, this novel method has demonstrated an increase in phase sensitivity in transmission, however, it can also be employed to boost the absorption or polarization contrast of weakly interacting objects in both transmission and reflection setups, thus, upgrading the sensitivity of various optical characterization methods.

Numerical Simulation of Tandem Shaped Charge Penetrating Armored Target under Explosive Loading

Tandem shaped charge is one of the hotspots in the research of antiarmor ammunition at present, but because of the shortcoming that the front-stage jet penetrates and detonates ERA (Explosive Reactive Armor), then it will affect the rear-stage jet to penetrate the main armor. Our research team proposes that the front charge of the tandem shaped charge can be designed as a low-density material charge liner so that the front-stage low-density jet can penetrate a small hole in the reactive armor without detonating. The rear-stage main charge forms a main jet through the small hole to penetrate the main armor. In this article, the damage of tandem shaped charge to armored target is studied when copper and new modified PTFE (polytetrafluoroethylene) materials are used as front-stage charge liner, and the numerical simulation results are verified by experiments. The numerical results show that the Cu (copper) jet formed by tandem shaped charge of double-copper material has strong impact performance. The interlayer explosive in reactive armor is detonated in the process of penetrating reaction armor, which weakens the penetration ability of the main rear-jet. However, the interlayer explosive in the reactive armor is not detonated during the penetration of the double-material tandem shaped charge. The front-stage modified PTFE jet and the rear-stage main jet are not affected by the explosion loading before penetrating the main armor. The experimental results show that the double-material tandem shaped charge overcomes the shortcoming of the influence of the front-stage explosion, and the penetration depth of the main armor reaches 703 mm, which is 14.3% higher than the penetration ability of the double-copper tandem shaped charge, which is basically consistent with the numerical simulation results. It provides a reference for improving the damage ability of tandem shaped charge to armored target.

Application of spatially hybrid fluid–kinetic neutral model on JET L-mode plasmas

Abstract We present a spatially hybrid fluid–kinetic neutral model that consists of a fluid model for the hydrogen atoms in the plasma grid region coupled to a kinetic model for atoms sampled at the plasma–void interfaces and a fully kinetic model for the hydrogen molecules. The atoms resulting from molecular dissociation are either treated kinetically (approach 1) or are incorporated in the fluid model (approach 2). For a low-density JET L-mode case, the hybrid method reduces the maximum fluid–kinetic discrepancies for the divertor strike-point electron densities and electron temperatures from approximately 150% to approximately 20% for approach 1 and to approximately 40% for approach 2. Although the simulations with purely fluid neutral model become more accurate for increasing upstream plasma density, we still observe a significant improvement by using the hybrid approach. When consuming the same CPU time in averaging the electron strike-point densities and temperatures over multiple iterations as for the simulations with fully kinetic neutrals, hybrid approach 1 reduces the statistical error with on average a factor 2.5. Hybrid approach 2 further increases this factor to approximately 3.3, at the expense of accuracy.

Research on Penetration Performance of Tandem Damage Elements to Oblique Reactive Armor

In order to study the penetration performance of the tandem warhead against the oblique reactive armor target, ANSYS/LS-DYNA finite element simulation software was used to simulate the penetration process of the tandem warhead against the oblique reactive armor target when the front liner of the tandem warhead was made of different materials. The results show that when the front liner material is copper, the formed jet penetrates the reactive armor and will detonate it. The rear jet is greatly affected by the reactive armor, and the final penetration depth is 550 mm; when the front liner material is material is polytetrafluoroethylene (PTFE), the formed low-density jet penetrates the reactive armor and does not detonate it. The rear jet directly penetrates the main target plate along the low-density jet penetration hole, and the final penetration depth is 646 mm, compared with the tandem warhead As far as the front liner uses copper jet, the penetration depth is increased by 17.45%.

The Dynamics of Globally Unstable Air-Helium Jets and Its Impact on Jet Mixing Intensity

The paper presents experimental investigations of the low-density air-helium jets. The paper is aimed at the analysis of the flow conditions promoting the local absolute instability leading to global flow oscillations. A number of the test cases are analysed with a wide range of the shear layer thickness showing conditions favorable for the global modes and also mixing intensity triggered by such a regime. It is shown that high mixing intensity is determined not only by the global regime but also by the vortex pairing process. The results are compared with a recently proposed universal scaling law for an onset into the global mode. The results turn out to be far from this scaling law and the reasons for such discrepancies are discussed. The measurements show also that if the shear layer at the nozzle exit is thin enough the global modes are suppressed. The mechanism leading to the global mode suppression under such conditions is carefully analysed.

The different flavors of extragalactic jets: The role of relativistic flow deceleration

We perform three-dimensional numerical simulations of relativistic (with a Lorentz factor of 10), non-magnetized jets propagating in a uniform density environment in order to study the effect of the entrainment and the consequent deceleration. Our simulations investigate the jet propagation inside the galaxy core, where the deceleration most likely occurs more efficiently. We compare cases with different density and pressure ratios with respect to the ambient medium and find that low density jets are efficiently decelerated and reach a quasi-steady state in which, over a length of 600 jet radii, they slow down from highly relativistic to sub-relativistic velocities. Conversely, denser jets keep highly relativistic velocities over the same length. We discuss these results in relation to the Faranoff Riley (FR) radio source classification. We infer that lower density jets can give rise to FR 0 and FR I radio sources, while higher density jets may be connected to FR II radio sources.