Slab Jets Research Articles

A numerical study on the role of instabilities on multi-wavelength emission signatures of blazar jets

Context. Blazars, a class of active galaxies whose jets are relativistic and collimated flows of plasma directed along the line of sight, are prone to a slew of magnetohydrodynamic (MHD) instabilities. These jets show characteristic multi-wavelength and multi-timescale variabilities. Aims. We aim to study the interplay of radiation and particle acceleration processes in regulating the multi-band emission and variability signatures from blazars. In particular, the goal is to decipher the impact of shocks arising due to MHD instabilities in driving the long-term variable emission signatures from blazars. Methods. To this end, we performed relativistic MHD (RMHD) simulations of a representative section of a blazar jet. The jet was evolved using a hybrid Eulerian-Lagrangian framework to account for radiative losses due to synchrotron process as well as particle acceleration due to shocks. Additionally, we incorporated and validated radiative losses taking into consideration the external Compton (EC) process that is relevant for blazars. We further compared the effects of different radiation mechanisms through numerical simulation of 2D slab jet as a validation test. Finally, we carried out a parametric study to quantify the effect of magnetic fields and external radiation field characteristics by performing 3D simulations of a plasma column. The synthetic light curves and spectral energy distribution (SEDs) were analyzed to qualitatively understand the impact of instability driven shocks. Results. We observed that shocks produced with the evolution of instabilities give rise to flaring signatures in the high-energy band. The impact of such shocks is also evident from the instantaneous flattening of the synchrotron component of the SEDs. At later stages, we observed the transition in X-ray emission from the synchrotron process to that dominated by EC. The inclusion of the EC process also gives rise to γ-ray emission and shows signatures of mild Compton dominance that is typically seen in low-synchrotron peaked blazars.

Impact of Boundary-Layer Cutting and Flow Conditioning on Free-Surface Behavior in Turbulent Liquid Sheets

The HYLIFE-II conceptual design uses arrays of high-speed oscillating and stationary slab jets, or turbulent liquid sheets, to protect the reactor chamber first walls. A major issue in thick liquid protection is the hydrodynamic source term due to the primary turbulent breakup of the protective slab jets. During turbulent breakup, drops are continuously ejected from the surface of turbulent liquid sheets and convected into the interior of the cavity, where they can interfere with driver propagation and target injection. Experimental data for vertical turbulent sheets of water issuing downwards from nozzles of thickness (small dimension) δ = 1 cm into ambient air are compared with empirical correlations at a nearly prototypical Reynolds number Re = 1.2 × 105. A simple collection technique was used to estimate the amount of mass ejected from the jet surface. The effectiveness of boundary-layer cutting at various “depths” into the flow to reduce the source term and improve surface smoothness was evaluated. In all cases boundary-layer cutting was implemented immediately downstream of the nozzle exit. Planar laser-induced fluorescence (PLIF) was used to visualize the free-surface geometry of the liquid sheet in the near-field region up to 25δ downstream of the nozzle exit. Large-scale structures at the edges of the sheet, typically observed for Re < 5.0 × 104, reappeared at Re = 1.2 × 105 for sheets with boundary-layer cutting. The results indicate that boundary-layer cutting can be used to suppress drop formation, i.e. the hydrodynamic source term, for a well-conditioned jet but is not a substitute for well-designed flow conditioning.

Flow Conditioning Design in Thick Liquid Protection

The HYLIFE-II conceptual design proposed using arrays of high-speed oscillating and stationary slab jets, or turbulent liquid sheets, to protect the reactor chamber first walls from damaging neutrons, ions and X-rays. Flow conditioning can be used to reduce turbulent fluctuations in these liquid sheets and thereby reduce surface ripple, or free-surface fluctuations, and delay jet breakup. Several flow conditioning configurations are studied experimentally for vertical turbulent sheets of water issuing downwards from nozzles of thickness (small dimension) δ = 1 cm into ambient air for Reynolds numbers Re = 5.0 × 104 and 1.2 × 105. In particular, the role of one or more fine screens in the flow conditioner was studied. As the flow conditioning element immediately upstream of the nozzle inlet, fine screens have been shown to have a major impact upon the sheet free-surface geometry. Planar laser-induced fluorescence was used to measure the free-surface geometry of the liquid sheet and its fluctuations in the near field at streamwise distances downstream of the nozzle exit x ≤ 25δ. Laser-Doppler velocimetry was used to quantify the impact of different conditioning configurations on the cross-stream velocity component and its fluctuations just upstream of the nozzle exit. The results indicate that minor differences in velocity and velocity fluctuations near the nozzle exit can lead to major variations in free-surface geometry, and that free-surface fluctuations are strongly affected by changes in flow conditioner design, even in the near-field region of the flow. A single-screen configuration was shown to produce the smoothest jets at both Reynolds numbers, with fluctuations of 3.3% at Re = 1.2 × 105 and x = 25δ.

The evolution of slab jets and self-similar models of extragalactic radio sources

Simple general arguments suggest that, like round jets, initially divergent hypersonic slab jets propagating through a uniform atmosphere are eventually recollimated by the high pressure of their cocoons and the subsequent evolution is approximately self-similar. Although slab jets are not directly relevant to astrophysical jets, they allow us to study the effects of enforced symmetry without resorting to expensive three-dimensional numerical simulations. In this paper we show that, although both jets with enforced mirror symmetry and those without eventually become self-similar, the flow depends significantly on the symmetry. In particular, the aspect ratio of the cocoon created by asymmetric jets seems to be independent of the initial jet opening angle for small opening angles, contrary to what is found for symmetric jets. This suggests that the hydrodynamics of the jet and its cocoon is much more complicated than has been assumed in current theoretical models of extragalactic radio sources and that results of two-dimensional axisymmetric numerical simulations can be rather misleading.

Studies of turbulent liquid sheets for protecting IFE reactor chamber first walls

The HYLIFE-II conceptual inertial fusion energy (IFE) reactor design uses stationary and oscillating slab jets, or liquid sheets, to create a protective pocket that allows target injection and driver beam propagation while protecting the chamber first walls from neutrons, X-rays and target debris. Thick liquid wall protection can, therefore, reduce reactor chamber size and increase chamber lifetime in commercial IFE reactors. Minimizing driver beam interference and irradiation of the final focus magnets places stringent requirements upon the surface smoothness of the stationary liquid sheets that shield the heavy-ion driver array. Experiments were carried out to determine how nozzle and flow conditioner design affect surface smoothness in turbulent liquid sheets at Reynolds numbers up to 130 000. The free-surface was directly imaged using planar laser-induced fluorescence (PLIF) at downstream distances up to 25 times the sheet thickness at the nozzle exit. The data are processed to determine the probability of finding fluid at any given spatial location. The surface smoothness of sheets of water issuing from nozzles with various contractions and rectangular or nearly elliptical exits into atmospheric pressure air were quantified and compared. The free-surface characteristics of liquid sheets issuing from unblocked and partially blocked (2.5% of total area) but otherwise identical flow conditioners were compared to investigate the robustness of flow conditioning elements over long operation times. These results on nearly prototypical turbulent stationary liquid sheets address a number of technical feasibility issues for thick liquid protection in IFE reactors.

Dynamics of Oscillating Turbulent Liquid Sheets

Recently, oscillating high-speed slab jets, or liquid sheets, have been proposed for shielding the first walls of inertial fusion energy (IFE) reactor chambers from damaging X-rays, neutrons and ions. The near-field dynamics of obliquely oscillating turbulent liquid sheets were investigated in scaled experiments. Results are presented for sheets at Reynolds numbers up to 37000 oscillated along various directions at frequencies from 0 to 11 Hz and amplitudes up to half the nozzle thickness (0.5δ). Data on maximum trajectories of oscillating sheets and growth rates of stationary sheets are presented for distances up to 90δ downstream of the nozzle exit. A model for predicting the maximum trajectory is presented. The bulk of the experimental data are in reasonable agreement with this model. These results can be used to provide design guidelines for thick liquid protection.

Direct Measurements of Free Surface Smoothness in Turbulent Liquid Sheets

The High-Yield Lithium-Injection Fusion Energy (HYLIFE)-II conceptual reactor design uses stationary and oscillating slab jets, or liquid sheets, of molten Flibe (Li2BeF4) to shield the chamber first walls from damaging neutrons, ions, and target debris. A lattice of stationary liquid sheets with the beams propagating through the lattice openings is used to protect chamber front and back walls. Extremely smooth sheets are required to effectively shield the chamber first walls without clipping the driver beams. Surface ripple and its growth are therefore a major concern in liquid protection design.In this study, a non-intrusive technique for directly visualizing and measuring the instantaneous free-surface geometry has been developed. Mean free-surface geometry and surface geometry fluctuation results for turbulent water sheets issuing vertically downwards into atmospheric pressure air are presented at Reynolds numbers based upon the nozzle thickness of 34000, at distances up to 25 nozzle thicknesses (25δ) downstream of the nozzle exit.

Stability and contraction of a rectangular liquid metal jet in a vacuum environment

Hydrodynamic similarity criteria and experimental data are reported here from experiments underway at UCLA that simulate the slab jets of the HYLIFE-II inertial fusion reactor. The current experimental setup produces a 2×20 mm slab jet, and reaches a maximum Reynolds number 5×10 4, corresponding to a jet velocity of 12 m/s. A high-speed camera is used to obtain and analyze data. Two major phenomena are observed, the inversion (axis-switching) of the slab jet owing to surface tension and corner vortices; and surface waves due to turbulent velocity profile relaxation. The main purpose of this series of experiments is the study of the rapid inversion (almost 50% of the jet over the length of interest) seen in previous experiments. It is shown here that this was due to secondary flows caused by features of orifice nozzle design. Current experiments show an inversion length considerably in excess of analytic correlations based on 2-D inviscid theory. For parameters approaching HYLIFE-II jets, inversion length is shown to be more than several hundred non-dimensional lengths.

Experimental Investigation of Free Liquid Metal Jets in Vacuum: Preliminary Results for IFE Chamber Wall Protection Applications

Experiments are under way at UCLA to simulate the liquid slab jets of the HYLIFE-II Inertial Fusion Reactor. Measurements of surface ripple and break-up length are made based on photographic images of the jet flow, and velocity data is obtained from an orifice-type flow meter. The experiment can be run with a selection of nozzles and upstream conditioners in order to determine the optimum configuration for suppressing disturbances. Preliminary data, taken while verifying the operation of the experimental system, indicate that the slab jets issuing from a nozzle comprisedslot cut in an orifice plate this nozzle type contract out of their initial rectangular shape more rapidly than would be expected from surface tension forces alone. Subsequent data are expected to aid in the proof-of-principle for thick liquid cavity designs, provide insight into design requirements of such systems, and increase the fundamental understanding of turbulent liquid jet flow in vacuum.

The Stability of Radiatively Cooling Jets. I. Linear Analysis

The results of a spatial stability analysis of a two-dimensional slab jet, in which optically thin radiative cooling is dynamically important, are presented. We study both magnetized and unmagnetized jets at external Mach numbers of 5 and 20. We model the cooling rate by using two different cooling curves: one appropriate to interstellar gas, and the other to photoionized gas of reduced metallicity. Thus, our results will be applicable to both protostellar (Herbig-Haro) jets and optical jets from active galactic nuclei. We present analytical solutions to the dispersion relations in useful limits and solve the dispersion relations numerically over a broad range of perturbation frequencies. We find that the growth rates and wavelengths of the unstable Kelvin-Helmholtz (K-H) modes are significantly different from the adiabatic limit, and that the form of the cooling function strongly affects the results. In particular, if the cooling curve is a steep function of temperature in the neighborhood of the equilibrium state, then the growth of K-H modes is reduced relative to the adiabatic jet. On the other hand, if the cooling curve is a shallow function of temperature, then the growth of K-H modes can be enhanced relative to the adiabatic jet by the increase in cooling relative to heating in overdense regions. Inclusion of a dynamically important magnetic field does not strongly modify the important differences between an adiabatic jet and a cooling jet, provided the jet is highly supermagnetosonic and not magnetic pressure-dominated. In the latter case, the unstable modes behave more like the transmagnetosonic magnetic pressure-dominated adiabatic limit. We also plot fluid displacement surfaces associated with the various waves in a cooling jet in order to predict the structures that might arise in the nonlinear regime. This analysis predicts that low-frequency surface waves and the lowest order body modes will be the most effective at producing observable features in the jet.

Simulation of the Kelvin‐Helmholtz Instability in the Magnetized Slab Jet

Two-dimensional magnetohydrodynamic simulations are performed to study the evolution of the Kelvin-Helmholtz instability for a slab jet moving parallel to the magnetic field. Surface waves are excited at each of the side boundaries, and the time evolution of the individual mode, as well as its interaction with the mode excited at the opposite boundary, is observed. The instability is very disruptive when the excited perturbations on the two side boundaries are antisymmetric. The long-term evolution of the antisymmetric perturbation shows a kinklike magnetic field structure after multiple reconnection events, while the symmetric perturbation yields wavy signatures only around the jet boundaries. Three different cases of the thin slab jet are considered with the antisymmetric perturbation: a hydrodynamic jet, a jet embedded in a uniform magnetic field, and a thermally confined magnetized jet. Although the imposed magnetic fields are weak, they show quite different evolutions. In the hydrodynamic case, the density and flow patterns associated with the vortices survive until the end of the simulation without significant changes after the initial development of the instability. For the jet embedded in a uniform magnetic field, the density pattern associated with the vortex is destroyed as magnetic reconnection develops, and a steady increase in density from the central jet region toward the side boundaries is seen at the final stage when the magnetic field becomes flat again. The density and flow structures in the thermally confined magnetized jet are similar to those of the hydrodynamic case, but they change slowly with the evolution of the magnetic field. Also, it is seen that the magnetic fields that originally reside in the jet are expelled out to the side boundaries. Similar results are obtained for both the transonic jet and the supersonic jet, but the density variation is more significant in the supersonic case.

Numerical simulation of the individual mode evolution in a slab jet

Numerical simulation of the individual mode evolution in a slab jet

3-D Simulations of Kelvin-Helmholtz Instabilities in Supersonic Jets

One of the key processes governing the structure and evolution of astrophysical jets is their interaction with the surrounding medium. A jet can deposit momentum and energy in the ambient medium, and entrain external material. The main physical process responsible for mixing between a jet flow and the ambient medium is the Kelvin-Helmholtz (KH) instability. We have previously analysed the 2D evolution of the axisymmetric modes of a cylindrical jet (Bodo et al 1994) and of the antisymmetric modes of a planar slab jet (Bodo et al 1995). These last are thought to give indications of the 3D evolution of the helical modes of a cylinder, since the linear behavior is very similar. In this contribution we present some preliminary results of fully 3D simulations comparing them with the mentioned 2D results.

Destabilization of Strongly Magnetized Jets

view Abstract Citations (15) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Destabilization of Strongly Magnetized Jets Hardee, Philip E. ; Clarke, David A. Abstract Simulations of strongly magnetized slab jets in uniform expansion in an external atmosphere have been performed and compared to an unmagnetized expanding slab jet. A strongly magnetized super-Alfvénic jet with lower magnetosonic Mach number but with sonic Mach number equal to a purely fluid jet destabilizes in a much shorter distance than the fluid jet. A strongly magnetized sub-Alfvénic jet remains stable until the jet becomes super-Alfvénic. At the Alfvén point the jet is submagnetosonic and destabilizes abruptly. Results from a spatial stability analysis of the magnetized slab jet are compared to the numerical simulations. In general, predictions based on the theory are borne out by the simulations. In particular, the stability properties of a super-Alfvénic jet are confirmed to be a function of the magnetosonic Mach number, and the theoretical prediction of rapid jet destabilization at the Alfvén point transition between sub-Alfvénic and super-Alfvénic flow is confirmed. However, nonlinear effects associated with decreasing the jet density relative to the external density and with increasing the magnetic field strength can increase the destabilization-decollimation length and reduce decollimation, respectively. Some of the implications for three-dimensional jets are considered. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/176037 Bibcode: 1995ApJ...449..119H Keywords: GALAXIES: INDIVIDUAL MESSIER NUMBER: M87; GALAXIES: JETS; HYDRODYNAMICS; INSTABILITIES; MAGNETOHYDRODYNAMICS: MHD full text sources ADS | data products SIMBAD (3) NED (1)

On jet response to a driving frequency and the jets in 3C 449

Hydrodynamical simulations of the response of an expanding slab jet to different driving frequencies have been performed. Amplitude growth of the resulting sinusoidal jet oscillation occurs when the driving frequency is lower than the resonant frequency at which sinusoidal wave growth associated with the Kelvin-Helmholtz instability is a maximum. In general, conditions leading to jet expansion result in the driving frequency being above the resonant frequency at some point along an expanding jet. At larger distances where the driving frequency is above the resonant frequency, growth ceases. If by this point sufficient amplitude growth of sinusoidal oscillations has not occurred, the jet is not decollimated. A consequence for astrophysical jets is that observed large-scale jet oscillation can be at wavelengths longer than to slightly less than the wavelength corresponding to the jet's lowest resonant frequency. This places an upper limit on a precessional frequency, orbital frequency, or random perturbation frequency at the central engine to which an astrophysical jet can helically respond at large scales. This result is applied to the radio source 3C 449 whose morphology suggests that orbital motion of the central engine is responsible for jet bending and an upper limit to the orbital frequency is obtained. Additionally, the observed trajectory of the southern jet along with information about the galaxy's atmosphere is used to estimate the orbital period and jet velocity.

Disc accretion onto magnetized compact objects

In a slab jet model, the influence of a strong magnetic field and density contrasts on the development of instabilities caused by velocity contrasts is studied in application to disc accretion onto a magnetized compact object. The perturbations propagation transverse to the magnetic field in external regions are shown not to be stable. Strong density contrast at the slab boundary (R = pJpJ cannot stabilize the instability of the acoustic resonance type (ARTI), the fundamental symmetric and antisymmetric modes being still unstable for any finite R. At the same time, a critical value of R exists (R - l/M2, M is the Mach number) at which higher reflection harmonics are stable. We present a comparative analysis of the ARTI and the Kelvin-Helmholtz instability which develops via surface modes at the interfaces between the disc material and magnetic field (magnetosphere). The ARTI can be responsible for the penetration of the accreting material into the magnetosphere.

Spatial stability of the magnetized slab jet

A spatial stability analysis of the Kelvin-Helmholtz instability of a magnetized slab jet is performed and compared to results of numerical simulations. Provided the jet is super-Alfvenic the dispersion relation describing the propagation and growth of Fourier components of some initial perturbation admits the same type of solutions as for a purely fluid jet. A growing fundamental sinusoidal mode at long wavelengths can be identified with an Alfven wave-like disturbance of the jet. Growing internal reflection modes at shorter wavelengths can be identified with fast magnetosonic waves reflecting off the jet's boundaries. Supermagnetosonic resonances comparable to the supersonic resonances of a strictly fluid jet exist where resonant wavelengths and growth lengths scale with the magnetosonic Mach number. The resonances disappear when the jet becomes transmagnetosonic but still super-Alfvenic and only the fundamental sinusoidal mode remains. The fundamental mode is stabilized when the jet is sub-Alfvenic, but for jet velocities slightly less than the slow magnetosonic speed reflection modes can grow even on a sub-Alfvenic jet. This behavior is qualitatively similar to the stability properties of a magnetized cylindrical jet. While the simulations reveal significant non-linear effects associated with increasing magnetic tension, the jet is not stabilized by nonlinear effects, and the linear analysis provides a reasonable description of the spatial stability properties of the jet. The results suggest that a jet which is initially sub-Alfvenic and stable to disruption will be doomed to disruption at the Alfven point if it becomes super-Alfvenic as a result of jet expansion.

Instabilities in astrophysical jets. II - Numerical simulations of slab jets

view Abstract Citations (26) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Instabilities in Astrophysical Jets. II. Numerical Simulations of Slab Jets Zhao, Jun-Hui ; Burns, Jack O. ; Norman, Michael L. ; Sulkanen, Martin E. Abstract In this paper, we describe numerical simulations of an unstable supersonic slab-symmetric jet. The instabilities within the jet are characterized by growing internal body waves and their coupled surface waves that are also predicted in linear perturbation theory. The characteristic theory of fluid dynamics is used to help interpret the wave morphologies. We demonstrate that these waves can be excited by imposing an arbitrary disturbance. From our numerical simulations, we find that the sound waves propagating against the flow slow down as they propagate outward, and they grow in amplitude. These waves eventually disrupt the jet at a certain length. This disruption length is related to the jet Mach number and the perturbation intensity. Thus, the Mach number of a jet observed with a radio telescope can be estimated by measuring the disruption length and estimating the perturbation intensity. The jet Mach numbers in radio tailed sources determined in this way agree quite well with estimates from ram pressure bending arguments. The wiggles and flares observed in many extragalactic jets, especially in tailed radio sources, appear to be intimately related to instabilities and the jet disruption process. Publication: The Astrophysical Journal Pub Date: March 1992 DOI: 10.1086/171062 Bibcode: 1992ApJ...387...83Z Keywords: Computational Astrophysics; Galactic Clusters; Shock Wave Interaction; Supersonic Jet Flow; Mach Number; Perturbation Theory; Radio Telescopes; Shock Wave Propagation; Surface Waves; Very Large Array (Vla); Astrophysics; GALAXIES: JETS; HYDRODYNAMICS; INSTABILITIES; METHODS: NUMERICAL; SHOCK WAVES full text sources ADS | data products SIMBAD (3) NED (1) Related Materials (1) Part 1: 1992ApJ...387...69Z

Instabilities in astrophysical jets. I - Linear analysis of body and surface waves

The instabilities of astrophysical jets are investigated in a new and more generalized way than in previous studies. The dispersion equations are solved for a thermally confined slab jet in a complex (k,ω) domain. A number of singularities related to certain wave modes in this system are found. It is shown that the internal jet flow in the system is characterized by two internal sound waves, propagating against and with the flow, respectively. These two body waves grow in amplitude. The one propagating against the flow is condensed, which is crucial to disruption of jets

Asymmetric morphology of the propagating jet. II - The effect of atmospheric gradients

Simulations of slab jets propagating in atmospheres containing pressure gradients, density gradients, and temperature gradients or temperature jumps have been performed for a range of jet velocities. The use of slab geometry allows study of asymmetric jet and lobe morphologies. Symmetry is broken by perturbing the jet at the inlet. The subsequent growth of the initial perturbation is the result of amplification by the Kelvin-Helmholtz instability.