Slab Jets Research Articles

Shear-layer instabilities in accretion discs around magnetized compact objects

In a slab jet model the influence of strong magnetic fields and density contrasts on the development of instabilities caused by velocity contrasts is studied and applied to disc accretion onto magnetized compact object.

Atmospheric gradients and the stability of expanding jets

Numerical simulations of adiabatically expanding slab jets in initial static pressure balance with an external atmosphere have been performed and compared to predictions made by a linear analysis of the stability of expanding jets. It is found that jets are stabilized by jet expansion as predicted by the linear analysis. It is also found that an expanding jet can be destabilized by a positive temperature gradient or temperature jump in the surrounding medium which lowers the Mach number defined by the external sound speed. A temperature gradient or jump is more destabilizing than would be predicted by a linear stability analysis. The enhanced instability compared to an isothermal atmosphere with identical pressure gradient is a result of the reduced external Mach number and a result of a higher jet density relative to the density in the external medium and higher ram speed. Other differences between predictions made by the linear theory and the simulations can be understood qualitatively as a result of a change in wave speed as the wave amplitude increases. 12 refs.

Numerical simulations of the nonlinear kink modes in linearly stable supersonic slip surfaces

We have performed high-resolution numerical simulations of supersonic slip surfaces to confirm and illuminate earlier analytic nonlinear stability calculations of such structures. This analytic work was in turn inspired by earlier computer simulations reported in Woodward (1985) and Woodward et al. (1987). In particular Artola & Majda (1987) examined the response of a supersonic slip surface to an incident train of small-amplitude nonlinear sound waves. They found analytic solutions which indicate that nonlinear resonance occurs at three angles of incidence which depend upon the Mach number of the relative motion. The two-dimensional simulations described here numerically solve this problem for a Mach-4 flow using the piecewise-parabolic method (Colella & Woodward 1984; Woodward & Colella 1984). The simulations show that sound waves incident at a predicted resonance angle excite nonlinear behaviour in the slip surface. At these angles the amplitude of the reflected waves is much greater than the incident wave amplitude (i.e. a shock forms). The observed resonance is fairly broad, but the resonance narrows as the strength of the incident waves is reduced.The nature of the nonlinear kink modes observed in the simulations is similar to that discussed by Artola & Majda. Most of the modes move in either direction with speeds near the predicted value. Speeds of other than this value are observed, but the disagreement is not serious in view of the strongly nonlinear behaviour seen in the simulations but not treated in the analytic work. The stationary modes seen in the analytic results are perhaps observed as transient structures. They may eventually dominate the flow at late times (Woodward et al. 1987).The role of the kink modes in the stability of slab jets is discussed, and it is argued that the stationary modes are more disruptive than the propagating modes.

Asymmetric morphology of the propagating jet

Simulations of slab jets propagating in constant atmospheres are reported for a range of jet velocities and Mach numbers. At early times, the jet maintains approximate axisymmetry within a backflowing cocoon. When the jet has penetrated farther into the external medium, the symmetry is broken by sideways oscillation and the leading edge of the jet moves about within a growing lobe. The oscillation results from nonlinear resonant amplification of the initial perturbation by the Kelvin-Helmholtz instability. Finally, the jet flaps chaotically within the growing lobe. The flapping is driven by turbulent vortices in the lobe. The basic picture of Scheuer's (1982) dentist's drill model of the physical processes underlying asymmetric morphologies in radio galaxies is confirmed. The fluid motions in the lobe are found to govern the location of the drill bit. The morphology is time-dependent on relatively short time scales. 32 refs.

Instability of Astrophysical Jets. II: Numerical Simulation of Two-Dimensional Choked Underexpanded Slab Jets

Lateral oscillations of choked underexpanded rectangular jets with a large aspect ratio are observed experimentally. We compare the experiments with the results of our numerical simulations of two-dimensional slab jets. A time-dependent technique of solution is applied to solve the Euler equations for a compressible ideal gas. The numerical results show that the jet does not oscillate laterally when the symmetric boundary condition is applied. On the other hand, the self-sustained asymmetric oscillation of the jet occurs under the asymmetric boundary condition. When an asymmetric jet oscillation appears in the computed domain, it continues even after the asymmetric boundary condition was replaced by the symmetrical ones. The oscillation of the supersonic jet is self-sustained. This is probably because of the feedback loop consisting of the sound waves propagating towards the nozzle exit and the coherent structures convected with jets.

Instability of Astrophysical Jets. II

Lateral oscillations of choked underexpanded rectangular jets with a large aspect ratio are observed experimentally. We compare the experiments with the results of our numerical simulations of two-dimensional slab jets. A time-dependent technique of solution is applied to solve the Euler equations for a compressible ideal gas. The numerical results show that the jet does not oscillate laterally when the symmetric boundary condition is applied. On the other hand, the self-sustained asymmetric oscillation of the jet occurs under the asymmetric boundary condition. When an asymmetric jet oscillation appears in the computed domain, it continues even after the asymmetric boundary condition was replaced by the symmetrical ones. The oscillation of the supersonic jet is self-sustained. This is probably because of the feedback loop consisting of the sound waves propagating towards the nozzle exit and the coherent structures convected with jets.

The role of oblique reflection shocks in astrophysical jets

view Abstract Citations (34) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Role of Oblique Reflection Shocks in Astrophysical Jets Hardee, Philip E. ; Norman, Michael L. Abstract Simulations of hydrodynamic slab jets show that the reflection modes predicted by the linear theory are not disruptive but saturate at finite amplitudes through the formation of weak oblique shocks. The properties of these shocks are investigated. Oblique shocks arising from the second reflection mode are weaker, propagate faster and propagate with different obliquity than oblique shocks arising from the first reflection mode. The point of intersection between first and second reflection mode shocks moves outward along the jet faster than the jet speed by a scissors effect. This simulation presents the first evidence for phase effects moving at greater than the jet's flow speed. That cylindrical astrophysical jets should contain similar features is suggested by the linear analysis and by numerical simulation of axisymmetric cylindrical jets. Implications for astrophysical jets and for apparent superluminal motions resulting from phase effects are discussed and evidence for these features in astrophysical jets is presented. Publication: The Astrophysical Journal Pub Date: July 1989 DOI: 10.1086/167629 Bibcode: 1989ApJ...342..680H Keywords: Plasma Jets; Radio Jets (Astronomy); Shock Wave Interaction; Computerized Simulation; Light Speed; Astrophysics; GALAXIES: JETS; HYDRODYNAMICS; SHOCK WAVES full text sources ADS | data products NED (3) SIMBAD (2)

Spatial stability of the slab jet. I - Linearized stability analysis. II - Numerical simulations

view Abstract Citations (96) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spatial Stability of the Slab Jet. I. Linearized Stability Analysis Hardee, Philip E. ; Norman, Michael L. Abstract A spatial stability analysis of the supersonic slab jet is performed, and expressions for the resonant frequencies, wavelengths, and wave speeds are determined. The results are compared to a spatial stability analysis of the supersonic cylindrical jet. Symmetric (pinch) and antisymmetric (sinusoidal) modes on the slab jet correspond to pinch and helical modes on the cylindrical jet. Higher order fluting modes on the cylindrical jet do not have a counterpart on the slab jet. On the slab jet growing harmonic components can propagate at any angle from 0^deg^ to +/- 90^deg^ with respect to the flow. On the cylindrical jet the propagation angle is uniquely determined by the mode number and the wavelength. The differences between slab and cylindrical geometry are most pronounced for long wavelengths associated with the sinusoidal and helical modes. However, for equal angles of propagation, solutions of the dispersion relation for the slab jet are nearly identical to the results for a cylindrical jet. The spatial stability of a supersonic slab jet provides a reasonable analog for the spatial stability of a supersonic cylindrical jet if the difference in propagation angle is taken into account. Publication: The Astrophysical Journal Pub Date: November 1988 DOI: 10.1086/166818 Bibcode: 1988ApJ...334...70H Keywords: Astrophysics; Magnetohydrodynamic Stability; Plasma Jets; Spatial Distribution; Astronomical Models; Linear Equations; Plasma Slabs; Reflected Waves; Supersonic Jet Flow; Astrophysics; GALAXIES: JETS; HYDROMAGNETICS; INSTABILITIES; WAVE MOTIONS full text sources ADS | Related Materials (1) Part 2: 1988ApJ...334...80N

Spatial Stability of the Slab Jet. II. Numerical Simulations

view Abstract Citations (72) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spatial Stability of the Slab Jet. II. Numerical Simulations Norman, Michael L. ; Hardee, Philip E. Abstract Results of numerical simulations of a supersonic slab jet wiggled at different frequencies are presented and compared with the linearized spatial stability analysis performed in Paper I. Slabs with different densities and Mach numbers have been simulated. In general, results from the numerical simulations compare favorably with predictions of the linearized analysis. In all simulations the slab jet is disrupted by sinusoidal oscillation at or near the resonant wavelength of the fundamental sinusoidal mode, independent of the driving frequency. The jet then propagates behind a jet front and forms a lobe, and the jet flaps within the growing lobe. The sinusoidal oscillation approaches a wavelength corresponding to the driving frequency except when the driving frequency is much less than or significantly exceeds the resonant frequency. In addition to sinusoidal oscillation, the simulations also show weak oblique shock waves which cross the jet channel at about the Mach angle. These oblique shock waves can be identified with the first and second sinusoidal reflection modes predicted by the linearized analysis. Publication: The Astrophysical Journal Pub Date: November 1988 DOI: 10.1086/166819 Bibcode: 1988ApJ...334...80N Keywords: GALAXIES: JETS; HYDROMAGNETICS full text sources ADS | data products SIMBAD (6) NED (6) Related Materials (1) Part 1: 1988ApJ...334...70H

Supersonic interface instabilities of accelerated surfaces and jets

The compressible Rayleigh–Taylor instability of a supersonic accelerated contact discontinuity between two gases is studied by numerically solving the two-dimensional Euler equations. The computed solutions exhibit a complicated set of nonlinear waves comprised of spike and bubble bow shocks, terminal shocks within the spike and bubble, Kelvin–Helmholtz rollup of the spike tip, and contact surface waves. The spike appears to attain a finite growth of aspect ratio approximately equal to 2. The propagation of a supersonic slab jet is also studied numerically, in order to compare and contrast the jet wave structure with that of the supersonic accelerated surface.