The hydrodynamics of astrophysical jets: scaled experiments and numerical simulations

Abstract

Context. In this paper we study the propagation of hypersonic hydrodynamic jets (Mach number >5) in a laboratory vessel and make comparisons with numerical simulations of axially symmetric flows with the same initial and boundary conditions. The astrophysical context is that of the jets originating around young stellar objects (YSOs). Aims. In order to gain a deeper insight into the phenomenology of YSO jets, we performed a set of experiments and numerical simulations of hypersonic jets in the range of Mach numbers from 10 to 20 and for jet-to-ambient density ratios from 0.85 to 5.4, using different gas species and observing jet lengths of the order of 150 initial radii or more. Exploiting the scalability of the hydrodynamic equations, we intend to reproduce the YSO jet behaviour with respect to jet velocity and elapsed times. In addition, we can make comparisons between the simulated, the experimental, and the observed morphologies. Methods. In the experiments the gas pressure and temperature are increased by a fast, quasi-isentropic compression by means of a piston system operating on a time scale of tens of milliseconds, while the gas density is visualized and measured by means of an electron beam system. We used the PLUTO software for the numerical solution of mixed hyperbolic/parabolic conservation laws targeting high Mach number flows in astrophysical fluid dynamics. We considered axisymmetric initial conditions and carried out numerical simulations in cylindrical geometry. The code has a modular flexible structure whereby different numerical algorithms can be separately combined to solve systems of conservation laws using the finite volume or finite difference approach based on Godunovtype schemes. Results. The agreement between experiments and numerical simulations is fairly good in most of the comparisons. The resulting scaled flow velocities and elapsed times are close to the ones shown by observations. The morphologies of the density distributions agree with the observed ones as well. Conclusions. The laboratory and the simulated hypersonic jets are all pressure matched, i.e. their axial regions are almost isentropic at the nozzle exit. They maintain their collimation for long distances in terms of the initial jet radii, without including magnetic confinement effects. This yields a qualitatively good agreement with the observed YSO jet morphologies. It remains to be seen what happens when non-axially symmetric perturbations of the flow are imposed at the nozzle, both in the experiment and in the simulation.

Astrophysical fluid simulations of thermally ideal gases with non-constant adiabatic index: numerical implementation

“…As the jet enters into the ambient medium, it immediately forms a bow-shock that pushes the ambient material to its sides. The overall morphology of the jet is determined primarily by the Mach number, the density contrast (a result that has also been confirmed in laboratory experiments (Belan et al 2013(Belan et al , 2014) and the magnetic field strength. The processed material gets heated and forms the cocoon as shown in Fig.…”

Hypersonic jets in astrophysical conditions: focus on spreading and asymmetric stability properties

“…The laboratory experiment makes use of facilities designed and built specifically for studying free hypersonic jets. The main features of these facilities, described in detail in the works by Belan et al (2008Belan et al ( , 2010Belan et al ( , 2011Belan et al ( , 2013, will be exposed in this section. The jets studied in the present work are generated by means of de Laval nozzles; they travel along the longitudinal axis of a cylindrical vacuum vessel, as shown in figure 1.…”

“…With the goal to address the propagation of YSO jets and their interaction with the ambient medium, Tordella et al (2011) (paper I) and Belan et al (2013) (paper II) studied in the laboratory and by numerical means hypersonic hydrodynamic (HD) flows. In paper I the authors considered as main jet parameters the Mach number M and the jet-to-ambient density ratio η and studied, both experimentally and numerically, representative cases of high and small values of density ratio.…”

Characterization of Supersonic Gas Jets for High-Resolution Laser Ionization Spectroscopy of Heavy Elements

“…Supersonic free jets, formed by the gas emitted from the exit section of a convergent nozzle to the ambient medium at a lower pressure, have been extensively studied over the past few decades [31,33,34,42,43,[58][59][60][61][62][63] with the research in this area stimulated by both the fundamental fluid mechanics and practical applications of such gas jets (e.g., rocket exhaust plumes, vehicle control jets, and jets emanating from young stars). The main details of a supersonic free jet structure are shown in Fig.…”

“…Besides that, a meticulous elimination of the light scattered from particles in the gas or nearby surfaces is required, since this light has the same wavelength as the Rayleigh signal. Electron beam-excited fluorescence enables the visualization and the extraction of temperature, velocity, and density flow fields, as demonstrated, e.g., for supersonic free jets and jets formed by de Laval nozzles using helium, neon, argon, and xenon [33,34]. Although electron beam-excited fluorescence is suited for the characterization of lowdensity flows [35], it cannot be applied for the extraction of the spectral broadening of the seeded atoms, since it characterizes the properties of the carrier gas.…”