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Shock wave

About: Shock wave is a research topic. Over the lifetime, 36184 publications have been published within this topic receiving 635848 citations. The topic is also known as: Shock waves & shockwave.


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TL;DR: In this article, an analytical prediction for the relation between the density variance and the rms Mach number in supersonic, isothermal, magnetized turbulent flows in molecular clouds is presented.
Abstract: It is widely accepted that supersonic, magnetized turbulence plays a fundamental role for star formation in molecular clouds. It produces the initial dense gas seeds out of which new stars can form. However, the exact relation between gas compression, turbulent Mach number and magnetic field strength is still poorly understood. Here, we introduce and test an analytical prediction for the relation between the density variance and the rms Mach number in supersonic, isothermal, magnetized turbulent flows. We approximate the density and velocity structure of the interstellar medium as a superposition of shock waves. We obtain the density contrast considering the momentum equation for a single magnetized shock and extrapolate this result to the entire cloud. Depending on the field geometry, we then make three different assumptions based on observational and theoretical constraints: B independent of ρ, B∝ρ1/2 and B∝ρ. We test the analytically derived density variance–Mach number relation with numerical simulations, and find that for B∝ρ1/2, the variance in the logarithmic density contrast, , fits very well to simulated data with turbulent forcing parameter b= 0.4, when the gas is super-Alfvenic. However, this result breaks down when the turbulence becomes trans-Alfvenic or sub-Alfvenic, because in this regime the turbulence becomes highly anisotropic. Our density variance–Mach number relations simplify to the purely hydrodynamic relation as the ratio of thermal to magnetic pressure β0∞.

222 citations

Journal ArticleDOI
TL;DR: In this article, the authors study the long term evolution of magnetic fields generated by an initially unmagnetized collisionless relativistic e+e− shock and show that magnetic fields start with magnetic energy density ~ 0.1-0.2, but rapid downstream decay drives the fields to much smaller values, below ~10−3 of equipartition after ~103 skin depths.
Abstract: We study the long term evolution of magnetic fields generated by an initially unmagnetized collisionless relativistic e+e− shock. Our two-dimensional particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle distributions are approximately isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially. The nonpropagating magnetic fields decay in amplitude and do not merge. The fields start with magnetic energy density ~ 0.1-0.2 of equipartition, but rapid downstream decay drives the fields to much smaller values, below ~10−3 of equipartition after ~103 skin depths. To construct a theory to follow field decay to these smaller values, we hypothesize that the observed damping is a variant of Landau damping. The model is based on the small value of the downstream magnetic energy density, which only weakly perturbs particle orbits, for homogeneous turbulence. Using linear kinetic theory, we find a simple analytic form for the damping rates for small-amplitude, subluminous electromagnetic fields. Our theory predicts that overall magnetic energy decays as (ωpt)−q with q ~ 1, which compares with simulations. However, our theory predicts overly rapid damping of short-wavelength modes. Magnetic trapping of particles within the highly spatially intermittent downstream magnetic structures may be the origin of this discrepancy and may allow for some of this initial magnetic energy to persist. Absent additional physical processes that create longer wavelength, more persistent fields, we conclude that initially unmagnetized relativistic shocks in electron-positron plasmas are unable to form persistent downstream magnetic fields. These results put interesting constraints on synchrotron models for the prompt and afterglow emission from GRBs. We also comment on the relevance of these results for relativistic electron-ion shocks.

221 citations

Journal ArticleDOI
TL;DR: In this article, a two-dimensional self-consistent numerical model of the discharge and gas dynamics in conditions similar to those of these experiments has been developed, which couples fluid discharge equations with compressible Navier?Stokes equations including momentum and thermal transfer from the plasma to the neutral gas.
Abstract: Surface dielectric barrier discharges (SDBDs) can modify the boundary layer of a flow and are studied as a possible means to control the flow over an airfoil. In SDBDs driven by sinusoidal voltages in the 1?10?kHz range, momentum is transferred from ions to the neutral gas, as in a corona discharge (ion wind), and the resulting electrohydrodynamic force can generate a flow of several m?s?1 in the boundary layer along the surface. In this paper we are interested in a different regime of SDBDs where nanosecond voltage pulses are applied between the electrodes. Recent experiments by the group of Starikovskii have demonstrated that such discharges are able to modify a flow although no significant ion wind can be detected.A two-dimensional self-consistent numerical model of the discharge and gas dynamics in conditions similar to those of these experiments has been developed. The model couples fluid discharge equations with compressible Navier?Stokes equations including momentum and thermal transfer from the plasma to the neutral gas. This is a difficult multi-scale problem and special care has been taken to accurately solve the equations over a large simulation domain and at a relatively low computational cost. The results show that under the conditions of the simulated experiments, fast gas heating takes place in the boundary layer, leading to the generation of a 'micro' shock wave, in agreement with the experiments.

221 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigate the interplay between different kinds of non-radial hydrodynamic instabilities that can play a role during the postbounce accretion phase of collapsing stellar cores.
Abstract: Performing two-dimensional hydrodynamic simulations including a detailed treatment of the equation of state of the stellar plasma and for the neutrino transport and interactions, we investigate here the interplay between different kinds of non-radial hydrodynamic instabilities that can play a role during the postbounce accretion phase of collapsing stellar cores. The convective mode of instability, which is driven by the negative entropy gradients caused by neutrino heating or by variations in the shock strength in transient phases of shock expansion and contraction, can be identified clearly by the development of typical Rayleigh-Taylor mushrooms. However, in those cases where the gas in the postshock region is rapidly advected towards the gain radius, the growth of such a buoyancy instability can be suppressed. In this situation the shock and postshock flow can nevertheless develop non-radial asymmetry with an oscillatory growth in the amplitude. This phenomenon has been termed “standing (or spherical) accretion shock instability” (SASI). It is shown here that the SASI oscillations can trigger convective instability, and like the latter, they lead to an increase in the average shock radius and in the mass of the gain layer. Both hydrodynamic instabilities in combination stretch the advection time of matter accreted through the neutrino-heating layer and thus enhance the neutrino energy deposition in support of the neutrino-driven explosion mechanism. A rapidly contracting and more compact nascent neutron star turns out to be favorable for explosions, because the accretion luminosity and neutrino heating are greater and the growth rate of the SASI is higher. Moreover, we show that the oscillation period of the SASI observed in our simulations agrees with the one estimated for the advective-acoustic cycle (AAC), in which perturbations are carried by the accretion flow from the shock to the neutron star and pressure waves close an amplifying global feedback loop. A variety of other features in our models, as well as differences in their behavior, can also be understood on the basis of the AAC hypothesis. The interpretation of the SASI in our simulations as a purely acoustic phenomenon, however, appears difficult.

221 citations

Journal ArticleDOI
TL;DR: In this paper, the formation of molecular gas behind shocks in atomic gas using a one-dimensional chemical/dynamical model was examined. But the most important result is to stress the importance of shielding the moleculargas from the destructive effects of UVradiation.
Abstract: Motivated by our previous paper, in which we argued for the formation of molecular clouds from large-scale flows in the diffuse Galactic interstellar medium, we examine the formation of molecular gas behind shocks in atomic gas using a one-dimensional chemical/dynamical model. In our analysis we place particular emphasis on constraints placed on the dynamical evolution by the chemistry. The most important result of this study is to stress the importance of shielding the moleculargas from the destructive effects of UVradiation. For shock ram pressures comparable to or exceeding typical local interstellar medium pressures, self-shielding controls the formation time of molecular hydrogen, but CO formation requires shielding of the interstellar radiation field by dust grains. We find that for typical parameters the molecular hydrogen fractional abundance can become significant well before CO forms. The timescale for (CO) molecular cloud formation is not set by the H2 formation rate on grains, but rather by the timescale for accumulating a sufficient column density or extinction, AV k0:7. The local ratio of atomic to molecular gas (4:1), coupled with short estimates for the lifetimes of molecular clouds (3‐5 Myr), suggests that the timescales for accumulating molecular clouds from atomic material typically must be no longer than about 12‐20 Myr. Based on the shielding requirement, this implies that the typical product of preshock density and velocity must benvk20 cm � 3 km s � 1 . In turn, depending on the shock velocity, this implies shock ram pressures that are a few times the typical estimated local turbulent gas pressure and comparable to the total pressures(gasplusmagneticpluscosmicrays).CoupledwiththerapidformationofCOonceshieldingissufficient, flow-driven formation of molecular clouds in the local interstellar medium can occur sufficiently rapidly to account for observations. We also provide detailed predictions of atomic and molecular emission and absorption that track the formation of a molecular cloud from a purely atomic medium, with a view toward helping to verify cloud formation by shock waves. However, our predictions suggest that the detection of the pre-CO stages will be challenging. Finally, we provide an analytic solution for time-dependent H2 formation that may be of use in numerical hydrodynamic calculations. Subject headingg ISM: clouds — ISM: evolution — ISM: kinematics and dynamics — ISM: molecules — shock waves — stars: formation

221 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023754
20221,519
2021986
2020989
20191,091
20181,064