Showing papers on "Rarefaction published in 1976"

The hydrodynamics of Type II supernove

Abstract: Observations of Type II supernovae indicate the presence of a moderately cool expanding photosphere. This situation can result from an explosion in a star with an extended envelope. The evolutionary phases of an explosion are described. Information on the propagation of the shock wave through the star can be obtained from ..gamma..=4/3 blast wave solutions. If the photon mean free path becomes large compared to the length scales of the flow, a thermal wave moves out from the shock wave and a dense shell is formed behind the shock. The arrival of the shock wave at the photosphere is accompanied by ultraviolet and X-ray burst. As the star expands, a rarefaction wave converts internal energy into kinetic energy. Detailed hydrodynamic models have been calculated, assuming an initial radius compatible with stellar evolution and an energy compatible with the observed velocities. The observed values of photospheric radius and temperature near maximum light are reproduced. Features of the models which are consistent with observation are: the ejection of a detached shell; the cooling of the photosphere from 10,000 K to 6000 K in tens of days after maximum visual light; the shape of the light curve around maximum; the decrease in themore » velocity of the gas at the photosphere in tens of days after maximum; and a photospheric radius of about 10/sup 14/ cm after several hundred days. (AIP)« less

Condensation of water vapor in rarefaction waves. I - Homogeneous nucleation

Abstract: A detailed theoretical investigation has been made of the condensation of water vapor/carrier gas mixtures in the nonstationary rarefaction wave generated in a shock tube. It is assumed that condensation takes place by homogeneous nucleation. The equations of motion together with the nucleation rate and the droplet growth equations were solved numerically by the method of characteristics and Lax's method of implicit artificial viscosity. It is found that, for the case considered, the condensation wave formed by the collapse of the metastable nonequilibrium state is followed by a shock wave generated by the intersection of characteristics of the same family. The expansion is practically isentropic up to the onset of condensation. The condensation front accelerates in the x,t plane. The results of the computations for a chosen case of water vapor/nitrogen mixture are presented by plotting variations of pressure, nucleation rate, number density of critical clusters, and condensate mass-fraction along three particle paths. Some consideration is given to homogeneous condensation experiments conducted in a shock tube. Although a direct comparison of the present theoretical work and these experiments is not possible, several worthwhile interpretative features have resulted nevertheless.

Dynamics of the cavitation zone during an underwater explosion near a free surface

Abstract: The problem of the development of the cavitation zone and the rarefaction wave profile in the region of regular reflection of the spherical shock wave of an underwater explosion from a free surface is analyzed for the axisymmetric formulation within the framework of a model of the two-phase medium consisting of a liquid with cavitation nuclei of the free gas uniformly distributed in it. An example of the calculation of the rarefaction wave profile and the zone of visible cavitation at different times is given for the case of the explosion of 1-g charge at depths of 3 and 5.3 cm for an initial volumetric gas concentration of 10−11 and an initial cavitation nucleus radius of 5 · 10−5 cm. The results of the calculation are compared with experiment.

Why Syrovatskii's mechanism of dynamic dissipation of magnetic fields does not work.

Abstract: Syrovatskii's mechanism of ‘dynamic dissipation of magnetic field’ is reinvestigated. In order to have this kind of ‘dynamic dissipation’ at a neutral line the ratio of current density to particle density must exceed a certain critical value. For conditions in the solar atmosphere near sunspots, this value can only be reached by a mechanism which produces a very large compression of the magnetic field as well as an extreme rarefaction of the density. Syrovatskii claims that his mechanism provides both these features. His enormous field compression, however, can only be obtained if one neglects the restoring Lorentz force (e.g. in Syrovatskii's model the compressed field near the neutral line is about one order of magnitude larger than the field of the sunspots which generates it). The second effect, i.e. the large plasma rarefaction around the neutral line, also is not real. This rarefaction is due to the particular flow field of Syrovatskii's model which allows for a free reconnection of the field lines across the neutral line; the magnetic field is treated like a vacuum field, the effects of the field accumulation near the neutral line being neglected. The aim of the present paper is to show how more realistic models modify Syrovatskii's results. Our numerical calculations lead to a maximum current to density ratio which is a factor of 106 smaller than the one obtained by Syrovatskii. Therefore one has to conclude that in the solar atmosphere one cannot produce in the way described by Syrovatskii the configurations which are necessary for ‘dynamic dissipation’.

Perpendicular explosive drive and oblique shocks

Abstract: Oblique shocks in various materials driven by Composition B-3, 9404, and TNT with the detonation wave perpendicular to the interface are investigated with flash radiographic techniques. The detonation products in the rarefaction behind the detonation front expand laterally as the explosive-sample interface bends under shock compression of the sample. With the products described by a polytropic gas equation of state, this expansion is shown to be adequately described in the vicinity of the detonation front by Prandtl-Meyer flow. Some new Hugoniot data for antimony are obtained in the course of the investigation. In some instances of perpendicular drive the compression of the sample is not accomplished strictly by strong shocks. This circumstance is exemplified by baratol driving aluminum, a case where the bulk sound speed exceeds the detonation velocity, and by 9404 driving beryllium, a case where it does not. Some experimental results are presented for both these systems.

Nonlinear skin effect of high‐power microwaves and a density‐rarefaction wave in a collisionless magnetoplasma

Abstract: Experimental results are presented which verify the nonlinear skin effect of high‐power microwaves incident on a collisionless, uniform plasma having a sharp boundary. The skin depth, observed by microwave reflection techniques, increases with the incident microwave power. The results are compared with the theoretical analysis in which the radiation pressure due to the electromagnetic field is taken into account. It is also found that a density‐rarefaction wave caused by an incident microwave propagates through an over‐dense plasma at a velocity closer to that of the ion acoustic wave.

Numerical Analysis of Converging Shock Waves in Concentric Solid Layer

Abstract: A theoretical analysis is made for the behavior of converging shock waves in a concentric two layer system. Finite difference method is used to solve the hydrodynamic equations with numerically convenient constitutive equations proposed. The results show that differences in material properties have appreciable effects on the feature, and that in particular the system having an inner rod of higher shock impedance produces a negative pressure region near the central axis by a converging rarefaction wave.

Observations of magnetohydrodynamic rarefaction waves

Abstract: Transverse magnetohydrodynamic rarefaction waves in a fully ionized deuterium plasma have been studied and measurements of the electron density as a function of time are compared with theoretical predictions. The agreement is good.

Compression shock in the transonic flow around a convex corner

Abstract: The interaction between mixed sub-and supersonic flow near a convex breakpoint of a profile with a rectilinear wall downstream of this breakpoint is investigated. If we start from the fact that the initial flow has the character of a singularity in the domain ahead of the last characteristic of the rarefaction node [1], then the solution in the interaction domain, obtained in the hodograph plane under the assumption of its continuity in the physical plane, is not realizable because of the presence of limit lines. This governs the hypotheses of the formation of the compression shock emerging from the corner point and having zero intensity there.

Theory of the detonation of weak condensed explosives

Abstract: The possibilities of detonation taking place in a material characterized by a shock adiabatic containing a sharp break (leading to a double shock-wave configuration) are examined. The range of possible velocities D of a self-sustaining detonation in the second shock wave is determined; D may be subsonic with respect to the original material. However, even for an arbitrarily low velocity of sound the range of subsonic D values above the break point on the adiabat is extremely limited: The minimum detonation velocity Dmin coincides (apart from a factor of 0.5–0.8) with the velocity of a longitudinal sound wave in the original material below the break point. This limitation with regard to D is associated with the formation of a wave of rarefaction in the reaction products, For D < Dmin the shock wave of rarefaction reaches the Jouguet point and breaks the steady-state complex of the detonation wave. The results obtained are valid not only for weak, but also for powerful, explosive substances, if (by virtue of any kind of losses) low-velocity forms of detonation are realized in these materials.

Density and temperature ahead of a cylinder with a thermally insulated wall and a cooled wall in a rarefied hypersonic system

Abstract: Results are presented of a measurement of density and temperature on the stagnation line of a cylinder in cross flow at Mach number M∞=5, Knudsen number Kn∞=0.06−0.33, and with temperature factor varying from 1 to 0.11. The effect of degree of rarefaction and the temperature factor on the structure of the perturbed region ahead of the cylinder has been investigated.