摘要:

It has been well established in the literature that the ratio of the shock wave thickness to the mean free path, (taken at some suitable conditions, e.g. in front of the shock) is independent of the gas density. Our recent experimental results for shocks in perfluoro‐n‐hexane (C6F14) show that this is not always the case.The experiments were performed partly in a large diameter, low‐density shock tube with the use of the electron‐beam densitometer, and partly in a smaller, heated shock tube designed for higher densities. Here the light reflectivity method was used. For higher densities the measured shock thickness, expressed in mean free paths, increases with increasing initial density.The results could be described qualitatively by assuming a preferable orientation of the linearly built C6F14‐molecules. This alignment, which represents a form of rotational relaxation, can be understood in the following way: In the velocity gradient of the shock the molecules are aligned with their longitudinal axis parallel to the direction of the flow. This leads to a broadening of the shock, which depends on its velocity gradient (Mach number) and on the relaxation time of molecule alignment (collision frequency or density).

ISSN:0094-243X    

DOI:10.1063/1.39445

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The shock waves generated by a sonic nose jet exhausting counter to a supersonic free stream of a Mach number 3 were visualized by means of a laser induced fluorescence method. The experiments were conducted for various values of the ratio of opposing jet total pressure to free stream total pressure. The ratio of a jet exit diameter to a body diameter was taken to be 0.2 and 0.4. The results show that the shock waves generated by the opposing jet are significantly affected not only by the ratio of jet total pressure to free stream total pressure but also by the ratio of the jet exit diameter to the body diameter. It was also observed that at low pressure ratios, there exists unstable flow regime. Simple analysis is applied to the prediction of the position of a free stream shock and a Mach disc, and compared with the experimental result. The comparisons show good agreement.

ISSN:0094-243X    

DOI:10.1063/1.39446

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We report on a 1‐D free expansion experiment for a mixture of two disparate gases, in analogy with the mechanism of shock formation following the collapse of matter in a supernova. The gas pressure and density are measured simultaneously. In the free expansion of a pure gas the pressure and density increase in such a manner that the temperature rises monotonically. A preexisting density inhomogeneity, on the other hand, steepens, and develops into discontinuities upon interaction with the endwall, accompanied by an instability.

ISSN:0094-243X    

DOI:10.1063/1.39447

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

It’s shown that the second viscosity can become negative in the wide class of nonequilibrium media. Such media are acoustically unstable. The nonlinear waves evolution was investigated in the media with the negative second viscosity.

ISSN:0094-243X    

DOI:10.1063/1.39448

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Kinetic theory approach is proposed for the plane thermomechanical problem. Tamm’s variational method, developed to describe shock wave propagation in the infinite medium, is extended to unsteady problems with the boundaries. A novel feature of the proposed procedure is the use of the stochastic form of the Boltzmann equation with the trial function constructed from the two local half‐Maxwellian distribution functions. In the collision integral, micro and macro variables are separated with the help of the auxiliary coordinate systems to facilitate integrations.

ISSN:0094-243X    

DOI:10.1063/1.39450

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A primary study was made experimentally and numerically of spherical underwater shock wave focusing and high pressure generation in an ellipsoidal cavity. Spherical shock waves were produced by microexplosion of silver azide or PETN of 10 to 100 mg. The ellipsoidal cavity has major and minor axes of 700 and 500 mm, respectively. The focused pressure was measured along the major axis by PVDF pressure gauges. By exploding a 100 mg PETN pellet the peak pressure at 8 kbar was obtained near the second focal point. The TVD finite difference numerical simulation was also conducted. Good agreement was obtained between the experiment and simulation.

ISSN:0094-243X    

DOI:10.1063/1.39451

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Spherical shock waves generated by an underwater sparkgap or an explosive pellet in the first focus of different ellipsoidal segments are focused after reflection in the second focus. Shadowgraphs give an impression of the converging process whereas the field measurement is done by self‐designed piezoelectric pvdf‐needleprobes.Two different series of reflectors are tested. The first is similar to spherical mirrors, the second are deep reflectors which surround the generation point. For both variations the parameters: size of reflector, focal length, reflector material and incident shock strength, show their influence on the focusing process and the location and amplitude of the maximum pressure. In the case of the shallow reflectors, which utilize only 5% of the primary shock energy, focusing is close to the geometrical acoustic and reaches high pressure amplitudes up to 1200 bar, compared to the deep types where the higher utilized energies of 50–98% induce increased nonlinear effects which lead to wider focal areas and lower focus amplitudes of only 500 bar but stronger effects on materials placed in this zone.However for both reflector types the maximal amplitudes are proportional to the angle of aperture.

ISSN:0094-243X    

DOI:10.1063/1.39452

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Numerical simulations have been carried out of initially weak (shock Mach number Ms≊1.1–2.1) cylindrical shock waves propagating radially inward, simulating the experiments of Takayamaet al. (1987). Although the code employs a finite‐element technique with an unstructured (triangular) grid and neither the system geometry nor the initial conditions have any rectangular symmetry, contours of the numerical solutions (particularly the vorticity) exhibit a pronounced enhancement of them=4 component. Solution of the linearized form of the equations of geometrical shock dynamics (Whitham, 1974) reveals no preference form=4. Possible explanations for the phenomenon are suggested.

ISSN:0094-243X    

DOI:10.1063/1.39453

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The results of experimental investigations of converging two‐dimensional cylindrical shock waves give rise to the question whether the observed features of shock propagation and reflection can be simulated by model computations meeting the actual experimental conditions. This work gives a discussion of existing models and presents a description of shock propagation and reflection which starts from a ‘‘snapshot’’ of measured physical quantities of the wave at a specific time. The computation yields the future time history of the shock wave to be compared with experimental data at later time steps, including the process of reflection.The initial conditions are derived from holographic interferometry pictures of the incoming shock. We take into account the two‐dimensional shape of the shock and model the downstream physical quantities in a suitable way. Accordingly, a two‐dimensional grid representation of discontinous (peak) initial conditions is given. Methods for the determination of shock front position and Mach number from two‐dimensional numerical profiles are reported, too. We discuss numerical conditions for the shock reflection at the axis since the particular choice of the reflection condition might substantially influence the numerical results.

ISSN:0094-243X    

DOI:10.1063/1.39433

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Ignition and detonation resulting from a collision of a shock wave with a 90° re‐entrant corner were examined using a 9 cm×9 cm shock tube. Schlieren photographs revealed that the collision of shock wave with a re‐entrant corner created a complicated shock structure in the vicinity of the corner, causing local strengthening (or focussing) of the shock wave. Depending on the incident shock strength, the compound reflections resulted in ignition of the gas mixture or direct initiation of detonation. The critical shock strength for ignition and detonation initiation, and thus the degree of focussing, were found to be very sensitive to the orientation of the re‐entrant corner. To better understand the gasdynamic processes involved, the interaction between the shock wave with the re‐entrant corner was simulated using a 2‐dimensional computer code. The scale dependence of the phenomena was also examined by performing experiments with re‐entrant corners of different sizes and with gas mixtures of different initial pressures.

ISSN:0094-243X    

DOI:10.1063/1.39434

出版商:AIP    

年代:1990

数据来源: AIP