摘要:

The reduction of test time in low pressure shock tubes, due to a laminar wall boundary layer, has been analytically investigated. In previous studies by Roshko and Hooker the flow was considered in a contact surface fixed coordinate system. In the present study it was assumed that the shock moves with uniform velocity, and the flow was investigated in a shock fixed coordinate system. Unlike the previous studies, the variation of free stream conditions between the shock and contact surface was taken into account. It was found that &bgr;, a parameter defined by Roshko, is considerably larger than the estimates made by Roshko and Hooker except for very strong shocks. Since test time is proportional to &bgr;−2, previous estimates of test time are too large, particularly for weak shocks. The present estimates for &bgr; appear to agree with existing experimental data to within about 10 percent for shock Mach numbers greater than 5. In other respects, the basic theory is in general agreement with the previous results of Roshko.

ISSN:0031-9171    

DOI:10.1063/1.1706887

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The rate of molecular dissociation behind strong shock waves is calculated with the assumption that dissociation can occur preferentially from the higher vibrational levels. An exponential probability of dissociation from the various vibrational levels is employed using an anharmonic oscillator model. Results for the dissociation of oxygen in an argon diluent are presented. Vibrational non‐equilibrium introduces aT−3temperature dependence into the oxygen dissociation rate constant in the range 4000°–8000°K. A dissociation lag‐time of the order of the extrapolated vibrational relax ation time is predicted immediately behind the shock front. The computed results are shown to be in agreement with available experimental results.

ISSN:0031-9171    

DOI:10.1063/1.1706888

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The application of microwave interaction with gaseous plasmas to the study of electrically driven shock waves in nonionized gases and gaseous plasmas is discussed. The microwave measurements are supplemented by observation of the visible light emitted from the shocked gas and by voltage induced signals on small metallic probes placed in the path of the propagating shock waves. The investigated shock waves were in the Mach 4 to Mach 14 velocity range in neon and argon gases at pressures from 0.1 to 5 mm Hg. It is demonstrated, in agreement with other workers, that the background gas through which the shock waves subsequently passes is ionized to a nonnegligible degree at the instant of the electric discharge by photons of adequate energy from the discharge chamber. It is also shown that at least in the particular shock tube used in this investigation, this preionization is consistent with ionization by soft x rays from the electron bombarded anode in the discharge chamber. The effect of the free electron constituent of the ionized background gas on the properties of the shock wave as well as its direct effect on some commonly used shock wave investigation techniques is discussed.

ISSN:0031-9171    

DOI:10.1063/1.1706889

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The velocity across a jet becomes nonuniform due to contraction or expansion of fluid in the presence of electric current within the nozzle from which it issues. The results obtained elsewhere on the instability of a jet of uniform velocity due to electric current and surface tension are corrected. It is further shown that velocity nonuniformity reduces this instability. The available data on the instability of a mercury jet issuing from a contraction are for small current density and, hence, low velocity nonuniformity. However, for reasons yet unknown, the data do not agree with the (corrected) theory for the case of uniform velocity.

ISSN:0031-9171    

DOI:10.1063/1.1706890

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The nonuniform expansion of a rigid, perfectly conducting piston into an infinitely conducting fluid wherein there exists a weak uniform magnetic field is considered. A solution for the state of the plasma and the magnetic field between the piston and the magnetohydrodynamic shock is obtained by a small perturbation method; the dependent variables are expressed in double expansions about a zero order (hydrodynamic) solution. The solution is valid for pistons (either cylindrical or spherical) whose expansion velocity is expressible by a small perturbation on a constant velocity. The development includes second order terms; however, numerical results are restricted to first order. Within the first order perturbation, it is shown that the fluid dynamic analysis is uncoupled from the magnetic field, and the shock shape retains its cylindrical or spherical symmetry. Calculations are carried out for a spherical piston, for several values of &ggr; (specific heat ratio), anda∞/v0wherea∞is the ambient speed of sound andv0is the uniform piston velocity. The results show that the shock velocity is nearly insensitive to changes in the piston velocity.

ISSN:0031-9171    

DOI:10.1063/1.1706891

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

Two point correlation and spectral equations for this case are derived from the equations of fluid and electrodynamics. Solutions are obtained by assuming that the turbulent field is homogeneous and weak enough for triple correlations to be negligible. For initial conditions, it is postulated that the turbulence is initially isotropic, and that the turbulent magnetic field fluctuations are initially zero. The interaction of the mean magnetic field with the turbulent velocity field then causes magnetic field fluctuations to arise at later times. In general, the turbulent energy in the mechanical and magnetic modes tends toward equipartition for large values of time or of mean magnetic field. However, when the kinematic viscosity is much less than the electrical resistivity (or magnetic diffusivity), as for liquid metals, equipartition is not approached before the turbulence is damped out by the mean magnetic field.

ISSN:0031-9171    

DOI:10.1063/1.1706892

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

A strong uniform magnetic field is suddenly imposed on a mass of conducting incompressible fluid initially in isotropic turbulent motion. Initial time derivatives of the spectrum are calculated through third order. After identification of terms, the simplifying assumptions of large mechanical and small magnetic Reynolds numbers are made, and the initial three‐dimensional spectrum assumed to be a Dirac shell; these permit the calculation and plotting of the one‐dimensional spectra. Interpretation of these spectra leads to the conclusion that the immediate effect of the field is to attenuate modes having cross‐field velocities and those having cross‐field node lines.

ISSN:0031-9171    

DOI:10.1063/1.1706893

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The distribution of molecular velocities in a strong shock wave is usually assumed to be bimodal, on the basis of mean free path arguments. Indeed, this assumption has been confirmed by Sakurai for the case of rigid sphere molecules. It turns out that such a confirmation is no longer possible when the intermolecular forces have sufficiently long ranges; in particular, it is shown that, for the case of a fully ionized gas, the Boltzmann equation is not satisfied by a bimodal distribution, regardless of the choice for the shock thickness parameter and Mach number. This breakdown of the bimodal solutions is ascribed to an inadequacy in the usual mean free path concept for a plasma, due to the dominance of weak collisions at high temperatures.

ISSN:0031-9171    

DOI:10.1063/1.1706894

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The reaction forces on a small charged body which traverses a low density plasma have been studied. The contributions to these forces are discussed with respect to individual particle and collective plasma action, with the relationships between these two contributions demonstrated. The reaction forces are obtained for a subsonic accelerating body, and, in various limiting cases, for a supersonic body undergoing a small degree of acceleration.

ISSN:0031-9171    

DOI:10.1063/1.1706895

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The equilibrium chemical composition and the thermodynamic properties of argon plasmas have been calculated for five pressures (0.1, 0.5, 1.0, 2.0, and 5.0 atm) at 100 K deg increments for the temperature range 5000° to 35 000°K. The argon plasma is assumed to be a perfect gas complex consisting of six components, namely electrons, argon atoms, and the first four argon ions. The partition functions for these have been calculated using tabulated data for observed atomic energy levels and estimated energies for energy levels which are predicted although not observed. The partition functions were terminated by application of the Debye cutoff criterion and a corresponding lowering of the ionization potential was included. The calculated data are presented in graphical form and the method followed in calculating the partition functions is evaluated.

ISSN:0031-9171    

DOI:10.1063/1.1706896

出版商:AIP    

年代:1963

数据来源: AIP