摘要:

Precursor waves have been studied in an electromagnetic shock tube of the T configuration. The precursor velocity, electron density, and current content were determined over a pressure range of 1 to 1000&mgr; Hg, for electrode discharge voltages of 3 to 17 kV in hydrogen and argon. A longitudinal nanosecond microwave technique was used in conjunction with a semi‐infinite microwave cavity measurement to measure the precursor velocity and attendant detailed electron density structure. The current associated with the precursor has been deduced from transverse magnetic field measurements made with an external pickup coil. The precursor exhibits a definite wave structure with front thicknesses of the order of 2 to 5 cm.

ISSN:0031-9171    

DOI:10.1063/1.1761961

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A coaxial electromagnetic shock tube has been used to study normal ionizing shocks propagating through hydrogen. An experiment is described which produced plane switch‐on ionizing shocks. Quantitative studies were conducted covering the sub‐Alfve´nic and trans‐Alfve´nic regimes. Data are presented that support the existence of switch‐on fronts in both these regimes, thereby demonstrating the significant effect of the initial electric field on the shock jump conditions. The results are interpreted by appeal to Taussig's theory of normal ionizing shocks. To choose from among the several theoretically predicted ionizing waves propagating at a given speed, Taussig's theory is supplemented by a simple physical criterion modeled on the Chapman‐Jouguet hypothesis of Gross and Kunkel. This criterion has strong intuitive support and results in a picture of ionizing shock propagation that is in general agreement with the experimental observations.

ISSN:0031-9171    

DOI:10.1063/1.1762001

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Turbulent spots have been identified as a transition mechanism in shock tube boundary layer flows for a range of weak shocks at initial pressures near one atmosphere in air, using thin‐film wall temperature gauges. The shape and growth rate of the spots is consistent with that found by other investigators in sub‐ and supersonic steady flows. A study of naturally occurring and artificially generated turbulent spots, and of flow tripping by two‐ and three‐dimensional roughness elements, has shown the existence of an unconditionally stable region behind the shock, in which finite disturbances will not cause flow breakdown or transition to turbulence. The limit of this stable region is given by Re&dgr;= 1.7 ± 0.3 × 103, based on boundary layer thickness at 99% of free‐stream velocity. In the absence of artifical perturbations, laminar flow is seen to persist for times as much as a factor 5 longer than those seen by previous investigators. These maximum transition times were limited by flow tripping at wall discontinuities, and hence represent only a lower bound on the ultimate duration of laminar flow attainable in shock tubes.

ISSN:0031-9171    

DOI:10.1063/1.1761968

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The configuration where three shocks separate three uniform regions is impossible. This has been proved by Courant and Friedrichs for materials having a polytropic equation of state. A proof independent of the form of the equation of state is presented. The conditions of mass and momentum conservation are incompatible for this configuration. For the configuration which occurs in Mach reflection the material flowing across the two shocks attains a greater velocity than that portion flowing across only one. This result holds even if the flow is considered isentropic.

ISSN:0031-9171    

DOI:10.1063/1.1761977

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The discontinuous forms of the full Navier‐Stokes equations are derived. A general method of solving linearized initial and boundary value problems is discussed. The following specific examples are solved: initial value problem in an unbounded domain, shock reflection from an insulating wall, shock evolution in a finite shock tube with insulating walls, and reflection of a sound wave from an isothermal wall. In all cases the dissipative parameters and the gas law are left arbitrary.

ISSN:0031-9171    

DOI:10.1063/1.1761987

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Expansions are given for isotropic distribution functions in a series of polynomials orthogonal with respect to a time‐dependent Maxwellian weight function. It is shown that the choice of the weight function can affect the rate of convergence or, indeed, whether the expansion will converge at all. For the case of the Fokker‐Planck equation describing the relaxation of electrons in a Lorentz gas, the infinite matrix determining the time dependence of the expansion coefficients is diagonalized by an appropriate choice of the weight function. The equilibrium expansion of Kahalas and Kashian and Osipov's result for an initial Maxwellian are special cases of our more general formulation. An alternative expansion method is discussed which can be expected to converge for all realistic distribution functions. Applications to the theory of plane shock waves are suggested.

ISSN:0031-9171    

DOI:10.1063/1.1761992

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Models are developed for the collision term in the kinetic equation for polyatomic gases. Assumptions are made which imply that the translational degrees of freedom undergo violent changes in collisions, while the internal degrees of freedom undergo relatively small transitions. The models consist of a term for the translational degrees of freedom which is closely related to the Krook models for monatomic gases and a term for the internal degrees of freedom which has the character of a diffusion expression. An explicit model is constructed for a gas of eccentrically loaded spherical molecules. Disposable parameters appearing in the model are evaluated in the limits of impulsive and adiabatic collisions. The existence of characteristic rotational energy relaxation times in these limits is briefly examined.

ISSN:0031-9171    

DOI:10.1063/1.1761993

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A simplified theoretical model is presented of axisymmetric thermal convection in an annulus of liquid of kinematical viscosity &ngr;, thermal diffusivity &kgr;, and thermal coefficient of cubical expansion &agr;, which rotates at &OHgr; rad/sec about a vertical axis and is subject to a horizontal temperature contrast of |&Dgr;T| C°. &OHgr; is such that although Coriolis forces are in (geostrophic) balance with horizontal pressure gradients throughout most (though not all) parts of the fluid, centripetal acceleration is everywhere much less than that due to gravity,g. When an appropriate Pe´cle´t number, proportional toX=g&agr;|&Dgr;T|&ngr;12/8&kgr;&OHgr;32, is very small, heat conduction dominates the temperature field and the governing equations can be linearized. Otherwise the governing equations are essentially nonlinear, and whenXis very large the temperature field is dominated by heat advection. The expressions obtained for the mean thermal structure and heat transfer agree satisfactorily with an analytical treatment of the smallXcase and with laboratory and numerical studies of the largeXcase. The expression obtained for the Brunt‐Va¨isa¨la¨ frequency, when combined with baroclinic instability theory, predicts that the axisymmetric flow should give rise to a definite type of nonaxisymmetric flow under conditions that agree remarkably well with laboratory studies.

ISSN:0031-9171    

DOI:10.1063/1.1761994

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The stability of a layer of a triply nonlinear liquid flowing down an inclined plane is investigated. It is found that the flow is always stable with respect to surface wave formation, if the liquid is predominantly non‐Newtonian. However, it is predicted that the instability of the flow is due to shear waves. The reduction of an angle of inclination of the plane and the second viscosity are all shown to be stabilizing factors.

ISSN:0031-9171    

DOI:10.1063/1.1761995

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The stability of the Helmholtz profile in an atmosphere with an unstable density gradient has been studied. It is found that the convective and shear instabilities are essentially separate. It is noted that the shear gives rise to an amplified mode at right angles to the direction of the mean flow.

ISSN:0031-9171    

DOI:10.1063/1.1761996

出版商:AIP    

年代:1967

数据来源: AIP