摘要:

In this paper, a numerical prediction is reported involving second‐order closure of a turbulent flow of a vertically burning, lean mixture of premixed combustible gases discharging from a pipe and developing into a turbulent combusting roundjet. Classical closures are used where available. Expressions for the chemical reaction rate term and other unclosed terms related to variable density flow in the Favre‐averaged turbulent transport equations are based on the Bray–Moss–Libby aerothermochemistry for premixed turbulent combustion, extended to variable enthalpy systems. Mixing of hot burned and cool ambient gases and the attendant buoyancy effects are found to be significant physical phenomena in the behavior of such lean premixed combusting jets. Results of the simulation are compared with experimental data of Yoshida [ProceedingsoftheEighteenthInternationalSymposiumonCombustion(The Combustion Institute, Pittsburgh, 1981), p. 931] with which reasonable numerical agreement is obtained. Reasons for discrepancies and possible lines for future research are discussed.

ISSN:0031-9171    

DOI:10.1063/1.866384

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Pseudo‐two‐dimensional wakes are generated by introducing spanwise cellular structures in the otherwise plane turbulent wake by means of the castellated blunt trailing edges of different configurations. The transverse growths of these coflowing cellular wakes are found to be independent of each other without any noticeable spanwise interaction. This wake growth is examined in the light of the plane equilibrium wake analysis. Though these wakes are not found to be exactly self‐similar, their growth shows a nonmonotonous approach toward the asymptotic state appropriate to that of a plane wake. The dye emission in the wakes illustrated a coherent vortical structure in the transverse plane, similar to that of the usual two‐dimensional wake, in spite of the initial spanwise irregularities.

ISSN:0031-9171    

DOI:10.1063/1.866385

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The computational limitations associated with the molecular dynamics (MD) method and the direct simulation Monte Carlo (DSMC) method are reviewed in the context of the computation of dilute gas flows with high vorticity. It is concluded that the MD method is generally limited to the dense gas case in which the molecular diameter is one‐tenth or more of the mean free path. It is shown that the cell size in DSMC calculations should be small in comparison with the mean free path, and that this may be facilitated by a new subcell procedure for the selection of collision partners.

ISSN:0031-9171    

DOI:10.1063/1.866386

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

In some inertial confinement fusion target designs a spherical shell collapses on a void or compresses a small amount of gaseous material. There can be a period during which both the outside (driver) pressure and the inside pressure have a negligible effect on the implosion dynamics, and the motion is essentially ballistic. The changes in the aspect ratio occur mainly because of geometrical convergence. For reasonable parameters the inner surface does not begin to decelerate until shortly before convergence is complete. An approximate description of this ‘‘coasting’’ phase has been developed and applied to study the evolution of perturbations on the inner and outer surfaces of the shell in the limit where the fluid is incompressible. The two surfaces are strongly coupled as long as the shell remains thin. When the shell becomes thick compared to the inner radius, the inner and outer surface perturbations decouple. Under these conditions the surface wave action is a good adiabatic invariant, which can be used to estimate the change in the amplitude of a perturbation as a function of the shell inner radiusR1. Detailed analysis confirms the adiabatic invariance argument and extends the results. The adiabatic invariant may also be applicable in the case of compressible fluids.

ISSN:0031-9171    

DOI:10.1063/1.866387

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Exact closed‐form solutions for finite amplitude sonic shocks are presented for the case of a van der Waals gas having a constant specific heat. Solutions are provided for both single and double sonic shocks. Sample calculations are presented that include sonic shocks embedded in smooth inviscid flows.

ISSN:0031-9171    

DOI:10.1063/1.866388

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A test‐particle method was developed to estimate the conditions that lead to collision‐dominated evaporation during pulse heating of a solid surface in vacuum. The method consists of calculating the number density of evaporated molecules above the surface neglecting collisions and then sending a test particle through this cloud. The probability that the test particle escapes from the cloud without making a collision is the measure of the importance of intermolecular collisions during the blow off. The method is applied to heating of a ∼1 cm diam spot on the surface with a heat pulse of ∼1 msec duration. These conditions characterize laboratory laser vaporization experiments intended to measure the vapor pressures of refractory solids at very high temperatures. The principal variable which determines the escape probability is the maximum surface temperature during the transient. When applied to laser heating of uranium dioxide, collisional effects in the vapor are predicted to begin at a maximum surface temperature between 2600 and 2800 K.

ISSN:0031-9171    

DOI:10.1063/1.866389

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Diffusion driven by a constant field, and opposed by a velocity‐dependent diffusion coefficient that decreases to zero at large velocity, leads to the phenomenon of ‘‘runaway.’’ It is studied here in the case of a one‐dimensional velocity space, when the Fokker–Planck equation is equivalent to an interesting Schro¨dinger equation. The runaway current is extracted from the smallest eigenvalue, after the equation has been solved by the method of matched asymptotic expansions. There is discussion of connections between our approach, the conventional approach, and the classical two‐dimensional results.

ISSN:0031-9171    

DOI:10.1063/1.866390

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The finite ion gyroradius effects on the stability of global tearing modes are studied using analytic and numerical techniques. It is found that in the collisional regime the modes are weakly stabilized compared to the zero ion temperature case. In the semicollisional regime, the stabilization is weaker than the polarization drift stabilization in the zero ion temperature case. There is overall a very minor finite ion temperature modification of the stability boundary in the plasma beta, collisionality space.

ISSN:0031-9171    

DOI:10.1063/1.866391

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

In this paper the nonlinear problem of the free boundary equilibrium of a plasma inside a conducting circular shell has been solved analytically in the high beta limit. The unknown function describing the plasma shape has been expanded in a Volterra functional series, the functional analogous to the Taylor series. The hierarchy of the linear integral equations obtained from the expansion is, at least in principle, analytically solvable, so that the solution of each equation can be given in a closed form. The analytical computations have been carried out up to the fourth order and the results compared with numerical computations.

ISSN:0031-9171    

DOI:10.1063/1.866523

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A set of three coupled nonlinear differential equations describing low‐frequency (∂/∂t≪&OHgr;i, &OHgr;ibeing the ion gyrofrequency), electromagnetic perturbations slowly varying (∂/∂z≪∇⊥) along the external magnetic fieldB0ezin a homogeneous plasma is derived using a two‐fluid description. The equations are valid over a wide range of &bgr; (ratio of the electron thermal pressure and the magnetic pressure) and in proper limits reduce to the Hasegawa–Mima and convective cell equations, respectively. A general traveling solution for the full set of equations is described and studied in detail for the axially antisymmetric (dipole) case, when the only possible solution propagating with a finite velocity perpendicular to the external magnetic field has the form of a double vortex. There are two modes of vortices. The first one reduces to the Hasegawa–Mima modon in the limitvz=0, while the other in the limitvz=cA(Alfve´n speed) has zero density perturbation and can be identified as a generalized convective cell.

ISSN:0031-9171    

DOI:10.1063/1.866392

出版商:AIP    

年代:1987

数据来源: AIP