摘要:

The convective stability range of nonreactive binary gas mixtures (Xe:He,Xe:Ar) in a closed vertical cylinder with conducting walls heated from below was investigated. The xenon mole fraction was varied fromXXe= 1 to 0.03 and from 1 to 0 for the helium and argon containing mixtures, respectively. Critical thermal Rayleigh numbersNRa1for the onset of convective flow were obtained from high resolution differential temperature measurements. Thermal diffusion ratios were calculated, based on Lennard–Jones potentials, over the whole composition range for both gas mixtures. Minima inNRa1occurred at the composition for which maximum relative mass separation of the components due to thermal diffusion is predicted. Specifically, the monocomponent valueNRa1≊200 for a cylinder with height‐to‐radius ratio of 6 was reduced to 25 atXXe≊0.03, and 165 atXXe≊0.3 for the Xe:He and Xe:Ar systems, respectively. Comparison with Rayleigh–Be´nard instability theory, including Soret and Dufour effects, shows that for (gas) mixtures, the lateral walls act convectively less stabilizing than for monocomponent systems. It is shown that Dufour contributions to convective stability in these gas mixtures are negligible.

ISSN:0031-9171    

DOI:10.1063/1.863381

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The onset of natural convection is examined for a fluid‐saturated permeable medium contained between vertical coaxial cylinders of inner and outer radiir*iandr*0. The annular space is of heighth*. The horizontal boundaries are isothermal, with heating from below and cooling from above. Both permeable and impermeable upper boundaries are considered. Critical Rayleigh numbers Racand the preferred convective modes are determined as functons of the geometric ratiosh*/r*iandr*0/r*i. The confining vertical walls of the annular space tend to increase Racabove the value for an infinite horizontal layer. The preferred modes are predominantly asymmetric.

ISSN:0031-9171    

DOI:10.1063/1.863382

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Published data from boundary layers on convex surfaces are used to assess the performance of a calculation method based on the solution of modeled transport equations for the Reynolds’ stresses and the dissipation rate of turbulence energy. For flows with large curvature, the model closely reproduces the suppression of turbulence and the diminished growth rate and skin friction. The recovery of flow distorted by curvature is also predicted with results broadly in accord with the measurements.

ISSN:0031-9171    

DOI:10.1063/1.863383

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Measurements have been made of the growth of a turbulently mixed layer in a linearly stratified fluid for Brunt–Va¨isa¨la¨ frequenciesNin the range 0.099–0.386 Hz. Turbulent mixing was induced by a horizontal grid undergoing vertical oscillations at frequenciesfin the range 5.3–20.5 Hz. The experimental results support a theory due to Long which predicts the size of the mixed layerDmeasured from a ’’virtual’’ source plane, as a function of timet, given byD∝f1/2 N−7/18 t1/9.

ISSN:0031-9171    

DOI:10.1063/1.863384

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The rotational distribution, temperature, and collision number of N2in the Ar free jet are studied using the rotational transition cross sections of the exponential gap and power laws. The relaxation of the rotational distribution along the axis of the free jet is described by the master equation, where the velocity distribution is taken as the local Maxwell distribution. The effects of the stagnation density leveln0dand temperatureT0are investigated. The rotational distribution indicates a non‐Boltzmann distribution with overpopulation at the higher rotational levels. The degree of deviation from the Boltzmann distribution at the same axial distance increases with decreasingn0dand decreases with increasingT0. The rotational temperature ratioTr/T0depends weakly onT0, which is consistent with the experimental result of Poulsen and Miller in the N2‐Ar system. The rotational collision numberZris neither constant nor a monotonically increasing function of the translational temperature; still, the downstreamZrdepends weakly onn0dandT0consistent with the experimental result of Poulsen and Miller and is in quantitative agreement with their N2‐Ar value of 2±1.

ISSN:0031-9171    

DOI:10.1063/1.863385

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Electromagnetically driven flows between infinite conducting cylinders subjected to a constant axial magnetic field B0are considered. The linear stability problem for axisymmetric stationary disturbances is treated. In contrast with Chandrasekhar’s conjecture, the present numerical results indicate that the critical wave‐number tends to infinity with the magnetic field. The nonlinear state of the flow, slighly above transition, is investigated by means of Stuart’s energy approach. Further theoretical results are compared with experiment. At criticality, excellent agreement is found between linear stability theory and experiment. Above transition, only the shape of the experimental curves is correctly predicted by Stuart’s energy method.

ISSN:0031-9171    

DOI:10.1063/1.863386

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The system of quasi‐linear equations is derived without making use of the random phase approximation. The fluctuation quantities are described by the autocorrelation function of the electric field using the techniques of Fourier analysis. The resulting equations possess the necessary conservation properties, but comprise new terms which hitherto have been lost in the conventional derivations.

ISSN:0031-9171    

DOI:10.1063/1.863387

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A steady‐state, three‐fluid model of the boundary layer in a rotating plasma is presented. Predictions of the model are compared with discharge voltage measurements from rotating plasma experiments. These measurements have previously been interpreted in terms of Alfve´n’s critical velocity interaction. The model here, which takes centrifuging in the rotating plasma into account, offers an alternative description.

ISSN:0031-9171    

DOI:10.1063/1.863388

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A unified formulation is presented to study the nonlinear low‐frequency electrostatic waves in a magnetized low‐b plasma. It is found that there exist three types of nonlinear waves; (i) nonlinear ion‐cyclotron periodic waves with a wave speedVp≳Cs(ion‐acoustic velocity); (ii) nonlinear ion‐acoustic periodic waves withVp<Cs cosq; and (iii) ion‐acoustic solitons withCs cosq<Vp<Cs, where q is the angle between the wave vector and the magnetic field.

ISSN:0031-9171    

DOI:10.1063/1.863389

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Nonlinear damping rates of finite amplitude electrostatic waves are obtained by using an invariant perturbation method. The result is valid in an initial phase defined by 0<t≲tb≡r/wb(wbis the bounce frequency). As e‖f‖/T(f is the wave potential, andTis the temperature) increases, the nonlinear initial damping becomes significant and it dominates the linear damping rate glwhen, for example,e‖f‖/T≳0.3 fork= 0. 2kD(kDthe Debye shielding constant) ande‖f‖/T≳0.7 fork= 0.3kD. The theory does not assume a constant slope of the velocity distribution functionf0at the phase velocityvpsince higher order derivatives off0atvp, ∂nf0/∂vpn(n= 3,5) play an essential role in enhanced damping. First, a dispersion relation is obtained, is solved for the Langmuir wave, and the theory is applied to an ion‐acoustic wave. A simulation study is carried out on the latter wave. The result confirms the validity of the theory.

ISSN:0031-9171    

DOI:10.1063/1.863380

出版商:AIP    

年代:1981

数据来源: AIP