摘要:

A theory is given to explain the observed dependence on Reynolds number of the decay of turbulent line vortices. It is argued that the turbulent vortex has a triple structure. There is an outer region forr > r 1(r1is the radius of maximum tangential velocity) with a logarithmic distribution of circulation, and forr < r1an inner region and viscous core in both of which the motion is close to solid body rotation. It is predicted thatr1 ∼ (&ngr;&Ggr; 1 t2)1/4, where&Ggr;1is the circulation atr1. Further,&Ggr;1 /&Ggr;0is predicted to be a slowly decreasing function of&Ggr;0/&ngr;, where&Ggr;0is the strength of the vortex. The development of an overshoot of circulation in the outer region is discussed, and the axial velocities produced by growth of a trailing vortex are calculated.

ISSN:0031-9171    

DOI:10.1063/1.1694496

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The Reynolds time‐averaged equations are adopted for fully turbulent two‐dimensional flow of an incompressible fluid through a channel with plane smooth walls. Closure is effected by means of so‐called second‐order methods based on the conservation of mean turbulent kinetic energy. In particular, generalizations of the turbulent shear flow models due to Bradshaw and co‐workers and to Spalding and co‐workers, are adopted as representative of recent differential closure models. The models are treated in the limit of large turbulent Reynolds number by means of limit‐process expansions. The principal division of the flow is into a relatively thick defect layer near the center of the channel (in which the channel half‐width characterizes the layer thickness, the velocity may be linearized about its centerline speed, and the Reynolds stress dominates the Newtonian stress to lowest order), and the relatively thin viscous sublayer near the wall (in which a viscous length scale is appropriate, the velocity is small relative to the centerline speed, and the Reynolds and Newtonian stresses are of comparable magnitude). Certain previously unnoticed constraints on the closure models are revealed by the analysis.

ISSN:0031-9171    

DOI:10.1063/1.1694497

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The distributions across a smooth wall layer of the skewness and flatness factor of the first‐order time derivatives ofuand&ngr;, the longitudinal and normal velocity fluctuations, respectively, and ofu&ngr;, the Reynolds shear stress fluctuations, are compared with the corresponding distributions in a boundary layer over a rough wall. Both the smooth and rough wall boundary layers are nearly self‐preserving under zero pressure gradient conditions. Although the probability density of the square of∂u / ∂t, the time derivative ofu, is reasonably represented by the log‐normal distribution, the skewness and flatness factor of∂u / ∂tin the inner region of the layers are found to remain approximately constant for a turbulence Reynolds numberR &lgr;range 150‐450.

ISSN:0031-9171    

DOI:10.1063/1.1694498

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Taylor instability is investigated in a series of experimental runs at an Atwood number of 0.095 (density ratio of 1.22). The fluids used are octyl alcohol and water. The expected bubble and spike formation is observed, and, for the first time, the spike is seen to flatten and curl over due to Kelvin‐Helmholtz instability. The results are compared with the analytical, numerical, and empirical findings of others.

ISSN:0031-9171    

DOI:10.1063/1.1694499

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A thermal torque sometimes observed in Scott effect measurements has been studied experimentally and an explanation for the thermal torque proposed. The magnitude of the thermal torque can be comparable to the Scott torque depending on geometrical and thermal anisotropies. The thermal torque is predicted to decrease with application of an axial magnetic field.

ISSN:0031-9171    

DOI:10.1063/1.1694500

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Reflection of a plane shock wave from a wall has been simulated on a microscopic scale using a direct simulation Monte Carlo technique of the type developed by Bird. A monatomic gas model representing argon was used to describe the fluid medium and a simple one‐parameter accommodation coefficient model was used to describe the gas‐surface interaction. The influence of surface accommodation was studied parametrically by varying the accomodation coefficient from zero to one. Results are presented showing the temporal variations of flow field density, and mass, momentum, and energy fluxes to the wall during the shock wave reflection process. The energy flux was used to determine the wall temperature history. Comparisons with experiment are found to be satisfactory where data are available.

ISSN:0031-9171    

DOI:10.1063/1.1694501

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A detailed theoretical study is made of radiatively driven acoustic waves inside a closed cylindrical tube. The formulation accounts for all the essential phenomena for a gas in vibrational equilibrium—namely, gas motion, longitudinal and radial viscous dissipation, longitudinal and radial heat conduction, and radiative transfer with spectral detail included. An approximate expression is derived for the pressure response. This solution is applicable to arbitrary spectral distribution of the radiative absorption coefficient, under conditions attainable in the laboratory. The solution includes both tuned and untuned acoustic conditions as well as the spectrophone limit—that is, the condition where the length of the tube is much smaller than the length of the acoustic wave. The influence of spectral detail on the pressure response is illustrated by an example that employs a single Lorentz profile to represent an absorption band.

ISSN:0031-9171    

DOI:10.1063/1.1694502

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Measurements are presented of the acoustic pressure resulting from the periodic absorption and emission of radiative energy by an infrared‐active gas contained in a closed, cylindrical tube. The acoustic waves are generated by means of an external, sinusoidally time‐modulated blackbody source. The pressure measurements are for both resonant and nonresonant conditions at acoustic frequencies from about 60 to 600 Hz. Comparison of the measurements with predictions shows that the response is due primarily to the modified‐classical acoustic wave, but that a small contribution from the so‐called radiation‐induced wave can be measured in certain of the tests. Strong effects of vibrational nonequilibrium, similar to those observed previously in spectrophone studies, were found in some of the tests. Generally, the agreement between theory and experiment is satisfactory and provides verification of the basic physical and mathematical models of the theory.

ISSN:0031-9171    

DOI:10.1063/1.1694503

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The statistical properties of the plasma fluctuations in a long plasma column produced by a hollow cathode arc discharge show that different states of weak plasma turbulence exist. The transition between these states occurs as a result of changes in the confining magnetic field, the background gas pressure, and the boundary conditions. Associated with such transitions are radical changes in the plasma confinement. A sudden jump in the radial particle loss rates is manifested by the increase of the current collected by Buchelnikova ring‐collectors and by the change in the radial density profiles measured by electrostatic probes.

ISSN:0031-9171    

DOI:10.1063/1.1694504

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The electrical conductivity of a partially to strongly (Coulomb collision‐dominated) ionized dc plasma in a dc magnetic field was investigated at low frequency by two experimental methods, one utilizing the Hall effect and the other a plasma interaction with an induced solenidal electric field. Electron‐ion and electron‐atom momentum transfer collision frequencies and ion mobility in the magnetic field (ion slip) were inferred. The helium abnormal negative glow plasma was used at pressures of 0.5 to 2.5 Torr, electron densities of 1 to 32 × 1017m−3and temperature≈ 600 °K; the magnetic field ranged up to 675 G. The data support Shkarofsky's theory of partial ionization. When ion motion becomes dominant, the resulting ion mobility is 75% larger that He+mobility in helium taken from drift tube measurements. This disagreement suggests that further theoretical work on ion mobility in a plasma is needed.

ISSN:0031-9171    

DOI:10.1063/1.1694505

出版商:AIP    

年代:1973

数据来源: AIP