摘要:

The stability of pipe Poiseuille flow to azimuthally periodic disturbances is examined experimentally. The disturbance is generated in the fully developed flow region to avoid extraneous effects of possible entrance flow instability. A Reynolds number spectrum to above 3200 was covered with a frequency range from13to1kc/sec. The peak disturbance velocity amplitude was maintained at a value less than 2% of the maximum steady‐state velocity in the pipe. Some measurements of spacewise changes in velocity and disturbance wavelength were made as well as radial surveys of disturbance amplitude and steady‐state profiles. The results indicate that the stability is both frequency and Reynolds number dependent to the first mode of azimuthal periodicity, with a minimum critical Reynolds number occurring at about 2150.

ISSN:0031-9171    

DOI:10.1063/1.1691740

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The effect of slight viscoelasticity on the hydrodynamic stability of a plane Poiseuille flow is investigated by the linearized method of small two‐dimensional disturbances. The resulting equation is that of Orr‐Sommerfeld, modified however by a non‐Newtonian term. The constitutive equation used to represent the response of the fluid is that of a second‐order fluid. The equations are solved numerically, and compared with the results of Thomas. The critical Reynolds number is found to be lowered as the magnitude of a non‐Newtonian parameter increased.

ISSN:0031-9171    

DOI:10.1063/1.1691778

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The problem of the onset of thermal instability in heated layers of fluid is investigated in a linear approximation, assuming the confining planes to be surfaces of constant heat flux. It is shown that in the absence of time dependence a limiting point of the neutral stability curve can be obtained exactly by analytical methods. This point is evaluated for a variety of boundary conditions on the fluid motion, and it is verified that the results give criteria for the onset of instability as stationary convection. Similar calculations are made for an idealized fluid in the time‐dependent case, and it is established that the combined effects of buoyancy and surface‐tension gradients may lead to overstable oscillations in certain circumstances.

ISSN:0031-9171    

DOI:10.1063/1.1691741

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The marker and cell technique for the calculation of time‐dependent incompressible flows is used to study density‐current surges at density ratios between 1.05 and 3.0. The study is performed using the full two‐fluid approach as well as a single‐fluid technique involving solute transport and a Boussinesq approximation to the Navier‐Stokes equation. A comparison of these results indicates that the Boussinesq approximation is reasonably accurate for density ratios less than 1.3. The results of free surface and confined flow calculations are compared and the effects of slope, viscosity, and surface tension on density current flows are examined. Complete descriptions are given for the first time of the techniques used for including solute transport and surface tension in marker and cell calculations, and an analysis of the calculational stability properties of the method is included. The numerical results, which are in good agreement with available experimental measurements, provide information about some previously univestigated aspects of density current flows. Of particular importance here is the variation of the front velocity and height with density ratio, for ratios in the range, 1.2‐3.0. New analytical treatments of the initial current surge, the steady flow, and the effects of viscosity are presented and compared with computed results.

ISSN:0031-9171    

DOI:10.1063/1.1691748

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A steady laminar diffusion flame burning in the core of a vortex is studied. Solutions of the full Navier‐Stokes equation and the mass‐transfer equation are obtained to study the effect of the swirling motion on the flame and its flow field for different values of the Schmidt number. The flow conditions depend on two parameters:&Ggr;∞, the circulation of the vortex and Re, the Reynolds number based on the velocity at the axis of symmetry. The velocity distribution is strongly affected by the circulation. As&Ggr;∞increases when Re remains constant the width of the jet flow in the core region also increases. However, the mass concentration distribution and the position of the flame depend principally on Re, and on&Ggr;∞only slightly.

ISSN:0031-9171    

DOI:10.1063/1.1691775

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The influence of velocity distribution on one‐particle dispersion is studied. The ensemble of particle releases is subdivided into subensembles characterized by particle velocity at the instant of release. Under certain simplifying assumptions, the motion of the particles of each subensemble is a systematic drift, combined with dispersal about the subensemble drift position. The displacement variance due to subensemble drift is dominant for small timetsince particle release, whereas that due to subensemble is the more important at larget. The assumption that subensemble dispersal is Gaussian yields an estimate of the probability distribution density function&thgr;(x, t)governing particle position which is correct both in the limits oftsmall andtlarge. The estimate is thus better than that given by the usual assumption that&thgr;is Gaussian, and appears useful fortvalues less than the Lagrangian integral scale. The subensemble approach offers some new insights into the problem of the instantaneous structure of the individual plume.

ISSN:0031-9171    

DOI:10.1063/1.1691776

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A theory of homogeneous, isotropic turbulence in an incompressible fluid is formulated. This theory provides a basis for the dynamics of fully developed turbulence. A hierarchy of equations to determine the time evolution of single‐time moments of the velocity field is derived. Various properties of this hierarchy are exhibited, such as positivity of the dynamically determined energy density. The theory is applied to the study of the inertial range, where the Kolmogoroff theory is justified. A generalized inertial‐range theory is formulated. This “semilocal” theory is characterized by local energy transfer and nonlocal relaxation of triple moments. The generalized theory is applied to the study of hydromagnetic turbulence, which is shown to possess a semilocal inertial range.

ISSN:0031-9171    

DOI:10.1063/1.1691777

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

An analytic solution of the linearized Boltzmann‐Bhatnagar‐Gross‐Krook equation for the slipvelocity problem is developed based on the discrete ordinate method. Analytic expressions of the perturbed distribution function, the velocity profile and the slip coefficient, particularly suitable to numerical calculations, are thus obtained. The numerical value of the slip coefficient is calculated for various orders of approximation. The result obtained for 6 discrete points is in excellent agreement with the numerically exact value.

ISSN:0031-9171    

DOI:10.1063/1.1691779

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A technique is described for making accurate, localized measurements of the molecular velocity distribution function in rarefied flows of helium. The method is based on the observation of the Doppler profile of a helium emission line that is excited by a beam of energetic electrons. It has been demonstrated experimentally that distribution‐function measurements can be obtained. Applicability of this technique to rarefied gas flows and its accuracy are discussed.

ISSN:0031-9171    

DOI:10.1063/1.1691780

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A theoretical study has been made of the structure of so‐called gas‐ionizing hydromagnetic shock waves propagating into “cold” argon in the presence of realistic nonequilibrium effects. To satisfy the definition of these waves, it must be postulated that electrons are created solely by thermal ionization within the front. Comparison of the magnitudes of viscous and magnetic Reynolds numbers indicates that if such structures exist, they will consist of a narrow hydrodynamic (viscous)shock, followed by a much larger region of ionization, relaxation and magnetohydrodynamic interaction. For fronts with this architecture, a modified Zeldovich‐von Neumann‐Do¨ring approximation applies. Moreover, in this approximation it is impossible to construct nontrivial steady‐state structures for “fast” gas‐ionizing shocks. Solutions are found, however, for “slow” shocks and these are presented for a family of hydromagnetically oblique gas‐ionizing fronts at a Mach number of 20 and Alfve´n number of2−12for parametrically varied values of the upstream electric field.

ISSN:0031-9171    

DOI:10.1063/1.1691781

出版商:AIP    

年代:1968

数据来源: AIP