摘要:

The linear stability of plane Couette flow composed of two immiscible fluids in layers is considered. The fluids have different viscosities and densities. For the case of equal densities, there is a critical Reynolds number above which the interfacial mode of the bounded problem is approximated by that of the unbounded problem for wavelengths that are not short enough to be in the asymptotic short‐wavelength range, as well as for short waves. The full linear analysis reveals unstable situations missed out by the long‐ and short‐wavelength asymptotic analyses, but which have comparable orders of magnitudes for the growth rates. For the case of unequal densities, it is found that the arrangement with the heavier fluid on top can be linearly stable if the viscosity stratification, volume ratio, surface tension, Reynolds number, and Froude number are favorable.

ISSN:0031-9171    

DOI:10.1063/1.865346

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Spheromaks are formed in a mesh flux conserver in the presence of an external dc bias magnetic field. The particle confinement is improved when the spheromak separatrix is put inside the metal mesh by the application of positive bias flux. The spheromaks remain stable to tilt instabilities with ratios of bias‐to‐spheromak flux of up to 47±7%.

ISSN:0031-9171    

DOI:10.1063/1.865296

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Techniques of plasma surface wave theory are introduced to obtain new results for magnetic electron waves at the interface of two plasma regions with distinct properties. The procedure generalizes and simplifies problems involving sharp density jumps often encountered in laser– or beam–plasma interaction. As an example, a new magnetic instability is discussed.

ISSN:0031-9171    

DOI:10.1063/1.865297

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The heat transfer from a constant‐temperature cylinder in a uniform, time‐dependent cross flow at low Peclet number is considered. The time dependence is allowed to be strong, so that the velocity fluctuations may be comparable to, or larger than, the mean flow. The first nontrivial term in the Oseen approximation is calculated using matched‐expansion theory, and its physical significance is discussed. As an illustration, the time‐dependent heat transfer is calculated for a steady cross flow with large sinusoidal perturbations.

ISSN:0031-9171    

DOI:10.1063/1.865298

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Generalized Taylor dispersion theory is used to analyze the convective and diffusive transport of sedimenting colloidal particles occurring within a Poiseuille flow in a horizontal circular pipe that is being rotated slowly about its symmetry axis. Such rigid‐body rotation serves to keep the particles permanently in suspension despite their non‐neutral buoyancy, thereby preventing deposition of the particles on the cylinder bottom. In the ‘‘large’’ particle limit, where transverse diffusion is small compared with sedimentation, expressions are derived for the mean axial velocity and Taylor dispersivity of the colloidal particles. A novel flow field fractionation (FFF) scheme based thereon is proposed for continuously separating particles of different sizes and/or densities.

ISSN:0031-9171    

DOI:10.1063/1.865299

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The steady thermocapillary motion in a square cavity with a top free surface in the absence of gravitational forces is considered. The cavity is heated from the side with the vertical boundaries isothermal while the horizontal boundaries are adiabatic. The relative change in the surface tension is very small, i.e., an appropriate capillary number tends to zero, so that the free surface is assumed to remain flat at leading order. A finite‐difference method is employed to compute the flow field. Numerically accurate solutions are obtained for a range of Prandtl numbers and for Reynolds numbers Re as high as 5×104. Surface deflections are computed as a domain perturbation for small capillary number. In addition, asymptotic methods are used to infer the boundary layer structure in the cavity, in the limit of large values of the Reynolds and Marangoni numbers. For a fixed Prandtl number Pr, it is shown that the Nusselt number, liquid circulation, and maximum vorticity are asymptotic to Re1/3, Re−1/3, and Re2/3, respectively. These results are in agreement with the computed solutions. The leading‐order solution for the free‐surface deformation is sensitive to the value of Pr. With Pr>1, the depression near the hot corner may exceed the elevation near the cold corner, while a secondary elevation may be induced near the hot corner when Pr<1.

ISSN:0031-9171    

DOI:10.1063/1.865300

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

An analytical and numerical study is reported of steady‐state natural convection in a two‐dimensional porous layer heated from the side. Contrary to previous investigations of the phenomenon, which were all based on the Darcy flow model, a vector generalization of Forchheimer’s one‐dimensional model is used in the present study, which is valid for all values of local Reynolds number based on pore size. A matched boundary layer solution of the type developed by Weber for Darcy flow is developed for the limit of large‐pore Reynolds numbers (the ‘‘non‐Darcy’’ limit). It is shown that the natural convection phenomenon in the non‐Darcy limit is governed by a new dimensionless group, the Rayleigh number for the higher Reynolds number limit, Ra∞. Numerical experiments are reported in the range 1.6×105≤Ra∞≤1.6×109, in a porous layer with height/thickness ratio equal to 2, and with a high value of Darcy modified Rayleigh number (Ra=4000). The numerical experiments confirm the flow features and scales anticipated by the matched boundary layer solution for the non‐Darcy limit. The experiments also document the transition from the well‐known Darcy flow to the large‐pore Reynolds‐number limit treated in this paper.

ISSN:0031-9171    

DOI:10.1063/1.865301

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Grad’s method of moments is employed to derive balance laws and constitutive relations for plane flows of a dense gas consisting of identical, rough, inelastic, circular disks. Two temperatures are involved; these are proportional to the kinetic energies associated with fluctuations in translational velocity and spin, respectively. When the single particle velocity distribution function is assumed to be close to a two‐temperature Maxwellian, two distinct theories are obtained. One applies when the particles are almost smooth and the collisions between them are nearly elastic; the other applies to nearly elastic particles that, in a collision, almost reverse the relative velocity of their points of contact. I both cases energy is nearly conserved in collisions.

ISSN:0031-9171    

DOI:10.1063/1.865302

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The mean and fluctuating velocities in a turbulent boundary layer on a cylinder have been experimentally characterized for the case where the boundary layer is thick compared to the radius of transverse curvature. The mean velocity measurements suggest a mixed scaling for the ‘‘log law of the wall’’ using the wall coordinateyU&tgr;/&ngr; and the ratio of the local boundary layer thickness to the radius of the cylinder &dgr;/a. A relation for the slope and intercept of the log law of the wall as functions of &dgr;/abased on empirical results and simple analysis is presented. Measurements of the Reynolds stress for &dgr;/aof order 10 show that the Reynolds stress drops off much more quickly with distance from the wall than for a turbulent boundary layer on a flat plate. Both the Reynolds stress data and the turbulent intensity in the mean flow direction data are functions of the inverse radial distance from the center of the cylinder.

ISSN:0031-9171    

DOI:10.1063/1.865417

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Transitional pulsating flow in a pipe is investigated experimentally in the frequency region where its behavior is assumed to be quasisteady. Instantaneous velocity measurements were performed at several radial locations at the exit plane of the pipe. The output of flush‐mounted hot wires at two upstream positions was also recorded simultaneously. The results indicate that the quasisteady assumption is valid in general. The flow behavior is the function primarily of the instantaneous Reynolds number, although the laminarization process and retransition to turbulence are qualitatively different. The laminarization at subcritical instantaneous Reynolds numbers in a time‐dependent flow is a gradual process, similar to the observed turbulence decay in flow geometries where Re is varied spatially. The retransition to turbulence occurs by means of the generation of turbulent slugs at various locations along the pipe. These slugs are convected downstream and coalesce, eventually resulting in fully developed turbulent flow in the whole pipe at higher instantaneous Reynolds numbers.

ISSN:0031-9171    

DOI:10.1063/1.865303

出版商:AIP    

年代:1985

数据来源: AIP