摘要:

The stretching of material lines in time periodic sequences of ‘‘weak’’ flows (i.e., characterized by a linear stretching for long times) in which streamlines change at the end of each periodic unit is quantified in terms of a mixing efficiency. Two different flows and two different strain distributions give nearly identical results in terms of the mixing efficiency and indicate the existence of an optimal operating condition for maximum stretch. The results are relevant to fluid mixing in chaotic flows.

ISSN:0031-9171    

DOI:10.1063/1.865824

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Results of direct numerical simulations of the decay of nearly isotropic turbulence exhibit clearly that much of the flow evolves in orientation to a state in which the vorticity vector is nearly aligned with the velocity vector.

ISSN:0031-9171    

DOI:10.1063/1.865825

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The null‐collision concept is introduced into the direct‐simulation Monte Carlo method in the rarefied gas dynamics. The null‐collision technique overcomes the principle fault in the time‐counter technique and the difficulties in the collision‐frequency technique. The computation time required for the null‐collision technique is comparable to that for the time‐counter technique. Therefore, it is concluded that the null‐collision technique is superior to any other existing techniques in the direct‐simulation Monte Carlo method.

ISSN:0031-9171    

DOI:10.1063/1.865826

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Plasmas with variable ratios of hot electrons to colder background plasma, but without significant neutral gas, are investigated in the multiple mirror experiment (MMX) [Phys. Fluids29, 1208 (1986)]. The device permits, sequentially, the injection of plasma (H2or He) into a stable mirror cell, and electron‐cyclotron resonance heating (ECRH) to an electron temperature of several keV. A second source then provides the cold‐electron component with arbitrarync/nh≤50. At any point in time, the field coils that provide magnetohydrodynamic (MHD) stability for the hot electrons may be pulsed off for times short in comparison to the time required for cold plasma density to change. Instantaneous destabilization, even for plasma parameters well in excess of the value ofnc/nhfor which a hot‐electron drift approximation would predict stability, was observed. This result suggests that a mechanism in addition to a cold plasma background is required to stabilize a hot‐electron distribution. Comparison is made with recent theoretical work.

ISSN:0031-9171    

DOI:10.1063/1.865827

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Chaotic advection can be produced whenever the kinematic equations of motion for passively advected particles give rise to a nonintegrable dynamical system. Although this interpretation of the phenomenon immediately shows that it is possible for flows at any value of Reynolds number, the notion of stochastic particle motion within laminar flows runs counter to common intuition to such a degree that the range of applicability of early model results has been questioned. To dispel lingering doubts of this type a study of advection in a two‐dimensional Stokes flow slowly modulated in time is presented. Even for this very low Reynolds number, manifestly ‘‘laminar’’ flow chaotic particle motion is readily realizable. Standard diagnostics of chaos are computed for various methods of time modulation. Relations to the general ideas of parametric resonance and adiabatic invariance are pointed out.

ISSN:0031-9171    

DOI:10.1063/1.865828

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Unsteady liquid film flow on a rotating disk is analyzed by asymptotic methods for low and high Reynolds numbers. The analysis elucidates how a film of uniform thickness thins when the disk is set in steady rotation. In the low Reynolds number analysis two time scales for the thinning film are identified. The long‐time‐scale analysis ignores the initial acceleration of the fluid layer and hence is singular at the onset of rotation. The singularity is removed by matching the long‐time‐scale expansion for the transient film thickness with a short‐time‐scale expansion that accounts for fluid acceleration during spinup. The leading order term in the long‐time‐scale solution for the transient film thickness is shown to be a lower bound for film thickness for all time. A short‐time analysis that accounts for boundary layer growth at the disk surface is also presented for arbitrary Reynolds number. The analysis becomes invalid either when the boundary layer has a thickness comparable to that of the thinning film, or when nonlinear effects become important.

ISSN:0031-9171    

DOI:10.1063/1.865829

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The cavitation pressure of spherical gas bubbles is correlated to the fluid viscosity in a theoretical investigation of the viscous Rayleigh–Plesset equation. The analysis is complicated by the absence of a first integral for the dissipative system. A qualitative normal form bifurcation analysis about a double‐zero codimension 2 singularity is used to guide a quantitative estimate of the cavitation pressure by numerical and Melnikov perturbation techniques.

ISSN:0031-9171    

DOI:10.1063/1.865830

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Numerical and asymptotic solutions are found for the steady motion of a symmetrical bubble through a parallel‐sided channel in a Hele–Shaw cell containing a viscous liquid. The degeneracy of the Taylor–Saffman zero surface‐tension solution is shown to be removed by the effect of surface tension. An apparent contradiction between numerics and perturbation arises here as it does for the finger. This contradiction is resolved analytically for small bubbles and is shown to be the result of exponentially small terms. Numerical results suggest that this is true for bubbles of arbitrary size. The limit of infinite surface tension is also analyzed.

ISSN:0031-9171    

DOI:10.1063/1.865831

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A theoretical treatment of the stability of miscible displacement in a porous medium is presented. For a rectilinear displacement process, since the base state of uniform velocity and a dispersive concentration profile is time dependent, we make the quasi‐steady‐state approximation that the base state evolves slowly with respect to the growth of disturbances, leading to predictions of the growth rate. Comparison of results with initial value solutions of the partial differential equations shows that, excluding short times, there is good agreement between the two theories. Comparison of the theory with several experiments in the literature indicates that the theory gives a good prediction of the most dangerous wavelength of unstable fingers. An approximate analysis for transversely anisotropic media has elucidated the role of transverse dispersion in controlling the length scale of fingers.

ISSN:0031-9171    

DOI:10.1063/1.865832

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Curves for cumulative recovery of driven fluid, as a function of pore volume injected of driving fluid, are computed using a previously developed Monte Carlo simulation of two‐fluid flow through porous media at finite viscosity ratio. Both the linear channel and five‐spot geometries are examined at various grid sizes. For a range of unfavorable viscosity ratios, the trend is toward greater recovery as grid size increases, in the opposite direction to the trend at infinite unfavorable viscosity ratio, which is toward zero recovery as grid size increases.

ISSN:0031-9171    

DOI:10.1063/1.865833

出版商:AIP    

年代:1986

数据来源: AIP