摘要:

Experimental results on probability distribution functions (pdf’s) of full dissipation &egr;, enstrophy &ohgr;2, and enstrophy generation &ohgr;i&ohgr;jsijin two different turbulent flows: turbulent grid flow (Re&lgr;=74) and turbulent jet center (Re&lgr;=880) demonstrate the possibility of universal behavior of the pdf’s of these quantities.

ISSN:0899-8213    

DOI:10.1063/1.858590

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Several experimentally measured energy spectra of fully developed incompressible turbulent flows with Taylor microscale Reynolds numbers ranging from 130–13 000 have been reexamined. It is shown that all spectra collapse to a universal curve after a simple rescaling of space and time. This rescaling does not involve any multifractal‐type transformation, and the rescaled wave numbers scatter around the Kolmogorov dissipation wave numbers derived from experimentally measured quantities. An empirical formula for the resulting universal form is proposed, indicating a nontrivial form of the energy spectrum near the viscous dissipation cutoff.

ISSN:0899-8213    

DOI:10.1063/1.858591

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An exact expression is obtained for the probability density function (pdf) of any quantity measured in a general stationary process, in terms of conditional expectations of time derivatives of the signal. This expression indicates that the conditional expectations of both the time derivative squared and of the second time derivative influence the shape of the pdf, including its tails. A previous result of Ching [Phys. Rev. Lett.70, 283 (1993)] for temperature measurements in turbulent flows corresponds to the particular case when the latter quantity is linear.

ISSN:0899-8213    

DOI:10.1063/1.858830

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The hydrodynamic equations governing the evolution of the interface of a gas/liquid ‘‘bubble’’ as it moves through the space between two plates are solved using a finite difference algorithm defined on a moving deforming surface grid. Particular attention is paid to the redistribution of surfactant as the bubble deforms on squeezing through a constriction. The surfactant redistribution, the surface tangential flows, and the surface normal velocity depend on factors such as bulk viscosity, the bulk/surface surfactant exchange rate, the surface tension dependence on surfactant concentration, and the bulk velocity pattern. Zones of surfactant depletion just behind the bubble front and an accumulation near the bubble front are observed; there are also depletion and accumulation zones associated with the constriction. A variety of other effects are investigated and a detailed analysis permits the generation of schematic representations of the interplay of the coupled bulk and surface hydrodynamic equations for the system.

ISSN:0899-8213    

DOI:10.1063/1.858831

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The flow of a long bubble in an otherwise liquid‐filled tube is a hydrodynamics problem with interesting practical applications in enhanced oil recovery, the coating of monolithic structures, and more recently the design and operation of three‐phase monolithic reactors. For intermediate to large capillary numbers the ‘‘bubble’’ is axisymmetric and the theoretical approach of Bretherton [J. Fluid Mech.10, 166 (1961)] can be applied if the velocity profile of the fluid flowing between the bubble and the square tube wall is known. The velocity profile can be computed using an infinite series expansion of harmonic polynomials. A film evolution equation—which includes all the terms that are important throughout the entire bubble profile—is integrated using a parametric representation as a function of arclength and free‐surface angle. The flow is analyzed and complete bubble profiles are generated. The results are shown to be in good agreement with experiment.

ISSN:0899-8213    

DOI:10.1063/1.858832

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Stable and unstable displacement experiments were performed in millstone and limestone cores. Concentration histories at ten locations along the core samples were obtained by acoustic measurements. Particle‐tracking simulations of the displacements were also made utilizing permeability distributions measured with a permeameter. The combination of experimental observations and simulations indicate that superstable (M<1) displacements suppress the influence of heterogeneity; this suppression was reflected in smaller apparent dispersivities as the mobility ratio decreased below unity. In the millstone, which exhibited random heterogeneity, two‐dimensional particle‐tracking simulations reproduce with reasonable accuracy the growth of the fingered region in unstable displacements. In homogeneous porous media, concentration histories obtained in three‐dimensional simulations did not differ significantly from their two‐dimensional counterparts. In the more heterogeneous limestone, unstable displacements accentuated the influence of heterogeneity leading to longer transition zones. Two distinct flow regimes were observed in unstable displacements: (1) an initial period of rapid transition zone growth and (2) a subsequent period in which leading and trailing edges of the transition zone travel at nearly constant velocities.

ISSN:0899-8213    

DOI:10.1063/1.858833

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The Marangoni–Be´nard instability for a symmetrical three‐layer system is examined theoretically. Linear stability analysis and nonlinear numerical simulations show that the ratio of the heat diffusivities determines the nature of the instability. Monotonic disturbances exist only when this parameter is far enough from one, the motion being driven by one interface. When the heat diffusivity ratio is close to one, oscillatory convection is observed. This is explained on a physical base: the oscillation rests on the coupling of both interfaces, which creates a flip–flop mechanism leading to a double inversion of the vortices rotation during one period of oscillation.

ISSN:0899-8213    

DOI:10.1063/1.858834

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The quasistatic thermocapillary migration of a chain of two or three spherical bubbles in an unbounded fluid possessing a uniform temperature gradient is investigated in the limit of vanishing Reynolds and Pe´clet numbers. The line of bubble centers is permitted to be either parallel or perpendicular to the direction of the undisturbed temperature gradient. The governing equations are solved by a truncated‐series, boundary‐collocation technique. Results are presented which demonstrate the impact of the presence of other bubbles on a test bubble. In the three‐bubble case, a simple pairwise‐additive approximation is constructed from the reflections solution, and found to perform well except when the bubbles are close to each other. Also, features of the flow topology in the fluid are explored. Separated reverse flow wakes are found in the axisymmetric problem, and other interesting structures are noted for the case in which the line of centers is perpendicular to the applied temperature gradient. The observed flow structure is shown to be the result of superposition of simpler basic flows.

ISSN:0899-8213    

DOI:10.1063/1.858835

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The thermocapillary motion generated within a spinning vaporizing droplet is described. Rotation induces a swirling flow in the surrounding gas. This in turn establishes a nonuniform vapor concentration and temperature at the droplet surface. An internal circulation is thus created from the interfacial temperature gradients. This internal motion, described in the limits of small Reynolds and Marangoni numbers, appears as a pair of toroidal vortices. Depending on whether the gaseous Lewis number, Le, is less than or greater than one, the temperature along the surface peaks at either the poles or the equator of the droplet. Consequently, the direction of the internal circulation is from the poles to the equator or vice versa.  

ISSN:0899-8213    

DOI:10.1063/1.858836

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The interaction and coalescence of small spherical drops in dilute, homogeneous dispersions are considered theoretically under conditions, where drop motion results from gravity settling and thermocapillary migration acting simultaneously. A trajectory analysis is used to predict pairwise collision rates, and population dynamics equations are solved to predict the time evolution of the droplet size distribution. The rate of droplet collisions and growth may be reduced dramatically by antiparallel alignment of the gravitational and thermocapillary velocities. For such antiparallel alignment with the gravitational relative velocity exceeding the thermocapillary relative velocity for two widely separated drops, there is a ‘‘collision‐forbidden region’’ in parameter space. This occurs because the gravitational relative velocity decays more rapidly with decreasing separation distance between the drops than does the thermocapillary relative velocity, and so the resultant relative velocity along the line‐of‐centers from these two sources combined becomes zero at a finite separation and the drops are unable to collide. As a result, small drops which initially collide and coalesce due to thermocapillary motion will only grow until they reach a critical size for which the oppositely directed gravitational motion balances the thermocapillary motion.

ISSN:0899-8213    

DOI:10.1063/1.858837

出版商:AIP    

年代:1993

数据来源: AIP