摘要:

Our recently‐published linear analysis [Schwartzetal., Langmuir11, 3690 (1995)] demonstrated that an initially rippled thin layer of Newtonian liquid with uniformly distributed surfactant may level in unexpected ways. While the presence of surfactant will, in general, slow the rate of leveling compared to that of a perfectly clean system, there was shown to exist a realistic parameter range where increasing, rather than reducing, the amount of surfactant present will hasten leveling. Here, for the two‐dimensional problem, we investigate the importance of nonlinearity though numerical solution of (i) the unsteady lubrication form of the evolution equations with surfactant, and (ii) finite‐element solution of the exact governing equations for slow viscous flow. Confirmation of the linear results is demonstrated and quantitative discrepancy only appears for large‐amplitude and short‐wavelength ripples. Surface tension gradient driven flow explains the anomalies; for moderate surfactants, the surface quickly ‘hardens,’ leading to a decay rate of one‐quarter of the clean‐surface rate, while for weak surfactants, leveling proceeds to a plateau level which decays much slower than the hard‐surface result. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868952

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The breakup of a round water jet by a swirling coaxial annular air jet, issuing from convergent jet nozzles has been studied experimentally. The intensity of the swirl and the water to air mass flux ratio have been varied over a large range. It was found that the liquid jet is little affected by the swirl when the swirl number, here defined by the ratio of tangential to axial air jet nozzle velocities, is below a critical value (Scr). Just above this value the liquid jet undergoes an explosive radial expansion and disintegration. A simple model shows that the physically relevant parameters are the gas to liquid momentum flux ratioMand the ratio of the nozzle’s diameters. For small momentum flux ratios,Scrwas found to depend onM−1/2until an asymptotic constant value is reached at largeM. Surface tension has no effect on the breakup when air velocities are large, however, membrane‐type breakup is dominant at the lower air velocities when the aerodynamic Weber number is of order 100 or less. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868981

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present a schlieren photographic sequence of a laser generated cavity in water interacting with a solid boundary as seen from the boundary’s perspective looking through the bubble. Previous studies have photographed the collapsing bubble from the side and it is unclear from these photographs once a bubble becomes toroidal how the central hole develops. Our schlieren sequence displays the jet first penetrating the lower bubble surface and then follows the evolution of the central channel as the bubble contracts and reexpands. It is felt that the bubble shapes recorded add further knowledge to the possible damage mechanisms of cavitation. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868953

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The Generalized Langevin model representations of two second‐moment closure models for the rapid pressure‐strain term, proposed by Fu and Launder and by Jones and Musonge, are obtained. This representation makes it possible to use these models in PDF calculations of turbulent flows. The implications of three realizability constraints for the relationship between Langevin models and these second‐moment closures are discussed. A Generalized Langevin model representation exists only if the rapid pressure‐strain model satisfies realizability at the 2D turbulence limit. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868954

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effective thermocapillary mobility of a suspension of viscous, heat conducting droplets in a fluid of different shear viscosity and thermal conductivity is studied in the framework of the theory of linear transport coefficients of heterogeneous media. The fluid velocity and the temperature field are combined conveniently into a single four‐vector field. This allows a multiple scattering analysis and calculation of the thermocapillary mobility in terms of a renormalized cluster expansion. A mean field result is derived that reduces to the Clausius‐Mossotti formula in the case of bubbles. Correlation corrections are calculated in pair approximation. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868955

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A method is presented for making finite Fourier mode truncations of the Rayleigh–Be´nard convection system that preserve invariants of the full partial differential equations in the dissipationless limit. These truncations are shown to have no unbounded solutions and provide a description of the thermal flux that has the correct limiting behavior in a steady‐state. A particular low‐order truncation (containing 7 modes) is selected and compared with the 6‐mode truncation of Howard and Krishnamurti [J. Fluid Mech.170, 385 (1986)], which does not conserve the total energy in the dissipationless limit. A numerical example is presented to compare the two truncations and study the effect of shear flow on thermal transport. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868956

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Results are presented for the effect of solute–fiber hydrodynamic interactions on the hindered diffusion of a spherical macromolecule in random media comprised of cylindrical fibers. Hydrodynamic interactions are calculated by representing the sphere as a collection of point singularities and accounting for the fibers by using a numerical version of slender‐body theory. Electrostatic and other nonhydrodynamic interactions are neglected. The calculations show that the hydrodynamic mobility of the solute decreases in an exponential‐like fashion as the fiber volume fraction is increased. Also, at a given volume fraction, a medium of thinner fibers hinders solute transport more than a medium of thicker fibers. The results compare well with experimental data, both for protein diffusion in solutions of the polysaccharide Dextran and for protein diffusion in cross‐linked agarose gels. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868884

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The interactions of deformable drops in cavity flows is studied numerically in the limit of low Reynolds numbers for a two‐dimensional model. Flow in a square cavity is driven by the steady motion of one of the walls. Deformable drops will migrate across streamlines until they reach an equilibrium trajectory or equilibrium position; the rate and direction of migration depend on both the viscosity ratio and capillary number. High viscosity ratio deformable drops have a tendency to aggregate and form clusters. The presence of a deformable dispersed phase results in an elastic behavior of the suspension. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868957

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The shape of a drop centered in an axisymmetric extensional flow is determined by the viscous stresses that deform the drop and surface tension &ggr;′ that resists the deformation. The ratio of these stresses is given by the capillary number, Ca. When Ca is small enough, the drop attains a steady shape. However, above a threshold value, Cacr, the drop elongates continuously, and no steady shape is attained. When surfactants are present on the drop interface, the surface tension is determined by the surface concentration profile, which varies throughout the deformation process. Initially, the surface tension is given by &ggr;eq′, in equilibrium with the uniform surface concentration &Ggr;eq′. When the flow is initiated, surfactant is swept toward the drop tips, reducing the surface tension there, and altering the interfacial stress balance tangentially through Marangoni stresses and normally through the Laplace pressure. In this paper, the effects of an insoluble surfactant on drop deformation are studied. In previous work, either a surface equation of state for the surface tension &ggr;′ that is linear in the surface concentration &Ggr;′ was used, an approximation that is valid only for dilute &Ggr;′, or &Ggr;′ sufficiently dilute for the linear approximation to be valid were studied. In this paper, a nonlinear surface equation of state that accounts for surface saturation and nonideal interactions among the surfactant molecules is adopted. The linear framework results are recovered for &Ggr;′ that are sufficiently dilute. As &Ggr;′ is increased, the effects of saturation and surfactant interactions are probed at constant initial &Ggr;eq′and at constant initial &ggr;eq′. Finally, the case of strong intersurfactant cohesion is treated with a first‐order surface phase transformation model. At moderate surface concentrations, these nonlinear phenomena strongly alter the steady drop deformations and Cacrrelative to the uniform surface tension and linear equation of state results. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868958

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The velocity distribution function,f1, for a (linear) shear flow of a system of rigid inelastically colliding disks in a plane is measured by applying a novel algorithm to results of (MD) simulations involving 200 000 particles. The need to consider such a relatively large system is explained. It is found thatf1is well fitted by an exponent of a second‐order polynomial in the norm of the fluctuating velocities with angle‐dependent coefficients (which also depend on the density and the granular temperature). Other characterizations of the system studied in this paper are presented as background material. A hitherto unnoticed property of systems with Lees–Edwards boundary conditions has been discovered and its origin is briefly explained. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868951

出版商:AIP    

年代:1996

数据来源: AIP