摘要:

Numerical simulations of the Stokes boundary layer over a three‐dimensional wavy wall are performed in order to investigate the role played by infinitesimal wall imperfections in triggering transition to turbulence. Our results show flow patterns qualitatively similar to those experimentally detected. In particular the laminar, disturbed‐laminar and intermittent turbulent regimes are recovered. The characteristics of the above flow regimes are analyzed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868940

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have studied experimentally the fingered splatter left behind after a liquid drop impacts a solid surface at high values of the Reynolds and Weber numbers. The viscosity and surface tension of the liquid was varied by using several different fluid mixtures. The surface chosen was a thick paper sheet, on which the drop left a clear signature of the impact pattern. The maximum spreading of the fluid and the number of fingers seem to scale with anImpactReynoldsnumber,U(&pgr;2&rgr;D3/16&sgr;)1/4/&ngr;1/2, whereUis the impact velocity, &ngr; the kinematic viscosity of the fluid, &rgr; the fluid density, &sgr; the surface tension andDthe drop diameter. The number of fingers is weakly dependent on the surface tension and depends primarily on the inertial‐viscous interaction. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868941

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

It has generally been assumed in previous studies that the normalized shear‐layer frequency (fSL/fK) versus Reynolds number (Re) varies as Re0.5. There has also been a great deal of scatter in quoted values of the critical Reynolds number (Recrit) for the onset of the shear‐layer instability, with values ranging from 350 to 3000. In the present work, we find that Recritis strongly sensitive to the mode of primary vortex shedding and may vary between 1200–2600 depending on the angle of oblique shedding. In the case of the shear‐layer frequency, we have conducted a careful reevaluation of all previous data in the literature, and find the relationshipfSL/fK=0.0235×Re0.67. Surprisingly, not one of the previous studies yields a relationship that is close to Re0.5. We suggest an explanation for the variation Re0.67based on simple physical grounds. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868942

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Two‐dimensional convection can become unstable to a mean shear flow. In three dimensions, with periodic boundary conditions in the two horizontal directions, this instability can cause the alignment of convection rolls to alternate between thexandyaxes. Rolls with their axes in they‐direction become unstable to a shear flow in thex‐direction that tilts and suppresses the rolls, but this flow does not affect rolls whose axes are aligned with it. New rolls, orthogonal to the original rolls, can grow, until they in turn become unstable to a shear flow. This behavior is illustrated through numerical simulations and low‐order models, and the sequence of local and global bifurcations is determined. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868943

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Results of an experimental investigation on the Richtmyer–Meshkov instability of a He–CO2interface are reported. A simultaneous three‐directional laser absorption technique is used to follow, at a single abscissa, the evolution of the created mixing zone before and after the interaction with the reflected shock, during the same run. CO2density profiles have been determined within both the incident and the compressed mixing zones. However, near the pure CO2, the wall boundary layer reflected shock interaction perturbs measurements and does not allow objective conclusions. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868948

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We study singularity formation in the lubrication model for the unforced Hele‐Shaw system, describing the breaking in two of a fluid droplet confined between two narrowly spaced glass plates. By varying the initial data, we exhibit four different scenarios: (1) the droplet breaks in finite time, with two pinch points moving toward each other and merging at the singular time; (2) the droplet breaks in finite time, with two asymmetric pinch points propagating away from each other; (3) the droplet breaks in finite time, with a single symmetric pinch point; or (4) the droplet relaxes to a stable equilibrium shape without a finite time breakup. Each of the three singular scenarios has a self‐similar structure with different scaling laws; the first scenario has not been observed before in other Hele‐Shaw studies. We demonstrate instabilities of the second and third scenarios, in which the solution changes its behavior at a thickness that can be arbitrarily small depending on the initial condition. These transitions can be identified by examining the structure of the solution in the intermediate scaling region. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868915

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of adsorbed surfactants on the flow that takes place in the film forming zone of the slot coater is examined. The numerical technique employed in this work, that is based on the finite element method, can suitably handle viscous free surfaces with locally varying interfacial properties. The complete information produced by the computational code allows to relate the interfacial dynamics to the mechanism involved in the mass transport process: convection and adsorption/desorption. The results presented show that the effect produced by both interfacial viscosity and interfacial elasticity are rather similar; however, the operating conditions that make them more noticeable are different. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868944

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The dynamical behavior of suspensions of rod‐like particles is explored in unsteady elementary flows characterized by the following special property: while there may be arbitrarily large excursions from an initial state by the carrier fluid, the fluid returns to the initial state at the end of the time interval. In such aclosedflow, it is shown that the orientation distribution of a dilute non‐Brownian suspension will also return to its initial state, after arbitrarily large excursions driven by the flow of the carrier fluid. In contrast, the orientation distribution in a Brownian suspension will not return to its initial state at the end of the interval in a closed flow. These effects are exploited in a constrained nonlinear optimal control problem for the orientation distribution of a Brownian suspension in an example closed flow. The result of the control problem is a family of protocols that are most effective at achieving various degrees of anisotropy of the suspension, within the class of flows under consideration. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868945

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An experimental study of the contact of a falling sphere with a neutrally buoyant sphere in a viscous fluid is described. The experiments used a mixture of polyalklene glycol and tetrabromoethane as the Newtonian fluid, together with a pair of equal‐sized Teflon&squflg;and nylon spheres of 6.355 mm diameter. The spheres have microscopic surface roughness, which allows them to make physical contact when in close approach. This contact breaks the symmetry of the relative trajectory of the two spheres and affects the rate at which the heavy sphere moves past the neutrally buoyant one. The experimental observations verify the roll/slip model of Davis [Phys. Fluids A4, 2607 (1992)] for the interaction of the two spheres in contact. This model assumes that contact prevents the nominal surfaces from approaching closer than a minimum separation equal to the effective roughness height, and that the tangential component of the contact force is described by solid friction theory. A friction coefficient of 0.28±0.02 provides the best agreement between theory and experiment, with some variation observed when the experiments were repeated for the same and different pairs of spheres. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868946

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The terminal structure of the unsteady boundary‐layer equations is investigated for the problem of impulsive flow past a circular cylinder. It is known that both the classical and interacting boundary‐layer equations for this problem are singular and the singular structure for the interacting boundary layer on a circular cylinder has not been properly resolved using the classical finite difference approach within an Eulerian framework. The objective of this paper is to resolve the singular structure for both the classical and interacting boundary layers for the impulsively started flow past a cylinder. An adaptive‐grid scheme, coupled to a panel method for the interacting case, is employed to resolve the extremely small time and length scales associated with a terminal structure defined by the emergence of a singularity. Calculations performed in an Eulerian coordinate system show that interacting boundary‐layer calculations terminate sooner than the classical boundary‐layer calculation, and as the Reynolds number decreases, the singularity occurs earlier in time and closer to the rear stagnation point of the circular cylinder. Comparisons with previous Lagrangian and fixed‐grid Eulerian results are discussed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868916

出版商:AIP    

年代:1996

数据来源: AIP