摘要:

Based on experiments a new law is proposed for the vortex shedding from a circular cylinder which describes in a consistent way the Strouhal–Reynolds-number dependency asSr=Sr*+m/Refrom the beginning of the vortex shedding atRe=47up to the laminar–turbulent transition of the cylinder boundary layer atRe>2×105.The various vortex shedding processes, occurring with increasing Reynolds number, are described by different coefficientsSr*andm.©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869675

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The axisymmetric creeping motion of multiple composite spheres is analyzed to investigate the hydrodynamic interactions among these particles. A composite particle referred to in this paper is a spherical solid core covered with a permeable shell, whose thickness can be arbitrary. The Stokes equation and the Brinkman equation are used to describe the flow fields outside and inside the particle, respectively. For two identical composite spheres with thin porous layers in near contact, a lubrication analysis is employed to examine their relative motion. Analytic expressions for the pressure and the drag force are obtained for the layers having high permeability. For general cases, a boundary collocation method is applied to numerically solve for the unknown coefficients in the series solutions for the flow behavior of the multiple particles. The resulting drag forces are in good agreement with the predictions from the lubrication analysis and the reflection method. In general, the strength of hydrodynamic interaction among composite particles lies between the values among permeable particles with the same permeabilities and among solid particles. The hydrodynamic behavior for composite spheres may be approximated by that for permeable spheres when the porous layer is sufficiently thick, depending on the permeability. When the particles undergo relative motion, the drag increases with decreasing distance between them. However, the drag on the particle with larger size or lower permeability may reach a minimum at a certain distance for a chain of dissimilar particles, rather than in contact, when they translate at the same velocity. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869676

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The dynamics of an interface between two incompressible, inviscid, irrotational, and immiscible liquids with densities&rgr;1and&rgr;2under the influence of a time-dependent gravitational fieldg(t)is investigated. A Hamiltonian formulation of the system is adopted leading to a perturbative expansion of the equations of motion for the canonical variables. Equations, accurate up to third order in the perturbation amplitude are derived. They are able to describe the initial stage of instability “saturation.” The latter equations are integrated iteratively for two standard limiting cases: constant gravity (classical Rayleigh–Taylor instability),g(t)≡g0,and impulsive Richtmyer–Meshkov loading,g(t)=v0&dgr;(t−t0).The comparative growth of various two-dimensional structures and rectangular and hexagonal cells is evaluated. Surface tension effects are considered. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869677

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The spontaneous spreading of a thin liquid film along the surface of a deep liquid layer of higher surface tension is a ubiquitous process which provides rapid and efficient surface transport of organic or biological material. For a source of constant concentration, the leading edge of a nonvolatile, immiscible film driven to spread by gradients in surface tension is known to advance ast3/4in time. Recent experiments using laser shadowgraphy to detect the advancing front of spreading films indicate, however, that immiscible but volatile sources of constant concentration spread with a reduced exponent according tot1/2. Using a novel technique whereby fluorescent lines are inscribed in water, we have detected the evolution of a thermal instability beneath the leading edge of volatile films which strongly resembles a Rayleigh-Be´nard roll. We propose that the increased dissipation from this rotational flow structure is likely responsible for the reduction in spreading exponent. This observation suggests a conceptual framework for coupling the effects of evaporation to the dynamics of spreading. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869678

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The feasibility of controlling flow patterns of Rayleigh–Be´nard convection in a fluid layer confined in a circular cylinder heated from below and cooled from above (the Rayleigh–Be´nard problem) is investigated numerically. It is demonstrated that, through the use of feedback control, it is possible to stabilize the no-motion (conductive) state, thereby postponing the transition from a no-motion state to cellular convection. The control system utilizes multiple sensors and actuators. The actuators consist of individually controlled heaters positioned on the bottom surface of the cylinder. The sensors are installed at the fluid’s midheight. The sensors monitor the deviation of the fluid’s temperatures from preset desired values and direct the actuators to act in such a way so as to eliminate these deviations. The numerical predictions are critically compared with experimental observations. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869679

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The onset of convection driven by centrifugal buoyancy in the annular region between two coaxial corotating cylinders is investigated when the height of the annular region not only depends on the distance from the axis as in most previous works on the problem but on the azimuthal coordinate as well. The onset of convection in the form of stationary flows or in the form of thermal Rossby waves is investigated in dependence on the parameters of the problem. The evolution of the various modes with increasing amplitude of motion is followed through integrations in time of the fully nonlinear basic equations. Various forms of periodic, quasiperiodic and aperiodic time dependence are found. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869680

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The transition from two-dimensional thermoconvective steady flow to a time-dependent flow is considered for a liquid with a high Prandtl number(Pr=105)in a liquid bridge with a curved free surface. Both thermocapillary and buoyancy mechanisms of convection are taken into account. The computer program developed for this simulation transforms the original nonrectangular physical domain into a rectangular computational domain. To solve the problem in body-fitted curvilinear coordinates, the time-dependent Navier–Stokes equations were approximated by central differences on a stretched mesh. For liquid bridges with a flat interface, the instability corresponding to an azimuthal wave number ofm=0is not found for the investigated range of Marangoni numbers. The instability corresponding to anm=0is found for relatively low Marangoni numbers only in liquid bridges with a nonflat, free surface, and nonzero Rayleigh number. The steady state becomes unstable to axially running waves. It is shown that the onset of instability depends strongly upon the volume of the liquid. The stability boundary is reported for the aspect ratio&Ggr;=height/radius=4/3and for a wide range of liquid bridge volumes. The physical mechanism of the oscillations is based on the temporal interaction of the temperature sensitive free surface with the small local disturbances, created by temperature distribution inside the liquid bridge. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869681

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper the generation and evolution of an edge-wave packet are studied experimentally and numerically. In the laboratory an edge-wave packet is first generated on a sloping beach by a hinge-type wave-maker. Both the free surface displacement and velocity field are measured along several on-offshore cross sections. Numerical results are also obtained by solving the linear shallow-water wave equations and are compared with experimental data. Numerically predicted wave evolution characteristics are in good agreement with those shown by laboratory data. Analyses of the wave amplitude density spectra of both numerical solutions and experimental data show that wave packets are indeed trapped in the nearshore region and consist of a mixture of Stokes and higher-mode edge waves. Furthermore, the Stokes mode dominates in the low frequency range. Two additional wave-maker designs, i.e., the piston-type and the reverse hinge-type, are investigated numerically. Away from the wave-maker the wave forms (time histories) of the wave packets are insensitive to the details of wave-maker movements. The effects of beach slope on the evolution of wave packets are investigated. The behavior of the velocity field and the attenuation rates of runup amplitudes are also discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869682

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Experiments on the onset of buoyant instabilities leading to periodic formation of vortical structures in planar buoyant plumes of helium and helium/air mixtures injected into quiescent air are reported for a range of nozzle widths(w=20–70 mm),plume fluid densities (pure helium to that approaching air), and velocities at the nozzle exit. First, the plume parameters corresponding to the onset of the oscillatory instability were experimentally determined by varying the nozzle exit velocity for different nozzle widths and plume fluid densities in two different nozzle configurations. These configurations corresponded to a freestanding rectangular nozzle and a rectangular nozzle surrounded by a flat plate in the plane of the nozzle exit. The observed plume behavior in the near field was characterized as nonoscillatory, transitional, or pulsatile. The onset of pulsations in the near field of these buoyant plumes (within a height of two nozzle widths) was best correlated in terms of the plume source Reynolds number and the plume fluid to ambient density ratio. It was also found that the boundary conditions surrounding the nozzle exit had an influence on the onset of plume instability in the near field. Specifically, at a given plume to the ambient density ratio, the plumes with flat plate surround were found to transition to the oscillatory state at a lower value of the threshold velocity and therefore are less stable than the plumes originating from freestanding nozzles. Subsequently, the plume oscillation frequencies were measured as a function of plume width, plume source velocity, and the density ratio for a range of these parameters. The plume oscillation frequency was found to correlate well in terms of the nondimensional parameters, Strouhal number,S=(fw)/Vp,and Richardson number,Ri=[(&rgr;∞−&rgr;p)gw]/&rgr;∞Vp2,yielding a correlationS=0.55Ri0.45determined for1<Ri<102.This correlation is somewhat different from that of the axisymmetric buoyant plumes, which can be attributed to the differences in mixing rates and the strength of the local buoyancy flux in planar and axisymmetric plumes. The vortical structures formed in the unstable plumes also exhibit several distinct vortex pair modes. The centers of the formed vortex pairs, in general, do not remain colinear and distort with respect to each other when compared with the axisymmetric plume vortex rings, which are toroidal. The convection speeds of the vortex pair centers were also measured and reported in this study. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869683

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In a recent experimental investigation, Webster and Longmire [Phys. Fluids9, 655 (1997)] reported that the large-scale jet structures from inclined nozzles, which consisted of continuous inclined vortex rings, would undergo breakdown if the inclined angle of the nozzle was sufficiently large. They attributed the breakdown to the presence of longitudinal vorticity, but did not elaborate on the mechanism involved. In this paper, we examined the above issue by focusing primarily on the large-scale structures of the inclined jet (i.e., the inclined vortex rings). To the author’s knowledge, this area of research remains relatively unexplored. A study of it would certainly help to shed light on the mechanism involved in the breakdown of the inclined jets. Here we investigated the effects of the Reynolds number, the nozzle’s inclined angle, andL/D(see below) on the evolution of inclined vortex rings. Nozzles with the inclined angle of 5°, 10°, 20°, and 45° were considered, and the Reynolds number of the flow ranged from 1447 to 4824. TheL/Dwas varied from 0.77 to 1.92, whereLis the length of the slug of fluid ejected through the nozzle of diameterD.The results showed that when inclined vortex rings were formed, they were subjected to a differential rate of vortex stretching, due in part to a nonaxisymmetric vortex roll-up. As a result, circumferential flow was produced in the vortex core which increased with the nozzle angle and the Reynolds number. If sufficiently large, the circumferential flow was found to lead to core breakdown by initiating a wavy instability in the vortex filament which subsequently developed into a “bubble-type” breakdown and then a “double-helix”-type breakdown before the ring disintegrated into a chaotic motion. A simple physical model describing this transition process is proposed. However, if the circumferential flow was low, core bulging (or core swelling) might occur instead. A breakdown chart plotted using the Reynolds number andL/Dfor different nozzle angles is presented. The chart enables one to determine the flow conditions under which the core breakdown would occur. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869684

出版商:AIP    

年代:1998

数据来源: AIP