ISSN:1070-6631    

DOI:10.1063/1.3480126

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experiments are described which demonstrate that particle velocity fluctuations and hydrodynamic self‐diffusivities of sedimenting non‐Brownian spheres do not vary as the inner width of the vessel varies by a factor of 4. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868727

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Optical flow refers to an image processing technique originally developed for computer and robot vision applications. In this Letter, optical flow algorithms are utilized to obtain two‐dimensional velocity fields corresponding to fluid flows. The performance of the algorithms with regard to three flows is discussed: computer‐simulated suspension flow, bubble detachment from a submerged needle, andinvivoblood flow in a microvascular network. For small changes in successive images, the algorithms yield good quantitative and qualitative information about the flows. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868729

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Instantaneous velocity field in the three‐dimensional flow of an unstable vortex ring (Re=1360) in water has been measured by employing a newly developed holographic particle velocimeter, IROV. The vorticity distribution and circulation as a function of radius from the vortex core center are also presented. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868741

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The motion of non‐Brownian spheres settling in the midst of a suspension of like spheres has been experimentally studied under creeping flow conditions. A few glass spheres, marked with a thin coating of silver, were tracked in a suspension of unmarked glass spheres, made optically transparent by matching the index of refraction of the suspending fluid to that of the glass spheres. Particles were tracked with a real time digital imaging processing system. Particle trajectories were examined in the bulk region of the suspension for particle volume fractions ranging from 0% to 40% in 5% steps. Statistical analyses of local particle velocities yield the mean settling velocity, the RMS of the fluctuations of the vertical and horizontal particle velocity and the particle velocity autocorrelation functions. The long time fluctuating particle motion is demonstrated to be diffusive in nature. Vertical and horizontal correlation times and self‐diffusivities are found as a function of particle volume fraction, and a strongly anisotropic diffusion noted. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868733

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Aqueous surfactant solutions were atomized using an air assist atomizer, and the effects of dynamic surface tension on atomization were studied. The solutions differ from one another in their dynamic behavior, but all have the same equilibrium surface tension. The constant equilibrium surface tension was achieved by choosing surfactant concentrations above the critical micelle concentration (CMC). The dynamic change of surface tension in aqueous Tergitol NP‐10 solutions of 1.5, 3, 6, 12, and 60 mM was measured using the oscillating jet technique. While atomizing the liquid jet, it was found that frequencies and amplitudes of the flapping motion of the liquid jet increased with surfactant concentration showing that surface tension decreases in the region between the nozzle exit and breakup. The length of the intact liquid jet was found to be insensitive to surfactant concentration, suggesting that, due to sudden increase of surface area, surface tension is near its initial value in the breakup region. Measurements in the spray region show a strong influence of the dynamic change of surface tension on drop size. Finally, using an analysis based on a critical Weber number, the actual time average of surface tension was estimated at an axial location of 1.5× the intact length downstream from the nozzle exit, and an overall profile of surface tension as a function of axial location was proposed. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868725

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The secondary instabilities on a falling film which cause a monochromatic wave to evolve into solitary waves are examined with a weakly nonlinear theory. The unique phase speed dispersion relation dictated by inertia and capillarity is found to favor a nonlinear, two‐wave subharmonic resonance that causes neighboring crests to coalesce. This occurs when the fundamental wave frequency is below a critical value &ohgr;cwhich can be approximated by a nonlinear resonant frequency. An entire band of secondary waves are excited by this static subharmonic mechanism such that the coalescence occurs nonuniformly. The lower half of this band of secondary waves travels faster than the fundamental and induces the coalesced waves to move faster than the slower fundamental. For monochromatic waves with frequencies above &ohgr;c, a three‐wave oscillatory sideband instability is triggered which generates two secondary waves that are slower than the fundamental. This sideband instability involves a long‐wave modulation that causes several crests to coalesce simultaneously. Both secondary transitions are important intermediate stages of the route to spatiotemporal chaos involving solitary waves. ©1995 American Institute of Physics. 

ISSN:1070-6631    

DOI:10.1063/1.868726

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Two‐dimensional (2‐D) interfacial waves on flowing films are unstable with respect to both two‐ and three‐dimensional instabilities. In this paper, several distinct three‐dimensional instabilities that occur in different regions of the parameter space defined by the Reynolds numberRand the frequencyfof forced two‐dimensional waves are discussed in detail. (a)Asynchronous3‐Dinstability, in which spanwise deformations of adjacent wave fronts have the same transverse phase, appears over a wide range of frequency. These transverse modulations occur mainly along the troughs of the primary waves and eventually develop into sharp and nearly isolated depressions. The instability involves many higher harmonics of the fundamental 2‐D waves. (b) A 3‐Dsurbharmonicinstabilityoccurs for frequencies close to the neutral curvefc(R). In this case, the transverse modulations are out of phase for successive wave fronts, and herringbone patterns result. It is shown that this weakly nonlinear instability is due to the resonant excitation of a triad of waves consisting of the fundamental two‐dimensional wave and two oblique waves. The evolution of wavy films after the onset of either of these 3‐D instabilities is complex. However, sufficiently far downstream, large‐amplitude solitary waves absorb the smaller waves and become dominant. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868782

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Nonlinear interaction between a Tollmien–Schlichting wave and longitudinal rolls resulting from Rayleigh–Be´nard instability has been investigated in an unstably stratified plane Poiseuille flow of infinite extent. Cubic order amplitude equations for the interacting modes are derived on a weakly nonlinear basis in the neighborhood of a crossover point at which both modes become unstable on a linear basis simultaneously. A bifurcation analysis based on use of the actual numerical coefficients obtained from the governing equations and evaluated for various values of the Prandtl number is performed, and the results, such as the effect of longitudinal rolls on the subcritical instability characteristic of a Tollmien–Schlichting wave, are discussed. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868728

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Spherical Couette flow is studied with a view to elucidating the transitions between various axisymmetric steady‐state flow configurations. A stable, equatoriallyasymmetricstate discovered by Bu¨hler [Acta Mech.81, 3 (1990)] consists of two Taylor vortices, one slightly larger than the other and straddling the equator. By adapting a pseudospectral time‐stepping formulation to enable stable and unstable steady states to be computed (by Newton’s method) and linear stability analysis to be conducted (by Arnoldi’s method), the bifurcation‐theoretic genesis of the asymmetric state is analyzed. It is found that the asymmetric branch originates from a pitchfork bifurcation; its stabilization, however, occurs via a subsequent subcritical Hopf bifurcation. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868730

出版商:AIP    

年代:1995

数据来源: AIP