摘要:

An experimental study on turbulent pipe flows was conducted with a view to reduce their friction drag by oscillating a section of the pipe in a circumferential direction. The results indicated that the friction factor of the pipe is reduced by as much as 25&percent; as a result of active manipulation of near-wall turbulence structure by circular-wall oscillation. An increase in the bulk velocity was clearly shown when the pipe was oscillated at a constant head, supporting the measured drag reduction in the present experiment. The percentage reduction in pipe friction was found to be better scaled with the nondimensional velocity of the oscillating wall than with its nondimensional period, confirming a suggestion that the drag reduction seem to be resulted from the realignment of longitudinal vortices into a circumferential direction by the wall oscillation. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869538

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A variety of wave patterns are found to form in a thin layer of sand inside a cylinder rotated about its horizontal axis of symmetry at constant angular velocity. In particular, we observe a spanwise instability characterized by serrated frontal shapes remarkably similar to those seen in Newtonian fluids. Within a certain parameter range, an accompanying spatial pattern forms on the rising side of the cylinder and travels along the cylinder span. The associated phase velocity is relatively constant, whereas the relevant wavelength increases quadratically with angular rotation speed. Standing waves appear at a critical rotation rate. Further, in some cases, a propagating cellular pattern forms on the surface of the medium. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869539

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper we use a lubrication approximation to investigate the locally unidirectional gravity-driven draining of a thin rivulet of Newtonian fluid down a slowly varying substrate. The work generalizes the recent study by Duffy and Moffatt [Chem. Eng. J.60, 141 (1995)] of gravity-driven draining down a locally planar substrate to include the effects of substrate variation transverse to the direction of flow. Asymptotic and numerical results are obtained for several simple convex and concave transverse substrate profiles. In all the cases investigated these results show a number of common features. In particular, they show that a single stable slowly varying rivulet running continuously from the top to the bottom of a large horizontal circular cylinder is possible only if the transverse substrate profile is a sufficiently shallow trough. If the profile is a deeper trough then no such rivulet is possible near the bottom of the cylinder, while if the profile is a ridge then no such rivulet is possible near the top of the cylinder. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869569

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The spontaneous spreading of a thin volatile film along the surface of a deep fluid layer of higher surface tension provides a rapid and efficient transport mechanism for many technological applications. This spreading process is used, for example, as the carrier mechanism in the casting of biological and organic Langmuir–Blodgett films. We have investigated the dynamics of spontaneously spreading volatile films of different vapor pressures and spreading coefficients advancing over the surface of a deep water support. Laser shadowgraphy was used to visualize the entire surface of the film from the droplet source to the leading edge. This noninvasive technique, which is highly sensitive to the film surface curvature, clearly displays the location of several moving fronts. In this work we focus mainly on the details of the leading edge. Previous studies of the spreading dynamics of nonvolatile, immiscible thin films on a deep liquid layer have shown that the leading edge advances in time ast3/4as predicted by laminar boundary layer theory. We have found that the leading edge of volatile, immiscible spreading films also advances as a power law in time,t&agr;, where&agr;∼1/2. Differences in the liquid vapor pressure or the spreading coefficient seem only to affect the speed of advance but not the value of the spreading exponent, which suggests the presence of a universal scaling law. Sideview laser shadowgraphs depicting the subsurface motion in the water reveal the presence of a single stretched convective roll right beneath the leading edge of the spreading film. This fluid circulation, likely caused by evaporation and subsequent surface cooling of the rapidly spreading film, resembles a propagating Rayleigh–Be´nard convective roll. We propose that this sublayer rotational flow provides the additional dissipation responsible for the reduced spreading exponent. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869546

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Discontinuous behavior of liquids between parallel and tilted plates in the absence of gravity is discussed. A principal finding, derived mathematically from the classical Young–Laplace–Gauss formulation for capillary free surfaces, is that in a large range of configurations liquid bridges between parallel plates are unstable with respect to small, even infinitesimal, tilting of one of the plates. Under a computationally based hypothesis of uniqueness of spherical bridges in a wedge, it is shown that the same discontinuous behavior prevails for all but very particular circumstances. The various liquid configurations, which form the basis for an experiment on board the Space StationMir, are characterized and illustrated. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869547

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The sedimentation of spheres in a Newtonian fluid is experimentally studied under creeping flow conditions. The mean particle settling velocity and the particle velocity fluctuations are measured across the width of the sedimentation cell. We show that there can be a global intrinsic convection of the suspension superimposed on the settling motion of the particles. Unlike the predictions of the dilute theories of intrinsic convection, this effect is found to be small and even to disappear with increasing concentrations. We also find that there is an ordering of the suspension near the wall, which may be responsible for the observed small magnitude of the convection. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869548

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The three-dimensional intrinsic convection in a monodisperse dispersion of spheres settling in a vertical container of arbitrary cross section is calculated using the simple model of point forces with excluded volume near the walls, proposed by Bruneau &etal; [Phys. Fluids8, 2236 (1996)]. An exact solution of the model equations for a container with rectangular cross-section shows that corners have no significant influence on the convection. A dense suspension is modeled by assuming an equilibrium particle distribution in the near wall region. It is predicted that as a result of near wall ordering, the intrinsic convection decreases with increasing particle volume fraction. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869549

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The lattice Boltzmann method (LBM) is used to simulate viscous fluid flow through a column of glass beads. The results suggest that the normalized velocity distribution is sensitive to the spatial resolution but not the details of the packing. With increasing spatial resolution, simulation results converge to a velocity distribution with a sharp peak near zero. A simple argument is presented to explain this result. Changes in the shape of the distribution as a function of flow rate are determined for low Reynolds numbers, and the large-velocity tail of the distribution is shown to depend on the packing geometry. The effect of a finite Reynolds number on the apparent permeability is demonstrated and discussed in relation to previous results in the literature. Comparison with velocity distributions from NMR (nuclear magnetic resonance) spectroscopy finds qualitative agreement after adjusting for diffusion effects in the NMR distributions. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869550

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Pulsed magnetic field gradient stimulated echo nuclear magnetic resonance (NMR) measurements are reported for the steady-state flow and diffusion of two and three phases (water, dodecane,N2gas) within a sample of a Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area,q,is used to derive the displacement probability,P&Dgr;(X)for fixed observation times&Dgr;, with the displacementsXbeing measured along the macroscopic pressure gradient. An extensive range of NMR experiments was carried out, starting with single-phase flow of either water (an aqueous solution of NaCl 3&percent; w/v) or oil (dodecane) for various relative saturation states. Following these experiments,P&Dgr;(X)were acquired for water and oil when both phases were forced to flow through the sandstone. Finally, NMR measurements were performed in which three phases (oil, water andN2gas) were flowing simultaneously. Using the NMR data it was possible to evaluate the physical importance of parameters such as wettability, spreading and phase saturations on the transport phenomena. To our knowledge, these experiments represent the first comprehensive NMR study of multiphase flow in porous media, and the extent of the information obtained is providing a strong experimental basis to validate and develop accurate modelling of fluid transport in porous solids. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869551

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The motion of non-neutrally buoyant prolate spheroidal particles in vertical shear flows is investigated. Using the generalized Faxen law, we calculate the hydrodynamic forces and moments acting on such inertial and inertialess particles, and their trajectories. The calculations are done for (i) freely rotating particles, and (ii) particles with orientations fixed by means of an external torque exerted by a strong orienting field. Inertial particles are found to migrate across the streamlines, and their trajectories differ considerably from those calculated for inertialess particles. Neutrally buoyant spheroids, inertial or not, which either freely rotate or have fixed orientations in shear flows, translate along the streamlines. Non-neutrally buoyant inertialess spheroids freely moving in simple shear flow translate along periodic trajectories with no net lateral drift. In contrast, inertial particles under similar flow conditions drift laterally toward locations characterized by higher local velocities in a direction opposing gravity. The motion of non-neutrally buoyant inertial particles with fixed orientations may be unstable with the drift velocity growing exponentially with time. Conditions for the occurrence of this unstable motion are formulated analytically in terms of particle and flow parameters. In general, the rate of drift depends on particle shape, via its aspect ratio, and its inertia. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869552

出版商:AIP    

年代:1998

数据来源: AIP