ISSN:0899-8213    

DOI:10.1063/1.3480470

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Surfactant molecules adsorb onto the interfaces of moving fluid particles and are convected to regions in which the surface flow converges. Accumulation of surfactant in these regions creates interfacial tension gradients that retard the surface flow. In this study it is argued theoretically and demonstrated experimentally that fluid movement on the surface of a drop or bubble can remain unhindered in the presence of a single adsorbed surfactant if, relative to the convective rate of transport of adsorbed surfactant along the surface, desorption is fast, and the bulk concentration is high enough so that diffusion away from the particle is fast. For this circumstance, a uniform surface concentration of surfactant is maintained, and no gradients in surface tension arise to retard the surface velocity. The fluid particle flow behaves as it would in the absence of surfactant save that it has a reduced, uniform surface tension. The remobilization of surfactant‐laden interfaces of fluid particles is demonstrated experimentally in a three‐phase periodic slug flow in a capillary tube in which a train of alternating air and aqueous slugs ride on an annular wetting film of fluorocarbon oil. Surfactant, dissolved in the aqueous slug phase, adsorbs onto and retards the aqueous–oil interface. The hydrodynamics of this flow is such that small changes in the mobility of this interface create large shear rates in the oil layer. This significantly increases the pressure drop required to drive the slug train at constant velocity. Three surface adsorbers are used to demonstrate surface remobilization: The polyethoxy, nonionic surfactants Triton X‐100 and Brij‐35, which have fast desorption kinetics and do not retard the surface flow at high concentrations and, as a counter example, the desorption hindered protein bovine serum albumin, which is shown to be unable to remobilize an interface even at high concentration.

ISSN:0899-8213    

DOI:10.1063/1.857862

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The drift of a deformable droplet of low viscosity (viscosity ratio &lgr;=0.08) in a Couette device is examined. The drift is measured both in the plane of shear (due to the rigid outer bounding walls of the Couette device) and also normal to the plane of shear (due to the upper bounding stress‐free surface). A general relationship between normal stresses induced by the deformation of a droplet in an arbitrary shear flow and the leading‐order drift normal to rigid and stress‐free plane surfaces is described theoretically. This relationship is consistent with previous theoretical predictions for droplet migration in shear flows, and is used to compare results from the drift measurement experiments with first‐order deformation theories. The measured drift velocities are in reasonable agreement with the theory of Schowalteretal. [J. Colloid Interface Sci.26, 152 (1968)].

ISSN:0899-8213    

DOI:10.1063/1.857856

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Rectangular supersonic free and coaxial jets were used to enhance mixing relative to a circular jet in a convective Mach number range of 0.5 to 2.2. The different convective Mach numbers were obtained by changing the central jet gas composition, the temperatures of the inner and outer flows, and the velocity of the coaxial flow. The experimental techniques used were schlieren photography, total pressure, and gas‐sampling measurements. For all test conditions the rectangular jets showed substantial improved mixing relative to a circular jet. The free jets showed high mixing in the circumferential region of the jet while the coaxial jet had a high mixing rate inside the central jet.

ISSN:0899-8213    

DOI:10.1063/1.857860

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Streamline patterns in experiments performed by Boyer and Biolley [Philos. Trans. R. Soc. London Ser. A318, 411 (1986)] on flow past a shallow ridge on the floor of a channel in a rotating, stratified fluid compare very poorly with two‐dimensional theory, even though the aspect ratio of the ridge is relatively large at 3. In the present study, it is shown that a fully three‐dimensional calculation agrees quite well with the experiments, both in terms of the qualitative form of the streamline patterns, and also with respect to a quantitative measure of streamline deflection. The equations are solved by means of Fourier series across the channel and Fourier integral in the streamwise direction. The resultant double Fourier series is truncated, and the partial sum computed by machine. The form of the solution is relatively easily extended to other obstacle shapes.

ISSN:0899-8213    

DOI:10.1063/1.857861

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Computer simulations of two‐dimensional rapid granular flows of uniform smooth inelastic disks under simple shear reveal a dynamic microstructure characterized by the local, spatially anisotropic agglomeration of disks. A spectral analysis of the concentration field suggests that the formation of this inelastic microstructure is correlated with the magnitude of the total stresses in the flow. The simulations confirm the theoretical results of Jenkins and Richman [J. Fluid Mech.192, 313 (1988)] for the kinetic stresses in the dilute limit and for the collisional stresses in the dense limit, when the size of the periodic domain used in the simulations is a small multiple of the disk diameter. However, the kinetic and, to a lesser extent, collisional stresses both increase significantly with the size of the periodic domain, thus departing from the predictions of the theory that assumes spatial homogeneity and isotropy.

ISSN:0899-8213    

DOI:10.1063/1.857863

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The processes that bring about the change of cell size in the evolution of salt‐finger convection are investigated with a numerical model of the convection in a Hele–Shaw cell. It is shown that the increase of cell width during the convection is produced by the vertical penetration of increasingly wider cells from the edges of the finger zone into the interior, as has been observed in a laboratory experiment. The increase of scale is also shown to occur through the merging process in which narrow finger cells merge to form wider cells. Occasionally, transition from wide to narrow scale can occur, in which case the wide finger cell splits to form two or more narrow cells. The scale transition produced by the merging, penetration, and splitting processes is shown to have the effect of maximizing the buoyancy flux generation in an evolving finger convection. This maximization is also interpreted in terms of the most rapidly growing finger mode. The effect of the scale transition on the actual magnitude of the buoyancy flux is related to the energy dissipation of fingers.

ISSN:0899-8213    

DOI:10.1063/1.857864

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A unified analysis delineating the conditions under which the equations of classical incompressible and compressible hydrodynamics are related in the absence of large‐scale thermal, gravitational, and field gradients is presented. By means of singular expansion techniques, a method is developed to derive modified systems of fluid equations in which the effects of compressibility are admitted only weakly in terms of the incompressible hydrodynamic solutions (hence ‘‘nearly incompressible hydrodynamics’’). Besides including molecular viscosity self‐consistently, the role of thermal conduction in an ideal fluid is also considered. With the inclusion of heat conduction, it is found that two distinct routes to incompressibility are possible, distinguished according to the relative magnitudes of the temperature, density, and pressure fluctuations. This leads to two distinct models for thermally conducting, nearly incompressible hydrodynamics—heat‐fluctuation‐dominated hydrodynamics (HFDH’s) and heat‐fluctuation‐modified hydrodynamics (HFMD’s). For the HFD case, the well‐known classical passive scalar equation for temperature is derived as one of the nearly incompressible fluid equations and temperature and density fluctuations are predicted to be anticorrelated. For HFM fluids, a new thermal transport equation, in which compressible acoustic effects are present, is obtained together with a more‐complicated ‘‘correlation’’ between temperature, density, and pressure fluctuations. Although the equations of nearly incompressible hydrodynamics are envisaged principally as being applicable to homogeneous turbulence and wave propagation in low Mach number flow, it is anticipated that their applicability is likely to be far greater.

ISSN:0899-8213    

DOI:10.1063/1.857865

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Long time asymptotic behavior resulting from nonlinear interaction between long and short waves is examined numerically by using the Fourier expansion method. A slightly modulated short wave is adopted as an initial condition. The results exhibit recurrence or chaotic motion, depending upon the magnitude of the control parameters involved in the governing equations. It is found that the chaotic motion is possible, even in a case for which only a single unstable mode exists in the Fourier modes.

ISSN:0899-8213    

DOI:10.1063/1.857866

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The result of an experimental investigation into the natural near‐field transition of an untuned planar jet at moderate Reynolds number is presented. Here the term ‘‘untuned’’ refers to the case where the ratio of the thin shear layer instability frequency to the jet column frequency is not given by an integer power of 2, so that a sequence of shear layer vortex pairing or amalgamation events is incapable of yielding the jet column frequency near the end of the potential core. This case is of interest because the jet shear layer instability must undergo more dramatic frequency and phase adjustments in order to satisfy the downstream jet column constraint. This provides a unique opportunity to investigate how instabilities that scale with the jet column interact with those that scale with the nascent shear layer instability to configure the initial evolution of the natural planar jet. Auto‐bicoherence spectra are used in conjunction with conventional power spectra in order to provide quantitative measurements of the nonlinear phase coupling between wave triads that characterizes the near‐field transition of the natural planar jet. These measurements are complemented by two‐point correlation, coherence, and phase spectra that document the streamwise evolution and cross‐stream symmetry of structural patterns in the flow throughout the initial, interaction, and early self‐preserving regions. These measurements indicate that the wave interactions that characterize the planar jet near‐field transition are quite different from the sequence of subharmonic instabilities that typically characterize the planar mixing layer. In particular, suppression of the subharmonic instability and the formation of modulating sidebands are observed. The modulation occurs at the jet column frequency and the measurements suggest that this has an origin that is due to a kinematic effect associated with the lateral oscillation of the nascent shear layers near the nozzle lip. The origin of this oscillation appears fully consistent with Biot–Savart induction from the downstream region of the flow associated with the loss of symmetry of the large‐scale vorticity field.

ISSN:0899-8213    

DOI:10.1063/1.857867

出版商:AIP    

年代:1991

数据来源: AIP