摘要:

It is now well‐known that capillary–gravity waves of solitary type exist in deep water. These waves bifurcate from a train of trivial periodic waves when the wave number corresponds to the minimum of the dispersion curve. Such solitary waves are characterized by decaying oscillations in their tail. In this Letter, it is shown how these recently discovered waves can be obtained as the limit of periodic waves as the wavelength is continuously increased. Moreover, the existence of new branches of solitary waves is conjectured.

ISSN:1070-6631    

DOI:10.1063/1.868174

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

This Letter observes collapse and intensification of the two parameter (b,c) ‘‘Lissajous‐elliptic’’ vortex ring. Laboratory and direct numerical studies of this ring are proposed to elucidate near‐singular and intermittent fluid phenomena at very high Reynolds number. Quantifications of single filament Biot–Savart numerical simulations withvariouscore ‘‘radii’’ show that collapse may benonmonotonicin time. In the collapsing region, the largest positive strain‐rate eigenvalue, &agr;, isoffthefilamentand exhibits self‐similar, singular‐like behavior. A signature of collapse is found in the local approach to zero of the filament energy density in the collapsing regions.

ISSN:1070-6631    

DOI:10.1063/1.868175

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The spectral transfer of passive scalars in a self‐similar state is studied by direct numerical simulations of stationary isotropic turbulence. At high wave numbers the dominant process is forward cascading local transfer as a result of nonlocal triadic interactions that couple together two similarly high wave‐number scalar modes by one low wave‐number velocity mode. This indicates that advection by the energy‐containing eddies transfers the scalar fluctuations towards smaller scales.

ISSN:1070-6631    

DOI:10.1063/1.868176

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The relative sliding motion of a smooth plate over a periodically finned plate is studied under the Stokes limit. The resulting harmonic or biharmonic equations with mixed boundary conditions are solved by eigenfunction expansions and collocation. The drag is determined as a function of fin height and fin spacing.

ISSN:1070-6631    

DOI:10.1063/1.868177

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

This paper employs computer simulations to test the theory of Jenkins [J. Applied Mech.59, 120 (1992)] for the interaction between a rapid granular flow of spheres and a flat, frictional wall. This paper examines the boundary conditions that relate the shear stress and energy flux at the wall to the normal stress, slip velocity, and fluctuation energy, and to the parameters that characterize a collision. It is found that while the theory captures the trends of the boundary conditions at low friction, it does not anticipate their behavior at large friction. A critical evaluation of Jenkins’ assumptions suggests where his theory may be improved.

ISSN:1070-6631    

DOI:10.1063/1.868178

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The influence of convection caused by horizontal heat transfer through the sides of a vertical Bridgman apparatus is studied analytically. The case when the heat transfer across the side walls is small is considered, so that the resulting interfacial deformation and fluid velocities are also small. This allows one to linearize the Navier–Stokes equations and express the interfacial conditions about a planar interface through a Taylor expansion. Using a no‐tangential stress condition on the side walls, asymptotic expressions for both the interfacial slope and radial segregation at the crystal–melt interface are obtained in closed form in the limit of large thermal Rayleigh number. It is suggested that these can be reduced by appropriately controlling a specific heat transfer property at the edge of the insulation zone on the solid side.

ISSN:1070-6631    

DOI:10.1063/1.868179

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

In this paper acoustic agglomeration of aerosols is studied in its simplest form, using a time‐averaged form of the coalescence equations. An acoustic coagulation kernel is introduced that neglects all nonlinear effects, as well as particle interactions. Based on this model, it is shown that an acoustic frequency exists that optimizes the coalescence process. This optimum frequency does not have a unique value for all distributions, but for size distributions that are not too wide, it is given by &ohgr;opt=1/&tgr;*, where &tgr;* is the relaxation time of a particle having a diameter corresponding to the mode count diameter of the initial size distribution function. The paper also includes the result of a numerical integration of the coalescence equations for typical aerosol distributions, which show their evolution under the influence of a sound wave.

ISSN:1070-6631    

DOI:10.1063/1.868180

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The connection between the large‐scale flows observed in thermal convection and the drift of the roll structures in large aspect ratios is examined. The analysis is based on a perturbative expansion of slightly deformed, finite‐amplitude straight rolls. The derivation assumes that the aspect ratio of the container is large, so that the flow is translation invariant. A scale separation technique is used to smooth out the small‐scale dynamics. If the unperturbed rolls cannot support a mean velocity field, the dynamics of the perturbations is diffusive and the mean flow is driven by roll bending, stretching, or pinching. If the unperturbed state supports a mean velocity field, the perturbations in the roll structure can be advected or even can propagate as a dispersive wave. In the present formulation, pattern defects arise from resonance and multivaluedness, with the implication that they can be advected or even travel.

ISSN:1070-6631    

DOI:10.1063/1.868181

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An experimental study has been performed on the structure of an electrostatic spray of monodisperse droplets. Such a spray is established when a liquid with sufficient electric conductivity and moderate surface tension, in the present case heptane doped with an antistatic additive, is fed through a small metal tube maintained at several kilovolts relative to a ground electrode a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up into charged droplets that disperse into a fine spray. Flash shadowgraph of the breakup region showed that the jet initially breaks into droplets of bimodal size distribution by varicose wave instabilities. The spray monodispersity is established farther downstream by a segregation process of electrostatic and inertial nature that confines the bulk of the mass flow rate (97%) and 85% of the total current in a core of nearly monodisperse primary droplets, with the remainder in a shroud of satellites. Droplet size, axial velocity, and concentration were measured throughout the spray by phase Doppler anemometry (PDA). The complementary use of these measurements permitted the determination of the electric field via the spray momentum equation.It was found that droplets are ejected from the jet at a relatively high velocity in a region characterized by a very intense electric field. They maintain this velocity farther downstream because of inertia, even though the field is precipitously decreasing, and ultimately decelerate under the action of the drag force and a progressively weaker electrostatic force. Velocity and concentration fields were shown to be self‐similar. Comparison between the external field, due to the potential difference applied between the electrodes, and the space charge field shows that the dropletaxialmotion is driven primarily by theexternalfield, whereas the dropletradialmotion and, consequently, the jet lateral spreading, is controlled primarily by thespacechargefield. The latter is typically at least one order of magnitude smaller than the external one, except at off‐axis locations near the breakup region of the spray, where the two fields can be comparable. The droplet charge distribution was also determined via the spray momentum equation and the simultaneous measurements of droplet size and velocity in a region where droplets experience negligible acceleration. The charge distribution was found to be narrow, with a ratio of standard deviation over mean of 0.15.

ISSN:1070-6631    

DOI:10.1063/1.868182

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Two‐dimensional spin‐up in a rectangular geometry is examined by a numerical computation of the Navier–Stokes equations. Cells are, in most cases, created by the vorticity generated on the top and bottom boundaries. Further, the movement and interaction of those vortices play a key role in establishing the cell pattern in the quasisteady state. The critical phenomenon in the cell‐pattern selection (a small perturbation can result in a completely different cell pattern) observed by previous investigators turns out to be caused by the critical movement of vortices; that is, a pair of vortices, for instance, can merge with or part away from each other by a small perturbation. The final quasisteady flow decays slowly at high Reynolds numbers having a linear relationship between the streamfunction and the vorticity in the core inviscid region.

ISSN:1070-6631    

DOI:10.1063/1.868183

出版商:AIP    

年代:1994

数据来源: AIP