摘要:

The spatial transport of Reynolds stresses by the pressure fluctuations, the so‐called pressure transport, is analyzed by investigating the potential flow equation for the fluctuating pressure. Adopting the convection velocity concept for the unsteady term in the Euler equation, and integrating over space, the pressure–velocity correlation vector is decomposed into a bulk convective transport term (rapid part) and a turbulent diffusion term (slow part). Subsequent modeling of the convection velocity in terms of the intensity of turbulence, skewness, and the velocity scale of large‐scale eddies results in a new computational model for the pressure transport.

ISSN:1070-6631    

DOI:10.1063/1.868408

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Previous observations of three flow patterns generated by shock acceleration of a thin perturbed, fluid layer are now correlated with asymmetries in the initial conditions. Using a different diagnostic (planar laser Rayleigh scattering) than the previous experiments, upstream mushrooms, downstream mushrooms, and sinuous patterns are still observed. For each experiment the initial perturbation amplitude on one side of the layer can either be larger, smaller, or the same as the amplitude on the other side, as observed with two images per experiment, and these differences lead to the formation of the different patterns.

ISSN:1070-6631    

DOI:10.1063/1.868447

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Gram quantities of molten aluminum droplets at temperatures up to 1973 K are forced to interact with water under sustained pressure pulses of up to 40.8 MPa in a hydrodynamic shock tube. Conditions are identified under which the thermal interaction (physical regime) develops into chemical ignition and total combustion events. This, and the absence of catastrophic breakup in the physical regime, raises some very interesting questions with regard to the sequence of events during large‐scale aluminum–water explosions.

ISSN:1070-6631    

DOI:10.1063/1.868409

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The expression for the pressure in lattice Boltzmann realizations of the Navier–Stokes equations involves compressibility effects. The pressure in two‐dimensional steady and unsteady cylinder flow is obtained numerically in a lattice Boltzmann scheme for the first time and compared with experimental and finite‐difference results.

ISSN:1070-6631    

DOI:10.1063/1.868410

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A time‐resolved dynamic light scattering technique is presented for measuring the velocity gradient in transient but repeatable flows. The experimental technique is verified via measurements for a Newtonian fluid undergoing a known time‐dependent flow. The method is then applied to the creeping flow of a Newtonian fluid in a corotating two‐roll mill. It is demonstrated that the flow near the stagnation point can be accurately described by an analytical creeping flow solution for a two‐roll mill in an unbounded fluid. The time dependence of the velocity gradient for a concentrated polymer solution in the startup of the two‐roll mill has also been measured, it is believed, for the first time. The measurement provides direct evidence of the modification of the flow for the viscoelastic polymer liquid, and will ultimately lead to significant insights into the polymer dynamics for concentrated solutions in strong, extension‐like flows. A second significant feature of the dynamic light scattering experiment is that the initial magnitude of the correlation function is related to the degree of optical anisotropy of the polymer molecules, i.e., to the geometric configuration of the polymer chains. Thus, it yields information on the time‐dependent degree of polymer orientation and stretch that is equivalent to birefringence, but is obtained at the ‘‘point’’ occupied by the scattering volume rather than as a two‐dimensional average across the whole fluid as in birefringence. This measurement of polymer configuration is compared with birefringence data for the exact same flow.

ISSN:1070-6631    

DOI:10.1063/1.868411

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

It is demonstrated that, for the slow advance of a viscous liquid onto a previously dry substrate, the well‐known moving contact line paradox is alleviated for liquids exhibiting power‐law shear‐thinning behavior. In contrast to previous models that allow contact‐line motion, it is no longer necessary to abandon the no‐slip condition at the substrate in the vicinity of the contact point. While the stress is still unbounded at the contact point, the equations of motion are shown to be integrable. A three‐constant Ellis viscosity model is employed that allows a low‐shear Newtonian viscosity, and may thus be used to model essentially Newtonian flows where shear thinning only becomes important in the immediate vicinity of the contact point. Calculations are presented for the model problem of the progression of a uniform coating layer down a vertical substrate using the lubrication approximations. The relationship between viscous heating and shear‐thinning rheology is also explored.

ISSN:1070-6631    

DOI:10.1063/1.868412

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Spin coating of two commercially used polymer solutions is studied both theoretically and experimentally. Physical and rheological characterization of these solutions indicates that under the spinning conditions currently used they behave as nonvolatile, viscoelastic fluids with constant viscosity and elasticity. The corresponding Reynolds (Re) and Deborah (De) numbers are up to order unity. The theoretical analysis demonstrates and explains why, at very short times after the inception of impulsive spinning, the velocity and stress fields in such fluids develop in an oscillatory manner. The amplitude of these oscillations increases with the ratio of the retardation parameter to the Deborah number, whereas their damping rate gets smaller as De increases. Since these oscillations dissipate very rapidly, and before substantial thinning of the film takes place, the thinning rate, velocity, and shear stress components do not deviate eventually from those of a Newtonian fluid. Such a complete explanation of similar experimental findings has not been offered before. The radial normal stress component does increase considerably over its Newtonian value, and this explains certain ‘‘experimental practices.’’ Similar oscillatory development early on occurs even at higher Re, as long as Re∼De, but it is dissipated again, this time because of the abrupt thinning of the film. The theoretical results are in good agreement with experimental measurements of ‘‘dry film’’ thickness and with dynamical measurements of ‘‘wet film’’ thickness during spinning, which are reported herein for the first time. Care must be taken in reporting ‘‘dry film’’ thickness because the commercial solutions under study retain part of the solvent after ‘‘soft baking’’ over a hotplate. Complete solvent removal produces dry films, but requires treatment in a vacuum oven, higher temperatures, and longer heating times.

ISSN:1070-6631    

DOI:10.1063/1.868413

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The static shape of a drop levitated and flattened by an acoustic standing wave field in air is calculated, requiring self‐consistency between the drop shape and the wave. The wave is calculated for a given shape using the boundary integral method. From the resulting radiation stress on the drop surface, the shape is determined by solving the Young–Laplace equation, completing an iteration cycle. The iteration is continued until both the shape and the wave converge. Of particular interest are the shapes of large drops that sustain equilibrium, beyond a certain degree of flattening, by becoming more flattened at a decreasing sound pressure level. The predictions for flattening versus acoustic radiation stress, for drops of different sizes, compare favorably with experimental data.

ISSN:1070-6631    

DOI:10.1063/1.868414

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Steady‐state acoustic streaming flow patterns have been observed during the operation of a variety of resonant single‐axis ultrasonic levitators in a gaseous environment and in the 20–37 kHz frequency range. Light sheet illumination and scattering from smoke particles have revealed primary streaming flows which display different characteristics at low and high sound pressure levels. Secondary macroscopic streaming cells around levitated samples are superimposed on the primary streaming flow pattern generated by the standing wave. These recorded flows are quite reproducible, and are qualitatively the same for a variety of levitator physical geometries. An onset of flow instability can also be recorded in nonisothermal systems, such as levitated spot‐heated samples when the resonance conditions are not exactly satisfied. A preliminary qualitative interpretation of these experimental results is presented in terms of the superposition of three discrete sets of circulation cells operating on different spatial scales. These relevant length scales are the acoustic wavelength, the levitated sample size, and finally the acoustic boundary layer thickness. This approach fails, however, to explain the streaming flow‐field morphology around liquid drops levitated on Earth. Observation of the interaction between the flows cells and the levitated samples also suggests the existence of a steady‐state torque induced by the streaming flows.

ISSN:1070-6631    

DOI:10.1063/1.868415

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The stability of dielectric liquid bridges between plane parallel electrodes when an electric potential difference is applied between them is studied for an axisymmetric configuration regarding arbitrary volume, axial gravity, and unequal coaxial anchoring disks attached to the electrodes. The stability is determined from the bifurcation diagrams related to the static problem. Two mathematical approaches are presented which are different in scope. First, the Lyapunov–Schmidt projection technique is applied to give the liquid bridge bifurcation diagrams for the bridge considered as an imperfect cylindrical one. The imperfection parameters, i.e., the relative difference of radii to the mean diameter, the deviation from the cylindrical volume, and the gravitational Bond number, are assumed to be small. Second, a Galerkin/finite element technique is used to obtain numerically bifurcation diagrams for arbitrary values of all the parameters. Agreement between both methods is good for small enough values of the imperfection parameters. The effect of the polarization charges existing at the free surface is highlighted. As in the absence of applied electric field, the gravitational Bond number and the relative difference of radii separately decrease the stability of the liquid column, but both effects conveniently combined may cancel out.

ISSN:1070-6631    

DOI:10.1063/1.868416

出版商:AIP    

年代:1994

数据来源: AIP