摘要:

A multiscale expansion approach is applied to the solution of the free‐boundary Stokes’ problem describing flow in the vicinity of a moving contact line. A solution free of singularities is obtained for the case of a liquid advancing into an inviscid medium. The force singularity reported in earlier studies is relaxed as the dynamic contact angle approaches &pgr; in the immediate vicinity of the moving contact line. This solution formally breaks down at a nonvanishing viscosity ratio of the receding and advancing fluids, but it still holds approximately for the case of a volatile liquid advancing into gas when Stefan flow in the gas phase is taken into account.

ISSN:0031-9171    

DOI:10.1063/1.863626

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

An asymptotic analysis of subcritical instability in double diffusive convection is presented. Using a modified perturbation method, a Landau equation that determines how the amplitude of the convection evolves in time is derived. From the Landau equation, it is found that in certain cases, stable finite amplitude convection can exist even when the rest state with no flow is locally stable. The perturbation analysis complements and unifies previous work which is primarily qualitative or numerical in character.

ISSN:0031-9171    

DOI:10.1063/1.863633

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The presence of shear instability can lead to the generation of turbulence in a stably stratified fluid. Using linear hydrodynamic stability theory, a turbulent kinetic energy budget is computed for an unbounded viscous shear layer for which the hyperbolic tangent shear layer is used to define the mean flow. For neutrally stable flows, the turbulent production due to the action of the Reynolds stress on the mean shear flow is balanced by viscous dissipation and the conversion to potential energy through the buoyancy term. The calculations show that maximum production of turbulent energy occurs at the center of the shear layer and that some of this energy is then transported to the edges of the shear layer where viscous dissipation occurs and some of the energy is converted to potential energy near the center of the flow. As the horizontal wavenumber decreases, the viscous dissipation becomes more important than the buoyancy term in removing energy from the flow. As the Reynolds number increases, the production term is concentrated over a narrower region at the center of the shear layer. Streamlines for neutral and stable conditions are presented to show the flow patterns of the disturbances in thex–zplane. Some possible atmospheric and oceanic applications are suggested in the last section.

ISSN:0031-9171    

DOI:10.1063/1.863614

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Taylor’s frozen turbulence hypothesis is investigated in a laboratory free‐convection mixed layer which can simulate buoyancy‐driven turbulence in an atmospheric mixed layer (of heighth) for mean wind speeds of up to several meters per second. For large turbulence intensities the temperature spectra at the height 0.1hare found to be spuriously enhanced at higher wavenumbers based on the frozen turbulence hypothesis, as deduced theoretically by previous investigators. The excess is borrowed from spectral intensities within the energy‐containing range. A theory based on the concept of longitudinal‐temporal isotropy, after proper scaling of the coordinates, is shown to predict a simple shift of the entire spectrum toward higher wavenumbers with increasing turbulence intensity. The failure of the observed spectra to behave this simply is associated with a complicated structure of the correlation coefficient as a function of longitudinal‐temporal lag coordinates when the former is measured relative to a frame moving with the mean flow speed. The temperature spectrum based on the frozen‐turbulence hypothesis appears to be a satisfactory representation of the true spectrum in wavenumber space, in tubulence dominated by thermal convection, if the mean wind speed exceeds 2.7–3.6 times the root‐mean‐square horizontal velocity fluctuation.

ISSN:0031-9171    

DOI:10.1063/1.863623

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Moments, up to order eight, of the structure function of the steamwise velocity fluctuation, have been measured in both laboratory and atmospheric turbulent shear flows. The Reynolds number dependence of structure functions evaluated for a separation equal to the Taylor microscale is closely approximated by both lognormal and &bgr; models, at least for moments up to order six. Both models predict identical inertial subrange behavior for the sixth‐order structure functions. This prediction is in good agreement with experiment when the value of 0.2 is used for the universal exponent &mgr;. This value has been obtained from correlations of the squared velocity derivative fluctuations, calculated for both laboratory and atmospheric measurements over the inertial subrange. The slope of the correlation is related to that of the sixth‐order structure function, in agreement with a conjecture by Frisch, Sulem, and Nelkin. When plotted against moments of ordern+2, moments of even ordernexhibit a power‐law behavior whose exponent is in closer agreement with the lognormal than the &bgr; model.

ISSN:0031-9171    

DOI:10.1063/1.863624

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The influence of the mixture density and the reaction zone length on the static and dynamic impulse of the detonations through wheat dust, RDX dust, and decane droplet air mixtures was studied. A numerical solution of the detailed model of the two‐phase detonation initiation and development in the medium, provided the basis for the study. Mixtures with a higher average density were found to produce a significantly greater static impulse, but a reduced dynamic impulse. The reasons for this behavior were discussed.

ISSN:0031-9171    

DOI:10.1063/1.863625

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The Chester, Chisnell, and Whitham approximation has been applied to the Sedov problem of an explosion in a medium of uniform density. It is noted that the error in the propagation parameterKis 70% or more. The reason for this discrepancy has been explained.

ISSN:0031-9171    

DOI:10.1063/1.863627

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The initial transient response of a collisionless plasma to a high positive voltage step is investigated. Four different pulses are observed. An electron plasma wave pulse is followed by an ion burst. The latter is overtaken and absorbed by a highly supersonic ion pulse. Thereafter, an ion rarefaction pulse with roughly the ion acoustic velocity propagates into the other direction. The supersonic pulse is tentatively explained as the result of a transient Buneman instability. Together with the rarefaction pulse, it forms the first cycle of an instability which is customarily considered a current‐ driven ion acoustic one, but is, in fact, a potential relaxation instability.

ISSN:0031-9171    

DOI:10.1063/1.863628

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A theoretical and experimental description of the curvature‐induced electrostatic interchange mode in a simple mirror confined low‐&bgr; plasma is given. The frequency, growth rate , and nonflute‐like effects have been measured and compared with theory. Effects due to radial electric field, finite ion Larmor radius, line‐tying, wall radius, and parallel electron response are discussed.

ISSN:0031-9171    

DOI:10.1063/1.863629

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Experimental results are presented on the formation and dynamics of a rotating energetic proton ring in a magnetic mirror field. An annular 430 keV proton beam from a magnetically insulated diode is injected through a cusp‐like magnetic field to form a rotating proton beam in a 2 m long solenoidal magnetic field. With 15–400 mT of neutral gas in the 40 cm diam experimental chamber up to 83% of the 350 J total beam energy is in rotation, the axial velocity dispersion is small, and the beam is sharply defined radially with inner and outer radii of 7 and 13 cm, respectively. The beam is 90%–100% axially current neutralized by currents induced in the beam‐generated plasma. Azimuthal plasma currents are observed in air, but not in hydrogen. In hydrogen a ring containing 5×1015protons is formed with sufficiently small axial velocity dispersion so that the protons are confined axially by their own 3% diamagnetic well as they propagate about 1.3 m in an 8 kG solenoidal field. In air, up to 15% of the beam axial energy is inductively coupled to the induced plasma currents, and ≳50% reflections are achieved from a 1.23 mirror ratio magnetic mirror located 2 m from the ion injector. About 1% inductive energy coupling to a wall resistor array was observed in both air and hydrogen. A peak ring diamagnetism of 875 G was observed 50 cm from the injector at a point where the applied field was 11 kG (near the peak of an upstream mirror); this ring contained about 1×1016protons.

ISSN:0031-9171    

DOI:10.1063/1.863630

出版商:AIP    

年代:1982

数据来源: AIP