摘要:

The flow close to separation is studied using a coordinate expansion in order to determine the effects of transverse curvature on the structure. The simplest expansion using the Goldstein variables leads to a Stewartson alternative in which either the curvature must vanish or the skin friction must be regular. This difficulty is overcome by the addition of logarithmic terms when the curvature is negative, but is unresolved for positive curvature. The analogy with the flat‐plate compressible boundary layer is pointed out (with heat transfer analogous to curvature), and it is concluded that it is misleading to suggest that the difficulties for a hot wall can be resolved by setting the heat transfer equal to zero.

ISSN:0031-9171    

DOI:10.1063/1.1693652

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

When molecular diffusion through a viscous surface boundary layer is the limiting factor in interfacial gas exchange, theory and experiment show that the periodic surface dilation produced by capillary ripples will decrease the average thickness of the boundary layer and increase gas transfer. This increase is independent of boundary layer thinning by turbulence. For surface dilations typical of the sea surface with fully developed capillary ripples, theCO2transfer may be 3.5 times the flat‐surface value.

ISSN:0031-9171    

DOI:10.1063/1.1693653

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

When vaporization occurs in the presence of a temperature gradient in the boundary layer, homogeneous nucleation and condensation of droplets may occur. This process is studied theoretically in a rotating disk flow system for iron vaporizing into an atmosphere of cold argon. The usual boundary‐layer conservation equations are provided with appropriate source or sink terms to account for the conversion of the gaseous diffusing species (monomer) to drops of liquid and vice versa. Conservation equations for the monomer and for drops of all sizes are required. The source and sink terms are developed from the classical theory of homogeneous nucleation. Simultaneous solution of the coupled monomer and droplet equations provides insight into the enhancement of the vaporization rate due to condensation and also into the structure of the nucleation, droplet growth, and re‐evaporation cycle which was found to take place in the boundary layer on the disk.

ISSN:0031-9171    

DOI:10.1063/1.1693654

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The stability of plane Couette‐Poiseuille flow between porous walls is considered. Injection of fluid through one wall and suction through the opposite wall leads to a nonparallel flow that satisfied the Navier‐Stokes equations. Neutral stability diagrams are presented for Poiseuille flow and for Couette‐Poiseuille flow. In both cases crossflow produces a significant increase in the critical Reynolds number.

ISSN:0031-9171    

DOI:10.1063/1.1693655

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Recent results on the central limit theorem for sums of dependent random variables are used to evaluate the asymptotic rates of turbulent stretching of material line and surface elements in the limitt→∞. The turbulence is assumed to be incompressible and statistically stationary and isotropic, and it is shown that the expectation value of the logarithmic rate of increase of the magnitude of the material elements vanishes att→∞. Possibilities for application to weak‐field magnetohydrodynamic turbulence are noted and some associated difficulties are discussed.

ISSN:0031-9171    

DOI:10.1063/1.1693656

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The homogeneous turbulence problem is formulated by first specifying the multipoint velocity correlations or their spectral equivalents at an initial time. Those quantities, together with the correlation or spectral equations are then used to calculate initial time derivatives of correlations or spectra. The derivatives are in turn used in time series to calculate the evolution of turbulence quantities with time. When the problem is treated in this way, the correlation equations are closed by the initial specification of the turbulence, and no assumption is necessary for closing those equations. An exponential series, which is an iterative solution of the Navier‐Stokes equations, gave much better results than a Taylor power series when used with the limited available initial data. In general, the agreement between theory and experiment was good.

ISSN:0031-9171    

DOI:10.1063/1.1693657

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Velocity derivatives obtained in atmospheric boundary layers over the open ocean are analyzed for correspondence to the lognormal distribution. The squared velocity derivative is found to be not lognormal, but its average over an interval within inertial subrange demonstrates excellent lognormality.

ISSN:0031-9171    

DOI:10.1063/1.1693658

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Using a simplified model, the governing equations of the motion of an isolated buoyant thermal are formulated. The general solutions which include both large and small density differences and both the initial accelerating motion and the final decelerating motion, are obtained. The asymptotic behavior for large time is given and compared with experimental results. The effect of various parameters, such as density difference, mass entrainment, and effective drag coefficient are also discussed.

ISSN:0031-9171    

DOI:10.1063/1.1693659

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The study of 62 wakes of 2.9‐mm water drops falling through a cloud of small droplets revealed approximately cylindrical disturbances 5 to 10 drop diameters wide and 1340 diameters long for drop speeds equivalent to 78% of their terminal speed of 8.0 msec−1. The wake disturbances lasted for about 9/16 sec at one place in space, in which time the air was displaced by about 0.5 cm in the direction of the moving drop. An evaluation of the flow pattern, based on velocity measurements showed that the averaged kinetic energy in the wake is extremely small and the work done by the drop on a layer of air with a cross section of the wake must be converted very rapidly into heat. The pattern and the decay of the measurable or gross vorticity, as well as the velocity gradients across the plane of observation are also given. The wake properties seem to agree with those of turbulent wakes; however, the development of a ringlike velocity maximum around the axis of symmetry is also possible. For rain it is concluded that the intensity of the small scale turbulence created by the falling drops is so small that, in general, no interactions are to be expected to affect their heat and mass transfer, their drag, or the collection of droplets by drops falling in the wake of another drop, at least for the wake ranges for which the results are applicable.

ISSN:0031-9171    

DOI:10.1063/1.1693660

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

An accurate theoretical analysis of the thermal creep slip velocityq˜T,asyfor arbitrary&agr;, the fraction of the molecules that are diffusely reflected from the surface, is carried out by applying the method of elementary solutions to the Bhatnagar‐Gross‐Krook model. Specifically,q˜T,asyis expressed in terms of two power series that, respectively, converge rapidly for&agr;→1and&agr;→0, and it is found thatq˜T,asydepends only slightly on&agr;. Some alternate forms of boundary conditions are also discussed, and it is shown that the thermal creep slip velocity does not depend on&agr;p, the momentum accommodation coefficient. These results confirm the essential accuracy of a very general variational expression given recently by one of the authors. Further, some consequences of the present results in the analysis of the thermal transpiration experiments are also discussed.

ISSN:0031-9171    

DOI:10.1063/1.1693661

出版商:AIP    

年代:1971

数据来源: AIP