摘要:

The minimum molecular scale mixing rate of the turbulent plume was found by measurements of the plume ‘‘flame length’’ under various initial conditions. A simple derivation based on the self‐similar properties of plumes provides power law relations for the mixing rate, which agree well with the experiments.

ISSN:0899-8213    

DOI:10.1063/1.857607

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A kinetic theory is presented that predicts the dispersion of particle orientation, solely as a result of hydrodynamic interactions, about the principal axis of extension in uniaxial and planar extensional flows. The mean‐squared displacement of the orientation vector is calculated in both the asymptotic dilute and semidilute concentration regimes. In the dilute regime (wherenL3≪1, withnthe fiber number density andLthe fiber length) it is found that the mean‐squared displacement isO[nL3/ln2(r)], whereris the fiber aspect ratio. For semidilute suspensions (wherenL3≫1) it is predicted that the dispersion isO[ln(nL3)/nL3]. Thus the dispersion increases as the concentration is increased from infinite dilution and then ultimately decreases in the semidilute regime. The physical reasons for this behavior are discussed and, in particular, the semidilute scaling is demonstrated to be a consequence of the short‐range particle screening that develops in semidilute suspensions [Phys. Fluids A1, 3 (1989);2, 7 (1990)]. The effect of this orientational dispersion on the material properties of fiber composites is also considered.

ISSN:0899-8213    

DOI:10.1063/1.857608

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

From recorded observations in oscillatory flows over a bed sparsely covered with sand, it has been suggested that the initial stage of ripple formation involves hydrodynamic interaction at a distance among particles which are far apart. This mechanism appears to be common to both a thick layer of sand bed and a monolayer of beads on a smooth plane. In view of these facts it is helpful to study first a simpler problem that is of relevance in laminar flows. Specifically, the case where the typical particle radius is much smaller than the spacing, which is in turn much smaller than the thickness of Stokes boundary layer, is considered. With a view to examining the far field, a quasisteady Oseen approximation is invoked, and the boundary‐layer flow is replaced by a linear shear and the particle by a localized force distribution. The results are used to infer the tendency of interaction between two particles.

ISSN:0899-8213    

DOI:10.1063/1.857609

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Effective boundary conditions are derived to be used with the classical Hele–Shaw equations in calculating the shape and motion of a Hele–Shaw bubble. The main assumptions of this analysis are that the displaced fluid wets the plates, and that the capillary number Ca and the ratio of gap width to characteristic bubble length &egr; are both small. In a small region at the sides of the bubble, it is found that the thin‐film thickness scales with &egr;2/5 Ca4/5, rather than the Ca2/3scaling that is valid over most of the thin film above and below the bubble.

ISSN:0899-8213    

DOI:10.1063/1.857610

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The maximum stable length of nonisothermal liquid bridges is predicted through a series of numerical computations. The results show that the maximum length of the bridges is less than the Rayleigh limit determined by the isothermal capillary stability analysis. It is enhanced by increasing the strength of the thermocapillary convection, while it is reduced for higher percentage change of surface‐tension force or stronger buoyancy‐driven convection. For the cases of small parameters, the predictions are in qualitative and quantitative agreement with the previous asymptotic results.

ISSN:0899-8213    

DOI:10.1063/1.857611

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Droplet formation from capillary streams of viscous low vapor pressure fluids in a vacuum has been studied. A new form of capillary stream breakup, which entails the use of an amplitude‐modulated sinusoidal disturbance, has been employed extensively in this work. The genesis of droplets formed due to an amplitude‐modulated disturbance on a stream is reviewed in some detail and illustrated pictorially. Because capillary stream breakup and subsequent droplet propagation took place in a vacuum there were no significant interactions with the surrounding atmosphere. Sensitive measurements of the relative speeds of each drop have been made by allowing them to travel 6 m in a vertical vacuum chamber, and measuring the time between drops using an optical method. Speed differences as low as 3.5×10−7times the average stream speed have been measured. New information about capillary stream breakup is inferred by comparing the speed variations for droplet streams generated with both amplitude‐modulated disturbances and single‐frequency disturbances. An autoregressive model is developed and found to represent the deviations in time between droplets with consistency and reproducibility.

ISSN:0899-8213    

DOI:10.1063/1.857612

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Computations of finite‐amplitude, spatially periodic wave growth on a cylindrical jet have been carried out using a boundary integral method. The initial wave growth is in agreement with Rayleigh’s linear theory. When followed to completion these waves pinch off large drops separated by smaller satellite drops (spherules) that decrease in size with decreasing wavelength. The computed sizes of both drops and satellites agree with experiment. It is found that satellites will form for all unstable wave numbers. The small satellites that are computed at wave numbers near the critical wave number were not predicted by near‐linear analysis but are observed in experimental photographs of jet breakup. Computation of the collapse of elongated satellites shows short waves propagating on their surfaces.

ISSN:0899-8213    

DOI:10.1063/1.857613

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The mechanism of the instability of a fluid flowing down an inclined plane is clarified by the recognition that the Reynolds number can also be interpreted as the Froude number.

ISSN:0899-8213    

DOI:10.1063/1.857614

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The three‐dimensional (3‐D) instability of a parallel mean flow and a two‐dimensional (2‐D) wave is examined. Such secondary instability is shown to exist for the Gaussian profile as a model for the wake. Numerical results are obtained based upon a recently proposed wave–mean‐flow interaction mechanism, with the assumptions of temporal problem and inviscid flows. Classical 2‐D instability is strong in the near wake but decays to zero in the far wake, where it is overwhelmed by 3‐D instability. It is conjectured that this will qualitatively explain the emergence of three‐dimensionality in the far wake.

ISSN:0899-8213    

DOI:10.1063/1.857615

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A numerical investigation of the eigenvalue problems related to both the energy and the linear theory of stability of a Newtonian liquid between sliding pipes is presented. An enlargement of the region of nonlinear stability is shown as the radii ratio is increased. On the other hand, the flow always turns out to be linearly stable.

ISSN:0899-8213    

DOI:10.1063/1.857616

出版商:AIP    

年代:1990

数据来源: AIP