摘要:

One‐dimensional and joint probability density distributions for longitudinal components of turbulent velocities as well as higher‐order correlations are measured in a turbulent boundary layer on a flat plate using hot‐wire anemometry and high‐speed computing methods. The effect of the nonlinear response of the hot‐wire is taken into account. Data pertaining to the general nature of the turbulent boundary layer are presented and comparison is made between the measured correlations and those corresponding to a Gaussian probability distribution of turbulent velocities as well as to non‐Gaussian distributions of the Gram‐Charlier type. Similar comparisons are made of the measured one‐dimensional and joint probability distributions. Probability distributions in the boundary layer are also compared to those measured downstream of a grid. The closure of the tails of the probability distribution and its effect on the accuracy of the measurements of higher‐order moments is considered.

ISSN:0031-9171    

DOI:10.1063/1.1694421

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Aquasi‐isotropiclocally homogeneous model for the two‐point double‐velocity correlation tensor is developed by the application of a plausible set of symmetry restrictions to this tensor. The two‐point correlation is related to the single‐point correlation by three independent functions, and reduces to the nonisotropic single‐point correlation and the isotropic two‐point correlation in the limiting cases. It is shown that the model implies that by the use of three independent microscales (and macroscales) it is possible to describe a matrix of 27 elements of microscales (and macroscales) which describe the nonisotropic field. The model is also used to calculate the dissipation rate of the components of the Reynolds stresses. Finally, a formula which expresses the redistribution of the velocity fluctuations due to pressure fluctuations is developed.

ISSN:0031-9171    

DOI:10.1063/1.1694422

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A simple model for the motion of a turbulent buoyant thermal in a calm, stably stratified atmosphere is proposed. Using this model, the average motion of the thermal can be described in terms of its initial density difference, velocity and effective radius, and the three nondimensional gross parameters: mass entrainment constant, effective drag coefficient, and turbulence dissipation rate. Relations between the local turbulence field and the three gross parameters are discussed. Also, the maximum height a thermal can reach in a stable atmosphere for various initial conditions and gross parameters can be approximated by a simple power law relation.

ISSN:0031-9171    

DOI:10.1063/1.1694423

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The wave disturbance set up by the uniform motion of a Lagerstrom “singular needle” on or below the free surface of an incompressible viscous fluid is analyzed. Since the motion of this singularity generates a viscous wake, the interaction of the latter with the free surface is automatically included in the analysis. Based on formal expressions derived in earlier work, asymptotic representations for the wave height are derived which are valid for large Reynolds number relative to wavelength and for large distances downstream of the singularity. These representations explicitly show the effect of the viscous wake on the amplitude and phase of the surface waves. A definition of wave resistance is formulated appropriate to the large Reynolds number case, and curves of the wave resistance of the singularity versus speed are calculated.

ISSN:0031-9171    

DOI:10.1063/1.1694424

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The instability of a vortex‐pair to infinitesimal disturbances is studied in the inviscid and incompressible fluid approximations. The cores of the vortices contain uniform axial‐velocity jets of fluid density different from that of the surrounding medium. In one model a nonrotating core with surrounding potential vortex is assumed, and in the other a uniformly rotating core. In the lowest order of a perturbation parameter, denoting the ratio of vortex core radius to the distance between the vortices, the long‐wavelength symmetric amplification factor of a vortex‐pair decreases with increasing jet axial‐velocity in either model, with decreasing core density in a rotating core model, and with increasing core density in nonrotating core model. The vortex‐pair is more unstable to negative mode of deformation than to the positive mode in the presence of an axial jet in both models considered. The surface tension has a destabilizing effect on the vortex pair in the long‐wavelength approximations. The effect of core density and jet axial velocity on long‐wavelength symmetric amplification factors of the vortex pair in higher orders of perturbation parameters have also been outlined.

ISSN:0031-9171    

DOI:10.1063/1.1694425

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Measurements have been performed with a laser‐Doppler velocimeter of the radial dependence of azimuthal velocity components, using liquid H2O, in cylindrical containers with aspect ratiosA(i.e., the ratio of heightlto radiusb) ranging from 0.212 to 1.64. The reciprocal Ekman number ranged from 980 to 60 000. The radius was fixed at 5.25 cm. The rotating disk was located at the top. Our data are in fair agreement with numerical results of Tomlan and Hudson. Oscillations, observed during steady flow, are inertial waves with frequencies and amplitudes depending on aspect ratio and disk angular velocity. Spin‐down parameters fit a theoretical solution given previously. Observed oscillations during spin‐down have been accounted for through a heuristic argument involving convection of boundary‐layer fluid downward along the axis near the just‐stopped disk.

ISSN:0031-9171    

DOI:10.1063/1.1694426

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A theory for the steady state viscosity&eegr;of thixotropic clay‐water suspensions is given. The numerical results of this analysis can be approximated byu + u2/10 = K/&ggr;˙whereu = (&eegr;r − 1)/C, &ggr;˙is the shear rate,&eegr;∞is the viscosity for&ggr;˙ → ∞, &eegr;r =&eegr;/&eegr;∞, andKandCare parameters depending on concentration and temperature. For large shear rates this expression reduces to the Bingham behavior&eegr;r = 1 + &tgr;0/&eegr;∞&ggr;˙, where&tgr;0is the Bingham yield stress. Experiments on a Wyoming bentonite at various concentrations and temperatures give good agreement with the theoretically predicted behavior. The comparison of experimental and analytical results yields the cohesive force per particle and the ratio of the major and minor dimensions of the bentonite particles.

ISSN:0031-9171    

DOI:10.1063/1.1694427

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A theory of the opacity of bentonite‐water suspensions as a function of the shear rate is given. The experimental opacity behavior confirms previous theoretical predictions of the aggregate size distribution. The extinction coefficient of bentonite was determined at two wavelengths.

ISSN:0031-9171    

DOI:10.1063/1.1694428

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The stability of the plane Poiseuille flow of a finite linear viscoelastic fluid is studied without restriction on the magnitude of the natural time. It is assumed that the relaxation function decays exponentially. Curves of neutral stability show the important destabilizing influence of the memory of the fluid.

ISSN:0031-9171    

DOI:10.1063/1.1694429

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A technique is presented for calculating all the forces, with the exception of aerodynamic forces, acting on a raindrop falling at terminal velocity in air and subjected to a vertical electrical field such as might occur in the base of a thundercloud. The ultimate aim is a prediction of the onset of hydrodynamic instability of the drop under electric stress. Since as yet no model of aerodynamic flow around the drops is available, this method of calculating the forces and the deduced aerodynamic pressure profiles is presented. These profiles are compared to previous work and are shown to be reasonable. The assumption of a model of aerodynamic flow would allow the prediction of raindrop instability and corona, believed to be responsible for the initiation of lightning strokes.

ISSN:0031-9171    

DOI:10.1063/1.1694430

出版商:AIP    

年代:1973

数据来源: AIP