摘要:

Experiments were conducted in neutral surroundings on turbulent vortex pairs produced with various initial conditions. A pair was observed to spread linearly along the direction of motion: This is expressed asb=0.26z, wherebis one half its maximum width andzis the elevation of its leading extremity. The advance of the pair was found to followZ/b0=2.3(Tw0/b0)1/3, whereZandTare elevation and time, respectively, with respect to the virtual origin; andb0andw0are the initial size and velocity of the pair. These results appear to indicate that the motion of the pair follows the similarity solution and relates only to the initial momentum and the entrainment coefficient, regardless of the conditions of its generation. An additional experiment was conducted with polymer solutions to illustrate the influence of turbulent mixing inside the pair on its spreading behavior.

ISSN:0031-9171    

DOI:10.1063/1.861826

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Results of investigations into numerical predictions of turbulent flows in plane two‐dimensional channels and circular pipes with and without polymer additives are presented. These predictions are based on an available computer program for two‐dimensional boundary layer flows and on thek‐&egr; turbulence model. The model takes into account the low Reynolds number effects in the immediate vicinity of the wall and is modified to take into account the influence of the polymer additives in smooth as well as rough conduits in accordance with experimental results available in the literature. Results of the predictions are presented and compared with experiments. Based on the reliability of the present predictions and the uncertainty in the available measurements, a practical approach is outlined to study turbulent flows with drag reducing polymer additives. Degradation of polymer molecules is predictable with the described theoretical model.

ISSN:0031-9171    

DOI:10.1063/1.861827

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The nonzero value of the measured skewness of the streamwise temperature derivative is not necessarily inconsistent with the concept of local isotropy of small scale turbulence, and may be estimated adequately from a ramp‐like model for the large scale structure of temperature.

ISSN:0031-9171    

DOI:10.1063/1.861828

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The pressure gradient‐velocity correlations as applied to curved and rotating turbulent flows are developed. A problem is raised for modeling pressure gradient‐velocity correlations for two‐dimensional turbulence. It is interesting to note how the behavior of the pressure gradient‐velocity correlations differs for the cases of two‐ and three‐dimensional turbulence near and away from the boundary. An explicit dependence of pressure gradient‐velocity correlations on rotation is shown to exist in the case of rotating flows.

ISSN:0031-9171    

DOI:10.1063/1.861829

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A fluid is heated from below and cooled from above in such a way that a frozen layer overlies a convecting layer. The lower boundary of the fluid is maintained at a constant temperature while the upper boundary of the ice transfers heat to an ambient atmosphere according to Newton’s law of cooling. The Boussinesq equations for convection are used in the mean field approximation and with a one‐mode, two‐dimensional representation to determine the steady state of the system in the limit of very large Rayleigh number. The fluid‐ice interface is corrugated and is a free boundary of the problem. The convective steady states are likely to bifurcate discontinuously from the conductive steady states. Analytical and numerical results are given for the temperature at the upper boundary of the ice, the mean depth of the fluid, and the amplitude of the corrugations.

ISSN:0031-9171    

DOI:10.1063/1.861830

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The character of the growth rates of the normal modes for Rayleigh–Taylor instability of superposed incompressible, viscous fluids is analyzed in terms of appropriately scaled dimensionless parameters and a particularly simple representation of the Rayleigh–Taylor dispersion relation. The chief feature that emerges is that the scaled growth rate is remarkably insensitive to the values of fluid densities and viscosities. To within a few percent, the physical growth rate depends only on the surface tension, the density‐weighted average viscosity, and the effective acceleration. Approximate formulae for the most unstable wavenumber and the corresponding maximum growth rate are given.

ISSN:0031-9171    

DOI:10.1063/1.861831

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The nonlinear stability of a viscous liquid film flowing steadily down an inclined plane is studied in the phase plane. Explicit expressions of the critical points of the governing differential system are obtained. The nature of the critical points and the integral curves corresponding to various flow parameters, initial conditions, and disturbance characteristics are determined numerically. Based on the phase plane analysis and the numerical results, the following conclusions are reached: The film which is unstable according to linear theory may be stable with respect to a finite three‐dimensional disturbance if the initial amplitude and the side‐band width are sufficiently small. The particular values of the initial amplitude and the side‐band width beyond which the film becomes unstable depend on the relevant flow parameters. The film which is stable according to linear theory is also shown to be stable with respect to three‐dimensional small finite amplitude disturbances with finite bandwidths.

ISSN:0031-9171    

DOI:10.1063/1.861832

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Microscopic observations of the flow of fresh canine blood in a separated region formed by a step in a polyurethane surface have been made. These observations show the formation of large clumps of freely floating aggregates of platelets. These clumps can easily leave the region and may be representative of disorders occurring in the distal circulation when artificial devices are used in the bloodstream. Measurements of the growth rate of these aggregates are compared with a theoretical analysis of the rate of platelet collisions. Some criteria defining conditions under which such free floating aggregates occur are also suggested.

ISSN:0031-9171    

DOI:10.1063/1.861833

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Boundary conditions are investigated for a model gas composed of identical particles with velocities restricted to a given finite set of vectors. A possible model of the gas surface interaction is given and anHtheorem for a gas in a vessel is obtained. Then, the steady Couette flow between two parallel plates is studied. The model with four coplanar velocities is used and the velocity set has arbitrary orientation. The exact kinetic solution is compared with the solutions of the associated Navier–Stokes problems.

ISSN:0031-9171    

DOI:10.1063/1.861834

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Two explicit solutions of the inverse problem of one velocity transport theory are given. The first involves quantitiesK(n,n) obtained from a Gelfand–Levitan equation. The second involves quantitiesA(n,n) obtained from a Marchenko equation. These solutions are complementary. The first is most convenient to find the coefficients of the low order Legendre polynomials in the expansion of the scattering function. The second is efficient for calculating high order coefficients. The explicit relations between theA(n,n) andK(n,n) are given. Since most of the special results needed to obtain these solutions have been published elsewhere, proofs are at most sketched. Appropriate references provide any further details desired.

ISSN:0031-9171    

DOI:10.1063/1.861835

出版商:AIP    

年代:1977

数据来源: AIP