The flow of plastic disperse systems in the presence of the wall effect

Summary The results of studies of the wall effect in the flow of lubricating greases of different types are presented. It has been shown that the wall effect substantially influences the flow curves obtained for smooth capillaries, especially at low shear rates; the magnitude of this effect is estimated as a function of the capillary radius and shear rate at the capillary wall. It has been proved experimentally that the use of grooved surfaces eliminates the wall effect completely.The properties of a wall layer were assessed on the basis of the concept of the activation nature of the viscous flow of disperse systems. The following parameters of the wall layer have been determined from the variations of the free activation energy in the wall layer and in the bulk of the grease: thickness, disperse phase concentration and viscosity characteristics. The mechanism of flow generation and development in the wall layer is explained on the basis of the data obtained.Good agreement of theory with experiment is shown.

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Zusammenfassung Es werden die Ergebnisse von Untersuchungen des Wandeffekts beim Fließen von Schmierfetten unterschiedlichen Ursprungs mitgeteilt. Dabei wird gezeigt, daß der Wandeffekt die Fließkurven in glatten Kapillaren wesentlich beeinflußt, besonders bei niedrigen Schergeschwindigkeiten. Die Größe dieses Effekts wird in Abhängigkeit vom Kapillarradius und der Schubspannung an der Kapillarwand ermittelt. Experimentell wird nachgewiesen, daß bei Verwendung gerillter Oberflächen der Wandeffekt vollkommen verschwindet.Die Eigenschaften der Wandschicht werden unter Zugrundelegung der Vorstellungen von der Aktivierungsnatur der Prozesse des viskosen Fließens der Dispersionssysteme beurteilt. Aus der Veränderung der freien Aktivierungsenergie in der Wandschicht und im Volumen werden die Parameter der Wandschicht ermittelt: Dicke, Konzentration der Dispersionsphase und Viskositätskennlinie. Aufgrund dieser Kennwerte wird der Mechanismus der Bildung und der Entwicklung der Strömung in der Wandschicht erörtert.Es wird gezeigt, daß eine gute Übereinstimmung zwischen Theorie und Experiment besteht.

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With 10 figures and 1 table     

The influence of sample extremes on the maximum of partial sums of independent random variables

Many papers exist dealing with the distribution of the maximum of partial sums of independent random variables and studying the relationship between the sums and the sample extremes (see [L. Takacs,Combinator Methods in the Theory of Stochastic Processes, Wiley, New York (1967)], [J. Calambos,The Asymptotic Theory of Extreme Order Statistics, Wiley, New York (1978)]). The present paper deals with the effect the sample extremes have on the distribution of the maximum of sums of independent random variables. Proceedings of the Seminar on Stability Problems for Stochastic Models, Moscow, 1993     

Supersonic Flow Past an Obstacle with a Clearance at the Base

The supersonic air flow structure and the pressure distribution in the vicinity of a vertical cylinder suspended over the surface of a plate with a turbulent boundary layer are studied experimentally. The effects of the free-stream Mach number and the width of the clearance between the cylinder base and the surface on the dimensions of the separated flow region and the pressure distribution in the latter are examined.     

Viscoelastic behavior of an amorphous polymer under oscillations of large amplitude

The influence of periodic shear deformation and steady flow on a typical amorphous polymer is discussed. Forced sinusoidal vibrations were applied and the complex viscosity was determined. The action of a vibration of finite amplitude is equivalent to steady flow with a definite finite shear rate. Both processes cause truncation of the long-time part of the relaxation specturm. It may be accepted to a first approximation that the long-time boundary of the remaining part of the relaxation spectrum conforms to the long-time part of the initial spectrum, even if the plateau region of the spectrum is truncated. The concept of limiting truncation of the short-time part of the spectrum is introduced, this corresponding to the minimum absolute value of the complex viscosity versus reduced frequency and the lowest values of the dynamic and apparent viscosities. With an approximate representation of the relaxation spectrum, calculations were made of the maximum values of the viscosity and the coefficient relating the first difference of normal stresses to the square of the shear rate, and also of the apparent viscosity and normal stresses as functions of the shear rate. The calculated values are compared with experimental measurements, and it is shown that the correlation of the apparent viscosity and the absolute value of the complex viscosity is distributed at high frequencies, being superseded by a correlation between the apparent and dynamic viscosities.