Institute of Fundamental Technological Research

The qualitative changes of dynamic lift and friction forces caused by small-amplitude harmonic vibrations superimposed on flows in a plane slider bearing are considered for simple viscous and viscoelastic lubricating fluids. Low- and high-frequency disturbances are analysed in greater detail and the most beneficial situations discussed.

It is shown that circular as well as elliptic shearing flows of incompressible simple fluids can be treated as a subclass of motions with proportional stretch history (cf. [3]). For circular shearings, the known results on circularly polarized shear waves are derived in a different way, while for more general elliptic shearings certain new results on low frequency elliptically polarized shear waves are discussed.

In this paper, we consider the problem of plane squeeze-film flows in a kinematics based on the formalism of convected (moving and deforming) coordinates. The flows discussed are treated as instantaneous motions with proportional stretch histories (cf. [16]). Certain simplifications in the constitutive equations of an incompressible simple fluid (cf. [10]) have been achieved for moderately low Deborah numbers.

Approximate solutions of plane flows are obtained either for slightly viscoelastic fluids or in a form valid in the vicinity of any arbitrarily chosen instant of time. The conditions of improved lubrication, leading to inequalities imposed on material constants or kinematic quantities, are discussed in detail. Also, the necessary conditions are discussed under which the time-dependent distance between the plates may decrease non-monotonically, showing some “bounces”.

Recent interests in various non-viscometric flows of viscoelastic fluids such as polymer melts and solutions result from the progress made in the field of theoretical rheology as well as from the needs for investigation of more complicated flows in new rheometers. Among all non-viscometric flows of simple fluids a particular position is filled by motions with constant stretch history. These motions differ significantly from the well known class of steady viscometric flows, characterized entirely by three material functions and include, among others, the types of flows realized in the Maxwell Orthogonal Rheometer, the Kepes Balance Rheometer etc. In the first part of our review the general theory of non-iscometric flows with constant stretch history is outlined. The second part deals with various classes of flows, especially those occurring in the recently constructed rheometers. The possibilities of determination of dynamic rheological characteristics as well as normal stress effects are discussed in greater detail.

The method of small additional deformations superposed on a fundamental motion of the Rivlin-Ericksen viscoelastic medium (cf. [1]) has been applied to the case of slow steady simple extension or compression of a circular cylinder. The closed form solutions are obtained for the first order approximation, and some types of the stability loss are discussed. It is shown that the considered cylinder is unstable if the characteristic ratio of the radius to the length of a cylinder takes the corresponding critical values.