Showing papers by "Kenneth Walters published in 1976"

Elastico-viscous squeeze films. part I

Abstract: This paper contains theoretical and experimental work on the squeeze-film situation for Newtonian and non-Newtonian liquids. To facilitate the interpretation of experimental results, a theoretical analysis is developed for an inelastic liquid having an arbitrary viscosity/shear rate relationship. In the process, it is found necessary to relax the common assumption that material planes which are initially horizontal remain so during the subsequent deformation. It is also shown how the inertia of the moving plate can be accommodated in the analysis. The experimental work contains a description of how a commercial rheometer can be adapted with ease to perform in a squeeze-film mode. Experimental data on polymer solutions indicate that under light-loading conditions, the behaviour of the liquids is predictable from a knowledge of the shear dependent viscosity only. However, under conditions of heavy loading ( i.e. high Deborah number), viscoelastic effects are in evidence. Under these conditions, the liquids behave as better lubricants than one would predict from viscosity considerations.

Secondary flows in pipes of square cross-section and the measurement of the second normal stress difference

Abstract: In this paper, we are concerned with the possibility of determining the second normal stress difference v2 for elastic liquids from a study of the direction and strength of the secondary flows in pipes of non-circular cross-section. Experimental results show that the direction of the secondary flows in pipes of square cross-section is the same for six different elastico-viscous systems, although there is some evidence to suggest that the sign of v2 may not be the same for all the test liquids. The situation is clarified by a theoretical analysis which involves the numerical solution of coupled partial differential equations. From the analysis, it is concluded unequivocally that the direction of the streamlines is no indication of the sign of v2.

Elastico-viscous squeeze films: Part 2. superimposed rotation

Abstract: The first part of this paper contains a reconsideration of the conventional squeeze-film situation. It is shown that the Part 1 concentration on the half-time t 1 2 sometimes hides interesting elastico-viscous effects, since these are most pronounced at short times after the load is applied. The bulk of the paper is concerned with the more general situation in which a rotational flow is superimposed on the basic squeezing flow. This is brought about by rotating the bottom plate. An approximate theoretical analysis is shown to lead to a satisfactory prediction of observed behaviour under conditions of light loading. The experiments show substantial transient stress-overshoot effects under some conditions and there is also a possible indication of long-range memory effects in some of the experiments.

Stress overshoot: Real and apparent

Abstract: We give theoretical and experimental consideration to the stress overshoot phenomenon and attempt to distinguish between (apparent) overshoot arising frominstrument effects and (real) overshoot which can be regarded as a property of the material. The experiments are performed on a Weissenberg Rheogoniometer and particular attention is paid to the effect of the natural frequency of the torquemeasuring system and also to edge effects. The experimental results for Newtonian liquids are found to be in excellent quantitative agreement with the theoretical predictions. In the case of elastic liquids, the experimental results are in qualitative agreement with the predictions for a simple elastico-viscous model. We do not find any significant evidence of aberrations due to edge effects. It is concluded that although stress overshoot can be significantly affected by instrument factors, it is nevertheless a true material property.

On using the torsional balance rheometer to measure normal stresses in lubricating greases

Abstract: A new instrument, a Torsional Balance rheometer, has been used to measure shear and normal stress functions of lubricating greases. The new rheometer is based on torsional flow between parallel plates, but differs from conventional torsional flow instruments in that the plate separation is not held constant but allowed to adjust until the normal force is balanced by the axial load. In dealing with materials that have a yield stress, such as greases, the Torsional Balance rheometer is superior to fixed-plate rheometers since the zero of the normal force measurement is not dependent on the yield stress. Results have been compared with those obtained on a capillary viscometer and a Weissenberg rheometer.

Elastico-viscous squeeze films: Part 3. the torsional-balance rheometer

Abstract: The observation made in Part 2 that squeezing flow with a superimposed rotation results in an equilibrium situation with the applied load just balancing the normal stresses generated in the test fluid is used to develop a new technique (the Torsional-Balance Rheometer) for measuring the viscometric functions of elastic liquids. The Rheometer utilizes conventional torsional flow and its novel feature is that the applied load is fixed and the associated shear rate at the rim determined, in contrast to the usual situation where the shear rate is fixed and the total normal force measured. It is argued that the Torsional Balance has significant advantages over other rheometers in the very high shear-rate range, since the normal stresses being measured themselves supply a mechanism for keeping the top plate (which is free to float on the test fluid) at a constant separation from the rotating bottom plate, hence allowing very small gaps to be considered. Consistent data are shown to be possible for shear rates in excess of 10 5 s −1 .