Measurement of Surface Rheological Effects on a Rotating Flow
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MEASUREMENT OF SURFACE RHEOLOGICAL EFFECTS ON A ROTATING FLOW
ROGER F. GANS AND TIMOTHY J. SINGLER Department of Mechanical Engineering, 14627, USA
University of Rochester,
Rochester,
NY
ABSTRACT We report measurement of azimuthal velocity as a function of radius near the boundary between a liquid annulus and (a) a rigid freely floating cylinder and (b) an air core contained in a rapidly rotating horizontal cylindrical Case (a) agrees with previous theoretical calcontainer. Case (b) demonstrates culations and verifies the method. (1) that the interface can support stress and (2) that there is a distinct difference in the scale of radial variation in the bulk liquid from that observed in case (a).
INTRODUCTION Fluid processes in space are relatively immune from gravity-induced convective motions because the local gravitational acceleration is greatly reduced. For the same reason it is also possible to eliminate containers, leading to soIn the absence of containing walls and gracalled containerless processing. vity, forces in the "free" surfaces, and reactions of these surfaces, dominate Study of the dynamics of interfaces, and their interthe fluid dynamics. actions with the bulk fluids they separate (or fluid they bound) becomes vital. Examples of disturbing influences include motions induced by surface tension gradients -- the Marangoni effect -- and the "rigidization" of rising bubbles attributed to the resistance to deformation of surface active films at the bubble-medium interface. The rigidization of bubbles changes the rate of mass transfer during bubble extraction processes and has been studied by numerous investigators, going This topic can be said to have spanned the back at least to Boussinesq [1]. discipline of surface rheology, the study of forces and deformation in or at the boundaries between fluids. Surface rheology, which one might call the postulation and parameterization of stress-strain relations in interfaces, is a complex and rapidly growing The reader is referred to field. A review is beyond the scope of this report. It is important to note two, not wholly satisfactory, review papers [2,3]. that there are at least two approaches to the problem, phenomenological and formal, and the language used in each approach is not necessarily interchangeable. We For the purposes of this paper we take a phenomenological point of view. are able to measure velocity profiles near a free surface, and from these we hope eventually to be able to deduce something about the stress that the surface supports. We are in the process of connecting this stress to viscoelastic constants in a formal theoretical model of the bulk flow and surface rheology, We feel the sort of thing begun by Boussinesq and carried on by Scriven [4,5]. publication of this work to be premature because of certain anomalies in the data. These anomalies could be explained by an effective reduction in the visSuch an effect has been noted before [6], but we cosity of the bulk fluid. prefer to be on firmer ground before attempting to separ
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