ALBERT

Beschreibung: The dislodging by dynamic pressure forces of a drop adhered by surface tension to a plane is analysed. An integro-differential equation describing the drop shape is solved numerically and the critical Weber number as a function of contact angle hysteresis is found. © 1988, Cambridge University Press. All rights reserved.

Beschreibung: A corrugated plate is translating in a rotating fluid. Assuming low Reynolds number and small amplitudes compared to the Ekman thickness, a perturbation solution is found to second order. The resistance and power due to drag depend on the relative orientation of the corrugations with the motion. In certain instances, it is easier to move a corrugated plate than a flat plate in a rotating fluid. © 1988, Cambridge University Press. All rights reserved.

Beschreibung: A theoretical and experimental investigation is reported dealing with the onset of buckling in a horizontal layer of highly viscous liquid. The layer floats on a heavier liquid with negligible viscosity, and at rest is stabilized by gravity and surface tension. When sheared at a sufficient rate, the flat configuration of the layer becomes unstable; and the aim of the investigation is to establish the relation between critical values of the shearing rate and values of the layer’s thickness and other physical parameters. A primitive theory based on membrane approximations is first reviewed and its deficiencies are appreciated. Then a more reliable theory is developed, providing estimates of values taken by a dimensionless shear stress f at the threshold of instability. The values fcare found to depend primarily on a dimensionless number H proportional to the thickness of the layer. Experiments on sheared layers of silicone oil with various high viscosities are then described. Measured values offc plotted against H over a wide range are shown to be in satisfactory agreement with the theory. Finally, discrepancies between previous experimental results and ours are discussed. © 1988, Cambridge University Press. All rights reserved.

Beschreibung: This paper deals with the Soret separation of a binary mixture in a cylinder subjected to an axial temperature gradient. The study is connected to an experiment designed to measure the Soret coefficient of an Agl-KI mixture corresponding to a moderate Prandtl number (Pr = 0.6) and a high Schmidt number (Sc = 60). In such an experiment the species separation is often hidden by a mixing effect due to the buoyancy-driven convection generated by a horizontal temperature gradient induced by some defect of the heating system. Here, such a defect is simulated by a slight misorientation of the cell with respect to the vertical; a small inclination (y = 1°) of the cell has been considered, but the results can be generalized for any other small y. For situations corresponding to a top heating and a positive Soret parameter, S, two quite different regimes have been exhibited depending on the value of S, For moderate S, the induced solutal buoyancy balances the imposed thermal buoyancy, slowing down the flow and giving a good separation rate. For small S this balance does not exist (except in the centre), leading to a remixing of the species and thus to poor separation (the separation would be still worse for negative S). The smaller the (positive) Soret parameter is, the smaller the cell misorientation y has to be to allow a good separation rate. © 1988, Cambridge University Press. All rights reserved.

Beschreibung: The consolidation or concentration of suspended particulate solids under the influence of gravitational forces is a problem of widespread practical and theoretical interest. The literature, which is scattered over several fields, contains most of the elements necessary for a complete understanding of gravity settling, but considerable controversy and confusion persists about their synthesis. Here we propose to construct a quantitative theory covering the full range of processes from transient settling of large, stable particles to the slow consolidation of flocculated suspensions of submicron particles. Conditions for the existence of shocks are identified and the basic equations describing the phenomena are solved numerically for several Peclet numbers. © 1988, Cambridge University Press. All rights reserved.

Beschreibung: We have investigated the modulated and unmodulated travelling azimuthal waves appearing on the toroidal Taylor—Görtler (TG) vortices in a fluid contained between two concentric spheres with the inner sphere rotating. For smaller-clearance cases, toroidal TG vortices appear at the equator, just as in the flow between two concentric cylinders with the inner cylinder rotating. When the Reynolds number of the flow increases quasi-statically, spiral TG vortices appear in addition to toroidal TG vortices, and no modulation occurs, even if the Reynolds number further increases quasi-statically. However, when the Reynolds number is increased from zero to a particular value with a specific acceleration of the inner sphere, modulated wavy toroidal TG vortices appear. We found that the necessary condition for occurrence of modulation is the prevention of spiral TG vortices. Using simultaneous flow-visualization and spectral techniques, and measuring the fluctuation of sinks and sources of vortex boundaries, we obtained the frequency f1 of travelling azimuthal waves passing a fixed point in the laboratory and the modulation frequencies f2and f2of these waves, as determined by an observer in the laboratory and an observer fixed in a reference frame that rotates in phase with the travelling azimuthal waves, respectively. The relationship among the characteristic frequencies, f1, f2 and f2, obtained by modal analysis and the experimental results, is (f2+ kfjm)/f2 = — 1, where k and m are a modulation parameter and the wavenumber of travelling azimuthal waves, respectively. © 1988, Cambridge University Press. All rights reserved.

Beschreibung: The stability of eddies with three-layer stratification is examined experimentally. When the difference in density between the upper two layers is much greater (or less) than that between the lower two layers baroclinic instability on two different lengthscales (the Rossby radii associated with the upper and the lower interfaces) is possible. The vortices are created using modifications of two techniques described by Griffiths & Linden (1981) in their study of two-layer eddies. ‘Constant-flux’ eddies are generated by the release of a constant flux of buoyant fluid from a small source positioned a t the surface of a two-layer fluid. In a second variation of this experiment, the source is positioned at the interface between two layers and fluid of intermediate density is injected. As the horizontal lengthscale increases, the vortices evolve from a stable to an unstable state. It is showns that the size a t which the vortices become unstable may be significantly altered by the presence of a second interface. The results agree qualitatively with the conclusions of a linear stability analysis of quasi-geostrophic three-layer flow in a channel (Smeed 1988), but it is necessary to examine the effects of horizontal shear and Ekman dissipation to explain the experimental results. ‘Constant-volume’ eddies are produced by the release of a volume of buoyant fluid, initially contained within a cylindrical barrier, a t the surface of a two-layer fluid. After the barrier is removed, the buoyant fluid spreads a distance of the order of the Rossby radius. Similarly, vortices are created by releasing a volume of fluid of density intermediate between the initial two layers. Within a few rotation periods the vortices become unstable to disturbances similar to those observed in two-layer experiments. Qualitative agreement is found between the observed wavelength and the fastest growing mode predicted by the linear stability theory (Smeed 1988). When the disturbances reach large amplitude a change in lengthscale is often observed. © 1988, Cambridge University Press. All rights reserved.

Beschreibung: An experimental investigation of double-diffusive convection in a two-layer, saltstratified solution destabilized by lateral heating and cooling has been performed. Initially, diffusive regime phenomena are observed as the two uniform salinity layers are thermally driven and behave somewhat independently. As salt is transferred across the interface separating the layers, salinity stabilization decreases and complicated flow structure is observed a t the interface. In the final stages before mixing, the stabilizing salinity gradient becomes small, the thermal/hydrodynamic boundary layers on the heated and cooled sidewalls penetrate the salinity interface and mixing, in the finger regime, occurs. The dimensionless mixing time is described with parameters associated with thermal and salinity buoyancy forces and the enclosure aspect ratio. Careful selection of the experimental conditions allows dimensionless interfacial salinity transport rates to be correlated with appropriatedimensionless parameters. © 1988, Cambridge University Press. All rights reserved.