Showing papers by "David S. Cannell published in 1985"

Time dependence of flow patterns near the convective threshold in a cylindrical container

Abstract: Digitally processed shadowgraph images revealed time-dependent flow patterns slightly above the convective onset in a cylindrical cell of aspect ratio $L\ensuremath{\equiv}\frac{D}{2d}=15.0$ ($D$ is diameter, $d$ is height). This time dependence was monitored for up to 200 horizontal thermal diffusion times. At larger Rayleigh numbers, the system reached time-dependent states after much shorter transients.

Pattern Formation and Wave-Number Selection by Rayleigh-Bénard Convection in a Cylindrical Container

Abstract: We describe an apparatus and procedures for simultaneous heat transport measurements and computer enhanced shadowgraph flow-pattern imaging in a shallow horizontal layer of fluid heated from below. The heat transport measurements have a resolution of better than 0.1%, and the shadowgraph technique can detect the flow field for ≡ (R – Rc)/Rc as small as 10-2 (Rc is the critical value of the Rayleigh number R for onset of convection). The apparatus and procedures were used to study pattern and wave-number evolution in a cylindrical layer of water with radius-to-height ratio L = 7.5 and Prandtl number σ = 6.1. We found that dynamic sidewall forcing during the early thermal transients after a change in the heat current from a subcritical to a supercritical value establishes a cylindrical flow pattern. Once created, this pattern is stable in our apparatus over the wide range 0.16 8 even after the transients have decayed. With changing , adjustment in the wave number k takes place discontinuously by hysteretic changes at the cell center in the number of convection roll pairs. When is increased, the discontinuous changes at the cell center are towards smaller k and are preceded by a continuous loss of cylindrical symmetry (the middle roll pair moves off center). The selected wave numbers coincide neither with the zig-zag instability of the infinite system, as once suggested, nor with a linear extrapolation to = 0(1) of the recent prediction to lowest order in of Manneville and Piquemal and of Cross. Comparison of the selected k with measurements by others reveals no dependence upon L and σ. For < 0.16, the cylindrical pattern is unstable and decays on a time scale much longer than a horizontal diffusion time to patterns of rolls which tend to be perpendicular to the sidewalls and which contain defects. Once formed, these latter patterns will persist at large values of . These patterns also undergo a wave-number adjustment process with hysteretic changes mediated mostly by focus singularities near the walls. In these cases, larger values of also tend to produce smaller values of k.

Critical behavior near a vanishing miscibility gap.

Abstract: We report measurements of the osmotic susceptibility and correlation length for the pseudobinary mixture of guaiacol ($o$-methoxy phenol) plus glycerol-water as a function of temperature and water content $x[x\ensuremath{\equiv}\mathrm{weight}\mathrm{of}\mathrm{water}/(\mathrm{weight}\mathrm{of}\mathrm{water}\mathrm{plus}\mathrm{weight}\mathrm{of}\mathrm{glycerol})]$. This system exhibits a limited two-phase region which vanishes for $xl{x}_{0}\ensuremath{\simeq}0.0137$. We observed a smooth transition from nearly Ising-type behavior for $x\ensuremath{\gtrsim}4{x}_{0}$ through a non-power-law regime to doubled exponents for $x\ensuremath{\simeq}{x}_{0}$ and saturating divergences with doubled exponents for $xl{x}_{0}$.

Effects of ‘‘Kalliroscope’’ flow visualization particles on rotating Couette–Taylor flow

Abstract: Experimental results are presented which show that Taylor vortex flow in a vertically mounted rotating Couette system, visualized with a ‘‘Kalliroscope’’ suspension evolves to a spatially nonperiodic flow which is also nonperiodically time dependent. These effects are most noticeable in systems of large aspect ratio and depend on the concentration of the suspension. Experimental evidence is presented that suggests that these phenomena are caused by the evolution of a periodic variation of the Kalliroscope particle concentration along the cylinder axis and parallel to the gravitational field.

Tricritical phenomena in rotating Couette-Taylor flow.

Abstract: On presente des mesures de la vitesse d'un fluide dans un systeme tournant de Couette-Taylor de rapport d'aspect L≃1. A une petite vitesse angulaire w du cylindre interieur, le systeme contient une paire de tourbillons symetriques

New evidence concerning the Griffiths first sum rule for tricritical light scattering.

Abstract: Des etudes experimentales de diffusion lumineuse dans la region tricritique de melanges d'ethane et d'hydrocarbures plus lourds montrent que les intensites et les longueurs de correlation ne satisfont pas a la forme asymptotique de la premiere regle de somme de Griffiths. Les deux sommes croissent brutalement a l'approche du point tricritique, indiquant soit une divergence non classique des sommes, soit le fait que de nombreux termes de la serie de puissance doivent etre retenus pour la constante de couplage