Patterns in TimeDependent RayleighBenard Convection Experiments.
Abstract
In this dissertation, we report on studies involving computerenhanced shadowgraph flowvisualization and heat flux measurements on patterns in convective flows within a thin fluid layer of depth d that is heated from below. The experiments were conducted in a cylindrical container of radius r and aspect ratio L equiv r/d. In one study, the effects of stochastic perturbations on pattern formation and survival were examined. Two sets of experiments were performed in a cell with L = 10. In one set, the temperature of the top plate of the container was held constant while the heat current through the fluid was linearly ramped in time, resulting in a temperature difference Delta T between the bottom and top plates. After initial transients ended, the reduced Rayleigh number epsilon equiv Delta T/ Delta T_{rm c}  1, where Delta T_{rm c} is the critical temperature difference for the onset of convection, increased linearly with ramp rate beta such that near the onset epsilon(t) = beta t, where the time t is in units of the vertical thermal diffusion time t_ {v} equiv d^2/ kappa(kappa is the thermal diffusivity of the fluid). The ramp rates were in the range 0.01 <=q beta <=q 0.30. Two novel sidewall designs were used to allow pattern formation to occur independent of cell geometry. In the earliest stages the patterns were composed of irregularly arranged cells and varied randomly between experimental runs. In the second set of experiments the heat current was modulated sinusoidally with dimensionless frequency omega = 1, where omega is scaled by t_ {v}, resulting in epsilon(t) = epsilon_ {o } + sigma sin( omega t). The second study was concerned with the competition between reproducible cellular and rolllike flow patterns when epsilon is modulated as described above, but with higher values of omega and delta. The experiments were performed in a cell with L = 11. The modulation parameters were within the ranges 8.0 <=q omega <=q 18.0, 0.4 <=q delta <=q 3.3, and 0.2 <=q epsilon_ {o} <=q 0.6. Measurements of the convective threshold shift due to modulation epsilon_ {c}(delta,omega) were performed, and were in good agreement with theoretical predictions. (Abstract shortened with permission of author.).
 Publication:

Ph.D. Thesis
 Pub Date:
 1989
 Bibcode:
 1989PhDT.......136M
 Keywords:

 Physics: Fluid and Plasma