A Geophysical Flow Experiment in a Compressible Critical Fluid

The first objective of this experiment is to build an experimental system in which, in analogy to a geophysical system, a compressible fluid in a spherical annulus becomes radially stratified in density through an A.C. electric field. When this density gradient is demonstrated, the system will be augmented so that the fluid can be driven by heating and rotation and tested in preparation for a microgravity experiment. This apparatus consists of a spherical capacitor filled with critical fluid in a temperature controlled environment. To make the fluid critical, the apparatus will be operated near the critical pressure, critical density, and critical temperature of the fluid. This will result in a highly compressible fluid because of the properties of the fluid near its critical point. A high voltage A.C. source applied across the capacitor will create a spherically symmetric central force because of the dielectric properties of the fluid in an electric field gradient. This central force will induce a spherically symmetric density gradient that is analogous to a geophysical fluid system. To generate such a density gradient the system must be small (approx. 1 inch diameter). This small cell will also be capable of driving the critical fluid by heating and rotation. Since a spherically symmetric density gradient can only be made in microgravity, another small cell, of the same geometry, will be built that uses incompressible fluid. The driving of the fluid by rotation and heating in these small cells will be developed. The resulting instabilities from the driving in these two systems will then be studied. The second objective is to study the pattern forming instabilities (bifurcations) resulting from the well controlled experimental conditions in the critical fluid cell. This experiment will come close to producing conditions that are geophysically similar and will be studied as the driving parameters are changed

Subcritical transition to turbulence in plane Couette flow

International audienceThe transition to turbulence in plane Couette flow was studied experimentally. The subcritical aspect of this transition is revealed by the stable coexistence of laminar and turbulent domains. By perturbing the flow, a critical Reynolds number has been determined, above which an artificially triggered turbulent spot can persist. The study of the spatiotemporal evolution of these spots shows, among other things, the existence of waves traveling away from the turbulent regions

Respect by Design: How Different Educational Systems Interact with Mutual Respect in Classrooms

This ethnographic and comparative study examines interactions between educational systems and mutual respect in classrooms. I define mutual respect as the work of intervening on those power asymmetries typically found in classrooms — both between teachers and students, and among students — by way of according children increased equality, autonomy, and equity. I partnered with four elementary schools, situated across two educational systems (i.e., Montessori and International Baccalaureate) and two national contexts (i.e., Washington, D.C. and Toronto). Through participant observation, interviews, video-cued multivocal ethnography, and document collection, I analyzed: 1) the ways in which educational systems design for mutual respect; 2) how teachers understand and manage mutual respect in practice; and 3) the experiences of teachers and leaders as they manage designs for mutual respect in practice. Overall, the findings from this study provide clearer understanding of how and why systems might design for mutual respect, and how and why approaches might differ across systems. What is more, the findings suggest that a system’s relationship with its environment can shape the trajectory of mutual respect within the designs of a system, the practical logics of teachers, and the social contexts of classrooms. This study contributes an analytic framework — informed by the literature and elaborated via empirical study — for describing, comparing, and reasoning about the relationship between educational systems and increased equality, autonomy, and equity for students in classrooms.PHDEducational StudiesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/167937/1/whegseth_1.pd

Growth of a dry spot under a vapor bubble at high heat flux and high pressure

International audienceWe report a 2D modeling of the thermal diffusion-controlled growth of a vapor bubble attached to a heating surface during saturated boiling. The heat conduction problem is solved in a liquid that surrounds a bubble with a free boundary and in a semi-infinite solid heater by the boundary element method. At high system pressure the bubble is assumed to grow slowly, its shape being defined by the surface tension and the vapor recoil force, a force coming from the liquid evaporating into the bubble. It is shown that at some typical time the dry spot under the bubble begins to grow rapidly under the action of the vapor recoil. Such a bubble can eventually spread into a vapor film that can separate the liquid from the heater thus triggering the boiling crisis (critical heat flux)

Bound Pairs of Fronts in a Real Ginzburg-Landau Equation Coupled to a Mean Field

Motivated by the observation of localized traveling-wave states (`pulses') in convection in binary liquid mixtures, the interaction of fronts is investigated in a real Ginzburg-Landau equation which is coupled to a mean field. In that system the Ginzburg-Landau equation describes the traveling-wave amplitude and the mean field corrsponds to a concentration mode which arises due to the slowness of mass diffusion. For single fronts the mean field can lead to a hysteretic transition between slow and fast fronts. Its contribution to the interaction between fronts can be attractive as well as repulsive and depends strongly on their direction of propagation. Thus, the concentration mode leads to a new localization mechanism, which does not require any dispersion in contrast to that operating in the nonlinear Schr\"odinger equation. Based on this mechanism alone, pairs of fronts in binary-mixture convection are expected to form {\it stable} pulses if they travel {\it backward}, i.e. opposite to the phase velocity. For positive velocities the interaction becomes attractive and destabilizes the pulses. These results are in qualitative agreement with recent experiments. Since the new mechanism is very robust it is expected to be relevant in other systems as well in which a wave is coupled to a mean field.Comment: 9 pages (RevTex), 9 figures (postscript