Non-oberbeck-boussinesq effects in gaseous Rayleigh-Bénard convection

Non-Oberbeck-Boussinesq (NOB) effects are measured experimentally and calculated theoretically for strongly turbulent Rayleigh-Bénard convection of ethane gas under pressure where the material properties strongly depend on the temperature. Relative to the Oberbeck-Boussinesq case we find a decrease of the central temperature as compared to the arithmetic mean of the top- and bottom-plate temperature and an increase of the Nusselt number. Both effects are of opposite sign and greater magnitude than those for NOB convection in liquids like water.     

Large-scale circulation model for turbulent Rayleigh-Bénard convection

A model for the large-scale-circulation (LSC) of turbulent Rayleigh-Bénard convection in cylindrical samples is presented. It consists of two physically motivated stochastic ordinary differential equations, one each for the strength and the azimuthal orientation of the LSC. Stochastic forces represent phenomenologically the influence of turbulent fluctuations. Consistent with measurements, the model yields an azimuthally meandering LSC with occasional rotations, and with more rare cessations. As in experiment, cessations have a uniform distribution of LSC orientation changes.     

Review: Experiments in Fundamental Physics Scheduled and in Development for the ISS

This is a review of those experiments in the area of Fundamental Physics that are either approved by ESA and NASA, or are currently under development, which are to be performed in the microgravity environment of the International Space Station. These experiments cover the physics of liquid Helium (SUE, BEST, MISTE, DYNAMX, and EXACT), ultrastable atomic clocks (PHARAO, PARCS, RACE), ultrastable microwave resonators (SUMO), and particle detectors (AMS and EUSO). The scientific goals are to study more precisely the universality properties of liquid Helium under microgravity conditions, to establish better time standards and to test the universality of the gravitational red shift, to make more precise tests of the constancy of the speed of light, and to measure the particle content in space directly without disturbances from the Earth's inner atmosphere.     

Positron emission tomography (PET) of radiotracer uptake and distribution in living plants: methodological aspects

The use of radioisotopes as tracers in biology was pioneered in 1923 by Hevesy in the study of the uptake and distribution of lead in the bean plant. This seminal work has spawned applications of nuclear analytical methods in the life sciences. In medicine, nuclear imaging has become a standard part of patient care. Positron emission tomography (PET) is a nuclear medicine imaging modality that allows assessment of biochemical processes in vivo, to diagnose and stage diseases and monitor their treatment. Going full circle to Hevesy’s original biological study, PET imaging can be applied to the assessment of radiotracer uptake and distribution in living plants non-invasively. In order to apply PET imaging to the study of plants, several methodological aspects were examined in this study with the goal of establishing protocols that would be useful to investigators with access to commercial small animal PET scanners. The methodological aspects examined in this study included (a) the selection of suitable plant models, (b) standardization of scanning strategies such as PET tracer “injection” into plants and (c) imaging protocols to optimize detection of annihilation gammas.     

Finite-size effects lead to supercritical bifurcations in turbulent rotating Rayleigh-Bénard convection

In turbulent thermal convection in cylindrical samples with an aspect ratio Γ≡D/L (D is the diameter and L the height), the Nusselt number Nu is enhanced when the sample is rotated about its vertical axis because of the formation of Ekman vortices that extract additional fluid out of thermal boundary layers at the top and bottom. We show from experiments and direct numerical simulations that the enhancement occurs only above a bifurcation point at a critical inverse Rossby number 1/Ro(c), with 1/Ro(c)∝1/Γ. We present a Ginzburg-Landau-like model that explains the existence of a bifurcation at finite 1/Ro(c) as a finite-size effect. The model yields the proportionality between 1/Ro(c) and 1/Γ and is consistent with several other measured or computed system properties.