141 research outputs found
Transport in Rayleigh-Stable Experimental Taylor-Couette Flow and Granular Electrification in a Shaking Experiment
This dissertation consists of two projects: Rayleigh-stable Taylor-Couette flow and granular electrification.
Taylor-Couette flow is the fluid flow in the gap between two cylinders rotating at different rates. Azimuthal velocity profiles, dye visualization, and inner cylinder torques were measured on two geometrically similar Taylor-Couettes with axial boundaries attached to the outer cylinder, the Maryland and Twente T3C experiments. This was done in the Rayleigh stable regime, where the specific angular momentum increases radially, which is relevant to astrophysical and geophysical flows and in particular, stellar and planetary accretion disks. The flow substantially deviates from laminar Taylor-Couette flow beginning at moderate Reynolds number. Angular momentum is primarily transported to the axial boundaries instead of the outer cylinder due to Ekman pumping when the inner cylinder is rotating faster than the outer cylinder. A phase diagram was constructed from the transitions identified from torque measurements taken over four decades of the Reynolds number. Flow angular velocities larger and smaller than both cylinders were found. Together, these results indicate that experimental Taylor-Couette with axial boundaries attached to the outer cylinder is an imperfect model for accretion disk flows.
Thunderstorms, thunder-snow, volcanic ash clouds, and dust storms all display lightning, which results from electrification of droplets and particles in the atmosphere. While lightning is fairly well understood (plasma discharge), the mechanisms that result in million-volt differences across the storm are not. A novel granular electrification experiment was upgraded and used to study some of these mechanisms in the lab. The relative importance of collective interactions between particles versus particle properties (material, size, etc.) on collisional electrification was investigated. While particle properties have an order of magnitude effect on the strength of macroscopic electrification, all particle types electrified with dynamics that suggest a major role for collective interactions in electrification. Moreover, mixing two types of particles together does not lead to increased electrification except for specific combinations of particles which clump, which further points towards the importance of collective phenomena. These results help us better understand the mechanisms of electrification and lightning generation in certain atmospheric systems
Data management for heterogeneous research environments with CaosDB -- Experiences from an MPDL Open Source development project
Experimental and theoretical scientists in the turbulence department at the MPI-DS in Göttingen produce a large variety of heterogeneous data and analyze it in a number of different environments. In an MPDL project, the open source research data management software CaosDB was enhanced to meet these needs and hopefully those of other research groups as well. We will show the results of this process: automated integration of data from metadata-rich raw HDF 5 files and a new API with language bindings for Octave, C++ and Julia. Additionally, the user documentation was overhauled, programming tutorials published and perfomance bottlenecks identified. We will also share insights about "soft" measures to increase the overall utility of semantic data management: practical guidelines for scientists to produce truly FAIR data and workshops to empower scientists to work with CaosDB
Azimuthal velocity profiles in Rayleigh-stable Taylor-Couette flow and implied axial angular momentum transport
We present azimuthal velocity profiles measured in a Taylor-Couette
apparatus, which has been used as a model of stellar and planetary accretion
disks. The apparatus has a cylinder radius ratio of , an
aspect-ratio of , and the plates closing the cylinders in the
axial direction are attached to the outer cylinder. We investigate angular
momentum transport and Ekman pumping in the Rayleigh-stable regime. The regime
is linearly stable and is characterized by radially increasing specific angular
momentum. We present several Rayleigh-stable profiles for shear Reynolds
numbers , both for
(quasi-Keplerian regime) and (sub-rotating regime)
where is the inner/outer cylinder rotation rate. None of the
velocity profiles matches the non-vortical laminar Taylor-Couette profile. The
deviation from that profile increased as solid-body rotation is approached at
fixed . Flow super-rotation, an angular velocity greater than that of
both cylinders, is observed in the sub-rotating regime. The velocity profiles
give lower bounds for the torques required to rotate the inner cylinder that
were larger than the torques for the case of laminar Taylor-Couette flow. The
quasi-Keplerian profiles are composed of a well mixed inner region, having
approximately constant angular momentum, connected to an outer region in
solid-body rotation with the outer cylinder and attached axial boundaries.
These regions suggest that the angular momentum is transported axially to the
axial boundaries. Therefore, Taylor-Couette flow with closing plates attached
to the outer cylinder is an imperfect model for accretion disk flows,
especially with regard to their stability.Comment: 22 pages, 10 figures, 2 tables, under consideration for publication
in Journal of Fluid Mechanics (JFM
Untersuchung von hydrologischen Parametern und Spektren der Induzierten Polarisation an Lockersedimentproben
Im Rahmen eines gemeinsamen Projektes des Instituts fĂŒr Geophysik und extraterrestrische Physik und des Instituts fĂŒr Geoökologie der TU Braunschweig werden ZusammenhĂ€nge zwischen bodenhydrologischen und elektrischen Parametern untersucht. FĂŒr die Messungen an verschiedenen Boden-proben wurde eine Messzelle entwickelt, die sowohl die Bestimmung der hydro-logischen Eigenschaften durch Multi-Step-Ausflussexperimente (MSO) als auch die Untersuchung mittels spektraler Induzierter Polarisation (SIP) an Locker-sedimenten ermöglicht. Das Proben-material verbleibt bei beiden Verfahren in derselben Messzelle. Eine VerĂ€nderung der Probeneigenschaften durch den Aus- und Wiedereinbau des Materials wird ausgeschlossen. Die Messzelle und Ergebnisse der ersten Untersuchungen werden vorgestellt
Dynamics of Weak First Order Phase Transitions
The dynamics of weak vs. strong first order phase transitions is investigated
numerically for 2+1 dimensional scalar field models. It is argued that the
change from a weak to a strong transition is itself a (second order) phase
transition, with the order parameter being the equilibrium fractional
population difference between the two phases at the critical temperature, and
the control parameter being the coefficient of the cubic coupling in the
free-energy density. The critical point is identified, and a power law
controlling the relaxation dynamics at this point is obtained. Possible
applications are briefly discussed.Comment: 11 pages, 4 figures in uuencoded compressed file (see instructions in
main text), RevTeX, DART-HEP-94/0
On the Strength of First Order Phase Transitions
Electroweak baryogenesis may solve one of the most fundamental questions we
can ask about the universe, that of the origin of matter. It has become clear
in the past few years that it also poses a multi-faceted challenge. In order to
compute the tiny primordial baryonic excess, we probably must invoke physics
beyond the standard model (an exciting prospect for most people), we must push
perturbation theory to its ``limits'' (or beyond), and we must deal with
nonequilibrium aspects of the phase transition. In this talk, I focus mainly on
the latter issue, that of nonequilibrium aspects of first order transitions. In
particular, I discuss the elusive question of ``weakness''. What does it mean
to have a weak first order transition, and how can we distinguish between weak
and strong? I argue that weak and strong transitions have very different
dynamics; while strong transitions proceed by the usual bubble nucleation
mechanism, weak transitions are characterized by a mixing of phases as the
system reaches the critical temperature from above. I show that it is possible
to clearly distinguish between the two, and discuss consequences for studies of
first order transitions in general. (Invited talk given at the ``Electroweak
Physics and the Early Universe'' workshop, Sintra, March 23-25, 1994.)Comment: 16 pages, 4 figures not included (can be obtained from
hep-ph/9403310, or by request) RevTeX, DART-HEP-94/0
Complex conductivity of soils
The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m21, NaCl, 258C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in-phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent
- âŠ