GAGa DropT - Granular Anisotropic Gases in Drop Tower
Fundamental Physics: Physics of plasmas and solid/liquid dust particles
Drop Tower Bremen - DYT2012
K. Harth (1), K. May (1), T. Trittel (1), S. Wegner (1), R. Stannarius (1)
|(1)||Department of Nonlinear Phenomena|
Faculty of Natural Sciences
Otto-von-Guericke Universität Magdeburg
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This experiment aims on the investigation of the behaviour of granular anisotropic gases in microgravity. Anisotropic granular gases are encountered in a number of situations - in avalanches, sandstorms, dust devils, the formation and motion of dunes, planetary rings, and the asteroid belt, to name a few. Study of anisotropic granular gases has, until now, been primarily restricted to numerical computer simulations.
We encounter granular materials every day, but they still offer a large variety of riddles and problems for scientists and engineers. During the past decades, the field moved from engineering and geosciences into the focus of contemporary physics. A general theory of granulates has not been developed yet. Similar to conventional materials, different aggregate states can be observed in granulates. Granular gases are loose, agitated ensembles of grains - like particles in a sandstorm or in the Saturn rings. In contrast to atomic gases, grains lose kinetic energy in collisions. Thus, energy must be constantly supplied to avoid clustering or sedimentation. A granular gas under stable conditions at low excitation is best maintained in microgravity. The dissipation is expected to affect the shape of the velocity and energy distributions in the gas and can lead to cluster formation. Literature comprises numerous analytical and numerical studies. Experiments are hard to realize and rare. Nearly exclusively granular gases of spherical or completely irregular grains are considered. In 3D investigations of granular gases of spherical grains, the Knudsen regime (mean free path larger than container size) or clustering were analysed.
During the last years, research interest in granulates composed of anisometric grains rose immensely. Various soft matter systems display an effective anisotropy due to anisometric shapes of the individual constituents. In granulates, flow alignment, particle ordering and giant density fluctuations were reported. Grains of non-spherical shapes as constituents of granular gases have only been little considered so far.
To date, only few simulations of granular gases of anisometric grains exist in literature. The specific characteristics of the dynamics will depend on, e.g., the aspect ratio, shape and mass distribution of the grains. In previous experiments on a sounding rocket flight (REXUS 10), the first experimental data on a 3D granular gas of rodlike grains were gathered. Rotational as well as translational motion of the rods is measureable. Morover, the mean free path is considerably reduces as compared to a granular gas of spherical grains at comparable filling fraction. Dynamics in the particle-particle collision dominated regimes thus becom accessible.
The results of the REXUS experiment motivated us to explore the granular anisotropic gas in a systematic quantitative study. We will obtain full 3D position, orientation and velocity data from recombination of video data recorded from 2 perspective views. We investigate granular gases of different particles and at different excitations. Our results will provide important insights into the fundamentals of granular dynamics.
GaGa project website in German
ESA webstory on the team and the progress during the drop campaign.
Launch Date: 23 February 2011
GaGa (Granular Anisotropic GAses)
One experimental setup basically consists of a container with 3 individually tuneable vibrating walls and 2 cameras monitoring the granulate in the container from two perspectives during the micro-gravity period. The moveable walls will be used to excite the granular gases in microgravity.
The catapult capsule contains all in all 3 shaker / camera systems, so that in principle three different parameter sets can be recorded per catapult shot.
Five catapult shots were performed in the week from 3 to 7 December 2012 in the ZARM Drop Tower in Bremen. Prior to each shot, the excitation parameters for the shakers were set and calibrated. The specific granular materials were distributed on the bottom of the containers. High-speed video recording is started prior to lift-off, videos are stored on-board. Immediately after lift-off, the rods start rising from the bottom of the containers towards their top. A strong excitation was applied to the walls so that the rods distribute in the shaker, before the measurement parameters were set. From the stereoscopic video data, 3D positions and orientations of the rods are reconstructed. The trajectories are thendetermined and a statistical analysis will be performed. The results will be compared to literature data.
The evaluation of the experimental data is in progress. (January 2013)