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Research Containing: Convection

Oscillatory cellular patterns in three-dimensional directional solidification

by on 22 August 2016in Biology & Biotechnology, Physical Sciences No comment

We present a phase-field study of oscillatory breathing modes observed during the solidification of three-dimensional cellular arrays in microgravity. Directional solidification experiments conducted onboard the International Space Station have allowed us to observe spatially extended homogeneous arrays of cells and dendrites while minimizing the amount of gravity-induced convection in the liquid. In situ observations of transparent alloys have revealed the existence, over a narrow range of control parameters, of oscillations in cellular arrays with a period ranging from about 25 to 125 min. Cellular patterns are spatially disordered, and the oscillations of individual cells are spatiotemporally uncorrelated at long distance. However, in regions displaying short-range spatial ordering, groups of cells can synchronize into oscillatory breathing modes. Quantitative phase-field simulations show that the oscillatory behavior of cells in this regime is linked to a stability limit of the spacing in hexagonal cellular array structures. For relatively high cellular front undercooling (i.e., low growth velocity or high thermal gradient), a gap appears in the otherwise continuous range of stable array spacings. Close to this gap, a sustained oscillatory regime appears with a period that compares quantitatively well with experiment. For control parameters where this gap exists, oscillations typically occur for spacings at the edge of the gap. However, after a change of growth conditions, oscillations can also occur for nearby values of control parameters where this gap just closes and a continuous range of spacings exists. In addition, sustained oscillations at to the opening of this stable gap exhibit a slow periodic modulation of the phase-shift among cells with a slower period of several hours. While long-range coherence of breathing modes can be achieved in simulations for a perfect spatial arrangement of cells as initial condition, global disorder is observed in both three-dimensional experiments and simulations from realistic noisy initial conditions. In the latter case, erratic tip-splitting events promoted by large-amplitude oscillations contribute to maintaining the long-range array disorder, unlike in thin-sample experiments where long-range coherence of oscillations is experimentally observable.

Related URLs:
http://www.ncbi.nlm.nih.gov/pubmed/26565251

DECLIC, First Results On Orbit

by on 22 August 2016in Physical Sciences No comment

DECLIC is a multi-user facility to investigate critical fluids behaviour and directional solidification of transparent alloys, developed in the frame of a joint NASA/CNES research program. The instrument is a miniaturized thermo optical laboratory in which one can plug inserts containing the materials to be studied.

Related URLs:
https://cadmos.cnes.fr/sites/default/files/migration/automne/standard/2014_10/p8782_a528d45ac3dd0799d3fe13b58fdaf027IAC_10_A2_5_1.pdf

Transition to chaotic thermocapillary convection in a half zone liquid bridge

by on 22 August 2016in Biology & Biotechnology, Physical Sciences

A series of fluid physics microgravity experiments with an enough long run time were performed in the ‘‘KIBO,’’ the Japanese Experiment Module aboard the International Space Station, to examine the transition to chaos of the thermocapillary convection in a half zone liquid bridge of silicone oil with a Prandtl number of 112. The temperature difference between the coaxial disks induced the thermocapillary-driven flow, and we experimentally demonstrated that the flow fields underwent a tran- sition from steady flow to oscillatory flow, and finally to chaotic flow with increasing temperature differ- ence. We obtained the surface temperature time series at the middle of the liquid bridge to quantitatively evaluate the transition process of the flow fields. By Fourier analysis, we further confirmed that the flow fields changed from a periodic, to a quasi-periodic, and finally to a chaotic state. The increasing nonlin- earity with the development of the flow fields was confirmed by time-series chaos analysis. The deter- mined Lyapunov exponent and the translation error indicated that the flow fields made transition to the chaotic field with the increasing temperature difference.

Related URLs:
http://www.sciencedirect.com/science/article/pii/S0017931015005323

Growth of InxGa1−xSb alloy semiconductor at the International Space Station (ISS) and comparison with terrestrial experiments

by on 22 August 2016in Physical Sciences No comment

BACKGROUND: InxGa1 − xSb is an important material that has tunable properties in the infrared (IR) region and is suitable for IR-device applications. Since the quality of crystals relies on growth conditions, the growth process of alloy semiconductors can be examined better under microgravity (μG) conditions where convection is suppressed.;AIMS: To investigate the dissolution and growth process of InxGa1 − xSb alloy semiconductors via a sandwiched structure of GaSb (seed)/InSb/GaSb(feed) under normal and μG conditions.;METHODS: InxGa1 − xSb crystals were grown at the International Space Station (ISS) under μG conditions, and a similar experiment was conducted under terrestrial conditions (1G) using the vertical gradient freezing (VGF) method. The grown crystals were cut along the growth direction and its growth properties were studied. The indium composition and growth rate of grown crystals were calculated.;RESULTS: The shape of the growth interface was nearly flat under μG, whereas under 1G, it was highly concave with the initial seed interface being nearly flat and having facets at the peripheries. The quality of the μG crystals was better than that of the 1G samples, as the etch pit density was low in the μG sample. The growth rate was higher under μG compared with 1G. Moreover, the growth started at the peripheries under 1G, whereas it started throughout the seed interface under μG.;CONCLUSIONS: Kinetics played a dominant role under 1G. The suppressed convection under μG affected the dissolution and growth process of the InxGa1 − xSb alloy semiconductor.

Related URLs:
http://www.nature.com/articles/npjmgrav201511

Analysis of a resonance liquid bridge oscillation on board of the International Space Station

by on 22 August 2016in Physical Sciences No comment

We study the singular event which took place when conducting an experiment with a liquid bridge aboard the International Space Station. The liquid bridge vibrated unexpectedly for several tens of seconds with an oscillation amplitude larger than 15% of its radius. At first glance, the analysis of the mass force measured by the accelerometer during the oscillation did not show any significant perturbation. However, our study reveals the existence of two small-amplitude vibrations of the experimental setup with practically the resonance frequency of the first lateral mode. These vibrations occurred a few tens of seconds before the liquid bridge oscillation reached its maximum amplitude, produced a mass force with a magnitude of the order of 10−5g. The numerical integration of the non-linear Navier–Stokes equations reproduces remarkably well the free surface oscillations measured in the experiments. It allows us to reconstruct the three-dimensional liquid bridge motion which took place in the experiment. The present study illustrates the sensitivity of liquid bridges in a microgravity environment, where tiny perturbations may produce significant vibrations which survive over long periods of time.

Related URLs:
https://www.researchgate.net/publication/295847814_Analysis_of_a_resonance_liquid_bridge_oscillation_on_board_of_the_International_Space_Station

Spatiotemporal dynamics of oscillatory cellular patterns in three-dimensional directional solidification

by on 22 August 2016in Biology & Biotechnology, Physical Sciences No comment

We report results of directional solidification experiments conducted on board the International Space Station and quantitative phase-field modeling of those experiments. The experiments image for the first time in situ the spatially extended dynamics of three-dimensional cellular array patterns formed under microgravity conditions where fluid flow is suppressed. Experiments and phase-field simulations reveal the existence of oscillatory breathing modes with time periods of several 10’s of minutes. Oscillating cells are usually noncoherent due to array disorder, with the exception of small areas where the array structure is regular and stable.

Related URLs:
http://www.ncbi.nlm.nih.gov/pubmed/23767735

Effect of Varying the Initial Diameter of n-Octane and n-Decane Droplets over a Wide Range on the Spherically Symmetric Combustion Process: International Space Station and Ground-based Experiments

by on 22 August 2016in Physical Sciences No comment

This study reports on an investigation of varying the initial droplet diameter (Do) over a very wide range (from 0.5 mm to 5 mm) on droplet combustion. The droplet burning history is examined in an environment of reduced convection as promoted by low gravity to achieve spherical droplet flames. The fuels examined are n-octane and n-decane. The long burning times for Do > 1.2 mm were accommodated in the Multi-user Droplet Combustion Apparatus (MDCA) onboard the orbiting International Space Station (ISS), while experiments for Do < 1 mm were carried out in a ground-based drop tower. The results reported encompass the widest range of Do examined in the history of droplet combustion experimentation for a given fuel. Both free floating (unsupported) and fiber-supported droplets are deployed and ignited. Quantitative data are obtained from digital analysis of the individual video images of the burning process for the droplet, flame and soot shell diameters. Results show that the droplet burning rate decreases with increasing Do throughout the Do range investigated. The mechanisms responsible include a combination of fuel molecule residence time effects and radiative losses from the flame, both of which influence soot formation to varying degrees. Assuming that increasing soot formation (e.g., from increasing residence times) would lower heat transfer to the droplet, "small" droplets (Do < 1 mm, with negligible radiation losses) will burn slower as Do increases in this initial droplet diameter range, which is consistent with the experimental results. For Do > 1.5 mm the droplet flames appeared less luminous and therefore less sooty, yet the droplets continued to burn progressively slower as Do increased. This effect is conjectured to be the result of increased radiative losses that would tend to reduce sooting that outweigh the longer residence times of the larger droplets that would tend to increase sooting. The experimental results reported also include the evolution of relative distances between the flame, soot shell, and droplet diameters, all of which are influenced by Do.

Related URLs:
http://www.che.utah.edu/~sutherland/USCI2013/PAPERS/2G11-070HE-0310.pdf

Instability and associated roll structure of Marangoni convection in high Prandtl number liquid bridge with large aspect ratio

by on 9 June 2015in Physical Sciences No comment

This paper reports the experimental results on the instability and associated roll structures (RSs) of Marangoni convection in liquid bridges formed under the microgravity environment on the International Space Station. The geometry of interest is high aspect ratio (AR = height/diameter ≥ 1.0) liquid bridges of high Prandtl number fluids (Pr = 67 and 207) suspended between coaxial disks heated differentially. The unsteady flow field and associated RSs were revealed with the three-dimensional particle tracking velocimetry. It is found that the flow field after the onset of instability exhibits oscillations with azimuthal mode number m = 1 and associated RSs traveling in the axial direction. The RSs travel in the same direction as the surface flow (co-flow direction) for 1.00 ≤ AR ≤ 1.25 while they travel in the opposite direction (counter-flow direction) for AR ≥ 1.50, thus showing the change of traveling directions with AR. This traveling direction for AR ≥ 1.50 is reversed to the co-flow direction when the temperature difference between the disks is increased to the condition far beyond the critical one. This change of traveling directions is accompanied by the increase of the oscillation frequency. The characteristics of the RSs for AR ≥ 1.50, such as the azimuthal mode of oscillation, the dimensionless oscillation frequency, and the traveling direction, are in reasonable agreement with those of the previous sounding rocket experiment for AR = 2.50 and those of the linear stability analysis of an infinite liquid bridge.

Related URLs:
http://scitation.aip.org/content/aip/journal/pof2/27/2/10.1063/1.4908042

Space experiment on the instability of Marangoni convection in large liquid bridge – MEIS-4: effect of Prandtl number

by on 9 June 2015in Physical Sciences No comment

Microgravity experiments on the thermocapillary convection in liquid bridge, called Marangoni Experiment in Space (MEIS), are carried out in "KIBO" of ISS. Three series of experiments, MEIS-1, 2, and 4, have been conducted so far. This paper reports the results obtained from MEIS-4, in which 20cSt silicone oil ( Pr = 207) is used to generate large liquid bridges. They are suspended between coaxial disks that are 50mm in diameter, with their maximum length equal to 62.5mm. MEIS-4 aims at (1) determining the critical temperature difference for the onset of oscillatory flow; (2) realizing high Marangoni number conditions for high Pr fluid; (3) clarifying the effects of volume ratio, heating rate, hysteresis, and cooled disk temperature; and (4) observing whether the hydrothermal wave with azimuthal mode number m = 0 appears or not. The main results are presented and compared with those obtained in MEIS-1 and 2, which utilized liquid bridges of 5cSt silicone oil ( Pr = 67).

Related URLs:
http://stacks.iop.org/1742-6596/327/i=1/a=012029

3-D Flow Measurement of Oscillatory Thermocapillary Convection in Liquid Bridge in MEIS

by on 9 June 2015in Physical Sciences No comment

Marangoni Experiment in Space (MEIS) has been conducted in the International Space Station (ISS) in order to clarify the transition processes of thermocapillary convection in liquid bridges. The use of microgravity allows us to generate long liquid bridges, 30mm in diameter and up to 60mm in length. Several flow visualization techniques have been applied to those large liquid bridges. 3-D PTV is used to reveal highly three-dimensional flow patterns that appear after the transition. Three CCD cameras are used to observe the motions of the tracer particles from different view angles through the transparent heated disk made of sapphire. Particle images are recorded in the HDD recording system in ISSand they are downloaded to the ground for data analysis. A conventional 3-D PTV technique and a newly-developed multi-frame particle tracking method are combined to obtain the results that can help better understanding of oscillatory 3-D flow fields in the liquid bridges. It is shown that the flow pattern changes from a 2-D axisymmetric steady flow to an oscillatory 3-D non-axisymmetric flow under the supercritical conditions.

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