PRELIMINARY RESULTS OF THE SOUNDING ROCKET EXPERIMENT ON OSCILLATORY MARANGONI FLOWS IN LIQUID BRIDGE

R. Savino, R. Monti, M. Lappa
Università degli Studi di Napoli "Federico II", Dipartimento di Scienza e Ingegneria dello Spazio "Luigi G. Napolitano", P.le V.Tecchio 80, 80125 Napoli (Italy)

L. Carotenuto, D. Castagnolo, R. Fortezza
MARS Center, Via Comunale Tavernola, 80144 Napoli (Italy)

This paper reports on the preliminary results of the experiment PULSAR (Pulsating and Rotating Instabilities in Oscillatory Marangoni Flows), performed on the MAXUS 3 Sounding Rocket launched last November from the Swedish base in Kiruna. Aim of the experiment was the study of the oscillatory Marangoni convection in a cylindrical liquid bridge of silicone oil with kinematic viscosity of 5cSt. The attention has been focused on the three dimensional flow structure that is established in the liquid bridge when the temperature difference between the end disks supporting the bridge exceeds a certain critical value. The experiment was motivated by preliminary on-ground numerical simulations and microscale experimental studies, that have pointed out that the oscillatory Marangoni instability appears at the beginning in the form of a pulsating regime, caused by a hydro-thermal standing wave, and then it turns to a rotating regime, caused by a traveling wave. The height of the bridge was equal to the disk diameter (20mm), and the imposed temperature difference was 15K during the first 460 s and 20K in the second part of the experiment, until the end of the microgravity period.
The preliminary analysis of the temperature profiles, measured by four thermocouples located at the same radial and axial coordinate but at different azimuthal coordinates (shifted at 90°), and the surface temperature distribution, measured by an infrared thermocamera, show that a pulsating and a mixed pulsating-rotating regimes have been established during the experiment.
The azimuthal wave number of the oscillatory regime, the oscillation period, the time for the onset of the oscillations are in good agreement with the numerical predictions obtained in the pre-flight analysis.
The experiment was fully controlled in Telescience mode by the MARS Center remote control room by using four dedicated ISDN lines.


BERNARD-MARANGONI INSTABILITY IN VISCOELASTIC MEDIA

G. Lebon¹ and S. Van Vaerenbergh²
1Liege University, Institute of Physics B.5, Sart Tilman, B-4000 LIEGE 1 (Belgium)
2Brussels Free Unlversity, MRC, B-1050 BRUSSELS (Belgium)

The Benard-Marangoni instability (coupled thermogravitational and thermocapillary effects) in a thin fluid layer of a viscoelastic fluid heated from below is studied. A linear and a weakly non linear analysis are successively presented. The viscoelastic medium is modelled by means of the upper convected Jeffreys or Oldroyd B model which is a rather popular model in rheology. In comparison with previous analyses, two new dimensionless numbers are introduced, namely the so-called gravitational and the rate of heating number instead of the classical Rayleigh and Marangoni numbers. The critical values for the temperature gradient, wavenumber and oscillation frequencies corresponding to the onset of convection are determined from a linear approach. After motion has set in, particular patterns are predicted taking the form of either rolls, hexagons or squares. By means of a non linear technique, restricted to steady Situations, it is determined under which specific conditions a specific pattern is preferred. The role of the Prandtl and the Biot numbers is emphasized and discussed.


FROM STEADY TO CHAOTIC THERMOCAPILLARY FLOW IN FLOATING ZONES
UNDER MICROGRAVITY

D. Schwabe
Physical Institute, University Giessen, Germany
E-Mail: Dietrich. Schwabe@physik.uni-giessen.de

We report on the preliminary results of a 14 h duration microgravity experiment MAUS G 141, conducted on STS 89 in January 1998 during its docking phase to the MIR space station.

Two NaNO3-melt floating zones (diameter = 6.00 mm, lengths 1 = 2.5 mm, 1 = 4.5 mm) have been established under ~-g by melting solid samples and up to 100 axial temperature gradients have been applied for 270 s, each. The temperature oscillations of oscillatory thermocapillary convection (TC) have been measured with three fine thermocouples of different azimutal positions and the flow has been visualised by tracers in a central vertical light sheet, recorded by video cameras. The crystal-melt interface, shaped by heat transport due to TC, flow-structures, oscillation frequencies for various Marangoni-numbers Ma and transition to chaos are reported.
The microgravity results are compared to those of the reference experiment at 1 g on ground. Whereas at 1 g mainly azimuthally travelling waves are found to constitute oscillatory TC, only pulsating oscillatory states (standing waves) are found under microgravity.
Selected Video-scenes of TC under microgravity are shown (VHS).


GEOPHYSICAL FLUID FLOWS BETWEEN TWO CONCENTRIC SPHERICAL SHELLS: THE ELECTROHYDRODYNAMIC TESTCONTAINER FOR THE FLUID SCIENCE LABORATORY

C. Egbers
Center of Applied Space Technology and Microgravity (ZARM), University of Bremen, 28359 Bremen, F.R.G.

Thermal convection in a spherical shell represents an important model in fluid dynamics and geophysics. This report summarizes concurrent experimental, theoretical and numerical studies for the preparation of a Space Station experiment inside the Fluid Science Laboratory (FSL). The special device (Fig. 1) for investigations of supercritical thermal convection in spherical shells under a central force field with respect to geophysical simulations is called electrohydrodynamic container (EHC). A central symmetric force field similar to the gravity field acting on planets can be produced by applying a high voltage potential between the inner and outer sphere using the effect of dielectrophoretic force field. To turn off the unidirectional gravitation under terrestrial conditions, these experiments require an environment of microgravity such as available in the Fluid Science Laboratory. Investigations on thermal convective instabilities occuring in the spherical gap flow under terrestrial conditions are of basic importance especially for the understanding of symmetry-breaking bifurcations during the transition to chaos. Microgravity experiments on thermal convection with a simulated central force field are important for the understanding of large scale geophysical motions as the convective transport phenomena in the earth's liquid outer core (Fig. 2).