Problems related to non-linear variations of surface tension

Generally, the tension at a liquid/gas interface is decreasing with temperature. For aqueous solution of alcohol, alkylammonium chloride in water, some binary metallic alloys and nematic liquid crystals the surface tension presents an extremum as a function of temperature. The influence of this property on the hydrodynamic stability and on the surface tension induced flows is studied as well theoretically as experimentally.     

Marangoni convection when the surface tension increases with the temperature in normal and low gravity conditions

Thermocapillary convection has been studied in n-heptanol aqueous solutions whose surface tension is increasing with the temperature.

The fluid was confined in a parallelipipedic enclosure and a thermal gradient was imposed parallel to the free liquid/gas interface. The motions induced by the thermocapillary forces have been studied under low gravity conditions during Texus sounding rocket experiments and during the Spacelab D1 mission.

The combined thermocapillary and buoyancy convection have been extensively studied during ground based experiments.

The influence of the aspect ratio and of the alcohol concentration were investigated. Detailed velocity field was determined at the steady state under normal gravity conditions by Laser Doppler Anemometry (LDA).

The aspect ratio was of 0.3 and the temperatures imposed to the lateral sides of the cell were respectively 45°C and 60°C. With these experimental conditions, two superposed contrarotative cells were observed with rapid motions in the surface from the cold to the hot side. This convective pattern was also observed during the normal and high gravity periods of parabolic flights but during the low gravity period of the parabola the motions stopped everywhere in the fluid even in the surface and reappeared immediatly at the pull out of the parabola.     

Influence of a surface tension minimum as a function of temperature on the marangoni convection

Aqueous long chain alcohol solutions have a surface tension which presents a minimum as a function of the temperature. At this particular temperature, the dimensionless Marangoni number is vanishing. It is thus expected that around the temperature of the minimum, Marangoni Convection would be slowered with a convective pattern deeply modified. An experiment to be performed in microgravity during the Texus 8 flight is described.     

Atmospheric pressure torque and axial rotation of Venus

The influence upon the axial spin rate of Venus of a ground pressure of atmospheric origin is investigated. This semi-diurnal sectorial pressure distribution leads to a deformation of the solid planet (pressure bulge). In addition to the classical (braking) torque due to the solar solid tide, other kinds of torques can possibly occur: the tidal torque upon the pressure bulge (braking), the pressure torque upon the pressure bulge itself (braking) and the pressure torque upon the tidal bulge (accelerating). The torque budget is estimated and shows that, for usual values of the phase delay angles of thermal origin or caused by internal friction, the main effect is to slow down the axial rotation of the planet.     

The optical bidirectional accelerometer

A new Optical Bidirectional Accelerometer (O.B.A.) is described in this paper.

This accelerometer gives in real time the acceleration level in two orthogonal axis.

The advantages of this instrument in microgravity environment during a parabolic flight experiment (e.g. in fluid mechanics) are explained. The capability of O.B.A. to detect variations of the gravity level is studied. Theoretical curves give the sensibility and accuracy of the accelerometer for different parameters.

We describe the prototype and the different tests realized on it. The experimental results of the ground investigations are compared with the theoretical curves described in the previous section. The second part of the tests has been realized during a ESA parabolic flight campaign with the NASA KC 135 aircraft and in collaboration with the Belgian Air Force on a Fouga Magister.     

The loss of stability in ground based experiments in liquid bridges

In this study our primary goal is to investigate the loss of stability of the steady convection in deformable liquid bridges. Definitely, the deformation plays an important role in the transition process from the steady axi-symmetric 2-D basic state to the 3-D periodical one. As it was shown experimentally by many researchers, the critical Marangoni number is very sensitive to the volume of liquid.

It seems to be proved^1–4, that the critical wave number for high Prandtl fluids (for example, silicone oil of different viscosities. Pr > 50) is m = 1 for aspect ratios Γ = height/radius ≥ 1.0 regardless of the free surface shape. But the data from our last experiments show, that by changing surrounding conditions around the liquid bridge we can change the critical wave number. Particularly, by placing the liquid bridge of diameter 2r = 6mm into another cylindrical volume of the diameter 2R = 12mm kept at constant temperature, the critical mode is switched from m = 1 to m = 2.     

Fluid physics experiments under microgravity conditions

The proposals for the detailed investigations of the Marangoni Bénard instability problem are described. It will be performed during the Spacelab D2 mission and prepared during Texus sounding rocket flight.     

Study of the Marangoni convection around a surface tension minimum under microgravity conditions

At equilibrium, aqueous fatty alcohol solutions presents a surface tension minimum versus temperature. The influence of such an extremum on the Marangoni convection is studied. Two experiments have been performed under microgravity conditions (Texus 8 (1983) and Texus 9 (1984) flights). The velocity fields are determined by following the paths of tracer particles and furthermore, in the Texus 9 experiment, differential interferograms have been recorded.     

Pulse detonation engines: Technical approaches

The paper contains analysis of the problems preventing from wide use of pulse detonation engines (PDE), and provides suggestions to overcome those problems. In particular, the results of theoretical investigations of basic operating cycle in PDE—deflagration-to-detonation transition (DDT) processes in combustible gaseous mixtures and transmission of detonation into large chambers—are presented. The paper investigates the effect of implosion shock waves on the onset of detonation in gases, and suggests the scheme of detonation transmission from a narrow gap into a wide chamber, which makes it possible to reduce the predetonation length thus shortening the necessary length of the engine.     

Preliminary results of Texus 8 experiments on effects of surface tension minimum

A stable, observable liquid-gas interface was realized in an experimental cell (3 × 2 × 1 cm) under microgravity conditions. The liquid used was an aqueous 6.24 10^?3 molal solution of n-heptanol. A temperature gradient, established at the interface, resulted in the build-up of a Marangoni convection cell with the liquid flowing in an unusual direction (i.e. at the surface the liquid flowed from the cold to the hot side).The same experiment on Earth gives rise to a Marangoni cell superimposed on buoyancy cells. The Marangoni and the buoyancy cells then turn in opposite directions.