R. Monti

Velocity measurement by self‐tracking correlator

A continuously variable time delay correlator was designed for the purpose of measuring velocity. The inverse transit time (velocity) between upstream and downstream detectors is continuously adjusted by a feedback loop which maintains constant the level of cross correlation at two equal intervals from the peak correlation. Variable delay is achieved by changing the clock frequency which controls a digital shift register delay line. The output of the system is linear and identical to that calculated with parallel logic cross correlation computers.

Oscillatory Marangoni convection in cylindrical liquid bridges

Oscillatory Marangoni convection in silicon–oil liquid bridges, sustained by two circular coaxial disks with prescribed time‐dependent temperature profiles and bounded by cylindrical free surfaces, is investigated by direct three‐dimensional (3‐D) and time‐dependent simulation of the model equations, using finite difference methods explicit in time and a staggered spatial mesh in cylindrical coordinates. It is shown that, for low enough values of the dimensionless rate of ramping, the time‐dependent nature of the boundary conditions becomes unimportant and the computed critical Marangoni numbers approach the values obtained with steady stability analyses. For typical microgravity experiments, involving unsteady boundary conditions, the computed critical Marangoni numbers and the oscillation frequencies agree with available experimental data of sounding rockets and Spacelab experiments. The 3‐D thermo‐fluid‐dynamic oscillatory regime structures are depicted, discussed, and compared with previous experiment...

Physics of fluids in microgravity

Fluid Science Relevance in Microgravity Research Mechanical Behaviour of Liquid Bridges in Microgravity Interfacial Phenomena Thermal Marangoni Flows Interfacial Patterns and Waves Fluid Mechanics of Bubbles and Drops Diffusion and Thermodiffusion in Microgravity Critical and Supercritical Fields and Related Phenomena Microgravity Two Phase Flow and Heat Transfer Transient and Sloshing Motions in an Unsupported Container Pool Boiling and Bubble Dynamics in Microgravity Combustion Phenomena at Microgravity Fluid Flow and Solute Segregation in Crystal Growth from the Melt Fluid Flows and Macromolecular Crystal Growth in Microgravity Fluid Dynamics Experiment Sensitivity to Accelerations Prevailing on Microgravity Platforms Facilities for Microgravity Fluid Science Research Onboard Appendix A: ISs Assembly Sequence Appendix B: Flight Control Positions and Their Call Signs in the International Space Station.

Three-dimensional numerical simulation of Marangoni instabilities in non-cylindrical liquid bridges in microgravity

Instability of Marangoni convection in non-cylindrical (convex or concave) liquid bridges of low Prandtl number fluids is investigated by direct three-dimensional and time-dependent simulation of the problem. Body-fitted curvilinear coordinates are adopted; the non-cylindrical original physical domain in the (r,z,φ) space is transformed into a cylindrical computational domain in a (ξ,η,φ) space. The geometry of the domain is transformed using a coordinate transformation method by surface fitting technique. The field equations are numerically solved explicitly in time and with a finite difference technique in a staggered grid. The numerical results are analyzed and interpreted in the general context of the bifurcations theory. The computations show that for semiconductor melts the first bifurcation is characterized by the loss of spatial symmetry rather than by the onset of oscillatory flow and that it is hydrodynamic in nature. The flow field azimuthal organization related to the critical wave number, depends on the geometrical aspect ratio A=L/D of the liquid bridge and on the shape factor S (convex S>1, concave S<1) of the free surface. The critical azimuthal wave number increases when the geometrical aspect ratio of the bridge is decreased and, for a fixed aspect ratio, can be shifted to higher values by increasing the volume (convex bridges) or to lower values by decreasing the volume (concave bridges). This behavior is explained on the basis of the relation between the typology of the azimuthal disturbances and the structure of the fluid-dynamic field. A generalized law is found to correlate the critical azimuthal wave number of the instability to the geometrical aspect ratio and to the shape factor. A second oscillatory (Hopf) bifurcation occurs when further increasing the Marangoni number. Experimental results available in literature on this second bifurcation are considered for comparison. The experimental and numerical results show a good agreement.

Buoyancy and surface-tension-driven convection in hanging-drop protein crystallizer

Abstract This paper deals with natural and Marangoni convection in hanging (or sitting) drop protein crystallizers. In the pre-nucleation phase the drop is modelled as a mixture of water, precipitating agent and protein, bounded by an undeformable interface with a surface tension exhibiting a linear dependence on the concentrations; axial symmetry is assumed with respect to the drop axis. The post-nucleation phase is modelled assuming a given location of the crystal and appropriate boundary conditions for the concentrations of protein and precipitating agent in the neighbourhood of the crystal and at the drop surface. The final state of the pre-nucleation is used as the initial condition for the post-nucleation phase. The field equations, written in a suitable spherical co-ordinates system, are solved, with appropriate boundary and symmetry conditions, by a numerical algorithm based on finite-difference schemes. The study cases refer to the crystallization of lysozyme in a solution of sodium chloride in water, for two configurations, full-size and half-size geometries. The computations indicate that for these configurations solute transport is dominated by convection and that the convection velocities are one or even two orders of magnitude larger than the characteristic diffusion velocities. In the pre-nucleation phase solute Marangoni effects are negligible for the half-zone geometry but in the full-size geometry they are comparable to buoyancy driven flows. Calculations of buoyancy flows around a growing crystal show that in ground conditions non-uniform concentration gradients may have a detrimental effect on the growth kinetics.

Experimental and Numerical Investigation of Martian Atmosphere Entry

A numerical and experimental investigation was performed to study the aerothermodynamic problems of entry into the Martian atmosphere. The mathematical and physical model used to study the flowfield around a capsule entering a CO 2 environment is described. Computational fluid dynamics tools have been applied to solve the system of governing equations. The importance of surface catalycity effects on the stagnation-point heat transfer and on the heat load in Martian atmosphere is highlighted. Stagnation-point heat flux levels applied to models of different materials in a plasma wind tunnel are shown, and numerical correlations are presented. The different role played by surface catalycity in Earth and Mars environments is shown.

INSTABILITY OF THERMOCAPILLARY CONVECTION IN LIQUID BRIDGES

It is well known that thermocapillary convection arises in liquid bridges when the support discs are heated differentially and uniformly. Upon increasing the temperature difference, the convective flow shows a transition from an axisymmetric to an oscillatory regime. This phenomenon has been investigated both experimentally and theoretically, but prior experimental results exhibit some discrepancies with respect to the predictions of stability analyses. The present paper discusses experimental results obtained under microgravity conditions, comparing them with previous experimental observations made on ground and in space and with recent theoretical models of the instability. The results agree with the description of oscillations in terms of superposition of hydrothermal waves. Finally, a possible mechanism for a “pulsating” instability is proposed, together with a new scaling law for the oscillation frequencies at onset; this law, which correlates the critical frequency to the fluid properties, the geome...

Influence of buoyancy forces on Marangoni flow instabilities in liquid bridges

The influence of buoyancy forces on oscillatory Marangoni flow in liquid bridges of different aspect ratio is investigated by three‐dimensional, time‐dependent numerical solutions and by laboratory experiments using a microscale apparatus and a thermographic visualisation system. Liquid bridges heated from above and from below are investigated. The numerical and experimental results show that for each aspect ratio and for both the heating conditions the onset of the Marangoni oscillatory flow is characterized by the appearance of a standing wave regime; after a certain time, a second transition to a travelling wave regime occurs. The three‐dimensional flow organization at the onset of instability is different according to whether the bridge is heated from above or from below. When the liquid bridge is heated from below, the critical Marangoni number is larger, the critical wave number (m) is smaller and the standing wave regime is more stable, compared with the case of the bridge heated from above. For the critical azimuthal wave number, two correlation laws are found as a function of the geometrical aspect ratio A.

Microgravity Experiment Acceleration Tolerability on Space Orbiting Laboratories

Residual gravity and g jitter aboard space orbiting laboratories induce disturbing effects on fluid and material science experiments. A nondimensional scaling analysis of the field equations, under the assumption of high frequencies and small amplitudes of g jitter, was carried out to assess the tolerability criteria for microgravity experiments in the presence of oscillatory and time-averaged fluid thermodynamic distortions induced by multiplefrequency excitation. The most important result is that the tolerability limits, imposed by the presence of timeaveraged distortions, are substantially lower than those corresponding to oscillatory distortions. The tolerability domains, in the plane of frequency vs acceleration, were numerically computed for a specific test case consisting of a directional solidification process of binary alloys or semiconductors. Nomenclature b = amplitude of the sinusoidal displacement, cm c = mass concentration measured with respect to its reference value c.v = solute concentration at the crystal-melt interface in a solid GO = solute concentration at the crystal-melt interface in a

Lymphocytes on sounding rockets.

The effect of gravity on human lymphocytes in-vitro has been extensively studied by our team in true microgravity in space /1-3/, in simulated lowgravity in the clinostat /4,5/ and in hypergravity in the centrifuge /5-7/. Lymphocytes are easily purified from peripheral blood and can be ictivated either chemically or by exposure to mitogens in culture. Thereby, cells start to proliferate, T—lymphocytes secrete lymphokines (IL-2, IL-3, IL—4 as well as interferon). The mechanism of the in-vitro activation is analogous to that occurring in—vivo when the immune system is challenged by antigens.