D. Paterna

Buoyancy and Marangoni Effects in an Evaporating Drop

Experiments and numerical simulations are carried out to study Marangoni and buoyancy effects in a hanging evaporating drop. The liquids investigated are n-octane, which exhibits Marangoni effect, and water, which does not exhibit thermal Marangoni effect. The disk sustaining the drop (diameter of a few millimeters) is held at a constant temperature. A temperature difference arises in the droplet as a consequence of the energy exchange with the ambient and of the evaporative cooling. In the presence of surface tension gradients (Marangoni effect), convective flows are established, and small surface temperature differences are measured at the drop-ambient interface. When the thermal Marangoni effect is absent (as in the water droplet), the surface temperature is stratified, and much larger surface temperature differences are established over the drop surface. The velocity field inside the droplet is evaluated by monitoring the motion of tracers within the drop, in the meridian plane, using a charge-coupled device (CCD) camera. The surface temperature distribution is detected by an infrared camera

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.

Marangoni flotation of liquid droplets

Flotation of liquid droplets on pool surfaces, in the presence of temperature differences, is studied experimentally and numerically. Coalescence or sinking of the droplet is prevented by the thermal Marangoni motion, owing to the surface tension imbalance at the pool surface. The mechanism is the same as that investigated in previous works on coalescence and wetting prevention in the presence of temperature differences. If the droplet is colder than the liquid surface, the flow is directed radially towards the drop; this radial flow field drags the ambient air under the drop, thus creating an air film and avoiding a direct contact between the droplet and the pool molecules. The surface velocities are measured visually with a CCD camera to image the motion of tracers floating on the pool surface; the surface temperature distributions along the pool and the droplet surfaces are measured by an infrared thermocamera. The experimental results are correlated by numerical results obtained under the assumption of spherical drop and axisymmetric flow regime

Thermodiffusion in nanofluids under different gravity conditions

A convective transport model is developed to study the role of thermal diffusion, or the Ludwig–Soret effect, in nanofluid systems with temperature gradients. The study deals with a fluid suspension of nanoparticles enclosed between two differentially heated horizontal, relatively closely spaced plates (Benard configuration). An order-of-magnitude analysis is performed to identify the relevant parameters of the problem. Three-dimensional simulations are performed taking into account different conditions, including normal or microgravity conditions, gravity orientation, and positive or negative Soret effect. Different modes of convective instabilities are shown to be present in the system, which are associated with the gravity force and the density differences induced by concentration gradients. The characteristic flow patterns and instability developments are in agreement with the experimental findings obtained by independent investigators on colloidal suspensions. The onset of instabilities, their charac...

Numerical and Experimental Investigation of PRORA USV Subsonic and Transonic Aerodynamics

The Italian unmanned space program PRORA USV (Programma di Ricerca Aerospaziale Unmanned Space Vehicle) is a technological program focused on the realization of flying test beds to demonstrate key vehicle and operational technologies applicable to future reusable launch vehicles. In this framework, the dropped transonic flight test is the first in a series of flight experiments the objective of which is the investigation of the final transonic part of the orbital return phase of a winged reentry vehicle. To prepare this mission, the development of the aerodynamic database is in progress, using both extensive wind tunnel tests and computational fluid dynamics. This study deals with investigations focused on the aerodynamic characterization of vehicle rudders in subsonic and transonic regimes for different angles of attack and rudder deflections. The aerodynamic characteristics of the vehicle are computed and discussed. Comparisons between numerical and available experimental data show a satisfactory agreement.

Counter-measures to mitigate residual-g effects on microgravity experiments on the space station ☆

Abstract The paper wants to approach the problem of the disturbances mitigation on the International Space Station (ISS) from a purely scientific view point by evaluating and trying to minimize the effects (fluid-dynamic disturbances) more than trying to minimize some of the causes (as achieved by Isolation Mounts). A study case, consisting in a cylindrical test cell filled with a silicone oil in presence of a temperature difference ΔT, has been identified to reproduce the ISS microgravity environment (when in presence of a steady residual-g) on ground by properly changing the fluid properties, the dimensions and the imposed temperature difference. The orientation of the cell with respect to the vertical direction has been changed using an appropriate device that corresponds, in the MGE, to different angles between the cell axis and the residual-g. The maximum distortion of the temperature field has been evaluated numerically and experimentally by measuring the maximum temperature difference, in the mid-cross-section, between points that would exhibit the very same temperature in a purely diffusive, quiescent state. Further experimental tests have been performed, using a different configuration, to measure the surface temperature distribution with an infrared thermocamera. The correlation between experimental and numerical results are in good agreement and the results indicate that the experiment facility can be properly oriented to minimize the convection disturbances. Another experiment for the study of the temperature/concentration distortions induced by residual-g and g-jitter during experiments on diffusion/thermodiffusion is proposed.

Compatibility of the microgravity environment of the international space station with fluid and material science experimentation

Abstract This paper deals with the effects of space non uniform (pendular) accelerations on fluid and material science experiments onboard the International Space Station. These motions, that can be generated by rigid body oscillations around the centre of mass of the ISS, or by elastic deformations of the structure, imply different effects on liquid behaviour onboard ISS (in particular, they generate convective motions even in isodense fluids). A very simple model of a cantilever beam is presented that shows the different roles played by linear and rotational accelerations in inducing fluid motion inside a uniform density liquid cell. Experimental data are presented for the low frequency and the high frequency (streaming) case, correlated by numerical results. Low frequency pendular motion of the Russian FOTON capsule is considered and its effects on fluid physics experimental results obtained in previous missions is analysed. The possibility of mean flow (streaming) generation due to high frequency non-translational vibrations in a isodense fluid is considered, and comparisons are presented between on-ground experimental and numerical results.

Linear and Pendular Acceleration Effects in Fluid Dynamic Experiments Under Low Gravity

To study the influence of residual acceleration on fluid physics experiments in microgravity environments, the different driving forces in the vorticity equation (leading to convective motion in a fluid cell) are considered. Steady and linear time-dependent accelerations generate convection in an otherwise quiescent fluid only in the presence of density gradients. Pendular acceleration may induce convective motion even in the absence of a density gradient induced by temperature and/or species concentration differences. Numerical simulations are carried out for different study cases, in the presence of low-frequency and high-frequency oscillations, based on realistic values of the accelerations onboard space platforms. The numerical results are compared with experimental results, obtained in microgravity and ground laboratories.