Antonio Viviani

Thermal and Concentrational Maragoni Convection at Liquid/Air Bubble Interface

The paper presents the experimental study of thermo- and solutocapillary Marangoni convection around a gas bubble in an inhomogeneous fluid with a vertical thermal or surfactant concentration gradient. The stationary bubble in the form of a short horizontal cylinder with a free lateral surface was placed into a vertically oriented thin liquid layer (Hele-Shaw cell). The evolution of thermal and concentration fields and fluid flows was studied applying the interferometric method. In contrast to a thermocapillary convection representing a stationary flow and stable temperature distribution, the periodic concentration disturbances around the bubble were observed in the solutocapillary case. The regularities of the discovered effect were revealed, and its interpretation was proposed.

The influence of the horizontal component of the temperature gradient on nonlinear convective oscillations in two-layer systems

The influence of the horizontal component of the temperature gradient on nonlinear oscillatory convective regimes, developed under the joint action of buoyant and thermocapillary effects in the 47 v2 silicone oil-water system, is investigated. Cavities with different lengths have been considered. Transitions between oscillatory flow regimes with different symmetry properties and steady flows have been studied. It is shown that under the action of the horizontal component of the temperature gradient, specific asymmetric oscillatory flow develops in the system.

Oscillatory marangoni convection around bubbles and drops in heterogenous solutions of surfactants

Experimental investigation was performed to study the concentration convection around stationary gas bubbles and insoluble drops in a thin liquid layer placed in a vertical Hele-Shaw cell. The bubbles or drops, squeezed between the two parallel cell walls, took the shape of short cylinders with free lateral surfaces. The cell was filled in with an aqueous solution of a surface-tension active fluid (surfactant) with vertically stratified concentration. A special wire frame prevented bubbles from rising up under the buoyancy force, thus modelling the microgravity conditions. A convective motion in the mixture develops at the bubble or drop interface, due to the solutocapillary Marangoni forces. Owing to a small thickness of the liquid layer (∼1mm), the arising flows and surfactant concentration distributions are nearly two-dimensional so that it is possible to investigate their structure and evolution by interferometric technique. The experiments revealed the development of oscillatory convection around the drop interface, which was similar to that observed in bubble tests. The period and duration of oscillations were determined in relation to time, surfactant concentration gradient and concentration Marangoni number. The analysis of bubble and drop behavior showed that the existence of self-oscillatory modes is related to the specific interaction between the solutocapillary and soluto-gravitational mechanisms of motion.

Nonlinear development of oscillatory instability in a three-layer system under the joint action of buoyancy and thermocapillary effect

The nonlinear development of oscillatory instability under the joint action of buoyant and thermocapillary effects in multilayer system is investigated. The nonlinear convective regimes are studied by the finite difference method. The calculations have been performed for two-dimensional flows. The interfaces are assumed to be nondeforming. Rigid heat-insulated lateral walls are considered. Transitions between the flows with different spatial structures are studied. Specific types of nonlinear flows—symmetric and asymmetric oscillations—have been found. It is shown that the oscillatory flow takes place in an interval of Grashof number values bounded both from below by the quiescent mechanical equilibrium, and from above by a convecting steady state. Cavities with different lengths are considered.

Nonlinear regimes of anticonvection, thermocapillarity, and Rayleigh–Benard convection in two-layer systems

The nonlinear regimes of anticonvection and Rayleigh–Benard convection in a two-layer system with periodic boundary conditions on lateral walls in the presence of the interfacial heat release are studied. The region where anticonvective and the Rayleigh–Benard instability mechanisms act simultaneously is considered. The influence of the thermocapillary effect on anticonvective and Rayleigh–Benard flows, is investigated. New types of nonlinear traveling waves and modulated traveling waves are found.

Aerodynamic Analysis of a Manned Space Vehicle for Missions to Mars

The paper deals with the aerodynamic analysis of a manned braking system entering the Mars atmosphere with the aim to support planetary entry system design studies. The exploration vehicle is an axisymmetric blunt body close to the Apollo capsule. Several fully three-dimensional computational fluid dynamics analyses have been performed to address the capsule aerodynamic performance. To this end, a wide range of flow conditions including reacting and nonreacting flow, different angles of attack, and Mach numbers have been investigated and compared. Moreover, nonequilibrium effects on the flow field around the entry vehicle have also been investigated. Results show that real-gas effects, for all the angles of attack considered, increase both the aerodynamic drag and pitching moment whereas the lift is only slighted affected. Finally, results comparisons highlight that experimental and CFD aerodynamic findings available for the Apollo capsule in air adequately represent the static coefficients of the capsule in the Mars atmosphere.

THERMOCAPILLARY BUBBLE MIGRATION IN CONFINED MEDIUM

Abstract The aim of this paper is to advance the numerical investigation of unsteady thermocapillary migration of gas bubbles in a fully confined non-isothermal liquid. The relevance of the study is twofold: to support space experiments such as the Italian experiment on board of the Spacelab IML-2 Mission and to fill a lack in thermocapillary migration studies mainly devoted to slow motion in infinite medium (so that no comparison is possible with experimental results). The motion due to surface tension gradients of one and two spherical bubbles inside a liquid filling a cylindrical box, with differentially heated ends disks, is herein analysed, both for linear and quadratic temperature dependence of the surface tension at the bubble-liquid interface. A Galerkin finite elements method is employed to solve the unsteady axisymmetric governing equations casted under a velocity, vorticity, temperature formulation. The flow field into the liquid medium, the net force exerted on the bubbles, the bubbles speed and location are determined as a function of the time; pressure and temperature fields around the bubble are reported.

Oscillatory Regimes of Solutocapillary Marangoni Convection

It is common knowledge that a fluid motion can be initiated both by the volume (buoyancy) forces, due to density variations in presence of a gravity field, and the surface (capillary) forces, due to variations of surface tension along a fluid/fluid interface. Involving in motion the surface and the near-surface layers the capillary forces gives rise to a volumetric flow generally known as the Marangoni convection. In turn, inhomogeneity of the surface tension may result from its dependence on temperature or concentration of the dissolved surfacetension active (surfactant) component. Such convection, thermocapillary or solutocapillary one, respectively, plays an important role in hydrodynamics and heat/mass transfer of inhomogeneous multi-phase media with the liquid/liquid interface or the free surface between liquid and gas. The Marangoni convection has a marked effect on the intensity of many technological processes encountered in food, chemical, oil, metallurgical and other industries, including those proceeding in microgravity conditions where the gravitational mechanisms of convective motion are weakened or absent. Particular interest on research in this field has quickened in recent years due to new achievements in the development of space technologies and life-support and survival systems for orbital stations. Marangoni phenomena is a problem of great application value for such areas as ecology (purification of water surface from petroleum products), meteorology, biology (motion of bacteria and microbodies), medicine (spreading of pulmonic surfactants during aerosol inhalation in treatment of lung diseases) and so on. Despite the global abundance of the Marangoni convection the direct experimental investigation of the Marangoni flows in the so-called pure form is a challenging task. The reason is that under normal terrestrial conditions the existence of temperature and concentration gradients in a fluid also leads to development of the gravitational Rayleigh convection with considerably higher intensity than that of the Marangoni convection. Hence the latter proves to be almost suppressed by a more severe convective driving mechanism or veiled by its action. At the same time, under certain conditions, the surface forces may dominate over the buoyancy ones. Such conditions are realized, for example, in shallow layers and fluid films, liquid bridges and zones confined between solid surfaces, and also for small insoluble drops or gas bubbles suspended in liquids. In the past decades, the Marangoni convection in such systems has been the subject of intensive investigation. However, most of the studies are concerned with thermocapillary variant of the Marangoni

Drops in a Surfactant Solution: Oscillatory Regimes of Mass Transfer

The paper presents the results of experimental investigation of interaction between the buoyancy convection and the Marangoni motion during saturation of a drop with surfacetension active substance (surfactant) dissolved in water. To gain a deeper insight into the nature of this interaction the mass transfer process was investigated for drops inserted in thin horizontal or vertical layers. The use of horizontal layer has the advantage of simulating the microgravity conditions under which the buoyancy flows are absent; the chief distinction of the vertical layer is that gravity convection reached the maximal intensity. In both cases the drops were surrounded by surfactant solutions with vertical stratification of concentration. Investigation into the structure and evolution of the concentration fields inside and around the drop was carried out using the Fiseau interferometer. Visualization of the convective flow was made by means of the opaque emulsion formed at the boundary of the drop during dissolution of the surfactant.

MAGNETIC EFFECTS ON NON-LINEAR MARANGONI FLOW IN PLANE LIQUID BRIDGE

Abstract Thermocapillary flow arises under zero-gravity in a plane layer of liquid metal bounded by two plane fixed interfaces with a passive gas and held by two differentially heated walls. On the interfaces the surface tension depends quadratically, with a maximum, on the temperature (non-linear Marangoni flow). The fluid considered has a low Prandtl number (Pr=0.01) and is electrically conducting with a magnetic Prandtl number negligible. The unsteady two–dimensional Navier–Stokes equations in the velocity–vorticity formulation are solved by a Galerkin Finite Element semi-implicit method. For high conditional Reynolds numbers Re the flow becomes oscillatory; by increasing further the Re the flow reaches a new steady state. To drag the oscillations a uniform and steady magnetic field is applied normal to the plane of the bridge. Critical Reynolds numbers are calculated for an aspect ratio A=4 for various values of the magnetic field.