Paolo Barbante

Flight Extrapolation of Plasma Wind Tunnel Stagnation Region Flowfield

Development of reusable space vehicles requires a precise qualification of their thermal protection system materials. The catalytic properties are usually determined in plasma wind tunnels for test conditions relevant to the flight mission program. Therefore, for such a situation, it is important to have a methodology that allows the correct extrapolation of the ground test conditions to the real flight ones and vice-versa. The local heat transfer simulation concept presented in this paper is a possible strategy for accomplishing this task. Computational results show that the ground test conditions are indeed correctly extrapolated to the flight ones and a simple method of accounting for possible discrepancies between the two configurations is presented.

Computation of nonequilibrium high-temperature axisymmetric boundary-layer flows

Efficient and accurate Hermitian-type multipoint finite difference methods are used to develop a general boundary-layer code for analyzing reacting flows around bodies of revolution. The main motivation is to build a reliable code that can be used for the investigation of the influence of different physico-chemical models for transport properties, chemical kinetics, and finite rate wall catalysis on practically relevant quantities like heat flux and skin friction at the body surface. Special care has been devoted to the correct modeling of diffusion fluxes, an aspect that is often neglected in literature. The exact Stefan-Maxwell equations are used to model the diffusion fluxes and are solved with an efficient iterative technique. Finite rate catalysis is an important aspect of thermal protection system (TPS) materials studies, for which a boundary-layer code is a very useful tool because it allows the computation of the heat flux at a cost that is a fraction of a Navier-Stokes approach. Wall catalyticity effects are taken into account by means of a model that allows one to express a suitable set of wall reactions with the associated reaction-rate probabilities

A comparison of molecular dynamics and diffuse interface model predictions of Lennard-Jones fluid evaporation

The unsteady evaporation of a thin planar liquid film is studied by molecular dynamics simulations of Lennard-Jones fluid. The obtained results are compared with the predictions of a diffuse interface model in which capillary Korteweg contributions are added to hydrodynamic equations, in order to obtain a unified description of the liquid bulk, liquid-vapor interface and vapor region. Particular care has been taken in constructing a diffuse interface model matching the thermodynamic and transport properties of the Lennard-Jones fluid. The comparison of diffuse interface model and molecular dynamics results shows that, although good agreement is obtained in equilibrium conditions, remarkable deviations of diffuse interface model predictions from the reference molecular dynamics results are observed in the simulation of liquid film evaporation. It is also observed that molecular dynamics results are in good agreement with preliminary results obtained from a composite model which describes the liquid film by a standard hydrodynamic model and the vapor by the Boltzmann equation. The two mathematical model models are connected by kinetic boundary conditions assuming unit evaporation coefficient.

A kinetic model for collisional effects in dense adsorbed gas layers

The effects of adsorbed gas layers on gas‐surface interaction is investigated by a kinetic model based on the Enskog‐Vlasov equation. The onset of collisional effects in the gas layer is studied by equilibrium simulations. The effects of adsorption on the gas‐wall friction forces is investigated in a simple Couette flow geometry. It is shown that gas adsorption might reduce tangential stresses, as indicated by molecular dynamics based studies. Finally an example is produced in which the two‐dimensional Enskog‐Vlasov equation is solved to simulate the formation of a liquid film on the walls of a planar channel.

A kinetic model for capillary flows in MEMS

A kinetic model for the study of capillary flows in micromechanical devices has been presented. The model is based on the Enskog-Vlasov kinetic equation and provides a reasonable description of two-phase flows and of fluid-surface interaction. The structure of liquid menisci between two hydrophilic walls has been studied and our results agree fairly well with the Laplace-Kelvin equation. Meniscus breakage has been computed and a possible explanation of the phenomenon has been given.

Simulations of condensation flows induced by reflection of weak shocks from liquid surfaces

The condensation of a vapor onto a planar liquid surface, caused by the reflection of a weak shock wave, is studied by three different simulation method. The first one is based on molecular dynamics (MD) simulations of the Lennard-Jones fluid which are supposed to provide reference solutions. The second method is based on a Diffuse Interface Model (DIM), consistent with the thermodynamic properties of the Lennard-Jones fluid as well as with its transport properties. The third method is based on a hybrid model (HM) in which the liquid is described by a purely hydrodynamic approach, whereas the vapor is described by the Boltzmann equation. The two phases are connected by kinetic boundary conditions. The results show that DIM fails to accurately predict the condensation rate when the vapor is dilute but becomes more accurate when the vapor phase gets denser. HM reproduces MD simulations of nearly ideal vapor condensations with good accuracy, assuming unit condensation coefficient.