An efficient IMEX-DG solver for the compressible Navier-Stokes equations for non-ideal gases

We propose an efficient, accurate and robust IMEX solver for the compressible Navier-Stokes equation describing non-ideal gases with general equations of state. The method, which is based on an $h-$adaptive Discontinuos Galerkin spatial discretization and on an Additive Runge Kutta IMEX method for time discretization, is tailored for low Mach number applications and allows to simulate low Mach regimes at a significantly reduced computational cost, while maintaining full second order accuracy also for higher Mach number regimes. The method has been implemented in the framework of the $deal.II$ numerical library, whose adaptive mesh refinement capabilities are employed to enhance efficiency. Refinement indicators appropriate for real gas phenomena have been introduced. A number of numerical experiments on classical benchmarks for compressible flows and their extension to real gases demonstrate the properties of the proposed method

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

Increasing the maturity of measurements of essential climate variables (ECVs) at Italian atmospheric WMO/GAW observatories by implementing automated data elaboration chains

In the framework of the National Project of Interest NextData, we developed automatic procedures for the flagging and formatting of trace gases, atmospheric aerosols and meteorological data to be submitted to the World Data Centers (WDCs) of the Global Atmosphere Watch program of the World Meteorological Organization (WMO/GAW). In particular, the atmospheric Essential Climate Variables (ECVs) covered in this work are observations of near-surface trace gas concentrations, aerosol properties and meteorological variables, which are under the umbrella of the World Data Center for Greenhouse Gases (WDCGG), the World Data Center for Reactive Gases, and the World Data Center for Aerosol (WDCRG and WDCA). We developed an overarching processing chain to create a number of data products (data files and reports) starting from the raw data, finally contributing to increase the maturity of these measurements. To this aim, we implemented specific routines for data filtering, flagging, format harmonization, and creation of data products, useful for detecting instrumental problems, particular atmospheric events and quick data dissemination towards stakeholders or citizens. Currently, the automatic data processing is active for a subset of ECVs at 5 measurement sites in Italy. The system represents a valuable tool to facilitate data originators towards a more efficient data production. Our effort is expected to accelerate the process of data submission to WMO/GAW or to other reference data centers or repositories. Moreover, the adoption of automatic procedures for data flagging and data correction allows to keep track of the process that led to the final validated data, and makes data evaluation and revisions more efficient by improving the traceability of the data production process

Kinetic theory aspects of non-equilibrium liquid-vapor flows

Kinetic theory of fluids plays an important role in understanding and modeling mass, momentum and energy transfer between the vapor and liquid phase in non-equilibrium two-phase flows, in which evaporation and/or condensation take place. The paper presents a review of the literature which focuses on kinetic modeling of the vapor-liquid interface. Starting from the studies of the Knudsen layer structure in evaporation and condensation, the problem of the formulation of kinetic boundary conditions is described and discussed. The formulation of models based on approximate kinetic descriptions of dense fluids is described and the model capabilities are assessed through the analysis of the results obtained by various author

A comparison of models for the evaporation of the Lennard-Jones fluid

The evaporation of a thin liquid film is studied by a diffuse interface model whose thermodynamic and transport properties are consistent with those of the Lennard-Jones fluid. Solutions are obtained for various liquid film temperatures and downstream vapor flow velocities. The results are compared with reference molecular dynamics simulations of a system of atoms interacting by the $6-12$ Lenard-Jones potential. It is shown that the diffuse interface model underestimates the temperature drop across the non-equilibrium vapor region next to the liquid-vapor interface but overestimates the density drop, thus predicting smaller evaporation rates. Results indicates that the discrepancies between molecular dynamics and diffuse interface model predictions become smaller when the liquid film temperature approaches the critical temperature and the vapor becomes a dense, non-ideal gas. Further successful comparisons of molecular dynamics results with the predictions of a hybrid model, combining the continuum description of the liquid with the kinetic description of the vapor, suggest that the observed discrepancies can be attributed to poor description of the Knudsen layer provided by the diffuse interface model when the vapor phase is dilute

A comparison of different approaches to compute surface tension contribution in incompressible two-phase flows

We perform a quantitative assessment of different strategies to compute the contribution due to surface tension in incompressible two-phase flows using a conservative level set (CLS) method. More specifically, we compare classical approaches, such as the direct computation of the curvature from the level set or the Laplace-Beltrami operator, with an evolution equation for the mean curvature recently proposed in literature. We consider the test case of a static bubble, for which an exact solution for the pressure jump across the interface is available, and the test case of an oscillating bubble, showing pros and cons of the different approaches