Sub-grid Scale Modelling and a-Posteriori Tests with a Morphology Adaptive Multifield Two-Fluid Model Considering Rising Gas Bubbles

The predictive simulation of gas–liquid multiphase flows at industrial scales reveals the challenging task to consider turbulence and interfacial structures, which span a large range of length scales. For simulation of relevant applications, a hybrid model can be utilised, which combines the Euler–Euler model for the description of small interfacial structures with a volume-of-fluid model as a scale-resolving multiphase approach. Such a hybrid model needs to be able to simulate interfaces, which are hardly resolved on a coarse numerical grid. The goal of this work is to improve the prediction of interfacial gas–liquid flows on a numerical grid with comparably large grid spacing. From the low-pass filtering of the two-fluid model five unclosed sub-grid scale terms arise. The convective and the surface tension part of the aforementioned contributions are individually modelled with multiple closure formulations. Those models are a-posteriori assessed in cases of two- and three-dimensional gas bubbles rising in stagnant liquid. It is shown, that the chosen closure modelling approach is suitable to improve the predictive power of the numerical model utilised in this work. Hence, simulation results on comparably coarse grids are changed towards results obtained with higher spatial resolution.

Computational Modeling of Planing Hull Dynamics and Slamming in Head Waves

Fast boats often operate in planing regimes when they skim on the water surface and their weight is supported primarily by hydrodynamic forces. In the presence of waves, such hulls may experience large nonlinear motions and hydrodynamic loads, which limit their operational capabilities. To predict hull motions and loads and to optimize the hull shape and structure, one can take advantage of computational fluid dynamics tools that simulate these complex nonlinear flow processes and provide detailed hydrodynamic data, including pressure distribution on the hull and water spray. However, validation of these modeling approaches is needed in order to confidently use numerical tools for the boat design. In this study, numerical modeling is accomplished for dynamics of a realistic hull previously tested in controlled wave environments in towing tanks. Time-domain simulations were first carried out in regular head waves. Mesh-verification studies suggested appropriate numerical grid resolution. The hull’s heave motions, drag forces and bow accelerations were captured and compared with experimental data. The formal validation procedure was applied to confirm suitability of the current numerical approach. In the investigated regular-wave conditions, very pronounced slamming phenomenon was observed, when the hull re-entered water and experienced peak hydrodynamic loads. Pressure distributions on the hull surface and water surface deformations are presented for several time instances around the slamming event. In addition, numerical simulations were also conducted for random waves with statistical sea-wave parameters resembling those of the studied regular waves. The statistical boat responses, such as bow accelerations, heaving motions and drag forces, are compared to the corresponding metrics obtained in regular waves.

Distinguishing Closed Circulations from the Satellite Maps of the Dynamic Topography

<p>An algorithm for distinguishing closed multicore circulations from digital maps of dynamic topography (DT) is described. The algorithm is based on the expansion of circulations over the area from their cores (local maxima/minima of the DT) until the DT thresholds corresponding to these cores are reached. The algorithm is performed in several iterations until the points belonging to the closed circulations are completely exhausted. The algorithm is an exact numerical solution of the problem of determining the value of the DT for a closed loop, the most distant from the core of circulation. The algorithm takes into account the problems of nesting circulations of different signs into each other, the possible intersecting of circulations with different signs on the numerical grid, and the possible existence of islands or floating ice inside the circulations. A method is described for merging smaller DT maps to larger maps with the circulations distinguished from the smaller maps.</p>

Plume spreading test case for coastal ocean models

We present a test case of river plume spreading to evaluate numerical methods used in coastal ocean modeling. The main characteristics of the plume dynamics are predicted analytically, but are difficult to reproduce numerically because of numerical mixing present in the models. Our test case reveals the level of numerical mixing as well as the ability of models to reproduce nonlinear processes and frontal zone dynamics. We propose an analysis of simulated plume spreading which may be useful in more general studies of plume dynamics. The major result of our comparative study is that accuracy in reproducing the analytical solution depends less on the type of applied model architecture or numerical grid than it does on the type of advection scheme.

Artificial diffusion in the simulation of micromixers: A review

False (artificial) diffusion provides an erroneous estimation of molecular diffusion during the simulation of liquid micromixing. The present review introduces discretization methods, numerical grid types, and numerical errors to address the effect of false diffusion on the prediction of mixing efficiency of microfluidic devices. False diffusivity is characterized by the grid Peclet number, grid type, and discretization scheme. This review demonstrates that most investigators have selected a grid resolution just for the grid independence test. It is revealed that the convergence criterion should not be quantitative values of mixing efficiency even when high-order schemes are employed to discretize the computational grid. Based on the previous publications in this field, a straightforward procedure is recommended to manage false diffusion in the numerical simulation of micromixers.

Type II critical collapse on a single fixed grid: a gauge-driven ingoing boundary method

We develop a numerical method suitable for gravitational collapse based on Cauchy evolution with an ingoing characteristic boundary. Unlike similar methods proposed recently (Ripley; Bieri et al. in Class Quantum Grav 37:045015, 2020), the numerical grid remains fixed during the evolution and no points need to be removed or added. Increasing coordinate refinement of the central region as the field collapses is achieved solely through the choice of spatial gauge and particularly its boundary condition. We apply this method to study critical collapse of a massless scalar field in spherical symmetry using maximal slicing and isotropic coordinates. Known results on mass scaling, discrete self-similarity and universality of the critical solution (Choptuik in Phys Rev Lett 70:9, 1993) are reproduced using this considerably simpler numerical method.

Characterizing the vertical concentration profiles of ship plumes with a microscale model - is it all Gaussian?

<p>Accurate modeling of ship emissions is a topic of increasing interest due to the ever-growing global fleet and its emission of air pollutants. With the increasing calculation power of modern computers, numerical grid models can nowadays be used to analyze effects of shipping emissions from global to local scales. However, modeling entire ports and larger domains still requires a good representation for the vertical concentration profile of single ship plumes. As the shape of the plume strongly varies depending on parameters like plume temperature, ship-induced turbulence and meteorological conditions, the plume dilution does not always appear to be represented by a simple Gaussian distribution. In this work, the microscale model MITRAS is used to calculate vertical concentration profiles of ship plumes under varying technical and meteorological scenarios. The resulting curves are fitted with different mathematical curves (e.g. Gaussian, Polynomial and Gamma distribution) by a least square minimization approach and the best representations for individual scenarios are discussed.</p>

And yet it flips: connecting galactic spin and the cosmic web

ABSTRACT We study the spin alignment of galaxies and haloes with respect to filaments and walls of the cosmic web, identified with DisPerSE , using the Simba simulation from z = 0 − 2. Massive haloes’ spins are oriented perpendicularly to their closest filament’s axis and walls, while low-mass haloes tend to have their spins parallel to filaments and in the plane of walls. A similar mass-dependent spin flip is found for galaxies, albeit with a weaker signal particularly at low mass and low-z, suggesting that galaxies’ spins retain memory of their larger scale environment. Low-z star-forming and rotation-dominated galaxies tend to have spins parallel to nearby filaments, while quiescent and dispersion-dominated galaxies show preferentially perpendicular orientation; the star formation trend can be fully explained by the stellar mass correlation, but the morphology trend cannot. There is a dependence on HI mass, such that high-HI galaxies tend to have parallel spins while low-HI galaxies are perpendicular, suggesting that HI content may trace anisotropic infall more faithfully than the stellar component. Finally, at fixed stellar mass, the strength of spin alignments correlates with the filament’s density, with parallel alignment for galaxies in high density environments. These findings are consistent with conditional tidal torque theory, and highlight a significant correlation between galactic spin and the larger scale tides that are important e.g., for interpreting weak lensing studies. Simba allows us to rule out numerical grid locking as the cause of previously-seen low mass alignment.

Incorporating Climate Nonstationarity and Snowmelt Processes in Intensity–Duration–Frequency Analyses with Case Studies in Mountainous Areas

Downscaled high-resolution climate simulations were used to provide inputs to the physics-based Distributed Hydrology Soil Vegetation Model (DHSVM), which accounts for the combined effects of snowmelt and rainfall processes, to determine spatially distributed available water for runoff (AWR). After quasi-stationary time windows were identified based on model outputs extracted for two different mountainous field sites in Colorado and California, intensity–duration–frequency (IDF) curves for precipitation and AWR were generated and evaluated at each numerical grid to provide guidance on hydrological infrastructure design. Impacts of snowmelt are found to be spatially variable due to spatial heterogeneity associated with topography according to geostatistical analyses. AWR extremes have stronger spatial continuity compared to precipitation. Snowmelt impacts on AWR are more pronounced at the wet California site than at the semiarid Colorado site. The sensitivities of AWR and precipitation IDFs to increasing greenhouse gas emissions are found to be localized and spatially variable. In subregions with significant snowfall, snowmelt can result in an AWR (e.g., 6-h 100-yr events) that is 70% higher than precipitation. For comparison, future greenhouse gas emissions may increase 6-h 100-yr precipitation and AWR by up to 50% and 80%, respectively, toward the end of this century.

On the revealing closed circulations on satellite maps of dynamic topography of the ocean surface

An algorithm for revealing closed multi-core circulations on digital maps of dynamic topography (DT) is described. The algorithm consists in the expansion of eddies over the area from their cores (local maxima/minima of the DT) until the DT sills corresponding to these cores are reached, and is carried out in several iterations until the points belonging to the closed circulations are completely exhausted. The algorithm is an exact numerical solution of the problem of determining the value of the DT for a closed loop, the most distant from the core of circulation. The algorithm takes into account the problems of nesting into each other circulations of a different sign, the possible intersection with each other of the circulation of a different sign on the numerical grid, as well as the possible existence of islands or floating ice inside the circulations. A method is described for gluing smaller DT maps with the circulations revealed on them to larger maps.