Simulation of flows with violent free surface motion and moving objects using unstructured grids

This is the peer reviewed version of the following article: [Löhner, R. , Yang, C. and Oñate, E. (2007), Simulation of flows with violent free surface motion and moving objects using unstructured grids. Int. J. Numer. Meth. Fluids, 53: 1315-1338. doi:10.1002/fld.1244], which has been published in final form at https://doi.org/10.1002/fld.1244. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.A volume of fluid (VOF) technique has been developed and coupled with an incompressible Euler/Navier–Stokes solver operating on adaptive, unstructured grids to simulate the interactions of extreme waves and three-dimensional structures. The present implementation follows the classic VOF implementation for the liquid–gas system, considering only the liquid phase. Extrapolation algorithms are used to obtain velocities and pressure in the gas region near the free surface. The VOF technique is validated against the classic dam-break problem, as well as series of 2D sloshing experiments and results from SPH calculations. These and a series of other examples demonstrate that the ability of the present approach to simulate violent free surface flows with strong nonlinear behaviour.Peer ReviewedPostprint (author's final draft

Kelvin-Helmholtz multi-spacecraft studies at the Earth's magnetopause boundaries

Copyright © 2010 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.The following article appeared in AIP Conference Proceedings Volume 1216, pp. 483-486, and may be found at http://dx.doi.org/10.1063/1.3395908Twelfth International Solar Wind Conference, Saint‐Malo, France, 21–26 June 2009The Kelvin‐Helmholtz (KH) instability can operate in various situations in the solar wind, but at the boundaries of planetary obstacles, for example the Earth’s magnetopause, it is most amenable to investigation. Reliable estimates of wave characteristics are essential for comparison with theoretical and numerical models and for understanding the nonlinear development of KH waves and their role in the plasma entry into the magnetosphere. After discussing their typical conditions of appearance in KH unstable domains at the magnetopause, both theoretically and observationally, we outline recent results of multi‐spacecraft analysis with Cluster giving accurate, albeit spatially limited, determination of surface wave characteristics. Those characteristics (wavelength and propagation direction), close to the terminator on the nightside, are likely to be prescribed by the 3‐D geometry and the bending of field lines developed by the KH waves, rather than by the magnitude and the direction of the magnetosheath or background flow. An unprecedented number of satellites provides now the opportunity to extend the analysis of source regions of KH waves and their domains of development

A low diffusive Lagrange-remap scheme for the simulation of violent air-water free-surface flows

36p. Submitted to Journal of Computational Physics.In 2002, Després and Lagoutiére proposed a low-diffusive advection scheme for pure transport equation problems, which is particularly accurate for step-shaped solutions, and thus suited for interface tracking procedure by a color function. This has been extended by Kokh and Lagoutiére in the context of compressible multifluid flows using a five-equation model. In this paper, we explore a simplified variant approach for gas-liquid three-equation models. The numerical scheme has two ingredients: a robust remapped Lagrange solver for the solution of the volume-averaged equations, and a low diffusive compressive scheme for the advection of the gas mass fraction. Numerical experiments show the performance of the computational approach on various flow reference problems: dam break, sloshing of a tank filled with water, water-water impact and finally a case of Rayleigh-Taylor instability. One of the advantage of the present interface capturing solver is its natural implementation on parallel processors or computers. In particular, we are confident on its implementation on Graphics Processing Units (GPU) with high speedups

Study of compressible turbulent flows in supersonic environment by large-eddy simulation

A Large-Eddy Simulation (LES) methodology adapted to the resolution of high Reynolds number turbulent flows in supersonic conditions was proposed and developed. A novel numerical scheme was designed, that switches from a low-dissipation central scheme for turbulence resolution to a flux difference splitting scheme in regions of discontinuities. Furthermore, a state-of-the-art closure model was extended in order to take compressibility effects and the action of shock / turbulence interaction into account. The proposed method was validated against fundamental studies of high speed flows and shock / turbulence interaction studies. This new LES approach was employed for the study of shock / turbulent shear layer interaction as a mixing-augmentation technique, and highlighted the efficiency in mixing improvement after the interaction, but also the limited spatial extent of this turbulent enhancement. A second practical study was conducted by simulating the injection of a sonic jet normally to a supersonic crossflow. The validity of the simulation was assessed by comparison with experimental data, and the dynamics of the interaction was examined. The sources of vortical structures were identified, with a particular emphasis on the impact of the flow speed onto the vortical evolution.Ph.D.Committee Chair: Menon, Suresh; Committee Member: Ruffin, Stephen; Committee Member: Sankar, Lakshmi; Committee Member: Seitzman, Jerry; Committee Member: Stoesser, Thorste

Aspects of Nonlinearity and Dissipation in Magnetohydrodynamics

We investigate two projects: (i) the weakly nonlinear evolution of two oppositely travelling waves and (ii) the dissipative instability of a tangential discontinuity. We show that the ponderomotive force is a basic nonlinear force, which is, to leading-order, proportional to the product of a wave quantity and a gradient of a wave quantity. The ponderomotive force of Alfvén waves corresponds to a magnetic wave pressure force. The motion of beads on a string and the fluid motions in an oscillating tube are shown to be good mechanical analogues for the weakly nonlinear evolution of bounded Alfvén waves, especially in plasmas with a low plasma β. We examine, analytically and numerically, the weakly nonlinear evolution of bounded fast magneto-acoustic waves in the cold plasma limit and show that the ponderomotive force moves plasma along the equilibrium magnetic field. The maximum density enhancement is proportional to α2/β, with α the wave amplitude and β of the order of the plasma β, We obtain the wave amplitude and frequency modulation and discuss the problems with the cold-plasma assumption. The weakly nonlinear interaction of Alfvén pulses is investigated in the cold-plasma limit and for finite-β plasmas. We find excellent agreement between analytical and numerical results. We describe a density enhancement, maximally of order α2β1/2 at the position of Alfvén pulse excitation, which splits into two slow pulses, We describe the shock-formation of the Alfvén and slow pulses. The dissipative instability of the tangential discontinuity is examined and applied to coronal hole boundaries. We derive a dispersion relation, which includes weak viscosity and thermal conduction, and is solved for a specific model, using perturbation theory. The effect of viscosity and thermal conduction on stability are discussed, It is shown that dissipation lowers the threshold flow speed for instability

Continental collision and slab break-off: numerical models and surface observables

Collision zones worldwide show a vast array of complexity that is difficult to relate to simple collision dynamics. Here we address the problem of linking surface observables to collision dynamics through the use of two and three-dimensional numerical models. We will focus on two specific observables, topography and the presence of ultra-high pressure metamorphic terranes. Our model topography predictions are used to explain the uplift and subsidence history for the Arabia-Eurasia collision. This allows us to relate the post-collisional marine deposits, found on the overriding plate, to a steepening of the subduction interface. Our three-dimensional model is used to explore possible exhumation mechanisms for the Western Gneiss Complex in Norway. From these models we show how an asymmetric collision can help drive exhumation of material that has experienced comparable conditions to the Western Gneiss Complex. The linking of upper mantle and lithosphere dynamics to both topography observation and exhumation patterns allow better understanding of the subduction and collision process

Cluster observations of surface waves on the dawn flank magnetopause

On 14 June 2001 the four Cluster spacecraft recorded multiple encounters of the dawn-side flank magnetopause. The characteristics of the observed electron populations varied between a cold, dense magnetosheath population and warmer, more rarified boundary layer population on a quasi-periodic basis. The demarcation between these two populations can be readily identified by gradients in the scalar temperature of the electrons. An analysis of the differences in the observed timings of the boundary at each spacecraft indicates that these magnetopause crossings are consistent with a surface wave moving across the flank magnetopause. When compared to the orientation of the magnetopause expected from models, we find that the leading edges of these waves are approximately 45° steeper than the trailing edges, consistent with the Kelvin-Helmholtz (KH) driving mechanism. A stability analysis of this interval suggests that the magnetopause is marginally stable to this mechanism during this event. Periods in which the analysis predicts that the magnetopause is unstable correspond to observations of greater wave steepening. Analysis of the pulses suggests that the waves have an average wavelength of approximately 3.4 <i>R<sub>E</sub></i> and move at an average speed of ~65km s<sup>-1</sup> in an anti-sunward and northward direction, despite the spacecraft location somewhat south of the GSE <i>Z=0</i> plane. This wave propagation direction lies close to perpendicular to the average magnetic field direction in the external magnetosheath, suggesting that these waves may preferentially propagate in the direction that requires no bending of these external field lines<br><br> <b>Key words.</b> Magnetospheric physics (magnetospheric configuration and dynamics; MHD waves and unstabilities; solar wind-magnetosphere interactions