A5084480345- Fluid Dynamics and Turbulent Flows
- Solar and Space Plasma Dynamics
- Ionosphere and magnetosphere dynamics
- Magnetic confinement fusion research
- Computational Fluid Dynamics and Aerodynamics
- Gas Dynamics and Kinetic Theory
- Navier-Stokes equation solutions
- Nonlinear Dynamics and Pattern Formation
- Geomagnetism and Paleomagnetism Studies
- Meteorological Phenomena and Simulations
- Nonlinear Photonic Systems
- Particle Dynamics in Fluid Flows
- Quantum chaos and dynamical systems
- Wind and Air Flow Studies
- Stochastic processes and financial applications
- Climate variability and models
- Advanced Thermodynamics and Statistical Mechanics
- Advanced Fiber Laser Technologies
- Geophysics and Gravity Measurements
- Complex Systems and Time Series Analysis
- Plasma Diagnostics and Applications
- Plant Water Relations and Carbon Dynamics
- Statistical Mechanics and Entropy
- Computational Physics and Python Applications
- Astro and Planetary Science
Ruhr University Bochum
2015-2024
Observatoire de la Côte d’Azur
1989-2005
Heinrich Heine University Düsseldorf
1991-2001
University of California, Santa Barbara
1990-1994
We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers the range Rλ∈[120∶740]. Lagrangian structure functions all are found to collapse onto each other wide time lags, pointing towards existence universal behavior, within statistical convergence, calling for unified theoretical description. Parisi-Frisch multifractal theory, suitably...
A precise characterization of structures occurring in turbulent fluid flows at high Reynolds numbers is one the last open problems classical physics. In this review we discuss recent developments related to application instanton methods turbulence. Instantons are saddle point configurations underlying path integrals. They equivalent minimizers Freidlin–Wentzell action and known be able characterize rare events such systems. While there an impressive body work concerning their analytical...
We investigate numerically the question of blowup in finite time for ``swirling flow'' three-dimensional incompressible Euler equations. Using rotational symmetry, equations reduce to a two-dimensional problem which is solved by differences. The elliptic equation relating vorticity velocity with multigrid method. Calculations were performed 896\ifmmode\times\else\texttimes\fi{}640 mesh points.
Abstract Sharp large deviation estimates for stochastic differential equations with small noise, based on minimizing the Freidlin–Wentzell action functional under appropriate boundary conditions, can be obtained by integrating certain matrix Riccati along minimizers or instantons, either forward backward in time. Previous works this direction often rely existence of isolated positive definite second variation. By adopting techniques from field theory and explicitly evaluating prefactors as...
The occurrence of a finite time singularity in the incompressible Euler equations three dimensions is studied numerically using technique adaptive mesh refinement. As opposed to earlier treatments, prescribed accuracy guaranteed over entire integration domain. A vorticity could be traced down five levels refinement which corresponds resolution ${2048}^{3}$ points nonadaptive treatment. growth fits power law behavior proportional $1/({T}^{*}\ensuremath{-}t)$ where ${T}^{*}$ denotes when occurs.
Abstract Synthetic turbulence is a relevant tool to study complex astrophysical and space plasma environments inaccessible by direct simulation. However, conventional models lack intermittent coherent structures, which are essential in realistic turbulence. We present novel method featuring conditional structure function scaling fieldline curvature statistics comparable magnetohydrodynamic Enhanced transport of charged particles investigated as well. This presents significant progress...
The process of deriving fluid equations from the Vlasov equation for collisionless plasmas involves a fundamental challenge known as closure problem. This problem consists fact that temporal evolution any particle moment—such density, current, pressure, or heat flux—includes terms depend on next higher-order moment. Consequently, truncating description at nth order necessitates approximating contributions (n+1)th moment within choice truncation level and assumptions...
Abstract Models for the transport of high-energy charged particles through strong magnetic turbulence play a key role in space and astrophysical studies, such as describing propagation solar energetic cosmic rays. Inspired by recent advances high-performance machine learning techniques, we investigate application generative diffusion models to synthesizing test particle trajectories obtained from turbulent magnetohydrodynamics simulation. We consider velocity increment, spatial transport,...
Abstract The impact of turbulent fluctuations on the forces exerted by a fluid towed spherical particle is investigated means high-resolution direct numerical simulations. measurements are carried out using novel scheme to integrate two-way coupling between and incompressible surrounding flow maintained in high-Reynolds-number regime. main idea consists combining Fourier pseudo-spectral method for with an immersed-boundary technique impose no-slip boundary condition surface particle. This...
We present a new method for sampling rare and large fluctuations in nonequilibrium system governed by stochastic partial differential equation (SPDE) with additive forcing. To this end, we deploy the so-called instanton formalism that corresponds to saddle-point approximation of action path integral formulation underlying SPDE. The crucial step our approach is an alternative SPDE incorporates knowledge solution such are able constrain dynamical evolutions around extreme flow configurations...
A fully kinetic Vlasov simulation of the Geospace Environment Modeling Magnetic Reconnection Challenge is presented. Good agreement found with previous simulations using particle in cell (PIC) codes, confirming both PIC and code. In latter complete distribution functions fk (k=i,e) are discretized on a numerical grid phase space. contrast to simulations, code does not suffer from noise allows more detailed investigation functions. The role different contributions Ohm’s law compared by...
We report on a comparison of high-resolution numerical simulations Lagrangian particles advected by incompressible turbulent hydro- and magnetohydrodynamic (MHD) flows. Numerical were performed with up to $1024^3$ collocation points 10 million in the Navier-Stokes case $512^3$ 1 MHD case. In hydrodynamics our findings compare recent experiments from Mordant et al. [1] Xu [2]. They differ Biferale [3] due differences ranges choosen for evaluating structure functions. turbulence intermittency...
We investigate the spatio-temporal structure of most likely configurations realizing extremely high vorticity or strain in stochastically forced three-dimensional incompressible Navier-Stokes equations. Most are computed by numerically finding highest probability velocity field an extreme constraint as solution a large optimization problem. High-vorticity identified pinched vortex filaments with swirl, while high-strain correspond to counter-rotating rings. additionally observe that for and...
We present measurements of conditional probability density functions (PDFs) that allow one to systematically bridge from Eulerian Lagrangian statistics in fully developed 3D turbulence. The transition is investigated for hydro- as well magnetohydrodynamic flows and comparisons are drawn. Significant differences the PDFs observed these traced back differing coherent structures. In particular, we address problem an increasing degree intermittency going coordinates by means involved this...
We address the question whether one can identify instantons in direct numerical simulations of stochastically driven Burgers equation.For this purpose, we first solve instanton equations using Chernykh-Stepanov method [Phys.Rev. E 64, 026306 (2001)].These results are then compared to by introducing a filtering technique extract prescribed rare events from massive data sets realizations.Using approach entire time history evolution which allows us different phases predicted Chernykh and...
Three-dimensional turbulent flows are characterized by a flux of energy from large to small scales, which breaks the time reversal symmetry. The motion tracer particles, tend lose faster than they gain it, is also irreversible. Here, we connect irreversibility in single tracers with vortex stretching and thus generation smallest scales.
We compare different approaches towards an effective description of multi-scale velocity field correlations in turbulence. Predictions made by the operator product expansion, so-called fusion rules, are placed juxtaposition to approach that interprets turbulent energy cascade terms a Markov process increments scale. explicitly show rules direct consequence property provided structure functions exhibit scaling inertial range. Furthermore, limit case joint gradient and increment statistics is...