3 exposés de 30min donnés par K. Schneider (I2M), S. Gomes (Unicamp) et F. Jacobitz (U. San Diego)

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Date/heure
Date(s) - 14/10/2016
14 h 00 min - 15 h 30 min

Catégories Pas de Catégories


Nous aurons le plaisir d’écouter {{3 exposés de 30min}} donnés par :
– K. Schneider (I2M) – Tomographic reconstruction using wavelet-vaguelette decomposition for inverting the helical Abel transform. Application to tokamak edge turbulence light emission from a single image.
– S. Gomes (Unicamp) – Multiresolution and adaptive mesh refinement schemes for Euler equations: a comparative study
– F. Jacobitz (U. San Diego) – On Multiscale Acceleration Statistics in Rotating and Sheared Homogeneous Turbulence

Organisateurs : Caroline Chaux (I2M) et François-Xavier Dupé (LIF)

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Title: Tomographic reconstruction using wavelet-vaguelette decomposition for inverting the helical Abel transform. Application to tokamak edge turbulence light emission from a single image.
{{Kai Schneider}}, I2M, AMU

Images acquired by cameras installed in tokamaks are difficult to interpret because the three-dimensional structure of the plasma is flattened in a non-trivial way. Nevertheless, taking advantage of the slow variation of the fluctuations along magnetic field lines, the optical transformation may be approximated by a generalized Abel transform, for which we propose an inversion technique based on the wavelet-vaguelette decomposition. After validation of the new method using an academic test case and numerical data obtained with the Tokam 2D code, we present an application to an experimental movie obtained in the tokamak Tore Supra. A comparison with a classical regularization technique for ill-posed inverse problems, the singular value decomposition, allows us to assess the efficiency. The superiority of the wavelet-vaguelette technique is reflected in preserving local features, such as blobs and fronts, in the denoised emissivity map.

Ref.: R. Nguyen van yen, N. Fedorczak, F. Brochard, G. Bonhomme, K. Schneider, M. Farge and P. Monier-Garbet. Tomographic reconstruction of tokamak edge turbulence light emission from a single image using wavelet-vaguelette decomposition. Nucl. Fusion, 52, 013005, 2012.

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Title: Multiresolution and adaptive mesh refinement schemes for Euler equations: a comparative study.
{{Sonia Gomes}}, Professor, Unicamp, Campinas, Brazil, currently visiting IHP.

We present some comparison results between two adaptive numerical methods, namely the adaptive multiresolution method and the adaptive mesh refinement method for the resolution of 2D and 3D compressible Euler equations. The results are compared with respect to accuracy and computational efficiency, in terms of CPU time and memory requirements, with the corresponding finite volume scheme on a regular fine grid. For both methods, we use second-order schock-capturing schemes for the space discretization, together with explicit second-order Runge-Kutta time integration.

Ref.: R. Deiterding, M. Domingues, S. Gomes and K. Schneider. Comparison of adaptive multiresolution and adaptive mesh refinement applied to simulations of the compressible Euler equations. SIAM J. Sci. Comput., 03/2016, arXiv:1603.05211, accepted.

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Title: On Multiscale Acceleration Statistics in Rotating and Sheared Homogeneous Turbulence.
{{Frank Jacobitz}}, Professor, U. San Diego, USA, currently visiting I2M

The acceleration statistics of sheared and rotating homogeneous turbulence are studied using direct numerical simulation results with different rotation ratios of Coriolis parameter to shear rate $f/S$. For the range of rotation ratios $0 \le f/S \le 1$, a destabilization of the flow due to rotation and growth of the turbulent kinetic energy is obtained. For other values of $f/S$, rotation stabilizes the flow and a decay of the turbulent kinetic energy is observed. The statistical properties of Lagrangian and Eulerian acceleration are considered and the influence of the rotation ratio and the scale dependence of the statistics is investigated. The probability density functions (pdfs) of both Lagrangian and Eulerian acceleration show a strong and similar dependence on the rotation ratio. The flatness further quantifies its dependence and yields values close to three for strong rotation. For moderate and vanishing rotation, the flatness of the Eulerian acceleration is larger than that of the Lagrangian acceleration, contrary to previous results for isotropic turbulence. A wavelet-based scale-dependent analysis shows that the flatness of both Eulerian and Lagrangian acceleration increases as scale decreases. For strong rotation, the Eulerian acceleration is more intermittent than the Lagrangian acceleration, while the opposite result is obtained for moderate rotation.

Ref.: F.G. Jacobitz, K. Schneider, W.J.T. Bos and M. Farge. Structure of sheared and rotating turbulence: Multiscale statistics of Lagrangian and Eulerian accelerations and passive scalar dynamics. Phys. Rev. E, 93, 013113, 2016.


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Olivier CHABROL
Posts created 14

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