Generating observation guided ensembles for data assimilation with denoising diffusion probabilistic model

This paper presents an ensemble data assimilation method using the pseudo ensembles generated by denoising diffusion probabilistic model. Since the model is trained against noisy and sparse observation data, this model can produce divergent ensembles close to observations. Thanks to the variance in generated ensembles, our proposed method displays better performance than the well-established ensemble data assimilation method when the simulation model is imperfect

Continuous data assimilation of large eddy simulation by lattice Boltzmann method and local ensemble transform Kalman filter (LBM-LETKF)

We investigate the applicability of the data assimilation (DA) to large eddy simulations (LESs) based on the lattice Boltzmann method (LBM). We carry out the observing system simulation experiment of a two-dimensional (2D) forced isotropic turbulence, and examine the DA accuracy of the nudging and the local ensemble transform Kalman filter (LETKF) with spatially sparse and noisy observation data of flow fields. The advantage of the LETKF is that it does not require computing spatial interpolation and/or an inverse problem between the macroscopic variables (the density and the pressure) and the velocity distribution function of the LBM, while the nudging introduces additional models for them. The numerical experiments with $256\times256$ grids and $10\%$ observation noise in the velocity showed that the root mean square error of the velocity in the LETKF with $8\times 8$ observation points ($\sim 0.1\%$ of the total grids) and 64 ensemble members becomes smaller than the observation noise, while the nudging requires an order of magnitude larger number of observation points to achieve the same accuracy. Another advantage of the LETKF is that it well keeps the amplitude of the energy spectrum, while only the phase error becomes larger with more sparse observation. We also see that a lack of observation data in the LETKF produces a spurious energy injection in high wavenumber regimes, leading to numerical instability. Such numerical instability is known as the catastrophic filter divergence problem, which can be suppressed by increasing the number of ensemble members. From these results, it was shown that the LETKF enables robust and accurate DA for the 2D LBM with sparse and noisy observation data.Comment: 27 pages, 14 figure

負磁気シアートカマクにおける微視的不安定性のジャイロ運動論

本文データは平成22年度国立国会図書館の学位論文(博士)のデジタル化実施により作成された画像ファイルを基にpdf変換したものである京都大学0048新制・課程博士博士(エネルギー科学)甲第8535号エネ博第19号新制||エネ||6(附属図書館)UT51-2000-J44京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻(主査)教授 若谷 誠宏, 教授 近藤 克己, 教授 前川 孝学位規則第4条第1項該当Doctor of Energy ScienceKyoto UniversityDFA

Impact of poloidal convective cells on transport processes with kinetic electrons

新古典輸送と乱流輸送を自己無矛盾に扱うfull-Fジャイロ運動論的シミュレーションコードGT5Dによって、ポロイダル対流セルの輸送への影響を評価する。発表では、ポロイダル対流セルの扱いが異なるハイブリッド電子モデルの比較を通じ、その新古典や乱流輸送への影響を示す。16th Technical Meeting on Energetic Particles in Magnetic Confinement Systems – Theory of Plasma Instabilitie

Introduction of a multi-species collision operator in GT5D and its application to heavy impurity transport

The use of tungsten to coat divertors or other plasma facing components, as will be the case for ITER, inevitably leads to the pollution of the fusion plasma by heavy and highly charged impurities. Several tokamaks (ASDEX Upgrade, JET) have reported on-axis accumulation of tungsten – which would be detrimental to fusion efficiency – and its exhaust by electron heating. However, this kind of neoclassical and turbulent transport of high-Z impurities is not yet fully understood. To better treat heavy impurities, the gyrokinetic full-f Eulerian simulation code GT5D was recently upgraded with an implicit collision solver based on a multi-species linear Fokker-Plank collision operator. In addition, a flux-coordinate version of the code was developed to accelerate neoclassical simulations and quasi-local turbulence simulations in shaped magnetic configurations. A prior neoclassical benchmark for bulk ions between GT5D and the Hirshman-Sigmar (HS) theory was extended to various impurities in all transport regimes. While a standard low order approximation of the HS theory showed good agreement for light impurities, large differences were observed in the presence of moderate or large temperature gradients. Instead, under such conditions, we tested an improved form of the HS theory, that takes into account higher order flows to more accurately recover the friction coefficients in the Pfirsch-Schlüter regime, showing good agreement with GT5D.Numerical Methods for the Kinetic Equations of Plasma Physics (NumKin2018