Investigation of the magnetic field oscillations and reversals

该研究方案侧重于湍流发电机和磁场感应的数值研究，以回答冯·卡曼钠实验(VKS)提出的科学问题。继在导电流体的约束流中观察到发电机作用(在里加的实验，Karlsruhe，2000)之后，自然下一步是扩展到约束较少的和湍流的流动，世界各地的几个小组正在积极研究这一点。VKS团队在CEA Cadarache建造的这台仪器产生了关于具有磁流体动力学湍流的发电机的新的和原始的数据。2006年，在这种设置下，第一次在不受约束的湍流状态下进行的发电机观测是一项举世瞩目的成功，有望为天体物理和地球物理相关方面提供新的线索。这些希望被最近第一次在湍流状态下对发电机磁场反转的实验观察所证实。关于发电机的公开问题，预期会有结果：湍流起伏对阈值有什么影响？磁场的饱和度如何依赖于动力学雷诺数？起爆前后磁场和速度场的统计特性是什么？特别是在这个新的项目中：磁场的反转和振荡是如何被触发的？在数值研究中，将首先探讨涨落和湍流对发电机阈值的影响。为了理解实验结果，我们将研究对外加磁场的响应，作为到发电机阈值的距离的函数。然后，将探讨发电机不稳定的饱和程度作为雷诺数的函数。最后，我们将研究磁场的反转和振荡。

The research proposal focuses on a numerical study of turbulent dynamo and magnetic field induction, so as to answer scientific issues raised by the Von Karman Sodiuna experiment (VKS). Following the observations of dynamo action in constraint flows of electrically conducting fluids (experiments in Riga, Karlsruhe, 2000), the extension to less constrained and turbulent flows was a natural next step, and it is actively studied by several groups worldwide. The Instrument built by the VKS team in CEA Cadarache produced new and original data about dynamo with magnetohydrodynamics turbulence.The first dynamo observation in an unconstrained, turbulent regime in 2006 in this setup was a worldwide noticed success expecting to shed new light on aspects relevant for astrophysics and geophysics. These hopes were confirmed by the recently first experimental observation of reversals of a dynamo field in a turbulent flow regime.Results are expected concerning open questions regarding the dynamo: what is the effect of turbulent fluctuations on the threshold value? How does the Saturation of the magnetic field depend on the kinetic Reynolds number? What are the statistical properties of the magnetic and velocity fields both before and after onset? And especially in this new particular project: How are the reversals and oscillations of the magnetic fields triggered?In the numerical studies, the influence of fluctuations and turbulence onto the dynamo threshold will be first explored. The response to an imposed magnetic field will then be studied, as a function of the distance to the dynamo threshold, in order to understand experimental results. Then the Saturation of the dynamo instability as a function of the Reynolds number will be explored. Finally, the reversals and oscillations of the magnetic fields will be investigated.