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arXiv 2007.01159physics.flu-dyn
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流体流动Koopman分析的理论框架,第2部分:从线性到非线性动力学

A theoretical framework for Koopman analyses of fluid flows, part 2: from linear to nonlinear dynamics

Wei Zhang, Mingjun Wei

摘要:

通过将动力学从状态空间转移到其对偶空间,得到了动力学的理论框架。其结果是,线性结构的动力学解析分解的子分量和不变子空间分解的基础上局部Koopman谱理论被揭示。然而,非线性动力学与线性动力学的区别在于局部指数动力学和无穷维,后者是由于非线性相互作用,其特征在于递归增殖的Koopman特征空间。新框架为动力分析技术,如全局稳定性分析(GSA)和动力模态分解(DMD)技术提供了基础。此外,通过Mercer特征函数分解的线性结构导出了众所周知的固有正交分解(POD)。利用DMD技术对固定圆柱绕流的Hopf分岔过程进行了数值分解。Koopman分解与GSA的等价性在初始失稳阶段得到了验证。当流动达到周期性时,极限环解附近的Fourier模、最不稳定的Floquet模及其高阶导模是可数无限个Koopman模的叠加.平均流的非线性调制效应是叠加的单调Koopman模式的饱和。非线性共振现象归因于无限多个Koopman谱的排列。上述非线性动力学过程的分析依赖于第一部分讨论的Koopman谱的连续性和Koopman模的状态不变性。发现相干结构与态不变模有关。

英文摘要:

A theoretic framework for dynamics is obtained by transferring dynamics from state space to its dual space. As a result, the linear structure where dynamics are analytically decomposed to subcomponents and invariant subspaces decomposition based on local Koopman spectral theory are revealed. However, nonlinear dynamics are distinguished from the linear by local exponential dynamics and infinite dimension, where the latter is due to nonlinear interaction and characterized by recursively proliferated Koopman eigenspaces. The new framework provides foundations for dynamic analysis techniques such as global stability analysis (GSA) and dynamic mode decomposition (DMD) technique. Additionally, linear structure via Mercer eigenfunction decomposition derives the well-known proper-orthogonal decomposition (POD). A Hopf bifurcation process of flow past fixed cylinder is decomposed numerically by the DMD technique. The equivalence of Koopman decomposition to the GSA is verified at the primary instability stage. The Fourier modes, the least stable Floquet modes, and their high-order derived modes around the limit cycle solution are found to be the superposition of countably infinite number of Koopman modes when the flows reach periodic. The nonlinear modulation effects on the mean flow is the saturation of the superimposed monotonic Koopman modes. The nonlinear resonance phenomenon is attributed to the alignment of infinite number of Koopman spectrums. The analysis of above nonlinear dynamic process relies on the properties of continuity of Koopman spectrums and state-invariance of Koopman modes discussed in part 1. The coherent structures are found related to the state-invariant modes.

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