Amplification of deterministic disturbances in inertialess shear-driven channel flows of viscoelastic fluids is examined by analyzing the frequency responses from spatio-temporal body forces to the velocity and polymer stress fluctuations. In strongly elastic flows, we show that disturbances with large streamwise length scales may be significantly amplified even in the absence of inertia. For fluctuations without streamwise variations, we derive explicit analytical expressions for the dependence of the worst-case amplification (from different forcing to different velocity and polymer stress components) on the Weissenberg number ($We$), the maximum extensibility of the polymer chains ($L$), the viscosity ratio, and the spanwise wavenumber. For the Oldroyd-B model, the amplification of the most energetic components of velocity and polymer stress fields scales as $We^2$ and $We^4$. On the other hand, finite extensibility of polymer molecules limits the largest achievable amplification even in flows with infinitely large Weissenberg numbers: in the presence of wall-normal and spanwise forces the amplification of the streamwise velocity and polymer stress fluctuations is bounded by quadratic and quartic functions of $L$. This high amplification signals low robustness to modeling imperfections of inertialess channel flows of viscoelastic fluids. The underlying physical mechanism involves interactions of polymer stress fluctuations with a base shear, and it represents a close analog of the lift-up mechanism that initiates a bypass transition in inertial flows of Newtonian fluids.
Weakly nonlinear analysis of the viscoelastic instability in channel flow for finite and vanishing Reynolds numbers
最近发现的直线粘弹性剪切流的中心模式不稳定性（Garg等人，Phy. Rev. Lett. 121，024502，2018）为发生在较低Weissenberg（$Wi$）数下的弹性惯性湍流（EIT）的起源提供了解释。为了支持这一点，我们使用弱非线性分析表明，在Page等人（Phys. Rev. Lett. 125，154501，2020）在中性曲线上是通用的，不稳定性仅在低雷诺数（$Re$）和高$Wi$时变为超临界。我们证明，不稳定性可以被看作是纯粹的弹性起源，即使是$Re=O(10^3)$，而不是“弹性惯性”，因为潜在的剪切不激发的不稳定。它也被发现，一个现实的最大聚合物延伸长度，$L_{max}$，在FENE-P模型中的引入移动中性曲线更接近惯性$Re=0$限制在一个固定的溶剂-溶液粘度比，$β$。在使用$L_{max} =O(100)$的稀释限度（$β\rightarrow 1$）中，与Khalid等人（arXiv：2103.06794）最近报告的Oldrophil-B液体在$β=0.994$下的阈值$Wi=O(10^3)$相比，线性不稳定性可降低至在$β=0.98$下更具有物理相关性的$Wi\gtrsim 110$。再次的不稳定性是亚临界的，这意味着无惯性的直线粘弹性剪切流是非线性不稳定的-即不稳定的有限振幅的干扰-甚至更低的$Wi$。
Nonhomogeneous elastic turbulence in the two-dimensional Taylor-Couette flow
Univ. Lille, CNRS, ONERA, Arts et Métiers Institute of Technology, Centrale Lille（里尔大学、法国国家科学研究中心、法国航空国防实验室、巴黎国立高等工艺学院、里尔中央理工学院）；Univ. Lille（里尔大学）；Laboratoire de Thermique et Energie de Nantes, CNRS, Nantes Université（南特热能与实验室、法国国家科学研究中心、南特大学）
Can Transformers overcome the lack of data in the simulation of history-dependent flows?
ESI Group-UZ Chair of the National Strategy on Artificial Intelligence（ESI集团-乌兹国家人工智能战略主席）；Aragon Institute of Engineering Research (I3A)（阿拉贡工程研究所（I3A））；Universidad de Zaragoza（萨拉戈萨大学）；ESI Group Chair（ESI集团主席）；PIMM lab., ENSAM Arts et Métiers Institute of Technology（PIMM实验室，ENSAM艺术与工艺技术学院）
Deformable bodies in a 3-dimensional viscous flow: Vorticity-Stream vector formulation
Center for Interdisciplinary Research in Biology (CIRB), Collège de France, CNRS, Inserm, Université PSL（跨学科生物研究中心，法兰西学院，法国国家科学研究中心，法国国家健康与医学研究院，巴黎文理研究大学）；Facultat de Física, Universitat de Barcelona（巴塞罗那大学物理学院）；Instituto de Investigación Tecnológica (IIT) and Grupo Interdisciplinar de Sistemas Complejos (GISC), Universidad Pontificia Comillas（技术研究所和复杂系统跨学科小组，科米利亚斯宗座大学）；Institute of Nanoscience and Nanotechnology (IN2UB)（纳米科学与技术研究所）
