Relativistic viscous hydrodynamics with angular momentum
Relativistic viscous hydrodynamics with angular momentum
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具有角动量的相对论粘性流体动力学
DOI：
10.1016/j.scib.2022.10.020
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发表时间：
2022
期刊：
影响因子：
18.9
通讯作者：
Jinfeng Liao
中科院分区：
文献类型：
--
作者：
Duan She;Anping Huang;Defu Hou;Jinfeng Liao
流体力学是描述宏观物理系统长时间大距离行为的一般理论框架。它在物理学的各个分支有许多重要的应用，从宇宙膨胀和星系/恒星演化在非常大的尺度到相对论核碰撞在非常小的尺度。流体力学的核心是受精确守恒定律保护的物理量，如能量、动量和守恒电荷。过去的流体力学研究几乎完全集中在能量动量守恒和电荷守恒上。直到最近，人们才对理解角动量守恒在流体力学中的作用及其对潜在组分自旋输运的影响产生了迅速增长的兴趣。对旋转物质的自旋极化的实验观察强烈地激发了这种兴趣，从凝聚态物质流系统到相对论核碰撞中的亚原子流体[1-6]。人们正在积极努力发展一种描述这类系统的水动力理论框架，参见[7-17]。在这个方向上取得了重要进展，但也出现了概念和技术上的挑战，特别是在相对论体系中，自旋和轨道分量的分离变得微妙，对能量-动量张量的性质以及流体动力梯度膨胀产生了混淆。让我们从一般流体系统的流体力学描述的概念讨论开始。首先假设宏观尺度L（例如系统大小）和微观尺度λ之间存在分离，这是由流体组分的自旋和轨道角动量之间的热松弛和平衡相关的潜在动力学相互作用决定的。这样就可以引入一种中间流体动力学标度l来定义局部流体单元，λ≪l≪l，如图1所示为一种粗粒度工艺。每个流体单元都应该接近局部热平衡，并且可以用局部定义的流体动力场/变量表示，例如温度T （xµ）（或等效的能量den-）
Hydrodynamics is a general theoretical framework for describing the long-time large-distance behaviors of macroscopic physical systems. It has many important applications in various branches of physics, from cosmic expansion and galaxy/star evolutions at the very large scales to relativistic nuclear collisions at the very small scales. The core of hydrodynamics is about physical quantities protected by exact conservation laws, such as energy, momentum and conserved charges. Past hydrodynamic studies almost entirely focus on the energymomentum conservation and charge conservation. Only very recently, there has been a rapidly increasing interest in understanding the role of angular momentum conservation in the hydrodynamic context and its implications for spin transport of underlying constituents. Such interest is strongly fueled by experimental observations of spin polarization in rotating matter, with examples ranging from condensed matter flow systems to subatomic fluids in relativistic nuclear collisions [1–6]. Active efforts are underway to develop a hydrodynamic theory framework for describing such systems, see eg [7–17]. Important progress has been made along this direction, while there also appear both conceptual and technical challenges especially in the relativistic regime where the separation between spin and orbital components becomes subtle and confusions arise about the property of energy-momentum tensor as well as the hydrodynamic gradient expansion. Let us begin with a conceptual discussion on the hydrodynamic description of a general fluid system. One starts by assuming a separation between the macroscopic scale L (eg the system size) and the microscopic scale λ, which is determined by underlying dynamical interactions relevant for the thermal relaxation and equilibration among both spin and orbital angular momentum of the fluid constituents. This allows introducing an intermediate hydrodynamic scale l for defining local fluid cells, with λ≪ l≪ L, a coarse-graining process as illustrated in Fig. 1. Each fluid cell is supposed to be close to local thermal equilibrium and can be represented by locally-defined hydrodynamic fields/variables such as temperature T (xµ)(or equivalently energy den-