方柱绕流尾迹中的涡结构演化及磁场对其的影响研究

The flow around square cylinder, as extensively found in the nature and industries, is a very important type of flows. With different flow conditions, flow instability is always observed in the wake of square cylinder, such that the Karman vortex street, the inversion of the vortex street, the 3D flow instability and the turbulence, which have great influences on the flow and heat transfer. Moreover, in the Tokomak fusion device, where the flows around square cylinder is viewed as a typical flow pattern in the complex blanket module, the flow characteristics will be greatly influenced by the external magnetic fields. In the available publications, the inversion of Karman vortex street, as well as the 3D flow instability is considered to be the main characteristics of the flows around the square cylinder because they greatly determine the velocity distribution and the temperature distribution in the wake of the cylinder. Through performing direct numerical simulations, the present research focus to study the inherent physical mechanisms inducing the inversion of the vortex street and the 3D flow instability, including the influences of the inlet conditions, the wall boundary conditions and the Reynolds number(Re) when magnetic field is absent, their impact on the flow field ,as well as on the heat transfer is also present. Furthermore, we will further investigate how the magnetic field and the electrical conductivity of the square cylinder influence the inversion of Karman vortex street and the 3D flow instability, aiming to present the MHD effect on the flow field and the temperature field, particularly focusing on studying the evolution process of the vortex structures under the influence of magnetic fields.

自然界和工业界广泛存在着方柱绕流运动，在不同的流动条件下，尾迹中可能出现卡门涡街、涡街转置、三维不稳定流动甚至湍流等现象，而它们对流场特性与传热有着重要的影响。另外在托克马克装置中，由于包层结构非常复杂，方柱绕流将会是液态金属重要的流动形式，而约束磁场也会显著改变这种绕流的流场结构。在已有的研究中，卡门涡街的转置和三维不稳定性被认为是方柱绕流的重要特征，对流场和热量分布有着重要影响。本项目旨在通过三维直接数值模拟，首先研究无磁场情况下入口效应、壁面效应、Reynold数(Re)等因素对卡门涡街转置现象和三维不稳定流动的影响规律，探索其形成条件，揭示其产生的内在机理，并分析其对流动和传热的影响。在此基础上，进一步研究磁场和方柱导电性对涡街转置现象和三维不稳定流动的作用效果，分析其对绕流尾迹流动和传热的影响，从磁场对涡结构重构的角度深入探讨磁场效应内在的物理机理。

自然界和工业界广泛存在着方柱绕流运动，在不同的流动条件下，尾迹中可能出现卡门涡街、涡街转置、三维不稳定流动甚至湍流等现象，而它们对流场特性与传热有着重要的影响。另外在托克马克装置中，由于包层结构非常复杂，方柱绕流将会是液态金属重要的流动形式，而约束磁场也会显著改变这种绕流的流场结构。本项目旨在通过直接数值模拟，研究无磁场情况下涡街转置形成的根本原因，分析涡街转置对流动和传热的影响。随后研究流向磁场对于涡街转置的影响及作用机理，分析磁场作用下的传热特性。对于强磁场情况，则通过准二维模型研究强磁场作用下的尾涡运动规律及传热规律。最后研究低Re下，轴向磁场对于尾迹三维不稳定性的影响，并从磁场对涡结构重构的角度深入探讨磁场效应内在的物理机理。通过以上研究获得如下结论：1.发现了涡街转置的根本原因在于尾迹主涡与壁面涡层或是来流涡足够强的相互作用；2.涡街转置现象不利于传热；3.流向磁场作用下，洛伦兹力对涡街运动具有抑制作用，随着磁场强度的增加，升力系数单调递减，阻力系数先减小后增加，平均回流区长度先增加后减小，而传热效率单调递减；4.强磁场下，建立了回流区长度，阻力系数、压力系数及Strouhal数与Re、Ha的关系式；5.在轴向磁场作用下，柱形绝缘障碍物后的涡街在一定的参数范围内持续存在。涡街在完全被抑制之前，流场会达到接近二维的状态。该项目的研究成果发表学术论文5篇，其中核聚变专业期刊Fusion Engineering and Design和传热专业顶级期刊International Journal of Heat and Mass Transfer各2篇。项目的相关研究成果为磁约束核聚变中液态包层的设计及其流动传热特性分析提供了丰富的理论参考，具有重要的意义。