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。

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.