一种模拟动边界绕流的锐利界面浸入边界法

为避免复杂贴体网格的更新和畸形对动边界流场计算效率、精度的影响，以充分掌握结构场的受力特性，采用一种改进的锐利界面（sharp-interface）浸入边界法模拟具有动边界绕流的流动问题。该方法将计算域中的固体视为流体，固体边界离散为若干个拉格朗日网格点，通过在界面单元处插值重构流动参数（速度），将其直接作为流动求解器的边界条件，由此来反映固体边界的影响。即通过构造“虚拟点—受力点—垂足点”的计算结构，借助双线性插值得到虚拟点的速度，再通过强制满足固体边界的无滑移条件计算出受力点的速度，以此为边界条件，最终求解基于浸入边界法的耦合系统方程，实现复杂动边界的流动数值模拟。采用C++编写该浸入边界法的数值程序，以单圆柱绕流为验证算例，通过与文献和实验结果的对比，验证了该方法的准确性和可靠性。在此基础上，对主动运动椭圆柱绕流问题进行了精细计算，探讨了不同轴长比(A_R)、不同攻角($ theta $)下的椭圆柱对尾涡结构分布特征和水力不稳定现象的影响。捕捉到了反对称S型、“P+S” Ⅰ型、“P+S” Ⅱ型尾涡脱落模态，漩涡强度、涡脱频率和升阻比随A_R和$ theta $的变化规律，以及确定了升阻比临界攻角(25°)。

A sharp-interface immersed boundary method for simulating flows around bluff body with moving boundary

When the structural wall moves over a fixed grid, the structure coverage will change, resulting in many dead and emerging elements. To avoid the influence of malformation and reconstruction of body-fitted grids on the calculation efficiency and accuracy of the fluid-structure interaction problems with coupled boundary movement on the fixed grid, an improved numerical method for describing the interaction between an immersed rigid body and fluid based on a sharp-interface is proposed. In this method, both the fluid and solid are regarded as pure fluid domains in the whole computational domain, and the solid boundary is divided into several Lagrangian grid points. The flow parameter or velocity is reconstructed by interpolation at the interface element, which is then directly used as the boundary condition of the flow field, thus reflecting the influence of the wall boundary conditions. The method constructs the calculation structure of “virtual point, force point and vertical foot point”, and the velocity of the virtual point is obtained by bilinear interpolation. Then, the velocity of the force point is calculated by forcing the solid boundary to meet the no-slip condition, and the equations of the coupling system based on the immersion boundary method are finally solved to realize the numerical simulation of the flow with a complex moving boundary. The numerical program for this immersed boundary method is established using C++, then the accuracy and reliability of the proposed method are validated by comparison with the literature and experimental results of the basic numerical example of flow around a cylinder. Furthermore, the effects of the structural shape and the angle of attack on the trailing vortex structure, the vortex shedding frequency, and the lift/ coefficient characteristics of the flow around the elliptical cylinder have been analyzed. The anti-symmetric S-type, “P+S” Ⅰ-type and “P+S” Ⅱ-type trailing vortex shedding modes, as well as the variation laws of the vortex structure size, vortex shedding frequency and lift-drag coefficients ratio with axis ratio and angle of attack, are captured. The critical angle of attack (25°) corresponding to the maximum lift-drag ratio is determined as 25°.