sdfibm: a signed distance field based discrete forcing immersed boundary method in OpenFOAM

In this paper we present the algorithm and implementation of an open-source immersed boundary code sdfibm, which is based on OpenFOAM v6 and written in C++. The immersed boundary method (“ibm” of the name) treats the velocity field as the volume average of solid and fluid velocities, and applies the volume-average discrete forcing to account for the solid-fluid interaction. The signed distance field (“sdf” of the name) representation of the solid shape, together with the proposed pyramid decomposition method, allow accurate calculations of the volume fraction field created by solids overlapping with an arbitrary unstructured fluid mesh. SDF removes the need of intersection test between the solid and fluid mesh, or the discretization and re-sampling of the shape. Users can freely combine different solid components (shapes, materials, and motion constraints) into new solids within the plain-text input file, and implement new shapes and motion constrains easily. sdfibm is an efficient and robust tool for exploring complex solid-fluid interactions in a fully-resolved sense, and can generate data for closure models in upscaling procedures.

本工作提出了一款基于OpenFOAM v6、采用C++编写的开源浸入式边界代码sdfibm的算法与实现方案。本代码名称中的ibm指代浸入式边界方法（Immersed Boundary Method，IBM），该方法将速度场视为固体与流体速度的体积平均，并通过体积平均离散力来描述固液两相相互作用。本代码名称中的sdf即有符号距离场（Signed Distance Field，SDF），通过实体形状的SDF表征结合所提出的金字塔分解方法，可精确计算任意非结构化流体网格与实体交叠产生的体积分数场。SDF无需进行实体与流体网格的相交测试，亦无需对实体形状进行离散化与重采样操作。用户可通过纯文本输入文件自由组合不同的实体组分（形状、材料与运动约束）以构建新实体，且可便捷地实现新形状与运动约束的添加。sdfibm是一款高效稳健的工具，可在全解析框架下开展复杂固液相互作用研究，并可为尺度升级流程中的封闭模型生成对应数据。