INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS Int. J. Numer. Meth. Fluids (2012) Published online in Wiley Online Library (wileyonlinelibrary.com/journal/nmf). DOI: 10.1002/fld.3691 Hybrid grid generation for viscous flow analysis Seyoun Park 1 , Byungduk Jeong 1 , Jin Gyu Lee 2 and Hayong Shin 1, * ,† 1 Department of Industrial and Systems Engineering, KAIST, Daejeon, South Korea 2 R&D Institute Agency for Defense Development, Deajeon, South Korea SUMMARY Cartesian grid with cut-cell method has drawn attention of CFD researchers owing to its simplicity. However, it suffers from the accuracy near the boundary of objects especially when applied to viscous flow analysis. Hybrid grid consisting of Cartesian grid in the background, body-fitted layer near the object, and transition layer connecting the two is an interesting alternative. In this paper, we propose a robust method to generate hybrid grid in two-dimensional (2D) and three-dimensional (3D) space for viscous flow analysis. In the first step, body-fitted layer made of quadrangles (in 2D) or prisms (in 3D) is created near the object’s boundary by extruding front nodes with a speed function depending on the minimum normal curvatureobtained by quadric surface fitting. To solve global interferences effectively, a level set method is used to find candidates of colliding cells. Then, axis-aligned Cartesian grid (quadtree in 2D or octree in 3D) is filled in the rest of the domain. Finally, the gap between body-fitted layer and Cartesian grid is connected by transition layer composed of triangles (in 2D) or tetrahedrons (in 3D). Mesh in transition layer is initially generated by constrained Delaunay triangulation from sampled points based on size function and is further optimized to provide smooth connection. Our approach to automatic hybrid grid generation has been tested with many models including complex geometry and multi-body cases, showing robust results in reasonable time. Copyright © 2012 John Wiley & Sons, Ltd. Received 22 November 2011; Revised 15 March 2012; Accepted 30 April 2012 KEY WORDS: mesh generation; computational fluid dynamics; FVM; hybrid grid; viscous fluid 1. INTRODUCTION Grid generation is a troublesome area in numerical analysis. This is particularly true of the field of CFD where automatic creation of high quality mesh in a robust fashion still remains a difficult and time-consuming step. The quality and density of the grid used in CFD directly affects the accuracy of the numerical solution. However, an unnecessarily fine grid slows down computation and the arrival at a solution [1]. Grid types in CFD analysis can be grouped in two categories: structured grid and unstructured grid. The pros and cons of each type are well discussed in the literatures such as Baker [2]. It is believed that a structured grid created in body-fitted fashion is superior for capturing viscous flow near the objects’ surface because of the orthogonality of elements to the dominant flow direction [3]. However, a structured grid makes unnecessarily dense elements in areas far from focal objects to achieve the resolution required in the critical part of the domain. Moreover, it is not a simple task to automatically create a body-fitted structured mesh for objects with complex geometry. For this rea- son, a hybrid mesh consisting of a body-fitted structured mesh layer and an unstructured background mesh has gained the attention of CFD researchers for viscous flow analysis in recent years [3–13] including its use in several commercially available software programs such as VisCART, Harpoon, and GDT. Chand [4] proposed a structured–unstructured hybrid mesh for overlapping multi-body *Correspondence to: Hayong Shin, Department of Industrial and Systems Engineering, KAIST, Daejeon, South Korea. † E-mail: hyshin@kaist.ac.kr Copyright © 2012 John Wiley & Sons, Ltd.