# Building Geometry Simplification for Improving Mesh Quality of Numerical Analysis Model
## 1 Introduction  
The finite volume method (FVM) is commonly used to perform numerical analysis in many fields, including fluid dynamics, owing to its advantages in flux calculations in terms of precision [1]. In the construction field, FVM is used for environmental analyses using computational fluid dynamics (CFD), fire safety analysis [2], and some cases of heat, air, and moisture (HAM) analysis [3, 4]. In particular, CFD is widely used in the analysis of aerodynamic environments, e.g., indoor ventilation and micro-environments around occupants [5].  
FVM is a method that discretizes and analyzes partial differential equations in the form of algebraic equations. For the computation of algebraic equations, there is a need to divide the target model into finite volumes (\"cells\"), i.e., to design the mesh. During the mesh design process, a discretization error can occur, and the size of the error is affected by the geometry of the finite volume. Discretization errors not only reduce the accuracy of the simulation, but also cause numerical diffusion, which interferes with the convergence of the simulation. The accuracy and stability of the numerical analysis are quantitatively evaluated using a mesh quality that is calculated based on the geometry of cells [6, 7]. The mesh quality of an FVM model is typically evaluated according to the non-orthogonality and skewness. Non-orthogonal cells and skewed cells that are generated near complex surfaces are the main sources of numerical errors. In particular, analyses of buoyancy-driven environments, such as those with natural ventilation, are significantly affected by the mesh quality [8, 9].  
As the fine features and complexity of the geometry adversely affect the mesh design, a geometry simplification is commonly used as a preprocessing step to improve the mesh quality of the model [10]. Among the numerical analysis processes, the simplification of a complex geometry requires a greatdeal of manual work and is time-consuming [11]. In addition, the geometry simplification needs to be repeated manually until the target accuracy is achieved. Moreover, the determination of unnecessary geometrical features, simplification methods, and the level of simplification may depend on an analyst's arbitrary judgment and experience. Therefore, there is a need for an objective automatic geometry simplification method for the FVM analysis and the evaluation of the mesh quality improvement achieved by the simplification. However, most studies that focus on the simplification of building geometries aim to reduce the computational cost in the visualization process, and there have been relatively few studies focusing on numerical analysis.  
This study proposes a building geometry simplification method for improving the mesh quality of FVM models. The rest of the paper is organized as follows. Section 2 discusses previous studies on geometry simplification in the construction industry, and Section 3 discusses the mesh quality metrics. Section 4 presents the proposed automatic geometry simplification method for FVM analysis, while Sections 5 and 6 respectively discuss the evaluation method and results obtained showing the effects of the proposed method. Section 7 presents the conclusions and recommendations for future research.