# Development of Building CFD Model Design Process Based on BIM
## 6 Conclusions  
This study analyzed a method for automatically designing an optimized CFD model based on the BIM model and validated the accuracy. This enables the integration of BIM and CFD and improves the efficiency of CFD analysis in the construction field by reducing the time required to design optimal CFD models. The proposed method consists of a five-step process: BIM data extraction, geometry simplification, grid optimization, attribute data matching, and CFD model extraction. An IFC file was used as the input target BIM model to design a CFD model for OpenFOAM. The geometry and attribute data extracted from the IFC file were used to design the CFD model. For geometry simplification, the degree of simplification was adjusted by using the distance threshold. For grid optimization, the optimal grid size for the analysis target was applied by using the optimal grid resolution for each characteristic length employed in a previous study. Based on the optimal grid size, grid optimization was performed by creating a hexahedral grid system. In the case of attribute data, the wall material name and thermal transmittance stored in the IFC were matched with the CFD model. Finally, it is output in the form of OpenFOAM analysis case. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **A-Type** & **B-Type** & **C-Type** & **D-Type** \\ \hline Working time using & & & & \\ Ansys (s) & 2132 & 1040 & 1740 & 2467 \\ Working time using the & & & & \\ proposed method (s) & 533 & 352 & 521 & 481 \\ Reduction rate (\%) & 74.9 & 66.2 & 70.1 & 80.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Evaluation of reduction in working time for optimal CFD model design. Figure 9: Difference in temperature distribution and vector filed at B-type according to optimization. Validation was conducted on four building models to evaluate the proposed method. Validation was conducted by comparing the simulation results for the temperature and flow velocity inside the target facilities. The improvement of the grid model was analyzed according to the optimization. The total number of grids changed according to omitting or adding volume in the optimization process. The number of grids decreased when objects were omitted to improve complex shapes, while the number of grids increased when objects were added to improve the geometry. In terms of grid quality indicators, there was no significant change in maximum skewness after optimization. However, the maximum non-orthogonality improved by 20.78% after optimization, thereby demonstrating the effect of optimization. To evaluate the accuracy, validation was conducted by comparison with ANSYS simulation. The modeling and simulation results using ANSYS were compared with the simulation results using OpenFOAM by applying the proposed method for the same target and operation conditions. A steady-state analysis of the temperature and flow velocity inside the building was performed for validation. The cross-sectional average, by height and the volume average, was calculated for each analysis target to evaluate the relative error rate to the ANSYS results. As a result, the relative error rates for temperature and flow velocity were less than 5% in all of the analysis cases. Therefore, the proposed method could automatically generate an optimized CFD model by using a BIM model and also produce analysis accuracy similar to conventional CFD tools. Meanwhile, there were cases where the flow velocity and temperature distributions in some areas were different from the original model, due to shapes omitted through optimization. Even in such cases, the average error rate was less than 5% and did not have a significant effect on the overall analysis results. However, the accuracy of the analysis results may be compromised in these cases if accurate analysis results are needed for the areas around the omitted shape. Therefore, additional studies will be performed to improve the algorithm so that the optimization process is not applied to areas specified by the user.