# An Experimentally Validated CFD Code to Design Coanda Effect Screen Structures
## 4 Discussion  
In this work, an experimentally compared CFD code to optimize CES is presented. This forms the basis for reducing the experimental effort in designing CES with CFD. Compared to [17], quantitative (Tables 2 and 3) and qualitative comparisons (Figures 6 and 7) between CFD multiphase simulations and the corresponding experiments are presented. This work is an extension of [19], in which the outlet boundary conditions at the diverted flow region were set very close to the Coanda rake elements. Therefore, it is questionable whether these assumptions are sufficient. In the presented paper, these assumptions are taken away from the Coanda rake elements to make the boundary conditions more realistic from a physical point of view. Furthermore, CFD results with the same procedure as [19], i.e., simplified boundary conditions, are generated. The results show that \(\Pi~{}=~{}\frac{m_{ bypass}}{m_{ inlet}}\) is less accurate with the simplified boundary conditions and the fill level is more accurate with the simplified boundary conditions compared to the newly generated results. The reason is that with simplified boundary conditions, the flow is not predicted accurately at the last three rakes (15 th, 16 th, and 17 th rakes) compared to the experiment. In contrast, the fill level RMSE (see Figure 8 and Table 3) is improved with respect to the experiment with simplified boundary conditions. At the diverted flow region, it could be shown that the CFD results are accurate with simplified boundary conditions, which means the results could be confirmed with those in [19]. In the new CFD results presented, an unrealistic flow behavior at the diverted flow region is simulated. The reasons might be because in CFD simulations with RANS modeling, smaller eddies than the mesh are not simulated but wrongly modeled with virtual viscosity. Since the Coanda effect is a complex physical effect, RANS turbulence modeling does not work with the Coanda rake elements. The state-of-the-art turbulence model RANS k-\(\omega\)-SST was used. Additionally, other RANS turbulence models (k-\(\epsilon\) and Spalart-Allmaras) and models without any turbulence were tested but not presented in this paper. The results with k-\(\omega\)-SST were chosen to be presented since the results for the fill level and the mass flow rate fit the experiments best. k-\(\omega\)-SST features advantages over the k-\(\omega\) model in calculating the velocity boundary layer and the k-\(\epsilon\) model in calculating the free stream flow with a higher accuracy [27]. Unfortunately, the computational resources for this were not available. The simulations and measurements do not consider solid impurities. This simplification was introduced to decrease the complexity of CFD. In the simulations, the volume of fluid model shows a region where both water and air are present (see Figure 6, white region). In contrast, in the experiments, such a region was hard to identify with a camera system. Therefore, two simulation datasets were compared with one experimental dataset. Compared to the already existing CFD simulations of CES, a higher spatial resolution and/or additional boundary layers were used [17; 18; 19]. Therefore, it is not surprising that the presented CFD simulation results are independent of the spatial resolution. Regarding the mesh independence study, two additional simulations with less spatial resolution were performed since the spatial resolution of the \"base\" simulation was already better than that presented in other studies.