# An Experimentally Validated CFD Code to Design Coanda Effect Screen Structures
## 3 Results  
Figure 6 shows a qualitative comparison of the simulation and experimental results for flow rates of 7 m\({}^{3}\)h\({}^{-1}\), 8 m\({}^{3}\)h\({}^{-1}\), and 9 m\({}^{3}\)h\({}^{-1}\). The simulation results are at the final (fifth) time step of the transient simulation. The experiments show a snapshot of the moment in time when the flow is developed. For a quantitative comparison, \(\Pi\) is evaluated for the experiments as well as for the simulations, see Table 2. Both show similar rake behavior, i.e., remaining water in the middle of the rake. The thorn shows that at 7 m\({}^{3}\)h\({}^{-1}\), there is no water at the 16 th rake in the middle of the CES; at 8 m\({}^{3}\)h\({}^{-1}\), there is water at the 17 th rake; and at a flow rate of 9 m\({}^{3}\)h\({}^{-1}\), water is even at the last rake in the middle of the CES. The same behavior is also visible in the three 2D-CFD simulations. A different flow regime is depicted in the diverted flow region. The CFD simulations overestimate the Coanda effect, which is illustrated by the fact that the water remains at the rake and does not fall in the negative y-direction. A different flow regime shows the experiment in the diverted flow region. The water falls and does not connect the rakes. The simulation results with simplified boundary conditions are shown in Figure 7. In the diverted flow region, the simulation shows similar flow behavior as in the experiment, whereas from a physical point of view, the conditions are not correct since the flow is close to the boundary and the viscous effects could not be simulated accurately. Figure 8 shows the extracted lines for the experiment (black), the CFD lower fill level (magenta, Figure 6), and the CFD upper fill level (green) for the flow rates of (a) 7 m\({}^{3}\)/h, (b) 8 m\({}^{3}\)/h, and (c) 9 m\({}^{3}\)/h. The dashed lines show the simulation results with simplified boundary conditions. For visualization purposes, the geometry of the rakes is inserted as a point cloud. The outcome of the statistical analysis shows good agreement between the simulation results and the experimental results. As an example, the RMSE between simulation and experiment for 7 m\({}^{3}\)/h is 2.19 mm, see Table 3. The lower fill level of the CFD simulations fits better with the experiments than the upper fill level. The root mean square error of the y-coordinate between simulation and measurement at the edges for the three flow rates 7 m\({}^{3}\)/h, 8 m\({}^{3}\)/h, and 9 m\({}^{3}\)/h is investigated. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**\(\Pi\) Simulations**} \\
**Flow Rate in m\({}^{3}\)/h** & **II Simulations in \%** & **Simplified Boundary** & **II Experiment in \%** \\  & & **Condition in \%** & **Ill Experiment in \%** \\ \hline
7 & 3.6 & 0 & 10.14 \\
8 & 5.0 & 0.3 & 13.15 \\
9 & 10.7 & 1.8 & 16.17 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Relative bypass flow comparison between simulations and measurements. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Fill Level, mm** & **Fill Level, mm** & **Fill Level, mm** \\  & **(for 7 m\({}^{3}\)h\({}^{-1}\))** & **(for 8 m\({}^{3}\)h\({}^{-1}\))** & **(for 9 m\({}^{3}\)h\({}^{-1}\))** \\ \hline RMSE (CFDLower, experiment)** & 2.19 & 1.52 & 2.62 \\ RMSE (CFDUpper, experiment) & 4.37 & 3.81 & 4.96 \\ RMSE (CFDLower SBC, experiment) & 1.54 & 0.71 & 2.19 \\ RMSE (CFDUpper SBC, experiment) & 3.44 & 2.58 & 4.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: RMSE calculation of simulated and measured edges of the flow level height. SBC: simplified boundary condition. Figure 6: Left column: VoF CFD simulation results. The geometry and boundary conditions are shown in Figure 2. Red: water and blue: air. Right column: experimental results. The white lines show the coordinate system (x = 10 cm and y = 7 cm). The origin is set at the 8 th rake. The thorn shows the empty water fill level in the middle of the rake. The simulation results with simplified boundary conditions are not shown (see Figure 7).