# Helicopter Blade-Vortex Interaction Airload and Noise Prediction Using Coupling CFD/VWM Method
## 3 Results and Discussions
### Analysis of BVI Airload and Noise  
As the rotor wake has been recognized as one of the most important parameters for generating BVI noise, the vorticity iso-surfaces calculated using the full CFD method and the coupling CFD/VWM method are shown in Figure 4, and the non-dimensional vorticity magnitudes \(|\overline{\omega}|\) of both methods are set as 0.4. With the rotor wake influenced by forward flow and induced velocity, the formation and rollup of the rotor tip vortex occur on both the advancing and retreating sides; two more intense vortex bunches form there. However, the prominent diffusion in the wake-developing zone is observed in the full CFD method, reflecting that the numerical dissipation problem causes the vorticity to diffuse rapidly. By contrast, in the coupling method, the rotor wake is well preserved when traveling downstream. This indicates that the coupling method is capable of capturing distortion and the wake boundary, effectively improving the numerical dissipation problem in the CFD domain, and enhancing the ability to capture the BVI characteristics. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Blade** & **Blade** & **Lateral** & **Longitudinal** & **Blade Coning** & **Longitudinal** & **Lateral** \\
**Motion** & **Collective Pitch** & **Cyclic Pitch** & **Cyclic Pitch** & **Angle** & **Flapping Angle** & **Flapping Angle** \\ \hline Control & \multirow{2}{*}{6.14\({}^{\circ}\)} & \multirow{2}{*}{0.9\({}^{\circ}\)} & \multirow{2}{*}{\(-1.39^{\circ}\)} & \multirow{2}{*}{0.5\({}^{\circ}\)} & \multirow{2}{*}{\(-1^{\circ}\)} & \multirow{2}{*}{0\({}^{\circ}\)} \\ Inputs & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Control and flapping angles of the OLS rotor for the BVI condition. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Numerical** & \multirow{2}{*}{**Blade Grid**} & \multicolumn{2}{c}{**Number of Blade**} & \multicolumn{2}{c}{**Number of Rotor Revolutions**} & \multirow{2}{*}{**Time Step Size**} \\ \cline{3-3}
**Condition** & & & **Aerodynamic Segments** & & **CFD** & **VWM** \\ \hline \multirow{2}{*}{Set Values} & CO type & \multirow{2}{*}{60} & \multirow{2}{*}{4} & \multirow{2}{*}{6} & \multirow{2}{*}{2\({}^{\circ}\)} \\  & & & & & & \\ \(223\times 45\times 101\) & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Numerical condition of the OLS rotor for the BVI condition. Figure 3: Observation locations in the OLS model test. (**a**) Acoustic radiation spherical arc; (**b**) Lambert projection of spherical arc. During descent flight, specific BVI events are observed at several azimuth angles. Utilizing the blade and tip vortex geometry equations in [PERSON] et al. 's research [27], we can capture the BVI loci. Figure 5 shows the locations of BVI events and the lift distribution on the rotor disk. Six BVI events are identified in Figure 5a, of which four on the advancing side are marked as: #A1, #A2, #A3, #A4, and the other two on the retreating side are marked as: #R1, #R2. Figure 5b shows that complicated lift fluctuations occur near the locations of BVI events. According to the relative position between the blade and the vortex filament, the interactions #A3 and #R2 in the outer part of the blade are referred to as parallel interactions, because each blade's leading edge is nearly parallel to the vortex filament, while others are referred to as oblique interactions. Figure 6 presents the miss-distance, which varies with the azimuth angle on the axial plane, and the interaction strength of BVI depends on both the interaction angle and the miss-distance. Figure 4: Predicted rotor wake structure. (**a**) Full CFD method; (**b**) CFD/VWM method. Figure 5: Locations of BVI events and lift distribution on the rotor disk. (**a**) BVI events occur on the rotor plane; (**b**) Lift distribution on the rotor disk. The contour plots of SPL on the 3.44 \(R\) hemi-spherical surface are shown in Figure 7a.