# Helicopter Blade-Vortex Interaction Airload and Noise Prediction Using Coupling CFD/VWM Method
## 3 Results and Discussions
### Analysis of BVI Airload and Noise
As shown, the high noise level (acoustic \"hotspot\") appears at 120\({}^{\circ}\) in azimuth and 45\({}^{\circ}\) below the rotor plane, implying that the typical BVI noise has strong directivity, approximately down and forward from the rotor. Figure 7b presents the predicted sound pressure distributions using the CFD/VWM/FW-H method at four observation locations (#2, #3, #7, #9). For comparison, the results of using the CFD/FW-H method are also included in the figures. BVI noise is found to dominate at #3, #7, and #9, and each noise signature consists of positive and negative impulses, which are respectively induced by interactions on the advancing side and the retreating side. We also observed that a large negative peak occurs at #2, attributed to the blade thickness noise mechanisms. Compared with the CFD/FW-H method, the hybrid method is found to better predict the noise peak in amplitude and phase, because the rotor wake simulation is more accurate, which demonstrates its capability for favorable accuracy. Moreover, Table 3 illustrates the comparison of the computation time between the two methods. As shown, about 148 h and 21 h are required, respectively, further demonstrating the developed method to have high computation efficiency. Figure 6: Miss-distance variance with the azimuth angle. Figure 7: _Cont._  
During the calculation of BVI noise, the surface blade load is stored along the azimuth angle at each spanwise location. The impulsive surface loads from one interaction are extracted, with a smooth processing applied to the impulsive loads from others. The propagation characteristics of six BVIs can be distinguished effectively from each other, and the peak noise level contours on the 3.44 \(R\) hemi-spherical surface are presented in Figure 8. It is clearly shown that the noise from each interaction has a definite radiation direction and noise intensity, which determines the spatial occurrence of acoustic \"hotspots.\" Interactions #A1 and #R1 are oblique interactions, the noise from which radiates to around 160\({}^{\circ}\) and 325\({}^{\circ}\) in azimuth and about 40\({}^{\circ}\) below the rotor plane, respectively. Despite the small miss-distance of #A1 and #R1, the interaction angle changes with the azimuth angle during the interaction process, which causes the offset of the generated noise phase, thus the noise intensity is \begin{table}
\begin{tabular}{c c c} \hline
**Method** & **CFD/FW-H Method (Unsteady Solution)** & **CFD/VWM/FW-H** \\ \hline Blade grid & \(1,013,535\times 2\) & \(1,013,535\times 2\) \\ Background grid/particle number & 9,175,040 & 24,439 \\ Total Time & 148 h & 21 h \\ \hline \end{tabular}
\end{table} 
Table 3: Comparison of the computation time between the two methods. Figure 7: Radiation direction and sound pressure signatures of the BVI noise. (**a**) Contour plot of SPL on the sphere surface; (**b**) Noise sound pressure at four observation locations. not strong. By contrast, interaction #A3 is a parallel interaction as well as #R2 at the tip of the blade, inducing a strong impulsive effect that causes stronger noise. It is also evident that interaction #A3 radiating to around 120\({}^{\circ}\) in azimuth and 45\({}^{\circ}\) below the rotor plane dominates six BVIs, the peak noise level contour of which determines the BVI noise radiation field. In addition, interaction #A2 radiating its acoustic energy to 170-200\({}^{\circ}\) in azimuth down the rotor is stronger than #A1 but weaker than #A3 in noise intensity because of the fact that the wake angle, the miss-distance, and the interaction angle lie between those of #A1 and #A3. The noise from interaction #A4 is weaker due to the large value of the miss-distance, radiating to 70\({}^{\circ}\) in azimuth down the rotor.