# Helicopter Blade-Vortex Interaction Airload and Noise Prediction Using Coupling CFD/VWM Method
## 4 Conclusions  
In this study, a high-resolution coupled method CFD/VWM was adopted to study the aerodynamic characteristics of blade-vortex interaction, and the FW-H equation was utilized to calculate the acoustic characteristics. The BVI condition of the AH-1/OLS rotor was analyzed using the new hybrid method. A parametric study was performed to examine the effects on the BVI airload and noise characteristics. The following conclusions have been drawn from this study:  
1. The rotor wake is an important parameter that contributes to the accuracy of BVI noise prediction. Compared with the CFD/FW-H method, the hybrid CFD/VWM/FW-H method predicts a more accurate rotor wake and the impulsive sound pressure agrees better with the available test data.  
Figure 23: Contour plots of SPL on the sphere surface for different advance ratios. (**a**) \(\mu=0.194\); (**b**) \(\mu=0.224\).  
Figure 24: Max BVISPL variance with \(\alpha_{tpp}\) at different advance ratios.  
The hybrid method is also capable of reducing the computation time and resources, given that the computation time is about one-seventh of the CFD/FW-H method.
2. Variation in the tip-path-plane angle changes the miss-distance, affecting the BVI noise. In a strong BVI state, BVI noise is dominated by the parallel interaction #A3 that radiates to 120\({}^{\circ}\) in azimuth and 45\({}^{\circ}\) below the rotor plane. With the rotor disk tilted forward, BVI noise becomes weaker, and even disappears due to the reduction in the parallel interaction #A3. Meanwhile, the noise intensity decreases with increases in the miss-distance.
3. Increases in thrust coefficient oppositely impact the vortex strength and miss-distance, which accounts for the little changes in the sound pressure and the peak noise level. However, the radiation direction of the BVI noise moves from around 120\({}^{\circ}\) to 200\({}^{\circ}\) in azimuth down the rotor as the oblique interaction #A2 becomes stronger and dominates the BVI noise propagation.
4. Increasing the tip Mach number has little effect on the interaction angle and miss-distance, but greatly increases the noise radiation efficiency and the dynamic pressure on the advancing side, enhancing interactions #A1 and #A2. Therefore, the BVI noise intensity increases and the radiation directions from #A1 and #A2 become the main propagation direction of BVI noise. When the tip Mach number reaches transonic, BVI noise will never dominate the acoustic field in space.
5. Increases in the advance ratio greatly changes the interaction azimuth and interaction angle such that the interaction locations move backward and interaction #A2 changes to a parallel interaction. Thus BVI noise dominated by the parallel interaction #A2 radiates to around 160\({}^{\circ}\) in azimuth down the rotor. Also, the BVI noise intensity increases provided that the angle of the backward tilt is small. When it is not, the increase in the inverse miss-distance reduces the BVI noise.
6. Reductions in BVI noise can be obtained by increases in the miss-distance of the parallel interaction with the proper tip-path-plane angle and forward speed adjustment or by decreases in the tip Mach number. Decreases or increases in thrust coefficient change only the propagation direction, not its magnitude.  
This work was supported by National Natural Science Foundation of China (Grant Number: 11302103).  
[PERSON], [PERSON] and [PERSON] contributed to the formulation and analysis of BVI noise problem; [PERSON] and [PERSON] performed the numerical simulation; [PERSON] offered useful suggestions for the writing thought; [PERSON] wrote the paper.  
The authors declare no conflict of interest.