The Physics behind BVI Noise Reduction in Descent Flight through the Use of Dynamic Active Twist Control

The paper investigates the influence of a dynamic active twist control on BVI noise in descent flight for a Mach-scaled Bo 105 model rotor. Therefore, a numerical study has been carried out with DLR's aeromechanics rotor code S4. In addition, sound pressure levels have been computed with DLR's acoustic code APSIM. The intensity of BVI noise is determined by different vortex parameters as well as the blade-vortex miss-distance and the BVI location. In this paper, all parameters that influence the intensity of the radiated BVI noise are analyzed. The analysis is carried out for a rotor shaft angle of attack of αᴿₒ = 4° for which the most intensive BVI noise was identified for the passive rotor blade. The study lays emphasis on the effect of different control amplitudes and phases but concentrates on a control frequency of the third multiple of the rotor rotational frequency for which highest reductions in BVI noise were identified.