Reduction of rotor noise has become an increasingly important research subject for the design of quieter helicopter. In the last twenty years, there has been some moderate success in prediction of the rotor BVI noise and the active noise controls. But the physical mechanisms of unsteady wake behavior are not fully understood and drastic reduction of noise has not been achieved. This BVI occurs primarily when the helicopter is in forward and descending flights and the active controls are used in those flight conditions for BVI noise reduction. But the characteristics of wake and rotor noise are very complex in those conditions. Therefore, the detail wake instability and noise characteristics in the hover flight should be investigated first.
The tip-vortex pairing, one of the rotor wake instabilities, is successfully predicted by using a time-marching free-wake method. In this method, the wakes are gradually generated at each time step with the initial condition of no wake. The wakes consist of vortex filaments. A slow starting condition and parabolic blending curves in the interpolation of the vortex filaments are used. The predicted results show good agreements with experimental data within 3% error of pairing point location. The computations to predict the tip-vortex pairing are needed up to 30.
Tip-vortex pairing is the strong interaction between tip vortices trailed from multi-bladed rotor and this interaction consists of rolling of the tip-vortices with each other. All wakes show aperiodicity beyond a certain bound location. The pairing and aperiodicity depend on the collective pitch angle of the blade and the climb rate.
The tip-vortex pairing is identified from the spectrum analysis of calculated tip-vortex noise, when tip-vortex pairing occurs repeatedly. The noise induced by the repeatable tip-vortex pairings has a quadrupole directivity pattern. And the peak components of noise spectrum represent pairing period and instability mode. But in case that t...