A Concise Ten-Year Survey on DLR's involvement in European Rotor Noise Research

Abstract

Significant advances in understanding and controlling helicopter rotor noise were made in the recent past thanks to several dedicated European/International research programs. The availability of appropriate wind tunnel facilities (the German/Dutch Wind Tunnel, DNW, in particular) in conjunction with the application of ever more sophisticated testing techniques beyond acoustics, such as visualization of the highly complex aerodynamic interaction processes between flow and rotor blades using the DNW-developed Laser Light Sheet (LLS) techniques (to determine e.g. vortex trajectories and blade/vortex miss distances), Laser-Doppler-Velocimetry (LDV) to quantitatively assess the impact of rotational flow phenomena near rotor blades (e.g. vortex distribution, core size and circulation strength), the DNW-developed Moire-technique, a photometric "projected grid" method to determine blade deflection and blade attitudes during rotation, and the DLR-Göttingen-developed, highly promising Particle Image Velocimetry (PIV) to quantitatively determine in 2-D (soon 3-D) the flow- and velocity patterns over - compared to the LDV-method- much larger regions in a very quick, almost real time manner. Moreover, progress in acquiring - directly on the rotating blade - unsteady blade surface pressure data (being strongly related to one of the important source mechanism, namely "loading noise"), simultaneously with blade-vibration information and radiated acoustics, has furthered the knowledge on rotor aeroacoustics which can now be applied to design new operationally "quiet" rotors and to devise technically feasible noise reduction methods through, for example, sophisticated global-harmonic or individual blade control mechanisms. - The application of such new, often highly complex and manpower-intensive testing techniques in wind tunnels allow the acquisition of the necessary detailed technical insights into the physics of rotor aeroacoustics, before applying and testing appropriate and promising techniques on real ("full-size") helicopters in the framework of flight testing, where conditions are inherently less ideal and controllable and, foremost, where the influence of rotor/rotor- and rotor/fuselage-interactions must be accounted for, let alone trying to validate wind tunnel experiments with real flight test results. The ever better understanding of the intricate aeroacoustic mechanisms of rotor noise generation and radiation on the basis of such wind tunnel and flight experiments is instrumental in furthering the computational capabilities where substantial effort is presently spent to develop accurate prediction schemes for helicopter flyover noise, not in the least for purposes of land use planning and noise certification. This survey will discuss recent progress made above areas.