Mechanisms of transonic blade-vortex interaction noise

Experimental investigations of aerodynamic sound generation due to vortex-airfoil interaction in two-dimensional transonic flow were performed using two different experimental set ups - a vacuum-operated transonic wind tunnel and a shock tube. Flow visualization was accomplished by a Mach-Zehnder interferometer as well as by a holographic interferometer. The interferograms obtained were evaluated by an image processing system. In addition, pressure measurements were carried out in the shock tube which allows for a correlation of time-dependent pressure distributions and the density distributions evaluated from the interferograms. Two mechanisms of aerodynamic sound generation are identified. These are different from the dipole-like mechanism known for subsonic flowfields. The newly found sound waves are called 'transonic waves' and 'compressibility waves.' For both of them simple models have been suggested which can partly explain their occurrence and their strengths.