otor noise reduction methods being investigated by DLR and NASA engineers
"Reduced pressure on the top of the blade draws air upwards producing a vortex. The blade tip vortex is then directed downwards," explains DLR researcher Karen Mulleners, who is carrying out the tests in collaboration with NASA colleagues. "When other rotor blades subsequently come into contact with these vortices, the 'chopping' or throbbing noise that is characteristic of helicopters is produced.""The special thing here is the simultaneous use of three different optical measurement techniques," Raffel explains. The density and the velocity fields in the vortices and the deformation of the rotor blades are recorded. As in medicine, multiple investigative methods help reach the correct diagnosis. Thanks to the latest high-speed cameras, filming at speeds of as many as 5,000 frames per second is possible. This enables the turbulent, constantly changing vortex to be observed continuously for the first time. Until now, the time resolution of cameras has only been sufficient to capture snapshots.