Application of stereoscopic PIV to helicopter rotor blade tip vortices

Stereoscopic PIV technique was applied to the blade tip vortices of helicopter model in order to acquire the information that is needed by the understanding of the tip vortex structure during the blade-vortex interaction (BVI). The measurements have been performed in the NAL 6.5 m×5.5 m low-speed wind tunnel. The test blades have a rectangular tip and twisted NACA 0012 profile. The model was operated in the condition of maximum blade-vortex interaction (BVI) noise. The stereoscopic PIV system consisted of two CCD cameras and a double pulse Nd:YAG laser (1 J/Pulse, 532 nm). PIV image data were recorded at three positions and two azimuthal angles on the advancing side. As a result, three-component velocity fields of blade tip vortices were obtained from the PIV measurements, and location of the vortex relative to the rotor plane, the vortex core size, and the peak to peak velocity were derived. Furthermore, qualitatively valid value of turbulence intensity can be measured using by the PIV system. It was found that due to the vortex wander effects, average flowfields computed in a coordinate moving with the vortex are more accurate in terms of the derived vortex parameters than simple ensemble average flowfields.