Aerodynamic analysis and measurement of a subwing blade tip shape for blade-vortex interaction noise reduction

Under the NASA advanced subsonic technology program's tiltrotor element, a program was developed for testing nonrotating blade tip shapes to investigate the potential for diffusing the tip vortex. A baseline rectangular tip blade and a subwing blade design were tested at the UTRC Main Wind Tunnel. A detailed hot-film velocity survey was obtained in the wake of each tip shape with sufficient detail to resolve the tip vortex region in multiple planes downstream of the model. In a parallel effort, the NASA OVERFLOW CFD code was used to investigate predictive capabilities necessary for determining the vortex core formation, strength, and intensity. Both the experimental and CFD data show that the subwing design is capable of developing two vortex cores, each one weaker than the corresponding baseline core. This research highlights the important difference between vortex core strength (Gamma) and vortex core intensity (gamma). The experimental and CFD results show that the tip vortex strength is preserved as it convects downstream from the model. However, CFD predictions of vortex core intensity suffer greatly due to the effects of numerical diffusion.