An experimental investigation of viscous aspects of propeller blade flow

An experimental investigation of the laminar/turbulent flow in the vicinity of a rotating propeller blade was conducted using laser doppler velocimetry. Details of the flow were measured to assess the viscous features relative to classical potential theory. Three-dimensional velocity component measurements were made of the propeller blade boundary layer and wake using laser doppler velocimetry with a phase averaging technique to account for blade rotation.;The propeller blade flow was characterized by streamwise and radial boundary layer profiles. Laminar boundary layers were initiated at the leading edge with transition to turbulence occurring at the mid-chord of the blade. The midspan streamwise boundary layer resembled typical two-dimensional behavior. The radial boundary layer exhibited large outward flow near the wall in regions of laminar flow which was reduced after transition. The outer blade boundary layer edge velocities along the blade were predicted by potential theory implying no significant viscous-invicid interactions. The tip vortex initially formed at the blade tip and convected over the blade surface locally distorting the blade surface boundary layer.;The propeller turbulent wake was dominated by individual blade wakes, hub and tip vortices. The radial attached boundary layer at the trailing edge of the blade convected into the wake, and produced significant outward, radial flow at the wake centerlines causing a redistribution of the classical sheet vortex. However, in the streamline direction, the measured wakes followed typical two-dimensional turbulent wake decay laws. Tip vortex roll-up was almost complete at the blade trailing edge, causing a reduction of the vortex sheet strength near the tip relative to moderately loaded propeller theory. With increasing downstream distance, the vortex and the blade wake system diverged through mutual induction and locally decayed and dispersed through turbulent dissipation.;This investigation of propeller flows supports and improves current empirical propeller wake models that incorporate distinct tip vortices and deformed vortex wake sheets. It is proposed that improvements in performance prediction could be made by considering the dispersion of the wake sheet, measured blade section drag coefficients, and modification of the vortex wake sheet when tip vortex formation occurs on the blade.