Abstract:
High-resolution flow visualization (FV) and particle image velocimetry (PIV) measurements were taken to document the development and evolution of a rotor wake in ground effect. In particular, the current work focuses on diffusing the tip vortices, through the use of centrifugal pumping, before they can fully form and reach the ground. A baseline non-pumping blade set and four different pumping blade designs were tested. All the pumping blade designs utilized the same centrifugal air flow mechanism, but the internal flow exited the blade at angles of 0°, 30°, 45°, and 60° above the horizontal axis. Detailed flow field measurements were performed for regions of interest near the rotor and in the near-wall region to understand the development and evolution of the trailed tip vortices in ground effect. All the blades sets were tested in a hovering state in ground effect at a blade loading coefficient, CT/σ, of 0.080. Additional measurements were also performed on the baseline and 0° pumping blade design at a reduced blade loading coefficient of CT/σ = 0.053 to examine the effect mass flow rate had on tip vortex diffusion. All the pumping blade designs were found to incur a power penalty, which resulted from the exit slots at the tip increasing the profile losses. The 30°, 45°, and 60° pumping blade designs were found to have a degraded performance as compared to that of the 0° pumping blade, which is believed to result from negative thrust effects associated with the mass flow of the air out the exit slots. When operating at the higher thrust condition, the pumping blade designs were found to merely prolong the initial formation of the tip vortices as opposed to completely diffusing them. However, when the 0° pumping blade was operated at the lower thrust condition the tip vortices were found to be more diffused, with significant reduction in the upwash velocity near the ground.