Single Rotor Inflow and Wake Characterization in Hover with Dynamic Pitch Excitation

Abstract

A hovering two-bladed, 2-m-diameter rigid rotor was used to investigate the effects of dynamic pitch inputs at multiple blade loading coefficients. Unsteady rotor loads and the flow field were measured using an internal load cell and time-resolved particle image velocimetry, allowing for inflow, tip vortex trajectories and strength to be measured. Two sets of measurements were performed with an image acquisition rate of 44/rev: constant pitch angle (CT/σ = 0.06, 0.08, 0.10), and measurements with dynamic pitch ramp inputs (Δθ0 = 1° amplitude change over four rotor revolutions) at CT/σ = 0.06 and 0.08. The measurements at constant pitch angle and with dynamic pitch inputs were performed over 670 rotor revolutions, in both cases yielding statistically converged averages. In addition, the experiment was simulated using a finite-volume CFD solver. The results were correlated to the experimental data to assess the accuracy of the flow simulations and to gain further insight into the flow phenomenology. Vortex trajectories and axial velocities (inflow and wake) correlated well, and the tip vortex strength was slightly underpredicted by the CFD. From measurements and predictions, it was found that (independent of the thrust setting) the axial velocity in the rotor plane steadily increased to the value after the pitch input. Furthermore, vortex trajectories were little affected by the increased thrust and inflow. The vortex strength revealed trends similar to the axial velocity and gradually increased to the values after the pitch ramp input.