A Combined Experimental and Computational Analysis of the Flow Field of a Coaxial Counter-rotating Rotor in Hover

Abstract

The flow fields of a 2m-diameter two-bladed single rotor and a 2�2-bladed coaxial counter-rotating rotor were measured using Particle Image Velocimetry, and computed using a coupled RCAS-VVPM analysis. Time-resolved measurements were performed on the single rotor at 44/rev with at least 552 flow realizations per azimuth. Similar measurements were performed for the coaxial, counter-rotating rotor at 64/rev with at least 260 flow realizations per azimuth. The goal of this study was to compare the flow features of these rotor configurations and validate current numerical analyses. Overall, there was good correlation between the measurements and calculations for the single rotor configuration. Numerical computations captured the axial velocity distribution across the blade radius and rotor azimuth. The radial distributions of axial velocity revealed less agreement in the tip region due to differences in the tip vortex trajectories. The tip vortex positions extracted from the measurements revealed a radial displacement of delta r/R = 0.112 and an axial displacement of delta z/R = 0.230 over one rotor revolution. The numerical computations captured the radial position for the first blade passage, but over predicted the radial position after blade passage. The axial positions were under-predicted over the entire azimuthal range. Comparison for the coaxial, counter-rotating rotor configuration revealed a consistent over-prediction in both radial and azimuthal velocity profiles for all spatial locations investigated. The axial position of the tip vortex were captured well with slight under-prediction for the upper rotor. The radial tip vortex positions were over-predicted over the entire azimuthal range investigated. Further validation of the numerical model is required before it can be used to extract state-space inflow models for single and coaxial, counter-rotating rotors.