Wake of a Coaxial Corotating Rotor in Hover

Abstract

A recent paper by Tinney and Valdez (AIAA Journal, Vol. 58, No. 4, 2020, pp. 1657–1667) reports the trade space between near-field acoustics and thrust produced by a laboratory-scale, coaxial, corotating rotor in hover by considering the effect of various index angles and stacking distances between the upper and lower rotor for a range of rotor speeds. The current effort is a follow-up campaign employing high-speed digital schlieren and single camera particle image velocimetry to measure the flowfield produced by the same stacked rotor. Particle image velocimetry measurements quantify the evolutionary behavior of the upper and lower blade tip vortices, and their interactions for various index angles and wake ages for a given rotor speed. Vortex tracking methods quantify the anisotropic wandering motions of the blade tip vortices and reveal patterns that are unique to both the upper and lower rotors. The findings reinforce the notion that the optimal index angle for the thrust produced by a coaxial, corotating rotor is one where the blade tip vortices produced by the upper rotor graze the low-pressure side of the lower rotor. If the miss distance is too small, collisions between the upper rotor vortex and the lower rotor ensue so that any gains in thrust are compounded by blade-vortex-interaction noise penalties.