The unsteady wake produced by a coaxial co-rotating rotor in hover

In a recent article by Tinney and Valdez (2020; AIAA J. vol. 58, no. 4), the trade space between near-field acoustics and thrust produced by a laboratory-scale, coaxial, co-rotating rotor in hover was investigated experimentally. The study considered the effect of various index angles and stacking distances between the upper and lower rotor for a range of rotor speeds and was motivated by growing interests in the use of stacked rotors for urban air mobility applications. It was reported how stacking distance was less significant at affecting acoustics and thrust and that the primary variable was the index angle between the upper and lower rotors. An analysis of the first few blade pass frequency harmonics demonstrated how the same thrust coefficient can be achieved with different rotor speed index angles and stacking distances, all the while producing uniquely different sound levels in as much as 15 dB(A). The current focus is a follow up campaign employing high-speed digital schlieren system and a single camera PIV system to measure the flowfield produced by the same stacked rotor as it relates to the performance trade space presented by Tinney and Valdez (2020).7 PIV measurements quantify the evolutionary behavior of the blade tip vortices produced by the upper and lower rotors, 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, co-rotating 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 and any gains in thrust are compounded by noise penalties.