Impact of Wake Structure Characteristics on Small-Scale Rotor Performance over a Range of Reynolds Numbers

Small-scale rotorcraft exhibit degraded aerodynamic efficiency, which has been linked to non-ideal losses within the wake. Unique high-frequency, broad-band features have also been observed, without a physical verification of their origin. This work seeks to gather insight into the behavior of the rotor wake structures as a function of Reynolds number (Re), relate this to performance capabilities and the corresponding far-field acoustic signature. Two-component particle image velocimetry (PIV), performance, and acoustic measurements were performed using three small-scale, NACA 0012 rotors operated over a range of low-Reynolds number conditions. Rotor geometry and operational speed (Ω) were varied to obtain the desired Re variation. Span-wise PIV has demonstrated an absence of tip vortex formation as the operational thrust coefficient (CT) is increased, suggesting outboard tip stalling. Phase-locked, chordwise PIV has confirmed this hypothesis, showing the development of large-scale separation and a highly turbulent downstream wake. The wake characteristics at moderate collective angles (θ) show the presence of periodic counter-rotating structures attributed to laminar boundary layer vortex-shedding. Significant broad-band frequency content was measured, and found to correlate well with a physical quantification of the shedding phenomenon.