Influence of Separation Distance on the Flow of Two Closely Spaced Rotors

This paper describes an experimental investigation of the interactions of side-by-side rotors at varying rotor spacings. The tests were performed in hover ([Formula: see text]) using stereoscopic particle image velocimetry (SPIV). The rotors used were DJI 9443 rotors rotating at 4860 RPM (81 Hz). Rotor phase-locking was employed to synchronize the rotation rate with the captured PIV images. An isolated rotor case, along with four tip-to-tip separation distances of [Formula: see text], 0.2, 0.1, and 0.05, were explored. The shed tip vortices, rotor wake velocity profiles, and overall momentum flow rate measurements were analyzed at each separation distance case. Decreasing the tip-to-tip separation resulted in a 28% increase in wake boundary contraction ratio and a faster initial decay in tip vortex magnitude, which is three times faster for the smallest separation distance than for the isolated rotor. Interactions from the adjacent rotor also yielded a 47% decrease in vorticity of initial tip vortices and a 35% decrease in circulation at the smallest tip-to-tip spacing. The position of the rotor wakes was altered and shifted toward the adjacent rotor by up to 0.1D as separation distance decreased. The dual-rotor cases had an increase in momentum flux when compared to the isolated rotor case, with a maximum increase of 5.1% for the smallest tip-to-tip spacing case.