LAUSR

This study investigated the small drone propeller noise, particularly the discrete blade passing frequency (BPF) tone and its harmonics at low frequencies less than 1000 Hz. The unsteady Reynolds-averaged Navier–Stokes equations were solved to investigate the steady and unsteady loading noise sources around the blades with a radius of 17 cm rotating at 5000 rpm (a blade tip Mach number is 0.264). The uRANS computations showed eccentric ellipsoidal isobaric surfaces on the upper and lower blade surfaces identified as a steady loading noise source of the drone propeller. A simple mathematical model of an ellipsoidal steady loading noise with the lattice Boltzmann method predicted the BPF tone and even-number harmonics comparable to NASA’s SPL measurement of two different APC 1147 SF and DJI 9443 CF drone propellers. The decaying rate of $$- 6$$-6 in the SPL spectrum was quite closely matched for the first two discrete tones. The transient pressure fluctuation characteristics on the upper surface of the rotating blades revealed that the unsteady loading noise by blade–vortex interactions is found most closely related to the third and sixth harmonics of the round per second noise. An unexpected fifth component also arose in some other rotational speeds because of the random nature of the phase difference of pressure fluctuations, even at the same positions of the two propeller blades.