Aeroacoustic radiation of low Reynolds number rotors in interaction with beams

The radiation characteristics of rotor-beam interaction noise are studied experimentally for low Reynolds number small-scale rotors in interaction with beams of different shapes, sizes, and downstream positions. The number of blades ranges from two to four. For the two-bladed rotor, the presence of the beam has no effect on the mean aerodynamic performance. Moreover, the blade passing frequency (BPF) and the high frequency broadband noise (BBN) appear not to be affected by the presence of the beam. On the contrary, the magnitude of the 2×BPF-25×BPF harmonics increases up to 30 dB compared to the case without beam, with an envelope consisting of two humps: one centered around 5×BPF and another around 20×BPF-25×BPF. For the first hump, a dipole-like pattern with minimal amplitude aligned with the beam can be observed, whereas another dipole-like pattern is observed for the higher frequency hump, but with a minimal amplitude over all the rotor disk plane. Compared to the two-bladed rotor, the presence of the beam has an effect on the mean aerodynamic performance of the three- and four-bladed rotors, increasing both the torque and the thrust at iso-rotational speed. This change leads to a change in the directivity of the BPF tone that decreases at a latitude angle of θ=0° and increases at a latitude angle of θ=40°. Moreover, the same two competing humps are observed on the BPF harmonics envelope. Interestingly, the frequency range over which an amplification of the harmonic magnitude is observed seems not to be influenced by the number of blades. Finally, the magnitude of the low frequency hump increases with the beam diameter, the rotational speed, and the number of blades but decreases with the rotor-beam distance. That of the high frequency hump increases also with the rotational speed and the number of blades, but not anymore with the beam diameter, and reaches a maximum value when the rotor-beam distance is at an intermediate distance of L = 25 mm. This hump is also influenced, to a lesser extent, by the shape of the beam. The two different evolutions permit us to conclude that the noise generation mechanisms leading to the two humps must be different. Scaling laws of the acoustical energy are derived for all those parameters. As already done for previous experiments without beam, all of the results are made available as an open database, at https://dataverse.isae-supaero.fr/.