The rotor wake aerodynamic characterization is a fundamental aspect for the development and optimization of future rotary-wing aircraft. The paper is aimed at experimentally and numerically characterizing the blade tip vortices of a small-scale four-bladed isolated rotor in hover conditions. The investigation of the vortex decay process during the downstream convection of the wake is addressed. Two-component PIV measurements were carried out below the rotor disk down to a distance of one rotor radius. The numerical simulations were aimed at assessing the modelling capabilities and the accuracy of a free-wake Boundary Element Methodology (BEM). The experimental and numerical results were investigated by the Γ₂ criterion to detect the vortex location. The rotor wake mean velocity field and the instantaneous vortex characteristics were investigated. The experimental/numerical comparisons show a reasonable agreement in the estimation of the mean velocity inside the rotor wake, whereas the BEM predictions underestimate the diffusion effects. The numerical simulations provide a clear picture of the filament vortex trajectory interested in complex interactions starting at about a distance of z/R = −0.5. The time evolution of the tip vortices was investigated in terms of net circulation and swirl velocity. The PIV tip vortex characteristics show a linear mild decay up to the region interested by vortex pairing and coalescence, where a sudden decrease, characterised by a large data scattering, occurs. The numerical modelling predicts a hyperbolic decay of the swirl velocity down to z/R = −0.4 followed by an almost constant decay. Instead, the calculated net circulation shows a gradual decrease throughout the whole wake development. The comparisons show discrepancies in the region immediately downstream the rotor disk but significant similarities beyond z/R = −0.5.

An investigation of twin corotating rotors’ interaction effects was performed by load (thrust and torque) measurements, flow field dynamics through Time-Resolved Particle Image Velocimetry, and acoustic emissions using a microphone array. Two rotors, each with a diameter of D = 393.7 mm and equipped with three blades, were investigated in a side-by-side configuration, to simulate a multirotor propulsion system. The mutual distance between the propellers is 1.02 D, and four different rotating speeds, i.e., 2620, 3500, 4360, and 5200 RPM, were explored. In such a configuration, thrust and torque undergo a reduction compared to that found for a single propeller configuration. The level of aerodynamic load fluctuations increases as well. The interaction of the wakes produces a recirculation region at the external periphery of the shear layers. An innovative approach involving the coupling of Proper Orthogonal Decomposition (POD) and Wavelet Transform has been employed to investigate the dominant structures within the flow and their mutual influence. The results reveal that the interacting wakes are dominated by a wave-like motion pulsating at Harmonics of the Blade Passing Frequency (HBPF) of 1/3. Higher orders of POD modes capture coherent vortical structures, including tip vortices pulsating at HBPF = 1. The aeroacoustic investigation shows that the noise level, in terms of the Over All Sound Pressure Level, presents a remarkable increment concerning that generated by the single propeller.