Abstract
Experimental and numerical results of a propeller of 0.3 m diameter operated at 5000 RPM and axial velocity ranging from 0 to 20 m/s and advance ratio ranging from 0 to 0.8 are presented as a preliminary step towards the definition of a benchmark configuration for low Reynolds number propeller aeroacoustics. The corresponding rotational tip Mach number is 0.23 and the Reynolds number based on the blade sectional chord and flow velocity varies from about 46000 to 106000 in the operational domain and in the 30% to 100% blade radial range. Force and noise measurements carried out in a low-speed semi-anechoic wind-tunnel are compared to scale-resolved CFD and low-fidelity numerical predictions. Results identify the experimental and numerical challenges of the benchmark and the relevance of fundamental research questions related to transition and other low Reynolds number effects.
Highlights
The development of tools for the design and optimization of propellers employed in multi-copter unmanned air vehicles and drones has to face two major difficulties
A preliminary step towards the definition of a benchmark problem for small UAV propeller aeroacoustics was accomplished through comparisons between measurements and low-/highfidelity predictions
LBM/VLES simulations were performed by triggering the boundary layer transition on the suction side, without an exact knowledge of the real flow regime in the untripped physical tests
Summary
The development of tools for the design and optimization of propellers employed in multi-copter unmanned air vehicles and drones has to face two major difficulties. The first one is the availability of reliable force, flow and noise data acquired for the same experiment in controlled conditions. The second difficulty is related to the intrinsic limitation of scale-resolved CFD methods to capture low Reynolds number phenomena like laminar to turbulent flow transition and the occurrence of laminar separation bubbles. Recent attempts to validate Lattice-Boltzmann Method / Very Large Eddy Simulation (LBM/VLES) results [1] revealed that the flow recirculation induced by a rotor operated in a confined environment, and the consequent interaction between blades and turbulent eddies, generates high-order Blade-Passing Frequency (BPF) loading noise harmonics. Similar observations have been made in other experiments [2]. Other sources of experimental uncertainties are: (i) the vibration of the test rig resulting in additional sources related to the random blade motion [3], (ii) the presence of electric motor noise, which is affected by the rotor torque [4]
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