Aerodynamic interaction of side-by-side propellers in forward flight operating at low-Reynolds numbers

• Experimental investigation of twin propellers configuration: aerodynamic loads (load sensors) and wake flowfield (stereoscopic particle image velocimetry (SPIV)). • Application of a phase-reconstruction methodology based on the propeller blade position extracted from the raw images obtained from the SPIV measurements. • In depth analysis of the propeller-motion correlated and uncorrelated flow fields deriving from triple decomposition to the measured test cases. • Discussion on the reduced propeller performance and on the flow field topology in a side-by-side propellers configuration. An experimental investigation is conducted to study the interaction of side-by-side propellers operating in forward flight at low Reynolds numbers. The effect on performance is first evaluated by means of load cell measurements, while the flow field is studied employing a stereoscopic particle image velocimetry (SPIV) setup. Three different configurations are tested: single propeller, co-rotating and counter-rotating cases at varying advance ratios. The results indicate that the performance of the single propeller is decreased due to aerodynamic interaction, leading to an average 3.2% reduction in propulsive efficiency, evaluated across all the tested operating conditions. The effect is stronger at lower advance ratios, owing to a greater interaction between the two streamtubes in such conditions. SPIV measurements indicate a widening of the wake as well as a reduction in the turbulence intensity for the cases with two propellers, with a stochastic fluctuations approximately 15% lower for the twin propeller cases than the single propeller. The data is then sorted and phase-ordered a posteriori via a data-driven approach, effectively reconstructing phase-averaged flow fields. This enables the decomposition of the velocity field into phase-correlated and purely turbulent components. The results show that the stochastic (turbulent) component of the velocity field increases when the propellers operate at advance ratios different from the maximum efficiency condition.