Challenges to accurate computation of propeller performances at low angular velocities

Abstract

The main topic of the presented study is the alteration (i.e. possible degradation) of aerodynamic performances of small-scale rotors (applicable to today so popular unmanned multi-rotor air vehicles) at particularly low angular velocities (corresponding to 15-30% throttle that also imply low Reynolds number flows), i.e. particularly off-design operating conditions. These ranges of working regimes have not been investigated much (although they incorporate various interesting flow phenomena) and available data for validation is scarce, even though small-scale multi-rotor aircrafts usually carry rotors of fixed geometry and are controlled solely by changing the rotor angular velocities (that in turn produces changes in generated forces and moments). Although flows around rotor blades (at off-design conditions) are predominantly turbulent, other, much simpler and computationally less expensive numerical methods, such as blade element momentum theory (BEMT), are still used today for a quick estimation of propeller thrust and torque (i.e. power). By comparing the results obtained by different computational models, it is possible to evaluate their applicability and accuracy. Here, in order to quantify the changes in propeller flow behavior, both hover and axisymmetric flight condition were numerically investigated by two computational methods differing in complexity − BEMT vs. (U)RANS, and compared against the available experimental data. The paper briefly describes both employed numerical models and accentuates differences between them. Furthermore, in hover, quasi-steady RANS approach (by multiple reference frames) is compared to the URANS realized by sliding meshes. In both cases, the flow equations are closed by the well-proven and much employed k-ω SST turbulence model. Presented results include thrust and torque curves, as well as different flow visualizations. In the end, given that rotor characteristics may have dramatic effects on overall aircraft performance, it can be concluded that the accurate estimation of rotor thrust and torque at low angular velocities is extremely important but also quite hard to make. The differences between experimental and numerical data can amount to 50%, while degradation of rotor performances in flight at low Re can reach 20%.