presented an optimization method based on the BET for a design of a quiet and efficient mini UAV rotor. For this reason, BET tools are commonly used as an initial loading estimation during the design process. Nevertheless, the reduction of computational cost remains significant compared to the higher-fidelity tools.Įven though URANS simulations offer the highest reliability for a wide range of conditions, the general experimental trends can be often successfully captured by low-fidelity methods, as shown by Cerny et al. Forward flight calculations, for which a simple momentum theory is no longer applicable, require corrections to the assumed inflow distribution or involve coupling the BET method with more complicated inflow models. The quality of BET calculations, however, depends on the quality of the airfoil polar data, which represents a particular challenge for low advance ratios. proved that for the simulation of axial flight, a simple blade element/momentum model offers good agreement with the measurements regarding performance prediction. From aerodynamic tools described in the literature, the ones based on the blade element theory (BET) are the most simplified and time efficient, yet for moderate flight conditions, they show accuracy comparable with higher-fidelity solvers. Ĭonsidering all the facts, separate studies dedicated for small rotors aerodynamics and aeroacoustics are necessary, which implies the demand for accurate calculation methods, capable of handling specifics of the forward flight operation. As a result, noise prediction for small-scale rotors poses a challenge for traditional methods developed for fully turbulent flows. Flow effects characteristic for these conditions like separation bubbles or boundary layer transitions, together with intensified wake interactions, increase unsteadiness at blade edges and thereby amplify higher frequency noise components. Though the main noise sources remain the same as those for helicopter rotors, UAVs typically fly in a laminar-transitional flow regime, which affects the balance between tonal and broadband noise contributions in the pressure signature. Additional complexity arises when rotors operate at low Reynolds numbers, which has impact on both performance and noise generation. As advancing and retreating side effects are not compensated, like in the case of hinged helicopter rotors, a fixed-pitch propeller in forward flight experiences high thrust fluctuations throughout the rotation, which also results in an asymmetry of the produced wake. The main difference comes from the blade planform shape, as multicopters are typically driven by rigid blades with nonlinear twist distribution and strong chord variation along the span. At the same time, the mechanisms acting on a small propeller in such conditions are not yet fully understood and cannot be directly derived from the full-size helicopter rotor. Although extensive studies have been done considering propellers operating under axial-flow conditions, their results are not applicable for forward flight with higher advance ratios. Whilst nowadays most multicopters are powered by electrical motors, it is the rotor that represents UAV’s main noise source. On the other hand, the developing UAV market is severely constrained by the public acceptance of the generated noise. Therefore, optimisation of aerodynamic rotor design in regard to flight in edgewise flow could help to meet an ongoing challenge of increasing flight duration for electric UAVs. Nevertheless, depending on the mission profile, the operation time of small UAVs may be mostly spent on forward flight. Unmanned aerial vehicles, like quadrocopters, have gained popularity due to their vertical take-off and landing (VTOL) and hover capabilities allowing operation in urban areas. The analysis of scattering effects showed that influence of ground and rotor platform on aeroacoustic results was observable even for low frequencies. The wake influence on the results was estimated based on vortex trajectories from simulations and those visible in background-oriented schlieren (BOS) pictures. The resulting forces and noise levels showed satisfactory agreement with experimental data however, differences in accuracy could be noticed depending on the computational method applied. Obtained loading distributions served as input for aeroacoustic codes delivering noise estimation for the blade passing frequency on a plane below the rotor. The range of aerodynamic tools included blade element theory, potential flow methods (UPM, RAMSYS), lifting-line method (PUMA) and Navier–Stokes solver (FLOWer). The study provided a base of comparison of known computational techniques with different fidelity levels for performance and noise prediction of a single, fixed-pitch UAV rotor operating with varying flight parameters.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |