Computational fluid dynamics of unsteady aerodynamic wake on helicopter main rotor-hub assembly

Abstract

Flow field on the helicopter is intricate and has puzzled aerodynamicists for decades. Tail shake problem has become an issue since the creation of helicopters, where it has caused tremors on the structure of the helicopter, performance, occupant’s comfort and interrupted the control, response and quality of the flight. It has been the continuous issues for designers and researchers in improving flight quality and better helicopter performance. However, previous researches focused on unsteady helicopter rotor hub wake at low range advance ratios merely up to 0.3 where the air load pressure was believed to be too small to influence the flow surrounding at the vicinity of tail parts. Therefore, simulation work beyond 0.3 is much required to investigate the unsteady flow characteristics at higher advance ratios. The aim of this research was to identify the aerodynamic characteristics elicited by the unsteady wake of the helicopter’s main rotor hub-assembly at higher advanced ratios beyond 0.3 through observations on static and dynamic analyses. The parameters investigated were the rotational speeds of 1200, 1400 and 1600 rpm, with two, three and four main rotor blades, two different fairing configurations and four different angles of attack (a). Rotor aerodynamics was modelled using Computational Fluid Dynamics by employing sliding mesh method to account for rotor rotation and k-w Shear Stress Transport for turbulent modelling. The results were collected in percentage and compared with calculation that had been done through experimental works by other researchers. The data collected were pressure fluctuation, turbulent kinetic energy, turbulent intensity and drag force. Dynamic analysis focused on the power spectral density, which showed the wake amplitude formation in the frequency domain. In general, turbulent kinetic energy for four blades rotor components showed higher values as compared to two blades, three blades and fuselage components. Turbulent kinetic energy recorded maximum value from 3.0 m2 /s2 to 4.0 m2 /s2 fuselage and 6.4 m2 /s2 to 10 m2 /s2 for rotor for elliptical fairing and 6.8 m2 /s2 to 13 m2 /s2 and 14 m2 /s2 to 22 m2 /s2 for rectangular fairing. Turbulent kinetic energy and turbulent intensity were effected by the number of blades, rotational speed, angle of attack and geometry of fairings. Drag force sourced out from the fuselage created 29% to 70% while the rotor produce 30% to 45% of drag. For dynamic analysis, turbulent kinetic energy of rectangular fairing showed a high wake amplitude of 7268.5 (m4/s4)/Hz, while turbulent kinetic energy of elliptical fairing showed wake amplitude of 7285.3 (m4/s4)/Hz, which showed the effect of complex geometry on the turbulent formation. Furthermore, the simulation conducted on the actual rotor hub indicated that a rotational speed of 1200 rpm has the highest value of turbulent kinetic energy of 42.7 (m4/s4)/Hz without the fairing employment. Employment of fairing has proven to reduce the formation of wake frequency. The results from 1200 rpm rotational speed were successfully validated with past researchers’ results in predicting the wake formation based on the frequency domain. In conclusion, the study successfully showed that the formation of unsteady wake sourced from a simplified model helicopter drawn and proved the presence of fairing does reduce the wake formation on the aft of the fuselage. Subsequently, this research proposes that three rotor blades with an elliptical fairing configuration is the best configuration with the lowest wake formation.