Vibration Reduction in Rotorcraft Using Active Flow Control

Abstract

This work presents a comprehensive computational analysis examining the potential of active flow control (AFC) for helicopter rotor vibration reduction in forward flight. The initial phase of the work focused on characterization of a novel fluidic actuator concept which employs pulsed air jets on the upper and lower surfaces of the blade near the trailing edge. Computational fluid dynamics (CFD) simulations were developed to evaluate the unsteady aerodynamic effects introduced by AFC on an airfoil in a two-dimensional setting. The CFD simulation approach was validated by comparison with recent wind tunnel experiments performed in a parallel experimental study at Georgia Tech. Subsequently, a reduced-order model (ROM) was developed for calculating unsteady aerodynamic loads induced by AFC at reduced computational cost when compared to CFD. The ROM was constructed based on training data obtained from CFD simulations performed across a broad range of flow conditions representing the helicopter rotor operating environment. Validation of the ROM against direct CFD simulations demonstrated the ROM's ability to accurately predict unsteady loads in a fraction of the computational run time.

Next, the effect of AFC on rotor blade vibrations was examined by incorporating the flow control ROM into a comprehensive aeroelastic rotor analysis code. The comprehensive code models the fully-coupled flap-lag-torsional rotor blade dynamics and features an unsteady aerodynamic model comprised of three elements: (1) a CFD-based rational function approximation model for calculating sectional blade loads in attached flow, (2) the ONERA dynamic stall model for calculating sectional blade loads in separated flow, and (3) a free-wake model for calculating the influence of the three-dimensional wake on the rotor inflow velocity distribution. Open-loop control simulations employing AFC actuation were used to gain insight into the types of actuation signals that are most effective for vibration reduction. Closed-loop control simulations based on the higher-harmonic control (HHC) algorithm were developed to account for the discrete operating characteristics of the AFC actuators, since each actuator, in practice, can only be on or off at a fixed jet strength. The sensitivity of the closed-loop vibration reduction to the actuation power available, enforced as a saturation limit in the HHC algorithm, was examined.

Results have demonstrated the effectiveness and control authority of the AFC concept for vibration reduction: up to 83% reduction of the vibratory hub loads is possible. Furthermore, the vibration reduction capability was shown to be comparable to that obtained using an electromechanically-operated microflap. The effect of AFC on the overall rotor performance during closed-loop vibration control was also calculated, to illustrate the fundamental trade-off that exists in rotorcraft applications. Finally, the closed-loop control approach was employed at several different rotor advance ratios. The simulations displayed a consistent level of vibration reduction in a variety of flow conditions, including high-speed flight where dynamic stall produces high vibratory loads.