Alleviation of Rotorcraft BV1 noise and vibrations through control algorithms based on efficient aeroelastic/aeroacoustic formulations

Abstract

Because of their peculiar ability to take off vertically and their excellent handling qualities in hover and low speed flight conditions, helicopters can play several roles beyond the capabilities of fixedwing aircraft. However, the high levels of vibrations and noise generated represent nowadays critical issues limiting the helicopters operations both in civilian and military cenarios. Among the several sources contributing to the overall levels of vibration and noise, the main rotor plays a fundamental role through complex aerodynamic and aeroelastic phenomena affecting its operation. The rotorcraft research community is currently involved in exploring several active and passive approaches suited to reduce these annoying effects, and a lot of attention is particularly given to noise aerodynamically generated by main rotor when blade-vortex interactions (BVIs) occur. In this context, the aim of the thesis is to provide efficient formulations for performing fast and accurate rotor aeroelastic and aeroacoustic analyses. The availability of numerically efficient tools is extremely useful, for example, in early stages of design, optimization, and control synthesis, due to the high number of simulations that might be required. Some applications of the numerical tools developed are also presented in the thesis, in order to investigate different active and passive approaches for reducing vibration and BVI noise due to the helicopter main rotor. In particular, a procedure of advanced blade optimization is investigated as passive approach. In this procedure, blade shape and its mechanical/structural properties are selected using a genetic optimization algorithm, in order to reduce annoying vibratory loads at different flight conditions. In examining active control approaches, benefits achievable through the active twist rotor (ATR) solution are evaluated, investigating control effectiveness of both high-frequency and low-frequency actuators. The optimal, multi-cyclic, control theory is applied to identify the control law driving the ATR actuation. In all the applications proposed, rotor simulations are obtained by combining aerodynamic, aeroelastic and aeroacoustic tools able to accurately capture wake-blade mutual positions, which plays a crucial role in vibration and noise generation. In particular, rotor aeroelastic behaviour is described by a non-linear, beam-like model, coupled with a quasi-steady sectional aerodynamic formulation, taking into account three-dimensional effects by a free-wake inflow correction. The evaluation of the wake inflow is obtained by a boundary element method (BEM) suited for the analysis of helicopter rotors in arbitrary flight conditions, including those affected by strong aerodynamic body-wake interactions. Concerning the noise emitted by the rotor, it is evaluated through solution of the well-known Ffowcs Williams and Hawkings equation, which governs the propagation of acoustic disturbances aerodynamically generated by moving bodies. Specifically, the boundary integral Formulation 1A developed by Farassat is used. In this thesis, an accurate and efficient approach is presented for predicting blade airloads used for the aeroacoustic solution. It relies on the Küssner-Schwarz aerodynamic sectional theory, coupled with wake inflow information obtained through three-dimensional, free-wake aerodynamic solutions of trimmed rotor aeroelastic responses. In order to provide inflow corrections to sectional formulations used for both the aeroelastic solution and the blade airloads prediction, surrogate inflow models are also introduced in the thesis, as good trade-off solution between accuracy and computational efficiency. Furthermore, different algorithms for blade optimization and active control synthesis, aimed at efficiently exploiting aeroelastic and aeroacoustic models for the specific considered applications, are described. Several numerical investigations are presented to demonstrate the capability of the proposed approaches and examine their performance in the optimization and active control applications considered. In particular, single-point and multi-point optimization procedures are successfully applied to define low-vibrating rotor blades at the flight condition(s) considered, showing difficulties to guarantee significant vibratory loads reductions in off-design flight conditions, even if multi-point optimization is considered. A sensitivity analysis of the results obtained to the aerodynamic models used is carried out, confirming suitability of the surrogate models introduced in optimization problems. A final acoustic assessment of the optimal rotor obtained is performed, confirming the more acoustically annoying nature of low-vibrating rotors. Then, the high-frequency ATR control application is numerically investigated. Two different closed-loop controls are obtained, leading to significant reductions of the higher-harmonic noise in some areas of the acoustic field through limited blade twist imposed. In both cases, vibratory levels appears to be unaffected by control actuation, confirming the advantage of high-frequency controllers in terms of drawbacks onset. Finally, attention is focused on low-frequency ATR control strategy. In a preliminary analysis, 2/rev frequency appears to be effective in reducing BVI noise, with limited drawbacks in terms of increase of vibratory loads and low-frequency acoustic disturbance with respect to other frequencies considered. Hence, a 2/rev closed-loop noise control is numerically applied, showing an overall satisfactory reduction of the BVI noise in the acoustic field, with limited increase of low-frequency noise and vibratory hub loads.