Massimo Gennaretti

Novel Boundary Integral Formulation for Blade-Vortex Interaction Aerodynamics of Helicopter Rotors

A direct panel method based on a novel boundary integral formulation for the velocity potential is presented and applied to helicopter rotors experiencing blade-vortex interaction. It avoids the numerical instabilities arising in the standard direct panel method in case of blade/wake impingement. This aerodynamic formulation yields a unified approach for the calculation of free-wake evolution and the blade-pressure field; it is fully 3-D, includes body-thickness effects, and can be applied to blades with arbitrary shape and motion. Blade-pressure predictions and the corresponding acoustic fields correlate well with wind-tunnel test data for helicopter rotors in descent flight, in which severe blade-vortex interaction occurs.

Aeroelastic response of helicopter rotors using a 3D unsteady aerodynamic solver

The prediction of blade deflections and vibratory hub loads concerning helicopter main rotors in forward flight is the objective of this work. They are determined by using an aeroelastic model derived through the coupling between a nonlinear blade structural model and a boundary integral equation solver for three-dimensional, unsteady, potential aerodynamics. The Galerkin method is used for the spatial integration, whereas the periodic blade response is determined by a harmonic balance approach. This aeroelastic model yields a unified approach for aeroelastic response and blade pressure prediction that may be used for aeroacoustic purposes, with the possibility of including effects from both blade-vortex interaction and multiple-body aerodynamic interaction. Quasi-steady aerodynamic models with wake-inflow from the three-dimensional aerodynamic solver are also applied, in order to perform a comparative study. Numerical results show the capability of the aeroelastic tool to evaluate blade response and vibratory hub loads for a helicopter main rotor in level flight conditions, and examine the sensitivity of the predictions on the aerodynamics model used.

Prediction of Tiltrotor Vibratory Loads with Inclusion of Wing­-Proprotor Aerodynamic Interaction

A numerical methodology for the prediction of vibratory loads arising in wing―proprotor systems is presented. It is applicable to tiltrotor operating conditions ranging from airplane to helicopter-mode flights. The aeroelastic formulation applied takes into account the aerodynamic interaction effects dominated by the impact between proprotor wake and wing, along with the mutual mechanical influence between elastic wing and proprotor blades. A boundary integral formulation suited for configurations where strong body―vortex interactions occur yields the aerodynamic loads, and beamlike models are used to describe the structural dynamics. A harmonic balance approach is applied to determine the aeroelastic solution. In the numerical investigation, first, the aerodynamic solver is validated by correlation with experimental and numerical results available in the literature, then the vibratory loads transmitted by the wing―proprotor system to the airframe are predicted, focusing the attention on the analysis of the different aerodynamic contributions.

Assessment of Computational Models for the Effect of Aeroelasticity on BVI Noise Prediction

This paper deals with the computational analysis of acoustic fields generated by helicopter rotors when Blade-Vortex Interactions (BVI) occur. The prediction procedure starts from the determination of the steady periodic blade deformations. Then, the BVI-affected, unsteady aerodynamics solution is obtained by a potential-flow boundary integral formulation suited for aeronautical configurations experiencing blade-wake impingements. It is applicable to blades with arbitrary shape and motion and evaluates both wake distortion and blade pressure field. Finally, the noise field radiated by the rotor is computed through an aeroacoustic tool based on the Ffowcs Williams and Hawkings equation. The numerical investigation examines the sensitivity of BVI noise prediction on the aeroelastic model applied for the calculation of blade deformations, and assesses the accuracy of the results through correlation with experimental data concerning a helicopter main rotor in descent flight. Noise predicted is examined in terms of both acoustic pressure signatures and noise radiation characteristics.

A new velocity decomposition for viscous flows: Lighthill's equivalent-source method revisited

A convenient decomposition for vector fields is presented that falls within the vast class of potential-vorticity decompositions of the type υ=V¯ϕ+w, which includes the classical Helmholtz and Clebsch decompositions. The distinguishing feature of the present decomposition is that the rotational velocity contribution w (obtained by a line-integration along a path normal to the boundary) vanishes in much of the irrotational region. As a consequence, in the irrotational region one obtains υ=V¯ϕ. The mathematical formulation of the problem and the application to the analysis of incompressible viscous flows around solid bodies are discussed. For flows in which the vortical region is concentrated in a thin layer surrounding the surface of the body and the wake mid-surface, the relationship between the present formulation and the classical Lighthills equivalent-source approach is addressed. In addition, an exact extension of the Lighthill formulation is presented. The equivalence of the two approaches is established. Numerical results for the reconstruction of the two-dimensional velocity field (from a prescribed vorticity field) are presented to illustrate the differences between various approaches.

Time-Dependent Coefficient Reduced-Order Model for Unsteady Aerodynamics of Proprotors

We present a methodology for the identification of a periodic-coefficient reduced-order model (ROM) for the prediction of perturbation aerodynamic loads on tiltrotor propellers in cruise flight. Although the result is a periodic-coefficient model, the process requires only frequency-domain aerodynamic solutions. Assuming the unperturbed proprotor in axial flow, the matrix that collects the aerodynamic transfer functions between blade perturbative boundary conditions and generalized aerodynamic forces is first derived. Then its rational matrix approximation, followed by combination with the equations describing the wing/pylon/proprotor kinetic coupling, yields the aerodynamic ROM. This ROM is expressed in terms of a set of linear equations that relate the time evolution of the aerodynamic loads acting on the proprotor blades to wing/pylon and deformable-blade degrees of freedom

Effects of Biodynamic Feedthrough in Rotorcraft/Pilot Coupling: Collective Bounce Case

This paper discusses the aeroelastic interaction between the helicopter and the pilot called collective bounce. The problem is mostly studied in the time domain, using the multibody system dynamics approach to model the dynamics of the vehicle and the aeroelasticity of the main rotor and a linear or quasilinear transfer function approach for the voluntary and involuntary dynamics of the pilot. Different models are considered for the aerodynamic forces acting on the rotor, ranging from blade-element/momentum theory to a boundary-element method used independently and in cosimulation with the multibody model. The problem is analyzed in hover and forward flight, highlighting modeling requirements and the sensitivity of the stability results to a variety of parameters of the problem.

A High Order Boundary Element Formulation for Potential Incompressible Aerodynamics

A high order boundary element formulation is presented and applied to the solution of potentials incompressible flows around non-lifting and lifting configurations. The high order numerical algorithm is based on a bicubic interpolation of both geometry and quantities over each element of discretisation of the boundary. Numerical validation of the formulation is performed by studying the aerodynamic solution around fuselages and wings, and making comparisons with existing numerical results

Optimal design of tonal noise control inside smart-stiffened cylindrical shells:

This paper deals with the abatement of noise inside cylindrical cavities bounded by stiffened shells that are impinged by external tonal sound waves. The problem is analysed in a multidisciplinary context, involving interactions among exterior noise field, elastic shell dynamics, interior acoustics and control system. The actuation in the noise control process is performed through piezoelectric patches embedded in the stiffened shell, driven by a control law obtained from an optimal LQR cyclic control formulation, coupled with a genetic optimization algorithm (GA). A modal approach is applied to describe the smart shell dynamics and the interior acoustics that are coupled in the acoustoelastic plant model, while the exterior acoustic scattering is predicted through a boundary element method formulation. Numerical results deal with an aeronautical problem concerning a general aviation aircraft cabin impinged by the pressure disturbances emitted by a pair of propellers; in particular, the effectiveness and robustness of the active control law synthesized through the GA is investigated.

Multiblade Reduced-Order Aerodynamics for State-Space Aeroelastic Modeling of Rotors

Reduced-order aerodynamic models are tools that may be conveniently applied in a wide range of research and design applications in the aeronautical and mechanical fields. This paper presents a methodology for the identification of a reduced-order model (ROM) describing the linearized unsteady aerodynamics of helicopter rotors in arbitrary steady flight, which is particularly suited for the derivation of the state-space perturbation aeroelastic operators and is hence useful for stability analysis and aeroservoelastic applications. It is defined in terms of multiblade coordinates and yields a (finite state) constant-coefficient, linear, differential form relating them to the corresponding multiblade aerodynamic loads. This approach requires the prediction of a set of harmonic perturbation responses by an aerodynamic solver. The accuracy of the identified ROM in describing unsteady aerodynamics phenomena is strictly connected to that of the aerodynamic solver. Complex aerodynamic effects (like wake roll-up and wake―blade interactions) are included in the ROM if they are taken into account in evaluating the harmonic responses. Numerical results concerning a flap-lag helicopter rotor in forward flight are presented. These examine the accuracy the aerodynamic ROM introduced both in terms of aerodynamic loads predictions and in terms of aeroelastic stability analysis. An aeroservoelastic application is also included in order to demonstrate the suitability of the ROM proposed for the design of controllers.