Giuseppe Quaranta

Solution of the Riemann problem of classical gasdynamics

The mere structure of the linearly degenerate characteristic field of the equations of gasdynamics provides the natural frame to build the exact Riemann solver for any gas satisfying the condition evvv(s,v) ≠ 0, which guarantees the genuine nonlinearity of the acoustic modes. Differently from single equation methods rooted in the γ-law ideal gas assumption, the new approach is based on the system of two nonlinear equations imposing the equality of pressure and of velocity, assuming as unknowns the two values of the specific volume, or temperature, on the two sides of the contact discontinuity. Newton iterative method is used. The resulting exact solver is implemented for van der Waals gas, including the treatment of nonpolytropic behavior with molecular vibrations at thermal equilibrium, as well as for Martin-Hou gas, as an example of the general applicability of the proposed approach. The correctness of the new Riemann solver is demonstrated by comparisons with other numerical techniques.

Arbitrary Lagrangian Eulerian formulation for two-dimensional flows using dynamic meshes with edge swapping

Abstract The dynamic modification of the computational grid due to element displacement, deformation and edge swapping is described here in terms of suitably-defined continuous (in time) alterations of the geometry of the elements of the dual mesh. This new interpretation allows one to describe all mesh modifications within the arbitrary Lagrangian Eulerian framework, thus removing the need to interpolate the solution across computational meshes with different connectivity. The resulting scheme is by construction conservative and it is applied here to the solution of the Euler equations for compressible flows in two spatial dimensions. Preliminary two dimensional numerical simulations are presented to demonstrate the soundness of the approach. Numerical experiments show that this method allows for large time steps without causing element invalidation or tangling and at the same time guarantees high quality of the mesh elements without resorting to global re-meshing techniques, resulting in a very efficient solver for the analysis of e.g. fluid-structure interaction problems, even for those cases that require large mesh deformations or changes in the domain topology.

Multibody Analysis of Controlled Aeroelastic Systems on Parallel Computers

The paper describes the application of parallel techniques to amultibody multidisciplinary formulation. The problem is stated interms of a system of nonlinear Differential-Algebraic Equations(DAE). The parallel solution is obtained using a sub-structuringdomain decomposition method, that is able to exploit thecharacteristic quasi-monodimensional topology that multibodymodels usually present. The presence of explicit constraints inform of algebraic equations requires particular care in thetreatment of the related unknowns, to avoid local singularityproblems. The code has been successfully tested on differentcomputer architectures. Special attention has been dedicated toproduce a code that will efficiently work on a cluster of PCs.Results of three test problems, regarding the simulation of anonlinear beam bending and of complex aeroservomechanical systemsas an helicopter rotor and a tiltrotor aircraft, are presented.

Active Flutter Suppression for a Three-Surface Transport Aircraft by Recurrent Neural Networks

approach for the design of a flutter suppression system by means of Recurrent Neural Networks (RNNs). The controller is used to move flutter instabilities outside the flight envelope of an unconventional three surface, transport aircraft configuration. The design process requires a comprehensive aircraft model, where flight mechanics, structural dynamics, unsteady aerodynamics and control surface actuators are represented in state-space form, according to the “modern” aeroelastic approach. The control system implemented for flutter suppression is based on two RNNs: one is trained to identify system dynamics; the other works as a controller using an indirect inversion of the identified model. Keeping the training of both RNNs “on line” leads to an adaptive control system. Extensive numerical tests are used to tune the neural network design parameters and to show how the neural controller increases system damping, widening the flutter-free flight envelope by more than 15% of the uncontrolled flutter velocity. lutter can often be a critical problem for current flight vehicle design, due to the increase of structure lightness which leads to high structural flexibility. Several approaches may be followed to solve this problem. The first, which may be denominated “passive”, basically goes through a re-design of the aircraft to increase its stability boundaries. The second, which may be called “active”, tries to exploit the capabilities of control systems to improve aircraft stability properties, usually with a smaller weight increment. Furthermore, such controllers can be used to improve the performances and the cruise comfort. Here, an active control strategy based on Recurrent Neural Networks (RNNs) for flutter suppression is designed to improve the stability boundaries and performances of a transport aircraft with an unconventional configuration, denominated X-DIA. The X-DIA, sketched in figure 1, is a conceptual design of a short range 70-seats jet liner based on the idea of using three main lifting surfaces: a front canard, a 15 deg. forward swept wing and a T-tail. The rear location of the main wing along the fuselage, allowed by the forward swept wing coupled with the canard surfaces, is expected to yield a positive cooperation between structures and aerodynamics, allowing a significant weight saving and drag reduction. This configuration is currently the object of numerous investigations at the Dept. of Aerospace Engineering of Politecnico di Milano (DIAPM), both numerical and experimental. 1‐3 A 1/10, Froude scaled, wind tunnel model has been built and tests

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.

Coupled Multibody/Computational Fluid Dynamics Simulation of Maneuvering Flexible Aircraft

This paper illustrates the application of multibody system dynamics coupled to computational fluid dynamics for the aeroelastic analysis of detailed aircraftmodels performing arbitrary free flightmotion. An efficient alternative to modeling different aspects of aeromechanics in a monolithic code consists in building computational aeroservoelasticity modeling capability using independent software components for each domain: structure, fluid and mechanism analysis. This partitioned approach relies on dedicated software exploiting the most appropriate techniques to address the dynamics of each specific field. Efficiency is guaranteed since each subsystem can be modeled independently; specific time and spatial scales of interest are considered. Model setup is flexible: the designer can choose the most appropriate tools, trading accuracy for computational costs, requiring higher-order fidelity methods only when simplified ones cannot be applied, or their validation is pursued. The combination of multibody system dynamics and computational fluid dynamics yields a highly accurate prediction tool, that can be crucial in the preliminary and intermediate design steps of unconventional configurations, for the investigation of loads, performance, stability and vibratory response of the vehicle at the boundaries of the flight envelope. Its application to the analysis of an aircraft maneuvering in transonic flight is presented.

Numerical Evaluation of Limit Cycles of Aeroelastic Systems

This paper focuses on the analysis of limit-cycle oscillations of aeroelastic systems with multiple lumped nonlinearities. It aims at a comprehensive investigation capable of identifying limit cycles and their stability. The goal is achieved by using an incremental complexity approach. At the beginning, a solution based on dual-input describing functions is sought, to find both symmetric and asymmetric cycles approximated to their first harmonic. The related stability is investigated afterward by extending the single-input describing function “quasi-static” method. Such an approach is simple and quite similar to well-established existing methods used to evaluate linear flutter conditions directly. If higher harmonics are required, an extended harmonic balance based on a numerical minimization in the frequency domain is adopted and the stability of the computed solutions is then determined by using Floquet theory. The presented approach is applied to several nonlinear aeroelastic examples and validated by comparing stable limit cycles with solutions obtained through direct time marching integrations.

DYNAMIC CHARACTERIZATION AND STABILITY OF A LARGE SIZE MULTIBODY TILTROTOR MODEL BY POD ANALYSIS

A great level of flexibility and accuracy can be gained by employing a multibody approach as the modeling strategy for servo-aeroelastic analyses of rotating wing aircraft. However, the resulting models are often large, in terms of degrees of freedom, and fully nonlinear, making the dynamic characterization phase cumbersome and computationally demanding. Moreover, nonminimal set formulations such as the one used here to express the equations of motion may cause the rise of spurious eigenvalues. This paper presents the application to a tiltrotor model of a technique which associates the POD (Proper Orthogonal Decomposition) methodology, to reduce the impact of the model large dimensions, with standard dynamic system identification methods, to efficiently circumvent the complexity usually associated with the development phase of flexible multibody models. The same technique is also employed to assess the periodic motion aeroelastic stability of the tiltrotor.Copyright

Finite-volume solution of two-dimensional compressible flows over dynamic adaptive grids

A novel Finite Volume (FV) technique for solving the compressible unsteady Euler equations is presented for two-dimensional adaptive grids over time dependent geometries. The interpretation of the grid modifications as continuous deformations of the underlying discrete finite volumes allows to determine the solution over the new grid by direct integration of the governing equations within the Arbitrary Lagrangian-Eulerian (ALE) framework, without any explicit interpolation step. The grid adaptation is performed using a suitable mix of grid deformation, edge-swapping, node insertion and node removal techniques in order to comply with the displacement of the boundaries of the computational domain and to preserve the quality of the grid elements. Both steady and unsteady simulations over adaptive grids are presented that demonstrate the validity of the proposed approach. The adaptive ALE scheme is used to perform high-resolution computations of the steady flow past a translating airfoil and of the unsteady flow of a pitching airfoil in both the airfoil and the laboratory reference, with airfoil displacement as large as 200 airfoil chords. Grid adaptation is found to be of paramount importance to preserve the grid quality in the considered problems.

Linear Reduced-Order Model for Unsteady Aerodynamics of an L-Shaped Gurney Flap

The purpose of the work is to investigate the mechanism that underlies the development of unsteady loads by a novel L-shaped Gurney flap conceived to perform vibration control on rotorcraft blades. The device is combination of a spoiler with a Gurney flap. Exploiting the capabilities of a Reynolds-averaged Navier–Stokes flow solver employing the overset mesh approach, several numerical simulations are carried out at low Mach number. These simulations are used to develop a physically based linear reduced-order model in the frequency domain for the unsteady lift and pitching moment of a NACA 0012 airfoil, considering as input the pitch and plunge harmonic oscillations of the airfoil, together with the oscillations of the L-shaped Gurney flap. The aerodynamic assessment of the L-tab shows that the behavior of the loads can be predicted using an equivalent flat-plate model to represent the airfoil composed by three segments: the first representing the fixed part of the airfoil, the second representing the lon...