Advanced aeroelastic models for the analysis of lifting surfaces made of composite materials

Aeroelastic phenomena can occur to many engineering structures: aircraft wings, rotor blades, wind turbines and slender bridges, for instance, are typical structures where severe, potentially catastrophic, aeroelastic events can take place. Current design trends are aimed to engineering solutions prone to aeroelastic phenomena. Typical examples are given by the design of slender wind turbines or aircraft wings. On the other hand, the increasing success of composite materials has opened new design scenarios where the occurrence of aeroelastic phenomena can be postponed by means of tailoring. Reliable aeroelastic analysis tools are needed in order to reduce the need of expensive and time consuming experimental tests. The development of reliable and computationally efficient analysis capabilities for aeroelasticity represents one of the most intriguing challenges of our times. Flutter is certainly the most known aeroelastic phenomenon. Catastrophic flutter events can occur to lifting surfaces (e.g. aircraft wings, rotor blades and wind turbines), bridges and missile panels. Depending on the flow condition, different flutter analysis capabilities are required. For instance, linear low-fidelity analysis can be sufficiently accurate for subsonic and supersonic flutter of lifting surfaces with no separations events, whereas transonic flutter requires nonlinear capabilities and high-fidelity tools taking into account flow viscosity effects. Computational costs can be significantly different from theory to theory. The trade-off between accuracy and computational efficiency is of primary interest. To date, the doublet lattice method (DLM) is one of the most powerful tools for linear flutter analyses in subsonic regime. DLM emerged in late 1960s and, more recently, an improved version of DLM has been proposed. The following main features are responsible of DLM success: 1. it offers good accuracy (unless transonic regimes are considered and/or separation occurs); 2. DLM is computationally competitive with respect to simpler methods such as strip theories; 3. fairly complex geometries can be analyzed including whole aircraft. One-dimensional (1D) structural models, commonly known as beams, are intensively exploited in many engineering applications. Beam theories are, in fact, used to analyze the structural behavior of slender bodies, such as columns, arches, blades, aircraft wings and bridges. In a beam model, the 3D problem is reduced to a set of variables that only depends on the beam-axis coordinate. One-dimensional structural elements obtained are simpler and computationally more efficient than 2D (plate/shell) and 3D (solid) elements. This feature makes beam theories still very attractive for the static, dynamic and aeroelastic analyses. Classical models (Euler-Bernoulli and Timoshenko) have intrinsic limitations which preclude their applications for the analysis of a wide class of engineering problems. This work is aimed to the development of an aeroelastic formulation based on DLM and higher-order structural models. The structural formulation is based on the Carrera Unified Formulation (CUF) which allows the use of any-order structural models in a unified manner with no need of ad hoc implementations. CUF 1D models are extremely cost competitive with respect to 2D (plate/shell) and 3D models with no accuracy loss. In other words, CUF 1D structural elements lead to shell- and solid-like solutions with computational cost comparable to those of classical beam formulations. Linear flutter analysis was chosen to show the benefits of the present formulation and its possible extension to more sophisticated aeroelastic capabilities. Results from purely mechanical analyses have shown the enhanced capabilities of the 1D structural formulation proposed. Shell- and solid-like accuracies have been obtained by using CUF 1D models for a number of different structural problems, such as: thin-walls structures undergoing point loads, shell-like natural modes of hollow cylinders, composite structures. The number of degrees of freedom of a CUF 1D finite element model is typically some 5-10 times lower than shells and 20-25 times lower than solids. Flutter analyses have been conducted on isotropic and composite wings and results have been compared with those from models based on plate and shell finite elements. Results have highlighted the high accuracy assured by 1D CUF models in detecting bending-torsion couplings of thin-walls and laminated structures. Excellent matches with results from plate models have been found with considerably lower computational efforts.