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A formulation to exactly integrate multiple discontinuities in 2D/3D finite elements by means of equivalent polynomials in XFEM analysis
Full text linkL'abstract è presente nell'allegato / the abstract is in the attachmen
High Order Harmonic Balance Applied to an Aeroelastic T-Tail Model with a Control Surface Freeplay
No full textFreeplay-Induced Limit-Cycle Oscillations in a T-Tail: Numerical vs Experimental Validation
Full text linkTo investigate the effect of control-surface freeplay, an aeroelastic wind-tunnel model of a T-tail with freeplay in the control chain was developed. The T-tail rig presents a conventional vertical fin represented only by its main structural component: the main spar. No aerodynamic sectors have been used to reproduce the aerodynamic contribution of a horizontal tail. This was chosen because of the limited size of the wind-tunnel chamber. Two numerical models were developed. The first one describes the dynamics of the tail by a state-space system with nonlinearity represented as a lumped element in the actuator feedback loop, and the second is described by the high-order harmonic balance approach, where the response of the nonlinear system is approximated with a periodic signal. The two approaches are then validated against experimental measurements collected during wind-tunnel testing. The results of this validation process as well the numerical and experimental approaches adopted are reported in this paper
Experimental and Numerical Investigation of the Behavior of a T-Tail with Control Surface Freeplay
No full textImplementation into OpenSees of XFEM for Analysis of Crack Propagation in Brittle Materials
Full text linkFracture propagation simulations by means of the traditional Finite Element Method require progressive remeshing to match the geometry of the discontinuity, which heavily increases the computational effort. To overcome this limitation, methods like the eXtended Finite Element Method (XFEM), in which element nodes are enriched through the medium of Heaviside step function multiplied by nodal shape functions, may be used. The addition of a discontinuous field allows the full crack geometry to be modelled independently of the mesh, eliminating the need to remesh altogether. In this paper OpenSees framework has been used to evaluate crack propagation in brittle materials by means of the XFEM method. Two shell-type XFEM elements have been implemented into OpenSees: a three-node triangular element and a four-node quadrangular element. These elements are an improvement of the elements with drilling degrees of freedom lately suggested by the Authors [6]. The implementation of XFEM elements implied some major modifications directly into OpenSees code to take into account the rise of number of degrees of freedom in the enriched element nodes during the analysis. The developed XFEM elements have been used to evaluate crack propagation into a plane shell subject to monotonically increasing loads. Moreover, with due tuning, the modified XFEM OpenSees code can be used to study also other problems such as material discontinuities, complex geometries and contact problems
Behaviour of an Aeroelastic T-Tail Model with Rudder's Freeplay: Experimental and Numerical Investigation
No full textFreeplay is one of the most important nonlinearities that affect the control surfaces of the aircraft; it can induce utter phenomena and limit the performances of the same airplane. To investigate the effect of control surface freeplay, an aeroelastic wind tunnel model of a T-tail has been developed. A variable amplitude freeplay has been introduced in the control chain by a specifically designed linkage. The numerical model was designed according to the modern aeroelastic approach, describing the dynamics of the tail by a state space system. The nonlinearity has been introduced has a lumped element in the actuator feedback loop. The paper presents the experimental setup together with the correlation obtained with the numerical model
EQP - A 2D/3D library for integration of polynomials times step function
Full text linkThe EQuivalent Polynomials library, EQP, herein provided is a powerful tool for the numerical integration, with classical quadrature rules (e.g. Gauss–Legendre), of a function given by the product of an arbitrary polynomial times a Heaviside step function. The library can handle a multiplicity of shapes for the integration domain in one, two and three dimensions. Originally developed by Ventura Ventura,2006) to overcome the long-standing problem of integrating discontinuous functions in the context of the eXtended Finite Element Method, EQP library has been recently generalized to meet the needs of very different fields, spanning from computational mechanics, to computer graphics, evaluation of geometric region (mass) properties and computer simulation in general
2D finite elements for the computational analysis of crack propagation in brittle materials and the handling of double discontinuities
Full text linkCrack growth simulations by way of the traditional Finite Element Method claim progressive remeshing to fit the geometry of the fracture, severely increasing the computational effort. Methods such as the eXtended Finite Element Method (XFEM) allow to overcome this limitation by means of nodal shape functions multiplied by Heaviside step function to enrich finite element nodes. Through the medium of a discontinuous field, the entire geometry of the discontinuity can be modelled regardless of
the mesh, avoiding remeshing. In this paper two shell-type XFEM elements (a three-node triangular element and a four-node quadrangular element) to evaluate crack propagation in brittle materials are presented. These elements have been implemented into the widespread opensource framework OpenSees to evaluate crack propagation into a plane shell subjected to monotonically increasing loads. Moreover, in the perspective of fracture propagation simulations, the problem of managing multiple cracks without remeshing or operating subdivisions on the integration domain has been investigated and a four-node quadrangular finite element
for the computational analysis of double crossed discontinuities by the means of equivalent polynomials is presented in this paper.
Equivalent polynomials allow to overcome inaccuracies on the results when performing standard numerical integration (e.g. Gauss-Legendre quadrature rule) over the entire domain of XFEM elements, without the need of defining integration subdomains. The presented work and the computational strategy behind it may be extremely useful not only in the field of fracture mechanics, but also to solve complex geometry problems or material discontinuities
Flow Field Around the Flapping Flag
Full text linkThe flapping flag is a canonical fluid-structure interaction problem that describes a cantilever plate with flow along its elastic axis. When the flapping flag loses stability it enters a large amplitude Limit Cycle Oscillation (LCO). While theoretical models can accurately predict the flutter velocity and frequency, there are still discrepancies between the experimental observations and the theoretical predictions of the post-critical LCO response. This note provides recent flow field visualizations in a single longitudinal plane for a cantilevered aluminum plate in axial flow during its LCO. Particle Image Velocimetry (PIV) techniques are used to show that the flow over the midspan of the plate is attached even during the violent LCO motion. This observation suggests that potential flow aerodynamic models may be able to capture the essential features in the flow field
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