A.S. Nobari

Comparison Between Dedicated Model Updating Methods and Hybrid Method

By investigation of the short comings of two model updating methods, a hybrid method is suggested that make use of the advantages of each method and eliminates the disadvantage that eigenbased methods suffer from, an incomplete modal model. The main advantages of these methods are the small amount of data required and experimental noise elimination (minimization). On the other hand, using frequency reduced function (FRF) methods, one is not required to perform any modal analysis, and the data can be used directly, but theses methods suffer from the insensitivity of eigenvalues to FRF variations.

Structural Nonlinearity Identification Using Perturbed Eigen Problem and ITD Modal Analysis Method

Identification of nonlinear behavior in structural dynamics has been considered here, in this paper. Time domain output data of system are directly used to identify system through Ibrahim Time Domain (ITD) modal analysis method and perturbed eigen problem. Cubic stiffness and Jenkins element, as case studies, are employed to qualify the identification method. Results are compared with Harmonic Balance (HB) estimation of nonlinear dynamic stiffness. Results of ITD based identification are in good agreement with the HB estimation, for stiffness parts of nonlinear dynamic stiffness but for damping parts of nonlinear dynamic stiffness, method needs some additional improvements which are under investigation.

Stability analysis of aeroelastic systems based on aeroelastic FRF and condition number

Purpose – The purpose of this paper is to analyze the stability of aeroelastic systems using aeroelastic frequency response function (FRF).Design/methodology/approach – The proposed technique determines the instability boundary of an aeroelastic system based on condition number (CN) of aeroelastic FRF matrix or directly from FRFs data.Findings – Stability margins of typical section and hingeless helicopter rotor blade in the subsonic flow regimes (quasi‐steady and unsteady models) are determined using proposed techniques as two case studies.Originality/value – The paper introduces a technique which is applicable not only when aerodynamic and structure analytical models are available but also when there are experimental models for structure and/or aerodynamics, such as impulse response functions data or FRFs data. In other words, the main advantage of the proposed method, besides its simplicity and low memory requirement, is its ability to utilize experimental data.

Application of the modified reduced-order aerodynamics modelling approach to aeroelastic analysis

Abstract This study presents the application of the Proposed Modified Reduced-Order Aerodynamics Modelling approach for aeroelastic analysis based on the boundary element method (BEM) as a novel approach. The used BEM has the capability to capture the thickness effect and geometric complexity of a general three-dimensional model. In this approach the reduced-order aerodynamic model is defined through the eigenvalue problem of unsteady flow based on the unknown wake singularities. Based on the used aerodynamic model an explicit algebraic form of the aeroelastic equations is derived that reduces computational efforts and complexity. This special feature enables us to determine the aeroelastic instability directly via eigenanalysis of the aeroelastic problem. The eigenanalysis and reduced-order modelling of unsteady flow over a NACA 0012 airfoil are performed as an example and the results are compared with those obtained from conventional reduced-order modelling (CROM) method. The obtained results demonstrate the accuracy and superior efficiency of the present method over the CROM method. The method was then applied to the aeroelastic analyses of a typical section, in order to show the procedure of driving the explicit algebraic form. Furthermore, to illustrate the capability of the method in the handling of the three-dimensional problems, aeroelastic results for a wing are presented and compared with the results of the Theodorsens aerodynamic theory.

Improvement of Nonlinear Single Resonant Mode method

Presently, the Nonlinear Single Resonant Mode (NLSRM) method is the most efficient method for extending standard linear modal analysis concept to nonlinear systems. In this method, it is assumed that the mode of vibration in the resonant condition is close to the nonlinear normal mode and only one mode has nonlinear behavior. Therefore, predicting which one of the modes will exhibit nonlinear behavior is very important especially for a large system. The main object of this paper is twofold: (1) to improve the NLSRM method, (2) to use the sensitivity analysis for the prediction of nonlinear mode. The results are compared to the results of harmonic balance (HB) and time domain (TD) methods. It is shown that there is a very good agreement between HB and TD results with those of improved NLSRM methods and also the methodology for prediction of nonlinear mode is well suited.

A Comparison of Two Reduction Techniques for Forced Response of Shrouded Blades with Contact Interfaces

Two reduction methods for dynamic analysis of structures with local nonlinearity are compared. Dual and primal formulation have the same projection basis including flexibility residual attachment modes and free interface modes, but there are significant differences in their implementation. Both methods can be applied to nonlinear forced response analysis of turbine blades with contact interfaces in shroud. In this study, the shroud contact elements are employed using the adequate description of friction and 3D tangential coupled contact forces considering the effect of normal load variation. In order to examine and compare the accuracy of the two formulations, a rod and a simplified shrouded turbine blade was considered as case studies.

Investigation of damage effect on the effective dynamic mechanical properties of an adhesive in linear and nonlinear response regimes

In this paper, the effect of damage on the effective dynamic Youngs and shear moduli of a viscoelastic adhesive is investigated, in both linear and nonlinear regions of adhesive behavior. The investigation method is based on the measurement of the nonlinear Frequency Response Function (FRF) of adhesive bonded structure and inverse eigen sensitivity identification technique. Several single-lap joint specimens are manufactured in healthy and damaged configurations and their linear and nonlinear FRFs are then measured, using the Optimum Equivalent Linear FRF (OELF) concept. The measured FRFs are then used to extract linear and nonlinear bending and shear modes of the bonded structure. The modes of healthy and damaged structures are further used for identifying of Youngs and shear moduli at different frequencies. The results show that, for both linear and nonlinear regions, damages as small as 5% in the adhesive joint can be detected by the proposed technique, through their effects on the identified effective mechanical properties.

Identification of mechanical properties of adhesive joints using ANN

Purpose This paper aims to present a new approach to the fast determination of the effective, dynamic, mechanical properties of an adhesive for linear and nonlinear regions of the adhesive response, for both healthy and damaged states of the bond. Design/methodology/approach The proposed approach is based on the measurement of the linear and nonlinear frequency response function (FRF) of adhesive-bonded structure and using artificial neural network identification technique. For this purpose, linear and nonlinear FRFs are measured for several single-lap joint specimens that are fabricated in healthy and damaged configurations of the bond. The measured FRFs of healthy and damaged specimens are then used to identify the natural frequencies of the specimens. The experimental natural frequencies, in turn, would be used to train artificial neural network (ANN) which would be able to predict the effective Young’s and shear moduli and damping of adhesive in healthy and damaged specimens, for any given excitation level and frequency, within the training domain. Findings Simultaneous identification of the effective mechanical properties of adhesive for linear and nonlinear response regions, as well as healthy and damages states of the adhesive bond. Practical implications The introduced method is effective to model the assembled structures with the viscoelastic adhesive joints, for linear and nonlinear regions. Originality/value A fast methodology, using ANN, for identification the effective mechanical properties of adhesives, compared to other methods for both linear and nonlinear regions.

Aeroelastic numerical approach of a wing based on the finite element and boundary element methods

The aim of this study is to provide a coupled finite element–boundary element method approach for aeroelastic investigations in incompressible flow fields. Hence, an efficient and applicable tool for aeroelastic response computations is developed which uses boundary element method to solve flow fields and finite element method to analyze structural domains. In unsteady flow solution, the potential flow assumptions are considered, while the small deformations are assumed in structural dynamics behavior. Both solution procedures are tightly related by a boundary interface in a sequential coupling procedure. To illustrate the performance of aerodynamic solver for computation of three-dimensional unsteady flow configurations, lift response of a wing–body combination is computed and validated with available results in literature. Also, two test cases are presented that illustrate the accuracy of the developed tool for aeroelastic response analysis in comparison with available experimental results.

Assessment of the Performance of Modal Based Decoupling Method in Identifying Structural Joints Characteristics

In this paper, the performance of modal based decoupling method in structural joint identification is investigated. This method does not need any iteration and is independent from updating techniques. Because of these advantages, in present work it is tried to assess the effect of experimental and numerical issues of using modal data in this method. Issues such as effect of number of digits of precision of modal pairs, effects of lower hand higher eliminated modes, effect of any normalization of modal vectors and effect of possible noisy experimentally extracted modal parameters on identified joint characteristics are studied through some case studies.