Free Interface Modal Synthesis Method Research Articles

Blade reduced model considering local contact and analyzing blade vibration characteristics

A dynamic model of a tenoned rotating blade considering the pressure distribution characteristics of the contact interface is established and the degree of freedom of the model is reduced by the double coordination free-interface modal synthesis method in this paper. Firstly, the finite element model of the blade is established by ANSYS and the finite element matrix is extracted. Subsequently, multiple contact pairs were established on the contact surface of the tenon to capture the micro-slip characteristics of the contact interface. The normal pressure of the contact pair is obtained by the ANSYS blade-disk contact prediction and load equivalence to take the pressure distribution characteristics into account in the blade dynamics analysis. Then, non-uniform aerodynamic excitation is applied to the blade to simulate the load on the blade under real working conditions. Finally, the double coordination free-interface modal synthesis method is used to reduce the degree of freedom. Due to the coordination condition of the force and displacement of the substructure connection interface, the degree of freedom of the substructure interface is directly reduced. The final reduced degree of freedom is only the sum of the retained modes of the substructure, which greatly improves the computational efficiency. The dynamic equation is solved by the time integration method, and the effects of the tenon angle, rounded corner, and friction coefficient are discussed. The results show that the double coordination free-interface modal synthesis method can greatly reduce the degree of freedom of the tenon-connected blade model under the premise of ensuring accuracy while retaining the local contact characteristics of the contact interface. The structural parameters of the dovetail structure have a great influence on the vibration reduction characteristics of the blade. When designing the tenon structure, it is necessary to pay attention to these parameters.

Application of Free Interface Modal Synthesis Method on Modal Test Boundary Transformation of One Missile Wing

The large deviation between test results and real conditions caused by the coupling of test fixture is unavoidable. The research on free interface modal synthesis method can gain the modal test boundary transformation of one missile wing. For this purpose, the low order normal mode spectrum matrix and mode shapes was gained by free modal test, the extra high orders modes can be obtained through FEM analysis, while the boundary constrains was equivalent as a kind of sub-structure, and the modal synthesis was used to extract the constrained modal parameters. The experiment results are consistent with numeric calculation results very well, which indicated that this extracting method is feasible.

Dynamic modeling and active control of a strap-on launch vehicle

Dynamic modeling and active control of a strap-on launch vehicle are studied in this paper. In the dynamic modeling, the double-compatible free-interface modal synthesis method is used to establish dynamic model of the system, and its model precision is compared with those of finite element method (FEM), fixedinterface modal synthesis method and free-interface modal synthesis method. In the active control, the swing angle of rocket motor is used as design variable, and the control law design based on the model of mass center motion is adopted to validate the system. Simulation results indicate that the double-compatible model synthesis method can properly approximate the FEM which is used as the benchmark solution, and the model precision of the double-compatible modal synthesis method is obviously higher than those of the fixed-interface and freeinterface modal synthesis methods. Based on the control law design, the deflection of mass center of the launch vehicle is very small.

Identification Technology of Modal Parameter of a Strap-on Launch Vehicle

It is generally needed to conduct ground modal tests over strap-on launch vehicle to provide modal parameters for the attitude control design and load calculating. These modal parameters can also provide basis of installing sense organs on the launch vehicle. This paper introduces a modal parameter identification method based on input-output data of the system, which can be used to check calculations of modal parameters after ground modal tests. In this paper, the double-compatible free-interface modal synthesis method is first used for the modeling of the system so as to get the input and output data of the system. Then the identification techniques of observer/Kalman filter identification (OKID) and eigensystem realization algorithm (ERA) are introduced. Finally, numerical simulations are carried out to demonstrate the validity of the presented identification method. Simulation results indicate that the modal parameter of the system can be effectively identified using OKID and ERA.

Free-Interface Modal Synthesis Based Substructural Damage Detection Method

Free-interface modal synthesis method is applied to civil structure, and a substructure method is proposed by introducing the method into global sensitivity method. The substructure expression of the derivatives of eigenvalues and eigenvectors with respect to elemental parameters is obtained. The accuracy of the application of free-interface modal synthesis method is evaluated with different retained modes in substructure, and then the effectiveness of the proposed substructure sensitivity method is illustrated through an 11-storey building under both single- and multidamage cases. Both the damage locations and the extent can be effectively identified. By comparing it with the identical results of global sensitivity method, the proposed method can be faster in detecting the damage location and more stable under multidamage cases. Since this substructure sensitivity method only needs to update sensitivity matrix in the substructure with relative small number of DOFs, it may save much computation effort and become more efficient.

A Two-Step Model Reduction Method for a Strap-on Launch Vehicle

In this paper, a two-step model reduction method is proposed using a strap-on launch vehicle as research object. In this method, the double-compatible free-interface modal synthesis method is first used for the modeling of the system, and the first step of reduction work is done by preserving the lower-order modes of flexible components of the system in the modeling process for the system. Then the second step of reduction is done for the established dynamic model of the system using the modal cost analysis method. After the two steps of order reduction, a low-order dynamic model of the system can be obtained. Simulation results indicate that this low-order model can reflect the characteristics of the original system effectively and the order is low enough as well.

Vibration Analysis Of Ribbed Plates With A Rigid Intermediate Line Support

A numerical model for dynamic responses of ribbed plates with a rigid intermediate line support is presented. The model is based on the free interface modal synthesis method, with the constraint conditions enforced at the intermediate support by means of Lagrange multipliers. The model is verified by comparison with test results for double-span ribbed floor systems. As a special case, the model is also applied to predict natural frequencies of double-span plates without ribs. The potential of the modelling procedure in dealing with non-rigid or multiple intermediate line supports is discussed.

Reply: Free flexural vibration analysis of one-way stiffened plates by the free interface modal synthesis method

Reply: Free flexural vibration analysis of one-way stiffened plates by the free interface modal synthesis method

Discussion: Free flexural vibration analysis of one-way stiffened plates by the free interface modal synthesis method

Discussion: Free flexural vibration analysis of one-way stiffened plates by the free interface modal synthesis method

Free flexural vibration analysis of one-way stiffened plates by the free interface modal synthesis method

A numerical model is presented for predicting the natural frequencies of one-way stiffened plates with ribs having high ratios of flexural to shear rigidity. The model is based on the free interface modal synthesis method. Experimental validation using floors with wood I-joists and wood-based sheathing showed that the model has good numerical accuracy in the predictions of natural frequencies and mode shapes if analyses include shear deformation and rotatory inertia effects in ribs. Neglect of these effects can lead to large errors in the predicted natural frequencies for plates with ribs having high ratios of flexural to shear rigidity. Large errors can also be encountered in natural frequency prediction for plates with fairly low ratios of flexural to shear rigidity. This occurs with mode shapes that have multiple curvature along ribs if shear deformation and rotatory inertia effects are neglected. Key words: free flexural vibration, natural frequencies, ribbed plates, flexural rigidity, shear rigidity, modal synthesis.

A Modal Synthesis Technique Based on the Force Derivative Method

In this paper a new free interface modal synthesis method for the dynamic analysis of linear structures by the finite element method is developed. The technique is based on the so called higher order modal combination methods. In particular, the second force derivative method is used to accurately include the effect of the higher truncated modes in the representation of the substructures’ response. It is shown that the method is capable of providing very accurate estimates of the nature frequencies of the combined structure as well as the associated modes of vibration and elastic forces.