Cognate Space Identification for Forced Response Structural Redesign

2003 ◽  
Vol 127 (3) ◽  
pp. 227-233
Author(s):  
Vincent Y. Blouin ◽  
Michael M. Bernitsas
Keyword(s):  
Direct Method ◽  
A Priori ◽  
Normal Modes ◽  
Forced Response ◽  
Response Amplitude ◽  
Computational Time ◽  
Practical Guidelines ◽  
Structural Redesign ◽  
Large Admissible Perturbations ◽  
Perturbation Equations

Large admissible perturbations (LEAP) is a general methodology, which solves redesign problems of complex structures with, among others, forced response amplitude constraints. In previous work, two LEAP algorithms, namely the incremental method (IM) and the direct method (DM), were developed. A powerful feature of LEAP is the general perturbation equations derived in terms of normal modes, the selection of which is a determinant factor for a successful redesign. The normal modes of a structure may be categorized as stretching, bending, torsional, and mixed modes and grouped into cognate spaces. In the context of redesign by LEAP, the physical interpretation of a mode-to-response cognate space lies in the fact that a mode from one space barely affects change in a mode from another space. Perturbation equations require computation of many perturbation terms corresponding to individual modes. Identifying modes with negligible contribution to the change in forced response amplitude eliminates a priori computation of numerous perturbation terms. Two methods of determining mode-to-response cognate spaces, one for IM and one for DM, are presented and compared. Trade-off between computational time and accuracy is assessed in order to provide practical guidelines to the designer. The developed LEAP redesign algorithms are applied to the redesign of a simple cantilever beam and a complex offshore tower.

Cognate Space Identification for Forced Response Structural Redesign

2002 ◽  
Author(s):  
Vincent Y. Blouin ◽  
Michael M. Bernitsas
Keyword(s):  
Direct Method ◽  
A Priori ◽  
Normal Modes ◽  
Forced Response ◽  
Response Amplitude ◽  
Computational Time ◽  
Incremental Method ◽  
Practical Guidelines ◽  
Structural Redesign ◽  
Perturbation Equations

LargE Admissibility Perturbations (LEAP) is a general methodology, which solves redesign problems of complex structures with, among others, forced response amplitude constraints. In previous work, two LEAP algorithms, namely the Incremental Method (IM) and the Direct Method (DM), were developed. A powerful feature of LEAP is the general perturbation equations derived in terms of normal modes, the selection of which is a determinant factor for a successful redesign. The normal modes of a structure may be categorized as stretching, bending, torsional, and mixed modes and grouped into cognate spaces. In the context of redesign by LEAP, the physical interpretation of a mode-to-response cognate space lies in the fact that a mode from one space barely affects change in a mode from another space. Perturbation equations require computation of many perturbation terms corresponding to individual modes. Identifying modes with negligible contribution to the change in forced response amplitude eliminates a priori computation of numerous perturbation terms. Two methods of determining mode-to-response cognate space, one for IM and one for DM, are presented and compared. Trade-off between computational time and accuracy is assessed in order to provide practical guidelines to the designer. The developed LEAP redesign algorithms are applied to the redesign of a simple cantilever beam and a complex offshore tower.

Integrated Redesign of Large-Scale Structures by Large Admissible Perturbations

2003 ◽  
Vol 125 (4) ◽  
pp. 234-241 ◽  
Author(s):  
Vincent Y. Blouin ◽  
Michael M. Bernitsas ◽  
Denby Morrison
Keyword(s):  
Degrees Of Freedom ◽  
Large Scale ◽  
Direct Method ◽  
Computational Effort ◽  
Forced Response ◽  
Response Amplitude ◽  
Computational Time ◽  
Performance Constraints ◽  
Large Admissible Perturbations ◽  
Design Selection

In structural redesign (inverse design), selection of the number and type of performance constraints is a major challenge. This issue is directly related to the computational effort and, most importantly, to the success of the optimization solver in finding a solution. These issues are the focus of this paper, which provides and discusses techniques that can help designers formulate a well-posed integrated complex redesign problem. LargE Admissible Perturbations (LEAP) is a general methodology, which solves redesign problems of complex structures with, among others, free vibration, static deformation, and forced response amplitude constraints. The existing algorithm, referred to as the Incremental Method is improved in this paper for problems with static and forced response amplitude constraints. This new algorithm, referred to as the Direct Method, offers comparable level of accuracy for less computational time and provides robustness in solving large-scale redesign problems in the presence of damping, nonstructural mass, and fluid-structure interaction effects. Common redesign problems include several natural frequency constraints and forced response amplitude constraints at various frequencies of excitation. Several locations on the structure and degrees of freedom can be constrained simultaneously. The designer must exercise judgment and physical intuition to limit the number of constraints and consequently the computational time. Strategies and guidelines are discussed. Such techniques are presented and applied to a 2,694 degree of freedom offshore tower.

Selection of Variables for Structural Redesign by Large Admissible Perturbations

2003 ◽  
Author(s):  
Bonhyung Koo ◽  
Michael M. Bernitsas
Keyword(s):  
A Priori ◽  
Structural Elements ◽  
Baseline Design ◽  
Selection Of Variables ◽  
Effective Selection ◽  
Performance Specifications ◽  
Structural Redesign ◽  
Large Admissible Perturbations ◽  
Improved Accuracy ◽  
Selection Of

Redesign or inverse design is the process of generating a new optimal design which satisfies performance specifications starting from a baseline design with undesirable performance. The LargE Admissible Perturbation (LEAP) methodology makes it possible to redesign a structure for large changes in performance objectives and redesign variables without trial and error or repetitive finite element analyses. The next level of challenge in redesign automation is to identify a priori the structural elements and their properties that have the biggest impact and use only those in redesign. Based on LEAP, guidelines are developed in this paper for identifying effective selection of redesign variables for improved accuracy and reduced CPU time. These guidelines enable the designer to define the elements to be redesigned, to partition those elements among redesign groups, and to specify redesign variables in each group. In the numerical applications, an offshore tower is used to verify the developed guidelines. Three models of this tower with 160, 320, and 480 elements are used.

Redesign of Submerged Structures by Large Admissible Perturbations

2001 ◽  
Vol 123 (3) ◽  
pp. 103-111 ◽  
Author(s):  
Vincent Y. Blouin ◽  
Michael M. Bernitsas
Keyword(s):  
Finite Element ◽  
Optimization Problem ◽  
Mass Matrix ◽  
Added Mass ◽  
Forced Response ◽  
Initial Structure ◽  
General Perturbation ◽  
Large Admissible Perturbations ◽  
Fluid Added Mass ◽  
Perturbation Equations

The method of large admissible perturbations (LEAP) is a general methodology, which solves redesign problems of complex structures without trial and error or repetitive finite element analyses. When forced vibration constraints are incorporated into the redesign problem, damping and added mass due to the presence of fluid must be included into the model. The corresponding terms introduce theoretical and numerical difficulties, which are treated in this paper. The LEAP method has been implemented into a Fortran computer code RESTRUCT, developed at the University of Michigan. The redesign process is mathematically formulated as an optimization problem with nonlinear constraints, called general perturbation equations. First, a finite element analysis of the initial structure is executed. Then, the results are postprocessed by code RESTRUCT using an incremental scheme to find the optimum solution for the problem defined by the designer. Accurate determination of nonstructural terms, such as fluid added mass, is generally detrimental as far as forced response analysis is concerned. In redesign problems, however, simple but realistic models can be used. A simple transformation of the structural mass matrix is used to compute the added mass matrix and its dependency on the redesign variables. The presence of non-structural terms in the general perturbation equations requires the development of a new LEAP algorithm for solution of the optimization problem. A simple cantilever beam with 100 degrees of freedom is used to validate the fluid added mass model. The developed method and algorithm are then applied to a partially submerged 4,248 degree of freedom complex structure modeled with beam elements.

Selection of Variables for Structural Redesign by Large Admissible Perturbations

2003 ◽  
Vol 127 (2) ◽  
pp. 112-121
Author(s):  
Bonhyung Koo ◽  
Michael M. Bernitsas
Keyword(s):  
A Priori ◽  
Structural Elements ◽  
Baseline Design ◽  
Selection Of Variables ◽  
Effective Selection ◽  
Performance Specifications ◽  
Structural Redesign ◽  
Large Admissible Perturbations ◽  
Improved Accuracy ◽  
Selection Of

Redesign or inverse design is the process of generating a new optimal design which satisfies performance specifications starting from a baseline design with undesirable performance. The LargE Admissible Perturbation (LEAP) methodology makes it possible to redesign a structure for large changes in performance objectives and redesign variables without trial and error or repetitive finite element analyses. The next level of challenge in redesign automation is to identify a priori the structural elements and their properties that have the biggest impact and use only those in redesign. Based on LEAP, guidelines are developed in this paper for identifying effective selection of redesign variables for improved accuracy and reduced CPU time. These guidelines enable the designer to define the elements to be redesigned, to partition those elements among redesign groups, and to specify redesign variables in each group. In the numerical applications, an offshore tower is used to verify the developed guidelines. Three models of this tower with 160, 320, and 480 elements are used.

Treatment of Damping in Structural Redesign by Large Admissible Perturbations

1999 ◽  
Author(s):  
Vincent Blouin
Keyword(s):  
Structural Characteristics ◽  
Forced Response ◽  
Mode Shapes ◽  
Perturbation Equation ◽  
Structural Damping ◽  
Design Problems ◽  
Damping Matrix ◽  
Structural Redesign ◽  
Large Admissible Perturbations ◽  
External Forces

Abstract A method to solve redesign (inverse design) problems of complex structures with forced response amplitude constraints is developed. It is assumed that a structure is excited by harmonic external forces at a given frequency. The problem is to find optimum values of structural characteristics in order to achieve a desired level of forced response at one or several locations on the structure. The method of LargE Admissible Perturbations (LEAP) is used. Damping is shown to have an important impact on the redesign solution and must be considered in the redesign process. Two different treatments of the damping, leading to two different algorithms, are studied. In the case of structural damping, the commonly used Rayleigh formulation allows the damping matrix to be diagonalized by use of the real mode shapes of the undamped structure. This leads to the derivation of an exact perturbation equation with no loss of accuracy. When damping cannot be approximated by the Rayleigh model, the damping matrix must be treated externally and the perturbation equation is solved by means of an iterative process introduced into the redesign algorithm. The two algorithms are compared in terms of accuracy and limitations.

Experimental Validation of Forced Response Methods in a Multi-Stage Axial Turbine

2018 ◽  
Author(s):  
Thomas Hauptmann ◽  
Christopher E. Meinzer ◽  
Joerg R. Seume
Keyword(s):  
Experimental Data ◽  
Vibration Amplitude ◽  
Turbine Blades ◽  
Service Condition ◽  
Sensitivity Study ◽  
Forced Response ◽  
Tip Clearance ◽  
Computational Time ◽  
Reference Case ◽  
Axial Turbine

Depending on the in service condition of jet engines, turbine blades may have to be replaced, refurbished, or repaired in the course of an engine overhaul. Thus, significant changes of the turbine blade geometry can be introduced due to regeneration and overhaul processes. Such geometric variances can affect the aerodynamic and aeroelastic behavior of turbine blades. One goal in the development of the regeneration process is to estimate the aerodynamic excitation of turbine blades depending on these geometric variances caused during the regeneration. Therefore, this study presents an experimentally validated comparison of two methods for the prediction of forced response in a multistage axial turbine. Two unidirectional fluid structure interaction (FSI) methods, a time-linearized and a time-accurate with a subsequent linear harmonic analysis, are employed and the results validated against experimental data. The results show that the vibration amplitude of the time-linearized method is in good agreement with the experimental data and, also requires lower computational time than the time-accurate FSI. Based on this result, the time-linearized method is used to perform a sensitivity study of the tip clearance size of the last rotor blade row of the five stage axial turbine. The results show that an increasing tip clearances size causes an up to 1.35 higher vibration amplitude compared to the reference case, due to increased forcing and decreased damping work.

Comparison of the Influence Coefficient Method and Travelling Wave Mode Approach for the Calculation of Aerodynamic Damping of Centrifugal Compressors and Axial Turbines

2017 ◽  
Author(s):  
K. Vogel ◽  
A. D. Naidu ◽  
M. Fischer
Keyword(s):  
Centrifugal Compressor ◽  
Travelling Wave ◽  
Wave Mode ◽  
Forced Response ◽  
Computational Time ◽  
Influence Coefficient ◽  
Average Deviation ◽  
Aerodynamic Damping ◽  
Influence Coefficients ◽  
Periodic Boundaries

The prediction of aerodynamic damping is a key step towards high fidelity forced response calculations. Without the knowledge of absolute damping values, the resulting stresses from forced response calculations are often afflicted with large uncertainties. In addition, with the knowledge of the aerodynamic damping the aeroelastic contribution to mistuning can be considered. The first section of this paper compares two methods of one-way-coupled aerodynamic damping computations on an axial turbine. Those methods are: the aerodynamic influence coefficient, and the travelling wave mode method. Excellent agreement between the two methods is found with significant differences in required computational time. The average deviation between all methods for the transonic turbine is 4%. Additionally, the use of transient blade row methods with phase lagged periodic boundaries are investigated and the influence of periodic boundaries on the aerodynamic influence coefficients are assessed. A total of 23 out of 33 passages are needed to remove all influence from the periodic boundaries for the present configuration. The second part of the paper presents the aerodynamic damping calculations for a centrifugal compressor. Simulations are predominantly performed using the aerodynamic influence coefficient approach. The influence of the periodic boundaries and the recirculation channel is investigated. All simulations are performed on a modern turbocharger turbine and centrifugal compressor using ANSYS CFX V17.0 with an inhouse pre- and post-processing procedure at ABB Turbocharging. The comparison to experimental results concludes the paper.

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