System Eigenvalue Identification of Mistuned Bladed Disks Using Least-Squares Complex Frequency-Domain Method

2017 ◽  
Author(s):  
Yuan Huang ◽  
Grigorios Dimitriadis ◽  
Robert E. Kielb ◽  
Jing Li
Keyword(s):  
Least Squares ◽  
Frequency Domain ◽  
Research Effort ◽  
Response Prediction ◽  
Forced Response ◽  
Frequency Domain Method ◽  
Common Denominator ◽  
Complex Frequency ◽  
Modal Parameter ◽  
Stabilization Diagram

This paper presents the results from a research effort on eigenvalue identification of mistuned bladed rotor systems using the Least-Squares Complex Frequency-Domain (LSCF) modal parameter estimator. The LSCF models the frequency response function (FRF) obtained from a vibration test using a matrix-fraction description and obtains the coefficients of the common denominator polynomial by minimizing the least squares error of the fit between the FRF and the model. System frequency and damping information is obtained from the roots of the denominator; a stabilization diagram is used to separate physical from mathematical poles. The LSCF estimator is known for its good performance when separating closely spaced modes, but few quantitative analyses have focused on the sensitivity of the identification with respect to mode concentration. In this study, the LSCF estimator is applied on both computational and experimental forced responses of an embedded compressor rotor in a three-stage axial research compressor. The LSCF estimator is first applied to computational FRF data obtained from a mistuned first-torsion (1T) forced response prediction using FMM (Fundamental Mistuning Model) and is shown to be able to identify the eigenvalues with high accuracy. Then the first chordwise bending (1CWB) computational FRF data is considered with varied mode concentration by varying the mistuning standard deviation. These cases are analyzed using LSCF and a sensitivity algorithm is developed to evaluate the influence of the mode spacing on eigenvalue identification. Finally, the experimental FRF data from this rotor blisk is analyzed using the LSCF estimator. For the dominant modes, the identified frequency and damping values compare well with the computational values.

scholarly journals The PolyMAX Frequency-Domain Method: A New Standard for Modal Parameter Estimation?

2004 ◽  
Vol 11 (3-4) ◽  
pp. 395-409 ◽  
Author(s):  
Bart Peeters ◽  
Herman Van der Auweraer ◽  
Patrick Guillaume ◽  
Jan Leuridan
Keyword(s):  
Parameter Estimation ◽  
Frequency Response ◽  
Least Squares ◽  
Frequency Domain ◽  
Real Life ◽  
Mode Shapes ◽  
Frequency Domain Method ◽  
Complex Frequency ◽  
Modal Parameter ◽  
Domain Method

Recently, a new non-iterative frequency-domain parameter estimation method was proposed. It is based on a (weighted) least-squares approach and uses multiple-input-multiple-output frequency response functions as primary data. This so-called “PolyMAX” or polyreference least-squares complex frequency-domain method can be implemented in a very similar way as the industry standard polyreference (time-domain) least-squares complex exponential method: in a first step a stabilisation diagram is constructed containing frequency, damping and participation information. Next, the mode shapes are found in a second least-squares step, based on the user selection of stable poles. One of the specific advantages of the technique lies in the very stable identification of the system poles and participation factors as a function of the specified system order, leading to easy-to-interpret stabilisation diagrams. This implies a potential for automating the method and to apply it to “difficult” estimation cases such as high-order and/or highly damped systems with large modal overlap. Some real-life automotive and aerospace case studies are discussed. PolyMAX is compared with classical methods concerning stability, accuracy of the estimated modal parameters and quality of the frequency response function synthesis.

Comparison of Modal Parameter Estimation Techniques for Electromechanical Rotordynamics of Cage Induction Motors

2007 ◽  
Author(s):  
Timo P. Holopainen ◽  
Seppo A. Aatola ◽  
C. Hunter Cloud ◽  
Guoxin Li
Keyword(s):  
Parameter Estimation ◽  
Least Squares ◽  
Frequency Domain ◽  
Theoretical Models ◽  
Complex Frequency ◽  
Modal Parameter ◽  
Electromagnetic System ◽  
Estimation Techniques ◽  
Modal Parameter Estimation ◽  
Parameter Estimation Techniques

Electromagnetic fields in the air gap of an electric motor produce electromagnetic forces between the rotor and stator. These forces couple the electromagnetic system to the mechanical one. This electromechanical interaction changes the vibration behaviour of the machine, and it may decrease the critical speeds, induce additional damping or cause rotordynamic instability. The experimental validation of theoretical models of these effects requires modal parameter estimation techniques which are reliable and robust. The main aim of this paper is to compare available techniques for the modal parameter estimation. The studied methods were: a) peak picking, b) prediction error, c) polyreference least-squares complex frequency-domain, d) multiple output backward autoregression, and e) polyreference least-squares complex exponential. Experimental data for the comparison was obtained using a standard six-pole 18 kW induction motor equipped with a long flexible shaft. In addition, the theoretical values using a simple electromechanical rotor model were calculated for the test cases. Comparison showed the short-comings of traditional frequency domain techniques and some advantages of the modern frequency- and time-domain techniques.

Finite-Difference Complex-Frequency-Domain Method for Optical and Plasmonic Analyses

2018 ◽  
Vol 30 (11) ◽  
pp. 1024-1027 ◽  
Author(s):  
Di Wu ◽  
R. Ohnishi ◽  
R. Uemura ◽  
T. Yamaguchi ◽  
S. Ohnuki
Keyword(s):  
Finite Difference ◽  
Frequency Domain ◽  
Frequency Domain Method ◽  
Complex Frequency ◽  
Domain Method

Blade Forced Response Prediction for Industrial Gas Turbines: Part 1 — Methodologies

2003 ◽  
Author(s):  
Stuart Moffatt ◽  
Li He
Keyword(s):  
Frequency Domain ◽  
Gas Turbines ◽  
Stokes Equations ◽  
Physical Space ◽  
Response Prediction ◽  
Forced Response ◽  
Navier Stokes ◽  
Loosely Coupled ◽  
Transonic Compressor ◽  
Industrial Gas Turbines

Forming the first part of a two-part paper, the methodology of an efficient frequency-domain approach for predicting the forced response of turbomachinery blades is presented. The capability and computational efficiency of the method are demonstrated in Part Two with a three-stage transonic compressor case. Interaction between fluid and structure is dealt with in a loosely coupled manner, based on the assumption of linear aerodynamic damping and negligible frequency shift. The Finite Element (FE) package ANSYS is used to provide the mode shape and natural frequency of a particular mode, which is interpolated onto the CFD mesh. The linearised unsteady Navier-Stokes equations are solved in the frequency domain using a single-passage approach to provide aerodynamic excitation and damping forces. Two methods of obtaining the single degree-of-freedom forced response solution are demonstrated: the Modal Reduction Technique, solving the modal forced response equation in modal space; and a new Energy Method, an alternative method allowing calculations to be performed directly and simply in physical space. Both methods are demonstrated in a preliminary case study of the NASA R67 transonic fan blade with excitation of the 1st torsion mode due to a hypothetical inlet distortion.

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