한국지진공학회 추계학술발표회 IMPROVED SENSITIVITY METHOD FOR NATURAL FREQUENCY AND MODE SHAPE OF DAMPED SYSTEM Hong-Ki Jo1), *Man-Gi Ko2) and In-Won Lee3) 1) Graduate Student, Department of Civil Engineering, KAIST 2) Professor, Department of Civil Engineering, KongJu National Univ. 3) Professor, Department of Civil Engineering, KAIST
OUTLINE INTRODUCTION PREVIOUS STUDIES PROPOSED METHOD NUMERICAL EXAMPLE CONCLUSIONS
INTODUCTION Objective of Study Applications of Sensitivity Analysis - To find the derivatives of eigenvalues and eigenvectors of damped systems with respect to design variables. Applications of Sensitivity Analysis - Determination of the sensitivity of dynamic responses - Optimization of natural frequencies and mode shapes - Optimization of structures subject to natural frequencies. - Stability of structures - Reanalysis of modified structures
Problem Definition - Eigenvalue problem of damped system (1)
- State space equation (2) - Orthonormalization condition (3)
- Objective Given: Find: * indicates derivatives with respect to design variables (length, area, moment of inertia, etc.)
PREVIOUS STUDIES Z. Zimoch, “Sensitivity Analysis of Vibrating Systems,” Journal of Sound and Vibration, Vol. 117, pp. 447-458, 1987. (4) - restricted to lumped systems with distinct eigenvalues.
Q. H. Zeng, “Highly Accurate Modal Method for Calculating Eigenvector Derivatives in Viscous Damping System,” AIAA Journal, Vol. 33, No. 4, pp. 746-751, 1995. (5) (6) - many eigenvectors are required to calculate eigenvector derivatives.
Sondipon Adhikari, “Calculation of Derivative of Complex Modes Using Classical Normal Modes,” Computer & Structures, Vol. 77, No. 6, pp. 625-633, 2000. (7) - applicable only when the elements of C are small.
I. W. Lee, D. O. Kim and G. H. Jung, “Natural Frequency and Mode Shape Sensitivities of Damped Systems: part I, Distinct Natural Frequencies,” Journal of Sound and Vibration, Vol. 223, No. 3, pp. 399-412, 1999. I. W. Lee, D. O. Kim and G. H. Jung, “Natural Frequency and Mode Shape Sensitivities of Damped Systems: part II, Multiple Natural Frequencies,” Journal of Sound and Vibration, Vol. 223, No. 3, pp. 413-424, 1999.
Lee’s method (1999) (8) (9) - eigenvalue and eigenvector derivatives are obtained separately.
PROPOSED METHOD Rewriting basic equations - Eigenvalue problem (10) - Orthonormalization condition (11)
Differentiating eq.(10) with respect to design variable (12) Differentiating eq.(11) with respect to design variable (13)
- the coefficient matrix is symmetric and non-singular. Combining eq.(12) and eq.(13) into a single matrix (14) - the coefficient matrix is symmetric and non-singular. eigenpair derivatives are obtained simultaneously. corresponding eigenpair only is required.
Numerical Stability The determinant property (15)
Then (16)
Arranging eq.(16) (17) Using the determinant property of partitioned matrix (18)
Numerical Stability is Guaranteed. Therefore (19) Numerical Stability is Guaranteed.
NUMERICAL EXAMPLE Cantilever Beam
Analysis Methods Comparisons Lee’s method (1999) Proposed method Solution time (CPU)
Eigenvalue derivative Results of Analysis (Eigenvalue) Mode Number Eigenvalue Eigenvalue derivative (Lee’s method) (Proposed method) 1 -0.001 - 2.625i -0.014 -52.496i 2 -0.001 + 2.625i -0.014 +52.496i 3 -0.014 -16.449i -5.411e-1 -3.290e+2i 4 -0.014 +16.449i -5.411e-1+3.290e+2i 5 -0.035 -26.236i 4.770e-7 -2.970e-8i 6 -0.035 +26.236i 4.770e-7 +2.970e-8i 7 -0.106 -46.056i -4.242e+0 -9.210e+2i 8 -0.106 +46.056i -4.242e+0+9.210e+2i 9 -0.407 -90.244i -1.628e+1 -1.804e+3i 10 -0.407 +90.244i -1.628e+1+1.804e+3i Same
Eigenvector derivative Results of Analysis (First eigenvector) DOF number Eigenvector Eigenvector derivative (Lee’s method) (Proposed method) 1 2 3 1.513e-05 +1.513e-05i -3.027e-04 -3.027e-04i 4 1.204e-04 +1.204e-04i -0.002 - 0.002i 5 157 158 159 0.014 + 0.014i -0.279 - 0.279i 160 0.002 + 0.002i -0.038 - 0.038i Same
CPU time for 160 Eigenpairs Method Ratio (sec) Lee’s method (1999) 223.33 1.00 Proposed method 164.89 0.74
Comparison for each operations Total Lee’s method Proposed Method Operations CPU time (sec) 33.89 61.01 47.09 81.34 223.33 53.62 40.60 70.67 164.89
CONCLUSIONS An efficient eigensensitivity technique ! Proposed method - is simple - guarantees numerical stability - reduces the CPU time. An efficient eigensensitivity technique !
Future works Proposed method is able to extend to asymmetric non-conservative systems - Needs for comparison with other method in CPU time
Thank you for your attention.
APPENDIX Differentiating eq.(1) with respect to design variable (20) Pre-multiplying each side of eq.(20) by gives eigenvalue derivative. (21)
Lee’s method (1999) Differentiating eq.(3) with respect to design variable (22) Combining eq.(20) and eq.(22) into a matrix gives eigenvector derivative. (23)