US 5,686,667 AGranted

Method for determining the approximate resonance frequency of a structure surrounded by a compressible fluid

AssigneeThe United States of America as represented by the Secretary of the Navy

InventorsMichele D. McCollum, Clementina M. Siders

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Abstract

A method for determining the approximate resonance frequency of a structure urrounded by a compressible fluid includes the steps of: (1) performing analysis to determine in-vacuo frequencies and mode shapes of the structure; (2) selecting an in-vacuo mode of interest, (3) computing an influence matrix at the eigenfrequency of the selected mode of interest; (4) combining the computed influence matrix with structural stiffness and mass matrices from a finite element program, and using the modified matrices computing eigenvalues of the structure, including eigenvectors; (5) selecting from the computed eigenvectors a computed mode having substantially the same displaced shape as the in-vacuo mode of interest; (6) determining the eigenfrequency of the selected computed mode; (7) determining any difference between the computed eigenfrequency of the selected computed mode and the in-vacuo eigenfrequency of the selected in-vacuo mode of interest. The process iteratively returns to step (3) until the eigenvalues of the selected computed mode of step (5) and the in-vacuo eigenvalue substantially converge.

A method for determining the approximate resonance frequency of a structure surrounded by a compressible fluid, the method comprising the steps of:

(1) using a finite element model, performing an in-vacuo eigenvalue analysis to determine in-vacuo frequencies and mode shapes of said structure;
(2) selecting one of said mode shapes as an in-vacuo mode of interest, along with the corresponding eigenfrequency of said in-vacuo mode of interest;
(3) using a boundary element program, computing an influence matrix at said eigenfrequency of said selected mode of interest;
(4) combining said computed influence matrix with structural stiffness and mass matrices from a finite element program to thereby produce modified stiffness and mass matrices, and using the modified stiffness and mass matrices, computing eigenvalues of said structure, including eigenvectors;
(5) selecting from said computed eigenvectors a computed mode having substantially the same displaced shape as said in-vacuo mode of interest;
(6) determining the eigenfrequency of said selected computed mode;
(7) determining any difference between said above determined eigenfrequency of said selected computed mode and said in-vacuo eigenfrequency of said selected in-vacuo mode of interest;
(8) wherein if said difference is equal to, or less than, a selected tolerance, said above determined eigenfrequency of said selected computed mode is the approximate resonance frequency of said structure surrounded by said compressible fluid;
and
(9) wherein if said difference is greater than said tolerance, repeating step (3) substituting said above determined eigenfrequency of said selected computed mode for said in-vacuo eigenfrequency for the in-vacuo mode of interest, and thereafter, repeating steps (4)-(9), substituting in step (3) a most recent computed eigenfrequency of said selected computed mode for the previously used above determined eigenfrequency of said selected computed mode.

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Abstract

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