Communications – electrical: acoustic wave systems and devices – Acoustic image conversion – With memory means
Reexamination Certificate
2000-08-07
2001-10-09
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Acoustic image conversion
With memory means
C367S131000
Reexamination Certificate
active
06301192
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention is a method for creating frequency dependent structural/acoustic meshes for three dimensional finite element problems in infinite or semi-infinite mediums. The method maps a three dimensional object to a rectilinear acoustic field that can easily be modified for changes in frequency.
(2) Description of the Prior Art
Finite element analysis of structures submerged in an infinite medium are used to determine the stresses acting on prototype undersea hardware. Simple two dimensional axisymetric models are often employed because three dimensional models are too large and cumbersome to create. The most difficult problems in creating meshes for conventional finite element solvers are: (1) the generation of a three dimensional mesh that can accommodate a range of frequency responses without overstepping the memory requirements of the computer; and (2) the creation of a mesh that can capture small details of a structure and also be transmitted to a larger size mesh suitable for low frequency evaluation.
Not all finite element software packages are capable of solving structural acoustic problems. Often preprocessors are used to create elements that can be imported into nonlinear solvers. The preprocessors become slow and inefficient when processing the large number of nodes to be used in a coupled fluid/structure infinite medium analysis. Also the input files they generate require editing before they can be imported into the solver. When the files get too large, they exceed the memory allotment of the editor on the computer. It is more efficient in these cases to generate meshes using the solver program mesh generator.
Finite element software requires that a structure in a free field be surrounded by one wavelength of acoustic medium on all sides. An absorption fluid impedance condition is then placed at the boundaries to prevent reflections in the medium that could affect the structure's response. Another requirement is that the acoustic element length be a minimum of one-sixth of a wavelength for a solution. Good finite element mesh construction dictates that mesh nodes of linear elements have maximum dimensional ratios of 3:1:1. It also dictates that the included angles on quadrilateral and brick elements be greater than 5 degrees and less than 135 degrees. Often a model is desired to be evaluated for a frequency range over 500 Hz for steady state operation. Using the above criteria, a mesh with one-eighth symmetry that could be used at a frequency of 500 Hz would have a maximum element length of 29.5 inches. If this mesh were also to be used at 10 Hz, the mesh would have to extend 5905 inches in three directions. This would result in a model with 7,919,529 elements. Solving a problem of this size at each frequency of interest becomes an expensive task.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for generating variable frequency rectilinear meshes in two and three dimensions.
It is a further object of the present invention to provide a method as above which can be implemented with any preprocessor software or solver software.
It is yet another object of the present invention to provide a method as above for generating the meshes more efficiently.
The method of the present invention attains the foregoing objects.
In accordance with the present invention, a method for efficiently generating meshes in two and three dimensions is provided. The method involves generating several meshes, each covering a range of frequencies. The method of the present invention broadly comprises the steps of: enclosing a structure to be analyzed in a block of fluid having each of its sides formed by a plurality of equally sized elements; determining a bias factor and coordinates for the mesh; and generating a two dimensional mesh using said coordinates and said bias factor to transition from said equally sized elements to frequency dependent sized elements. After the two dimensional mesh has been created, the method of the present invention goes on to continue the transition by generating a three dimensional mesh using said coordinates.
Other details of the method of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following description and the accompanying drawings wherein like reference numerals depict like elements.
REFERENCES:
patent: 5581489 (1996-12-01), Groothuis et al.
patent: 5664064 (1997-09-01), Nakao et al.
patent: 5729670 (1998-03-01), Strumolo et al.
patent: 5768156 (1998-06-01), Tautges et al.
patent: 6219440 (2001-07-01), Schaff et al.
Gauthier Robert W.
Lall Prithvi C.
McGowan Michael J.
Pihulic Daniel T.
The United States of America as represented by the Secretary of
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