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frequency response analysis with prestressed normal modes
- Thread starter david1982
- Start date
BlasMolero
Mechanical
Hello!,
Specifying the load in a static subcase and including the STATSUB case control in the dynamic subcase, this will cause the differential stiffness to be
computed and included in the normal mode solution.
Best regards,
Blas.
~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director
IBERISA
48011 BILBAO (SPAIN)
WEB: Blog de FEMAP & NX Nastran:
Specifying the load in a static subcase and including the STATSUB case control in the dynamic subcase, this will cause the differential stiffness to be
computed and included in the normal mode solution.
Best regards,
Blas.
~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director
IBERISA
48011 BILBAO (SPAIN)
WEB: Blog de FEMAP & NX Nastran:
- Thread starter
- #3
Thanks blas!, but i would like to use the prestressed normal modes in a frequency response analysis.
I've got prestressed normal modes in the normal mode solution but i don't know how to introduce this normal
normal modes in a frequency response.
Thanks for all!
Regards.
I've got prestressed normal modes in the normal mode solution but i don't know how to introduce this normal
normal modes in a frequency response.
Thanks for all!
Regards.
BlasMolero
Mechanical
Dear David
Modal Frequency Response Analysis (SOL 111) uses the mode shapes of the structure to uncouple the equations of motion (when no damping or only modal damping is used). It uses the mode shapes of a structure, then modal frequency response analysis is a natural extension of normal modes analysis (SOL103), then accounting for prestressing in the normal mode analysis this is the way to consider this effect in Modal frequency response analysis.
I suggest to run first the prestressed normal modes analysis (SOL103) and use the option "save database for restart", you need to extract the normal modes to enable NX Nastran to select solution frequencies automatically in order to create a solution frequency function. Next setup the Modal Frequency Response Analysis (SOL111) to use the option "restart previous analysis" and select the NASTRAN Database (*.master) saved previously. You will take advantage of the analysis restarts option to transition from normal modes analysis to frequency response analysis.
Solution Frequencies can be defined in two separate ways. One way is to create a function which gradually raises the frequency in defined increments in a given frequency range. If you have not extracted the normal modes in a previous analysis, this is a way to create a frequency table to excite the structure. The disadvantage of this method is that it will give you a large number of data points to review in general and not concentrate the results around the natural modes, where the response should be the highest.
If you run a normal modes analysis by separate, and due that NX NASTRAN has the Analysis Restart capability, then you can use a different and more effective method to create the frequency table inside FEMAP. Utilizing the known results of the normal modes analysis you ran first, FEMAP can create a much more accurate frequency table for Modal Frequency Response, automatically, using standard values like 5 points per Existing Mode and +/-10% Frequency Band Spread.
Best regards,
Blas.
~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director
IBERISA
48011 BILBAO (SPAIN)
WEB: Blog de FEMAP & NX Nastran:
Modal Frequency Response Analysis (SOL 111) uses the mode shapes of the structure to uncouple the equations of motion (when no damping or only modal damping is used). It uses the mode shapes of a structure, then modal frequency response analysis is a natural extension of normal modes analysis (SOL103), then accounting for prestressing in the normal mode analysis this is the way to consider this effect in Modal frequency response analysis.
I suggest to run first the prestressed normal modes analysis (SOL103) and use the option "save database for restart", you need to extract the normal modes to enable NX Nastran to select solution frequencies automatically in order to create a solution frequency function. Next setup the Modal Frequency Response Analysis (SOL111) to use the option "restart previous analysis" and select the NASTRAN Database (*.master) saved previously. You will take advantage of the analysis restarts option to transition from normal modes analysis to frequency response analysis.
Solution Frequencies can be defined in two separate ways. One way is to create a function which gradually raises the frequency in defined increments in a given frequency range. If you have not extracted the normal modes in a previous analysis, this is a way to create a frequency table to excite the structure. The disadvantage of this method is that it will give you a large number of data points to review in general and not concentrate the results around the natural modes, where the response should be the highest.
If you run a normal modes analysis by separate, and due that NX NASTRAN has the Analysis Restart capability, then you can use a different and more effective method to create the frequency table inside FEMAP. Utilizing the known results of the normal modes analysis you ran first, FEMAP can create a much more accurate frequency table for Modal Frequency Response, automatically, using standard values like 5 points per Existing Mode and +/-10% Frequency Band Spread.
Best regards,
Blas.
~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director
IBERISA
48011 BILBAO (SPAIN)
WEB: Blog de FEMAP & NX Nastran:
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