Difference in modal optimization between compliance index and frequency?
Hello. I noticed that Inspire will use minimize COMB response for modal optimization where maximize frequency (edit: stiffness) is selected as the objective. I see that COMB is defined as
Is my understanding here correct?
- In a modal analysis, compliance will be zero since there are no forces applied.
- The natural frequencies are proportional to the square root of its eigenvalues.
- So with zero compliance, equal weighting, what we are essentially doing is maximizing the square of the sum of the frequencies of all modes requested. With 6 modes that would be:
- objective = max(f1^2+f2^2+f3^2+f4^2+f5^2+f6^2)
- This approach may produce different results to that of maximizing frequency of first mode with FREQ resposne.
Thanks.
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correct.
Compliance is only valid for static loadcases, not modal.
For modal analysis you could create multiple frequency responses, one for each mode, and then create an equation combining/summing them, manually.
COMB actually creates automatically this equation and all the necessary sum.
COMB is useful when both static and modal cases are present and you wnat the optimization to work on both the static compliance and frequencies combined.
Great, thanks for explaining!
Altair Employeecorrect.
Compliance is only valid for static loadcases, not modal.
For modal analysis you could create multiple frequency responses, one for each mode, and then create an equation combining/summing them, manually.
COMB actually creates automatically this equation and all the necessary sum.
COMB is useful when both static and modal cases are present and you wnat the optimization to work on both the static compliance and frequencies combined.
correct.
Compliance is only valid for static loadcases, not modal.
For modal analysis you could create multiple frequency responses, one for each mode, and then create an equation combining/summing them, manually.
COMB actually creates automatically this equation and all the necessary sum.
COMB is useful when both static and modal cases are present and you wnat the optimization to work on both the static compliance and frequencies combined.