I have a questiong regarding Uniform Base Excitation. I have been given the "Y Displacement data" (Displacement Vs Hz data) to be applied on an enclosure fixed at 3 ends (i.e. its base) to perform the Linear Dynamic analysis and study the response. What value should I specify under my Displacements (with respect to directions in Y direction) after I add the given curve?
I usually just use "1" to specify direction, then define the actual value with my time variation curve, but am not 100% sure that is the best way to do it. Maybe someone more experienced can verify this.
Thanks for your reply. I thought that the value specified under direction is the maximum load value and the time curve specified represents the fraction of the load and 1 being the max. Again, I am not sure and I need to confirm this.
It turns out that it doesn't matter which way you do it. I figured that because the units for the frequency dependent curve were Hz vs.dimensionless that this might be the case, but I just ran a study to verify it. It seems that the initial value put in the direction field is just multiplied by the value in the PSD curve. I ran one study (Test1) with a value of 1 mm^2/Hz, and a PSD curve from 5-500 Hz with break points as follows:
5 Hz, 1e-5
100 Hz, 2e-5
200 Hz, 3e-5
300 Hz, 3e-5
400 Hz, 2e-5
500 Hz, 2e-5
The second study that I ran (Test 2) used a value of 1e-4 mm^2/Hz, and a PSD curve as follows:
5 Hz, 0.1
100 Hz, 0.2
200 Hz, 0.3
300 Hz, 0.3
400 Hz, 0.2
500 Hz, 0.2
The results are identical -- I've attached the .jpeg for each one. Hope this clears things up.
That clears my doubt. So it does consider the multiplier with the value under directions. I really appreciate your help.
Yes, I too have found that it acts as multiplier. It's useful if you have Maximum Predicted Environment (MPE) and need to, say, jump up 6 dB on the PSD by multiplying everything by 2.
I have a similar question regarding the uniform base excitation. I have designed a satellite which will be fixed in the longitudinal direction inside the launch vehicle. I was wondering if the constraint / fixed geometry must be both in the same place or not. In the other words, My fixed geometry and base excitation geometry are the same (for example on the bottom side based on the figure I have posted).
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