November 19, 2018 at 5:34 pmAugusto13Subscriber
the structure in question is a fixed-free beam. I apply a pulse called Ricker (figure 1) of displacement at the fixed end of a beam. The expected path of the impulse is that a wave reflection occurs, but this does not happen in the time history (figure 2).
Figure 1. Impulse
Time history in the middle of the beam:
Figure 2. Time history (ANSYS)
I do not understand why the impulse disappears quickly. I'm using the controlled damping (0,005) of ANSYS, I'm late, I got some help.
November 19, 2018 at 7:36 pmsk_cheahSubscriber
I'm guessing your beam is extremely stiff with the first natural frequency far exceeding the impulse frequency.
November 21, 2018 at 7:23 pmAugusto13Subscriber
Hello, I think your analysis is correct.
I changed the excitation frequency to 5 Hz (axial excitation), the natural frequency of the beam is 4.9 Hz (axial direction). So I got this:
Figure 1. Time history
but this seems to me unexpected. I would expect behavior similar to Figure 2 (ignore the captions and numbers):
Figure 2. expected behavior.
November 21, 2018 at 10:34 pmsk_cheahSubscriber
Exciting the system very near the natural frequency with low damping will cause the system to ring at that frequency for a long time. Like a bell...
You've now tried two cases. Try the last one: very high impulse frequency relative to natural frequencies.
November 23, 2018 at 11:23 pmAugusto13Subscriber
organizing the tests (natural frequency of the beam is 4.9 Hz):
1st test - excitation frequency of 79.36 Hz
2nd test - excitation frequency of 5 Hz
3rd test - excitation frequency of 0.666 Hz
in test 3 I got this:
when the excitation frequency is far from the natural frequency, the wave quickly dies. But when both frequencies are close, a sinusoidal behavior is observed. Would you have any other alternative? thank you!
November 24, 2018 at 2:41 pmpeteroznewmanSubscriber
I recommend you plot
relative tip displacement = tip displacement - base displacement
which might show that test 3 is mostly rigid body motion.
If I hold a ruler at one end and go up>down>up, the other end will follow and plotting relative tip displacement will show a flat line at zero.
November 24, 2018 at 4:21 pmsk_cheahSubscriber
For 4.9Hz beam, try excitation impulse of 500Hz. Be sure your simulation time step is about 5000Hz. Also, do a hand calculation on when you expect the wave to reflect back to make sure your simulation time is long enough to capture the reflection.
November 26, 2018 at 8:06 pmAugusto13Subscriber
with 500 Hz ansys did not provide solution (I think the period was very small). I applied an excitation of 300 Hz (= 3,3e-3 s natural period), the behavior was identical to test 1 (see figure 1). I always take care that the time step is much smaller (in this case 10x less).
Figure 1. Time history (excitation of 300 Hz).
In my calculations we have:
1. time for wave to enter beam = 3,3e-3 s;
2. time to pass in the middle of the beam = 0.028 s;
3. time to reflect and return to the middle of the beam = 0.079 s;
item 2 does not occur in the right time, there is something strange. In the time of 3,3e-3s the maximum peak of displacement occurs in every beam, as if the wave was so fast that all the points reach their peak at the same time.
In item 3 the wave does not appear (stagnation of the graph to zero).
the wave velocity is well calibrated. The beam has 16 m and I use a data input that give me a speed of 316 m/s.
November 26, 2018 at 8:12 pmAugusto13Subscriber
Peter, I think the result of ANSYS is already with relative displacement, when I add the time history in the fixed support, the result is always zero at all times. So I believe that at the tip (or at any point) the displacement is already relative, or I'm wrong ?
November 26, 2018 at 9:26 pmpeteroznewmanSubscriber
Augusto, yes, you are correct. I was thinking of another member's model that used a base displacement. Sorry for the mistake.
November 27, 2018 at 2:36 pmsk_cheahSubscriber
I ran a simple model and plotted midspan displacement showing the reflection is quite close to the initial wave similar to your Fig 2 in your first post:
Animation of the fixed-free beam: https://i.imgur.com/nFEjE4P.mp4
Attached is v19.0 archived file of the model.
November 29, 2018 at 1:08 amAugusto13Subscriber
thanks for listening =]
I use ANSYS 17. I'm going to download from 19 and try to check where I'm going wrong. When I can, I'll post my mistake here.
December 5, 2018 at 2:50 pmAugusto13Subscriber
I finally got the final model. I only obtained the desired model when modeling without discretizing the thickness of the beam (model 2d). It seems to me that this experiment is only satisfied with this consideration, I still do not know the physical reason for this difference.
thank Jason and Peter for the answers!
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