- [Narrator] In this video I'm going to talk about
controlling transient analysis.
And that's in terms of the time step calculation,
both the number of time steps
and also the time step size.
We're gonna go through a logical process
to calculate both of those.
Now, when we prepare the time step calculation,
what we need to have is
a small enough time step for accuracy
and a long enough duration to ensure
damping can take effect.
We want to have confidence
that the peak responses are included,
we don't want to guess at the time step and duration.
If the duration is too long, it means we're wasting resource
if the time step is too small, we're wasting resource.
On the other hand, if the time step is too big,
it might not capture the frequencies we're after
and if the duration is too short,
it might truncate the signal,
so that we're not confident that it actually will have
a bigger overswing or a bigger response
past the end of the calculation point.
So here's the plate that's undergoing the transient analysis
and the first part of the time step calculation
is to look at the highest frequency of interest.
I'm showing the highest frequency of interest here,
I've assumed is mode six at 255 hertz
and it's second order bending in both directions
at the same time.
Though it may be that I've got to have a higher frequency
than this, in which case the time step is going to go down.
But, for the moment, this is my assumption.
Now mode six is 255 hertz,
if I take the reciprocal of the natural frequency,
that gives me the time period,
so that's .0039 seconds.
Now, that's not sufficient fidelity to capture that
highest frequency of interest.
I need at least ten points per wave
or ten points per cycle in my transient response,
so I divide by ten and I have
.00039 seconds as my time step.
And that's fairly straight forward,
so just remember highest frequency of interest,
reciprocal of that gives the time period,
divide by ten as a minimum
and that gives me my time step.
If possible, we'd like more points than that,
but that's kinda like the bare minimum there.
Now, the next step is to look at the
lowest frequency of interest,
which is almost certainly going to be the first mode.
And here we have 16 hertz, bending one,
so the second stage of the time step calculation
let's find that lowest frequency of interest,
in my particular case mode one, 16 hertz,
again turn it upside down,
the time period is one over 16,
which is .0625 seconds.
I want to do something different with this.
I want to have four cycles, or four swings,
to make sure I capture the highest response.
After four cycles, there should be enough
damping in the system so that the response is decaying,
so I'm assuming four cycles will capture the peak response.
Now, these four cycles are going to represent
the free motion duration.
So this is, after the loading has finished,
this is the free response,
where we'll see that decay occurring.
So, in the third stage of the calculation,
we look at the impulse load that's being applied
and the end of that load input time is .0102 seconds.
It's ten milliseconds and as I've shown in a previous video,
I put a slight slope in there,
which extends it to 10.2 milliseconds.
Now one thing I need to do here
is to check the fidelity of the load signal.
If we had a very high frequency content,
just say a 500 hertz frequency content driving,
then my time steps would have to capture that fidelity
and that would override the highest frequency of interest
from my structural response.
In this particular case, fidelity of the load signal is okay
I've got plenty of time steps throughout that impulse.
Now I want to calculate the total duration of the analysis.
This is the .25 seconds at the lowest frequency of interest
to make sure that I get four swings
or four cycles of that
and now I've added my load input total time.
So .25 seconds plus .102 seconds
gives me a duration of .262 seconds.
The number of steps is the duration divided by the time step
so it's .262 divided by .00039 we calculated earlier
and, rounded up, that gives me 670 time steps.
Now, it's a fairly straight forward and logical
approach we can take there,
I recommend you follow this process
for all transient analysis.
I often see people guessing at the time step or the duration
and the results are what I described
at the beginning of the video.
We either waste resource or we get inaccurate results.
I think the method is pretty straight forward and logical,
it's something I'd recommend you carry out.