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How to impose an initial displacement
Posted Jun 3, 2010, 4:45 a.m. EDT 3 Replies
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Hi,
I need to impose a initial displacement to a violin string. So I use:
Physics -> Equation system -> Point setting -> init, on the point I need.
But it seems that then the displacement of my point is put to zero for all the rest of the modelling.
Did I do it wrong? What is the best way to do it?
Thanks a lot!
I need to impose a initial displacement to a violin string. So I use:
Physics -> Equation system -> Point setting -> init, on the point I need.
But it seems that then the displacement of my point is put to zero for all the rest of the modelling.
Did I do it wrong? What is the best way to do it?
Thanks a lot!
3 Replies Last Post Jun 4, 2010, 8:09 a.m. EDT
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Posted:
1 decade ago
Hi
I wounder if you (or I ?) are not confusing "initial displacement" and "forced displacement" hereafter, I leave it to you to sort it out:
for me
An initial displacement = initial condition (tab) = position at time = 0, but it will not necesarily stay there as time go on.
A forced displacement = a point/region where we enforce a displacement: u=cte, v=cte w=cte (or just a few of these) and this remains like that however time is passing.
The exeption is if u=f(t) (and or v,w,p...) a function of time and not a constant.
If you are using eigenfrequency analysis you will not see the "time effect" as we are in the modal domain, and the "inital condition) will dissapear, but not necesarily the "forced diplacement"
If your cord is modelled as a line then by defining a point somewhere and you set it fixed u=v=0 (I assum in 2D, or Rx,Ry naming depends on module used) then you really fix it there.
For you I assume that you should try to use once the ALE and have a "moving " fixed point to simulate the change in frequency as the string is made shorter.
But do it in steps, setting up ALE or moving mesh is slightly trick the first times.
Have fun Comsoling
Ivar
I wounder if you (or I ?) are not confusing "initial displacement" and "forced displacement" hereafter, I leave it to you to sort it out:
for me
An initial displacement = initial condition (tab) = position at time = 0, but it will not necesarily stay there as time go on.
A forced displacement = a point/region where we enforce a displacement: u=cte, v=cte w=cte (or just a few of these) and this remains like that however time is passing.
The exeption is if u=f(t) (and or v,w,p...) a function of time and not a constant.
If you are using eigenfrequency analysis you will not see the "time effect" as we are in the modal domain, and the "inital condition) will dissapear, but not necesarily the "forced diplacement"
If your cord is modelled as a line then by defining a point somewhere and you set it fixed u=v=0 (I assum in 2D, or Rx,Ry naming depends on module used) then you really fix it there.
For you I assume that you should try to use once the ALE and have a "moving " fixed point to simulate the change in frequency as the string is made shorter.
But do it in steps, setting up ALE or moving mesh is slightly trick the first times.
Have fun Comsoling
Ivar
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Posted:
1 decade ago
Hi,
I just wants to see my cord moving after impose it a initial displacement. I know that an initial displacement is juts at time=0, but is it not what I am observing on my model.
It seems that the cord displacement is impose to zero whatever the initial displacement I choose first.
(For that I use the transient analysis, I'am not concerning by the eigenfrequency anymore, I prefer using a FFT then.)
So after that conclusion I chose to impose a load for a very short step.
What is the best solution?
Thanks a lot!
I just wants to see my cord moving after impose it a initial displacement. I know that an initial displacement is juts at time=0, but is it not what I am observing on my model.
It seems that the cord displacement is impose to zero whatever the initial displacement I choose first.
(For that I use the transient analysis, I'am not concerning by the eigenfrequency anymore, I prefer using a FFT then.)
So after that conclusion I chose to impose a load for a very short step.
What is the best solution?
Thanks a lot!
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Posted:
1 decade ago
Hi
I have some difficulties to follow your explanations, but if you impose a Boundary Condition (BC) its there, and it is not always easy to change with time (but still possible if you check your equations), or you impose a force, or a force(t), but then the position is string reaction force dependent.
Have fun Comsoling
Ivar
I have some difficulties to follow your explanations, but if you impose a Boundary Condition (BC) its there, and it is not always easy to change with time (but still possible if you check your equations), or you impose a force, or a force(t), but then the position is string reaction force dependent.
Have fun Comsoling
Ivar