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post #331 of 337 Old 09-06-2016, 12:27 PM
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Originally Posted by Bigus View Post
Not sure how experienced you are with FEM, from your question I assume you are at least to some degree. Proper constraints are of course hugely important to getting realistic results from a model.
30 yr at Lockheed, about 1/4 of my time.

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The use of symmetry is just the easiest way to get it right IMO. I suppose you could constrain the midpoint node of each face/edge against translation along its length, but them you've just utilized symmetry and the rest of the elements are redundant.
In my experience, if the model size is manageable it's a lot easier to apply a handful of restraints to a full model (w/o overconstraining and adding artificial stiffness) at the bottom corners.

Issues with symmetric models:

1) If you the FEM is based on a solid model, you have to add the cut features to create the symmetry model

2) The solid model and FEM coordinate systems may be different, potentially giving incorrect symmetric restraints

3) Decisions need to be made at the planes' intersections where their restraint requirements conflict

4) You end up with a lot of constraints at the plane of symmetry cluttering the display. You can turn them off, but that's another step

5) Not all modes are symmetric about planes of symmetry (though possibly all important ones are)

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you do have to constrain against rotation at planes of symmetry... inplane rotation though, right?
Yes, allowing only in-plane rotation is the same has restraining it out-of-plane

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I don't think out of plane rotation is possible in a 2D simulation inherently.
Right, I thought 3D element when you said 3D model

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Ad for my comment about 3d models, its simply an acknowledgement that 3d models often get complicated and lose planes of symmetry in trying to accurately model real geometry. That's all. If you still have a symmetric model, by all means, take advantage of it.
2d modeling of a speaker box will only let you analyze one side or feature (i.e. brace) at a time and not show interactions between them.

Noah

Last edited by noah katz; 09-06-2016 at 12:55 PM.
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post #332 of 337 Old 09-06-2016, 01:15 PM
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Well, yeah, your objections to symmetry are mostly about 3D which is why I initially said may not help with 3D models. My experience was that it was often more of a pain than it was worth, assuming the geometry was even symmetric to begin with.

Not sure why there should be conflicts at the intersection of planes of symmetry. This case doesn't have a node on that intersection so it doesn't matter, but if it did, y plane constrained x translation and z rotation, x plane constrained y translation and z rotation, intersection constrained both x, y translation and z rotation. Don't see where the conflict lies.

I may have my terminology mixed up or we may be talking about two different things regarding rotational constraints. I was speaking about that only in the case of a 2D model. No need to constrain against out of plane rotation as it isn't possible. But your comment made me realize I had initially oversimplified a bit... the plane of symmetry (at least for this case) has to be constrained for in plane rotation and not just translation as I had said. Otherwise you have simulated a pinned condition, not fixed. The center of a plate or beam in bending is essentially a fixed constraint, no rotation allowed as each end opposes the bending moment of the other.

Or I could be way off my rocker. Its only been about a decade since I did a career uturn and stopped looking at computers in a dark room... so I could look at computers in a dark room. Lol.
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post #333 of 337 Old 09-06-2016, 03:19 PM
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Nice to see some proper discussion.

So what do you think about if I added some feet to the whole enclosure model and simulated the bottom of the feet as rigid constraints in all axis and rotation? The top of the enclosure would then be free to move as it would with absolutely no restraints (maybe gravity?). I think that's the way to get accurate resonance data at least.

Do you have any opinions on whether or not I should apply the force as 1000 pascals on one side. Or atmospheric pressure on one side and then atmospheric pressure plus 1000 pascal on the other side?

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #334 of 337 Old 09-06-2016, 03:22 PM
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Originally Posted by Augerhandle View Post
Meanwhile, while someone was and is still working on simulations, 2,304 subwoofer boxes have been built with cross-bracing and are thumping away in HTs all across the world.
They probably sold 2304 sets of cable risers as well though Argument from popularity is recognized.

On a serious note, they probably now have to spend a few hours on a treadmill to burn off the calories they didn't waste on needlessly complicated ultra-low noise enclosure designs.

cms, aka driver diaphragm suspension mechanical compliance: 0.000065 meter/Newton or in standard form 6.5e-05 m/N. (smaller number is better)
rms, aka driver diaphragm suspension mechanical resistance: 6.41 Newton.sec/meter. (higher number is better)
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post #335 of 337 Old 09-06-2016, 10:45 PM
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They probably sold 2304 sets of cable risers as well though Argument from popularity is recognized.
Nope, it's called sarcasm
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On a serious note, they probably now have to spend a few hours on a treadmill to burn off the calories they didn't waste on needlessly complicated ultra-low noise enclosure designs.

"The wise understand by themselves; fools follow the reports of others"-Tibetan Proverb
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post #336 of 337 Old 09-06-2016, 10:49 PM
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Originally Posted by Bigus View Post
Not sure why there should be conflicts at the intersection of planes of symmetry.
For example, in the case of needing to allow in-plane rotation, in the perpendicular plane it's out-of-plane, so do you allow it or restrain it?

Not that hard to decide, but you have to think about it.

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Or I could be way off my rocker.
Nope, we were just not quite getting what the other was saying

Noah
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post #337 of 337 Old 09-06-2016, 11:04 PM
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So what do you think about if I added some feet to the whole enclosure model and simulated the bottom of the feet as rigid constraints in all axis and rotation? The top of the enclosure would then be free to move as it would with absolutely no restraints (maybe gravity?).
That'll work if the legs aren't too stiff; say 1/4" dia. and 4" long

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Do you have any opinions on whether or not I should apply the force as 1000 pascals on one side. Or atmospheric pressure on one side and then atmospheric pressure plus 1000 pascal on the other side?
doesn't matter; the equal/opposite atmospheres will cancel

Noah
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