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Negative prescribed acceleration on a 3D geometry part

Posted 30 mars 2017, 09:54 UTC−4 Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 5.2 9 Replies

Remi Shah
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I am trying to model an impact test on a polymer cylinder by defining the deceleration of the impact tup as it compresses the part. It is a negative acceleration but using the 'prescribed acceleration' method I cannot define the acceleration as negative. Putting a minus only defines it as being in the downwards direction on the z axis.

Does anyone know any way I could model this deceleration?

Thank you,

Remi Shah
9 Replies Last Post 30 mars 2017, 11:06 UTC−4
Jeff Hiller COMSOL Employee

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Posted: 8 years ago 30 mars 2017, 09:58 UTC−4
A deceleration is an acceleration in the opposite direction of the velocity, isn't it?
Jeff
A deceleration is an acceleration in the opposite direction of the velocity, isn't it? Jeff
Remi Shah
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Posted: 8 years ago 30 mars 2017, 10:00 UTC−4
Yes it is
Yes it is
Remi Shah
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Posted: 8 years ago 30 mars 2017, 10:01 UTC−4
How would I define the direction of the acceleration in relation to the velocity?
How would I define the direction of the acceleration in relation to the velocity?
Jeff Hiller COMSOL Employee

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Posted: 8 years ago 30 mars 2017, 10:17 UTC−4
My point was that, if your velocity is in the positive z direction, an acceleration in the negative z direction is exactly what you want.
Jeff
My point was that, if your velocity is in the positive z direction, an acceleration in the negative z direction is exactly what you want. Jeff
Remi Shah
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Posted: 8 years ago 30 mars 2017, 10:23 UTC−4
Oh I see what you mean. My only concern is that if I was to make a prescribed velocity in the positive z direction acting on the top surface of the cylinder part (and then the negative z direction acceleration) why would the velocity have any effect on the part because it is acting on the top layer but moving up, away from the part.

Thank you
Oh I see what you mean. My only concern is that if I was to make a prescribed velocity in the positive z direction acting on the top surface of the cylinder part (and then the negative z direction acceleration) why would the velocity have any effect on the part because it is acting on the top layer but moving up, away from the part. Thank you
Jeff Hiller COMSOL Employee

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Posted: 8 years ago 30 mars 2017, 10:33 UTC−4
I am not following you.
Jeff
I am not following you. Jeff
Remi Shah
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Posted: 8 years ago 30 mars 2017, 10:44 UTC−4
Sorry its a bit hard to describe. Basically I'm thinking that if I make a prescribed velocity act on the top boundary of the part in the positive z direction it will simply act upwards from the top of the part and have no effect on the part. It is the same reason I can't just achieve a deceleration effect by making the prescribed acceleration act upwards from the top of the part because it will have no effect on the part.

However if I make the prescribed velocity act upwards from the bottom of the part and the prescribed acceleration act down from the top of the part do you think this will have my desired effect. As in the upwards velocity through the part and the downwards acceleration acting together will result in an overall deceleration on the part.

I hope this is a bit clearer.

Remi
Sorry its a bit hard to describe. Basically I'm thinking that if I make a prescribed velocity act on the top boundary of the part in the positive z direction it will simply act upwards from the top of the part and have no effect on the part. It is the same reason I can't just achieve a deceleration effect by making the prescribed acceleration act upwards from the top of the part because it will have no effect on the part. However if I make the prescribed velocity act upwards from the bottom of the part and the prescribed acceleration act down from the top of the part do you think this will have my desired effect. As in the upwards velocity through the part and the downwards acceleration acting together will result in an overall deceleration on the part. I hope this is a bit clearer. Remi
Jeff Hiller COMSOL Employee

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Posted: 8 years ago 30 mars 2017, 10:57 UTC−4
I don't understand why you think it would have no impact on the part. Your part is a solid, so if one of its boundary moves, the part has to "follow".
Jeff
I don't understand why you think it would have no impact on the part. Your part is a solid, so if one of its boundary moves, the part has to "follow". Jeff
Remi Shah
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Posted: 8 years ago 30 mars 2017, 11:06 UTC−4
OK yes you're correct. I forgot to mention the base of the part is a fixed constraint so a velocity on the top layer acting upwards will actually stretch the part. If this 'stretching' velocity and a compressing downwards acceleration are modelled together will it result in a eventual 'equilibrium' where the acceleration causes the velocity to diminish to zero at some point?
OK yes you're correct. I forgot to mention the base of the part is a fixed constraint so a velocity on the top layer acting upwards will actually stretch the part. If this 'stretching' velocity and a compressing downwards acceleration are modelled together will it result in a eventual 'equilibrium' where the acceleration causes the velocity to diminish to zero at some point?

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