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Evaluate total force acting on a body

Posted Nov 8, 2011, 10:26 a.m. EST Fluid & Heat, Results & Visualization Version 4.2 7 Replies

Antonio Stanziola
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Hi everybody!
I made a simulation of a rigid 3d body moving inside a fluid at constant velocity.
I used the Stationary Laminar Flow study.
Everything works fine, but now how i can find the total force acting on the body because of the relative motion between body and fluid?

Thanks in advance,
Antonio
7 Replies Last Post Dec 7, 2012, 4:15 p.m. EST
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Nov 8, 2011, 11:28 p.m. EST
You can integrate the stress over the surface of the body or sum the reaction forces (search for reacf in the COMSOL documentation).

Nagi Elabbasi
Veryst Engineering
You can integrate the stress over the surface of the body or sum the reaction forces (search for reacf in the COMSOL documentation). Nagi Elabbasi Veryst Engineering
Antonio Stanziola
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Posted: 1 decade ago Nov 9, 2011, 8:17 a.m. EST
Thank you for your answer :)

I have just a last question: in this case, why the integration of the total stress along a coordynate (say, z) and the integration of the viscous stress along the same coordinate, give different results?
Thank you for your answer :) I have just a last question: in this case, why the integration of the total stress along a coordynate (say, z) and the integration of the viscous stress along the same coordinate, give different results?
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Nov 9, 2011, 9:12 a.m. EST
The viscous stress is only part of the total stress. It does not include the pressure term.
The viscous stress is only part of the total stress. It does not include the pressure term.
Antonio Stanziola
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Posted: 1 decade ago Nov 9, 2011, 10:26 a.m. EST
But, if i'm not wrong, the integral of the total force due to a pressure on a body, along a certain axis, should be 0.

It can be different from 0 only if the pressure force is considered without his sign, is the total stress calculated in this way?

Thanks in advance!
But, if i'm not wrong, the integral of the total force due to a pressure on a body, along a certain axis, should be 0. It can be different from 0 only if the pressure force is considered without his sign, is the total stress calculated in this way? Thanks in advance!
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Nov 9, 2011, 10:34 a.m. EST
The integral of the pressure term is not zero because the pressure field is not uniform.
The integral of the pressure term is not zero because the pressure field is not uniform.
Antonio Stanziola
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Posted: 1 decade ago Nov 9, 2011, 10:42 a.m. EST
So you're talking about the effects of form drag? I thought it was neglectable because the Reynold's number in every point of the fluid is very low (the maximum is of about 18), I guess I have to study the problem better :)

Thanks for your help!
So you're talking about the effects of form drag? I thought it was neglectable because the Reynold's number in every point of the fluid is very low (the maximum is of about 18), I guess I have to study the problem better :) Thanks for your help!
Lina Castano
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Posted: 1 decade ago Dec 7, 2012, 4:15 p.m. EST

Hi,

I just ran into this chain while looking for rigid body displacement of an object. How and where did you define the velocity of the object? and are you just using the laminar flow module or are you also using the fsi module? I have seen other chains where there is an angular velocity assigned to a rigid connector. However, all I need is a translational velocity but I do not know how to implement it since there seems to be no analogue for the rigid connector for translational displacements. I also tried to make the geometry time dependent but that doesn't work either. There is also a 'prescribed velocity' feature for solid mechanics but not sure if this is how you assigned the velocity to your object.

If you can shed some light to this problem, which you have resolved, I would greatly appreciate it,

Thanks very much,

Lina
Hi, I just ran into this chain while looking for rigid body displacement of an object. How and where did you define the velocity of the object? and are you just using the laminar flow module or are you also using the fsi module? I have seen other chains where there is an angular velocity assigned to a rigid connector. However, all I need is a translational velocity but I do not know how to implement it since there seems to be no analogue for the rigid connector for translational displacements. I also tried to make the geometry time dependent but that doesn't work either. There is also a 'prescribed velocity' feature for solid mechanics but not sure if this is how you assigned the velocity to your object. If you can shed some light to this problem, which you have resolved, I would greatly appreciate it, Thanks very much, Lina

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