# Discussion Forum

## solid.pw

Hi all,

I have some doubts regarding the variable solid.pw ; In particular, which is its meaning and which is the difference between solid.pw and solid.pm?

Thanks a lot, Maria

5 Replies Last Post Nov 22, 2019, 5:23 AM EST

Posted: 4 weeks ago
Updated: 4 weeks ago

This thread explains how to find the mathematical definition for a varaible whose name you know.

Best,

Jeff

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Jeff Hiller

Posted: 3 weeks ago

Hi Maria,

solid.pm is the 'pressure' as obtained by dividing the trace of the stress tensor by 3 (and reversing the sign). This variable always exists.

solid.pw is the pressure degree of freedom introduced when you select a mixed formulation (that is not only displacements as degreed of freedom).

Regards,
Henrik

Posted: 3 weeks ago

Hi Maria,

solid.pm is the 'pressure' as obtained by dividing the trace of the stress tensor by 3 (and reversing the sign). This variable always exists.

solid.pw is the pressure degree of freedom introduced when you select a mixed formulation (that is not only displacements as degreed of freedom).

Regards,
Henrik

Thank you so much for your support Henrik.

My problem is that I have a breast model within which I inserted a prosthesis. I want to analyze which is the pressure on the surface of the prosthesis which is in contact whit the biological tissue. I think that my interesting variabile is solid.pm. Is it correct?

Maria

Posted: 3 weeks ago

Hi Maria,

If I understand you right, you are looking for the pressure excerted by one object on the boundary of another. If this is the case, solid.pm is not what you are looking for. You want the normal component of the traction; that is

Here n is the normal to the boundary and is the full stress tensor.

Regards,
Henrik

Posted: 3 weeks ago

Hi Maria,

If I understand you right, you are looking for the pressure excerted by one object on the boundary of another. If this is the case, solid.pm is not what you are looking for. You want the normal component of the traction; that is

\sigma_n = \mathbf n^T \boldsymbol \sigma \mathbf n

Here n is the normal to the boundary and \boldsymbol \sigma is the full stress tensor.

Regards,
Henrik

Hi Henrik,

Thank you so much for your precious support. So, in order to obtain this normal component of the traction, should I insert that expression in Comsol? Where? I'm a beginner with this software, so I'm not sure to make correct assumptions.

Thank you!