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Hi

are you sure you are not mixing up constant mass and constant density ?.
In COMSOL we define the mass of a domain (or several as the sum over the individual domains) as the integration of the density over the MATERIAL frame (undeformed) volume.

If you integrate over the SPATIAL frame in structural, it is deformed, hence it's integrated volume might have changed so if you integrate the same density "solid.rho" and not "1" (which would give you only the volume) you will get a different total mass, An apparent violation of the mass conservation.

So it's all a question of reference

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Good luck
Ivar

Hi Ivar,

Thanks for your reply. Let me give you a piece of information to better understand my question. This is the same problem which i asked a question previously.

I am working on a chemical reaction problem where the reactant is converting into product. The reactant subdomain is dissolving/disappearing where as my product subdomain is forming/appearing over a period of time. The density of the reactant is constant and the product density is a function of space.

so In this case, i want to calculate both the mass of the reactant and mass of the product at any intermediate time to check the mass balance.

at any time 't': rc-core radius, rp-product radius (those are tracked correctly using intergration boundary coupling variables)

mass of the reactant is int(4*Pi*r^2*density of reactant) with limits 0 and rc (note that rc is varying w.r.t 't' and constant at any given time). I am not using those limits in comsol since it should take automatically from my reactant subdomain.

mass of the product is int(4*Pi*r^2*density of product) with limits rc and rp (note: both rc and rp are varying w.r.t 't' and constant at any given time, also density is varying w.r.t 'r and t'.

I am getting the mass balance wrong and i don't believe that it is doing the right thing. So, i just used a simple way to check the procedure by doing the following:

To check wheather comsol is giving the correct answer:

i took subdomain integration over reactant and product subdomain seprately

if i use 1 instead of the above mentioned expressions, that means it is nothing but int(1) with limits 0 to rc so the value is rc and i got that value. for the product i got rp-rc.

if i use r, that means it is int(r) with limits 0 and rc for reactant subdomain and int(r) with product subdomain. the answers should be rc^2 and rp^2-rc^2, but i am not getting those values.


Sorry about the long description...

Need some comments.........

Thank you,

Manohar