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question on coefficient form PDE or general form PDE

Posted Sep 3, 2012, 3:57 a.m. EDT Version 4.2, Version 4.2a 6 Replies

Manish Keswani
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I would like to have the following terms in coefficient form PDE or general form PDE: c*del(density) and
c*del(deldotvelocity), where c is a constant and del is the del operator. Can anyone please suggest how to do this?

Thanks,
Regards,
Manish
6 Replies Last Post Sep 6, 2012, 5:22 a.m. EDT
Suresh Seetharam
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Posted: 1 decade ago Sep 3, 2012, 5:44 p.m. EDT
Hi,


From what I see both your terms are just vectors with x and y components. But what is your conservation equation?


Suresh
Hi, From what I see both your terms are just vectors with x and y components. But what is your conservation equation? Suresh
Manish Keswani
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Posted: 1 decade ago Sep 3, 2012, 7:12 p.m. EDT
Thanks Suresh for your response.

Example of an equation is attached.

Manish
Thanks Suresh for your response. Example of an equation is attached. Manish
Suresh Seetharam
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Posted: 1 decade ago Sep 5, 2012, 3:34 a.m. EDT
Hi,


Your equation is confusing to me as it is mixed with scalars and vectors. To be specific, your LHS does not have a dot product, is this correct? The first and last term of your RHS is also a vector. Whereas the remaining term in the RHS is a scalar.

I may be wrong but you can check.


Suresh
Hi, Your equation is confusing to me as it is mixed with scalars and vectors. To be specific, your LHS does not have a dot product, is this correct? The first and last term of your RHS is also a vector. Whereas the remaining term in the RHS is a scalar. I may be wrong but you can check. Suresh
Manish Keswani
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Posted: 1 decade ago Sep 5, 2012, 3:48 a.m. EDT
Hi Suresh,

In the equation, u is a scalar and F is a vector. Sorry I had a typo in the WORD file where I said u and F are vectors.

Manish
Hi Suresh, In the equation, u is a scalar and F is a vector. Sorry I had a typo in the WORD file where I said u and F are vectors. Manish
Suresh Seetharam
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Posted: 1 decade ago Sep 6, 2012, 4:01 a.m. EDT
Hi,


Sorry, I am still unable to help you. You still have mixed scalars and vectors in your conservation equation. You need to have scalars on both lhs and rhs of the equation. When you have u as a scalar, then grad(u) becomes a vector.

Perhaps you can send the original source of your equation.


Suresh
Hi, Sorry, I am still unable to help you. You still have mixed scalars and vectors in your conservation equation. You need to have scalars on both lhs and rhs of the equation. When you have u as a scalar, then grad(u) becomes a vector. Perhaps you can send the original source of your equation. Suresh
Jesus Lucio
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Posted: 1 decade ago Sep 6, 2012, 5:22 a.m. EDT
Hi, Manish,

First, I think you need another (constitutive? or conservative?) equation, or perhaps one of the variables (u?) is known a priori, or it's a function of the other.

Then I would expand your vector equation (let's suppose you apply it in Cartesian 3D) in 3 scalar equations, in the dependent variables (u?,) FX, FY and FZ.

Then define a coefficient form PDE (in 'Add Physics'/'Mathematics'/'PDE Interfaces') with those three or four dependent variables.

Finally you will have to identify the coefficients of COMSOL PDE equations as function of yours.

Bye.

Jesus.
Hi, Manish, First, I think you need another (constitutive? or conservative?) equation, or perhaps one of the variables (u?) is known a priori, or it's a function of the other. Then I would expand your vector equation (let's suppose you apply it in Cartesian 3D) in 3 scalar equations, in the dependent variables (u?,) FX, FY and FZ. Then define a coefficient form PDE (in 'Add Physics'/'Mathematics'/'PDE Interfaces') with those three or four dependent variables. Finally you will have to identify the coefficients of COMSOL PDE equations as function of yours. Bye. Jesus.

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