
From:  Konstantinos Poulios 
Subject:  Re: [EXT] Re: adding buoyancy forces to Biot poroelastic equations 
Date:  Sat, 20 Nov 2021 01:33:37 +0100 
md.add_linear_term(mim9, 'alpha*Grad(p).Test_ug*(rho_tissuerho_air*Heaviside(X(1))rho_water*Heaviside(X(1)))*Test_u(1)')
you are right, it should bemd.add_linear_term(mim9, '(alpha*Grad(p)g*(rho_tissuerho_air*Heaviside(X(1))rho_water*Heaviside(X(1))))*Test_u(1)')
BRKostasOn Fri, Nov 19, 2021 at 11:47 PM Lesage,Anne Cecile J <AJLesage@mdanderson.org> wrote:Dear Konstantinos
I understand the use of the Heaviside function but I do not get how you impose that the gravity is along the x axis
Thank you
Regards
AC
From: Konstantinos Poulios <logari81@googlemail.com>
Sent: Wednesday, November 17, 2021 1:37 AM
To: Lesage,Anne Cecile J <AJLesage@mdanderson.org>
Cc: getfemusers@nongnu.org
Subject: [EXT] Re: adding buoyancy forces to Biot poroelastic equations
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Dear AnneCecile,
The GetFEM model object does not differentiate between left and right side of an equation (there is no reason for splitting equations like this), all equations are assumed to be in the form R(...)=0. So just move all terms on the same side.
The equation you provide is in strong form, you need to convert it to the respective weak form and apply the necessary integration by parts to get rid of higher order derivatives as you showed in your add_linear_term _expression_. After all these steps you should have
md.add_linear_term(mim9, 'G*Grad(u):Grad(Test_u)+G/(12*nu)*Div(u)*Div(Test_u)+(alpha*Grad(p)g*(rho_tissuerho_air*Heaviside(X(1))rho_water*Heaviside(X(1)))).Test_u')
you can also choose to split the term in two lines
md.add_linear_term(mim9, 'G*Grad(u):Grad(Test_u)+G/(12*nu)*Div(u)*Div(Test_u)')
md.add_linear_term(mim9, '(alpha*Grad(p)g*(rho_tissuerho_air*Heaviside(X(1))rho_water*Heaviside(X(1)))).Test_u')
but there is no reason for not keeping everything just in one term.
BR
Kostas
On Tue, Nov 16, 2021 at 11:12 PM Lesage,Anne Cecile J <AJLesage@mdanderson.org> wrote:
Dear all
To implement the building of my fem matrix for the mechanical equilibrium, I presently write
md.add_linear_term(mim9, 'G*Grad(u):Grad(Test_u)+G/(12*nu)*Div(u)*Div(Test_u)+alpha*Grad(p).Test_u')
How can i add an additional buoyancy terms to the equation (see righthand side attached equation picture)?
gravity is vector g =  9180 N along the x axis for my mesh
rhot (density tissue is constant) but rhof = rho water for x<0 and rhof = rho air for x>0
Thank you
AnneCecile Lesage
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