# Lagrange multiplier applied to functions in different FE spaces

Say,

mesh Th = (solid + fluid) regions ; //This mesh will be updated
mesh Thf = trunc(Th, region==fluid); //This is the fluid mesh which will be updated consequently
mesh Ths = trunc(Th, region==solid); //Solid mesh, but not used

int[int] labs = [2, 4];
meshL ThL = extract(Th, label=labs); //This is the interface \Gamma between the fluid and solid, which will also be updated

mesh Ths0 = Ths;  // Solid equations are formulated on this reference domain

fespace Dh(Ths0, P2);
Dh d;
fespace Vh(Thf, P2);
Vh u;
fespace Lh(ThL, P1);
Lh lambda;


If we use the Lagrange multiplier to enforce the continuity of velocity at the interface, we introduce something like:

\int_\Gamma \lambda( \frac{{\bf d}-{\bf d}_n}{\Delta t} - {\bf u})

in the energy function. My question is: because d and u are in different FE spaces, how do we implement this in FreeFEM?

Thanks very much for your help in advance!

For simplicity, consider

-\Delta u_s = f \quad \text{in }\Omega_s, \\ -\Delta u_f = 0 \quad \text{in }\Omega_f, \\ u_s = u_f \quad \text{on }\Gamma, \\ u_s = 0 \quad \text{on }\partial\Omega_s\setminus\Gamma, \\ u_f = 0 \quad \text{on }\partial\Omega_s\setminus\Gamma,

where \Gamma is the interface between the two domains \Omega_s (solid) and \Omega_f (fluid).

Let V_s=\{ u \mid u=0 \text{ on } \partial\Omega_s\setminus\Gamma \}, V_f=\{ u \mid u=0 \text{ on } \partial\Omega_f\setminus\Gamma \}, and V_\Gamma = \{ \text{functions on }\Gamma \}. Then, the weak form is: find (u_s,u_f,\lambda)\in V_s\times V_f\times V_\Gamma such that

\int_{\Omega_s} \nabla u_s\cdot \nabla v_s dx + \int_{\Omega_s} \nabla u_f\cdot \nabla v_f dx + \int_\Gamma \lambda (v_s-v_f) ds + \int_\Gamma \varphi(u_s-u_f)ds = \int_{\Omega_s} fv_s dx

for all test functions (v_s,v_f,\varphi)\in V_s\times V_f\times V_\Gamma. To solve this, we need multiple FE spaces corresponding to V_s, V_f, and V_\Gamma. A possible way is to use composite finite element spaces. Here is an example:

load "msh3"

// solid
mesh Ths = square(50,50, [-1+x, -0.5+y]);
// fluid
mesh Thf = square(50,50, [x, -0.5+y]);

// interface
int[int] labs = [2];
meshL ThL = extract(Ths,label=labs);

fespace Vhs(Ths, P1);
Vhs us;
fespace Vhf(Thf, P1);
Vhf uf;
fespace Lh(ThL, P1);
Lh lambda;

{
// Test functions
Vhs vs;
Vhf vf;
Lh phi;

func f = 1; // source term

solve a(<[us],[uf],[lambda]>,<[vs],[vf],[phi]>) =
int2d(Ths)(dx(us)*dx(vs)+dy(us)*dy(vs))
+ int2d(Thf)(dx(uf)*dx(vf)+dy(uf)*dy(vf))
+ int1d(ThL)(lambda*(vs-vf))
+ int1d(ThL)(   phi*(us-uf))
- int2d(Ths)(f*vs)
+ on(1,3,4,us=0)
+ on(1,2,3,uf=0);
}


I hope this answer fits your question.

@k-matsushima-19 , Thank you for looking at my question.
I can understand what you presented.

However, what would happen if Thf and Ths are moving?

Case1, no problem if both weak forms are formulated on the current nesh.

Case2, there is a problem if one weak form (usually the solid) is defined on the initial mesh, and the other (usually the fluid) is defined on the current mesh.

@k-matsushima-19 Thank you for looking at my question.
I can understand what you presented.

However, what would happen if Thf and Ths are moving?

Case1, no problem if both weak forms are formulated on the current nesh.

Case2, there is a problem if one weak form (usually the solid) is defined on the initial mesh, and the other (usually the fluid) is defined on the current mesh.
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