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# Composite finite element spaces NEW!

As of FreeFEM v4.13, we introduce the notion of composite finite element spaces in the language in order to generalize the definition of a variational form for coupled problems. This means that coupled variables are allowed to be defined on different meshes or even mesh types. This can be useful for example for domain coupling, surface-volume coupling, FEM-BEM coupling, etc.

## Limitations of the current syntax

In Freefem, you can define scalar and vector finite element spaces:

1fespace Vh(Th, P1); // scalar space
2fespace Uh(Th, [P2,P2,P1], periodic=[[2,y],[4,y]]); // vector space


The current definition of FE spaces and variational problems with multiple variables or components is subject to two limitations:

• When defining periodic vector spaces, all components are considered periodic (see Uh above)

• All variables/components have to be defined on the same mesh

The introduction of composite spaces aims at lifting these limitations and facilitates the definition of coupled problems, allowing to easily define and solve more general problems mixing different meshes or mesh types.

## Definition of composite spaces

A composite FE space is defined as a cartesian product of two or more “standard” FE spaces:

(49)$X_{h} = U_{h}^{1}(T_{h}^{1}, {FE}^{1}) \times U_{h}^{2} (T_{h}^{2}, {FE}^{2}) \times \cdots \times U_{h}^{n} (T_{h}^{n}, {FE}^{n}),$

where $$T_{h}^{i}$$ is the mesh of the $$i\text{th}$$ FE space and $${FE}^{i}$$ is the type of finite element:

• scalar FE element P1, P2 $$\dots$$

• vector FE element [P1,P1], RT0 $$\dots$$

• FE element with periodic boundary P1,periodic=[[2,y],[4,y]]

Each FE space $$U_{h}^{i}$$ can be defined on a different mesh $$T_{h}^{i}$$ ; meshes $$\left( T_{h}^{i} \right)_{i=1,n}$$ can even be of different types (mesh, mesh3, meshL, meshS).

### First examples

Defining a composite space as a product of FE spaces can be done by writing the product directly with * or by using the angular bracket syntax < and >. For example:

• vector FE space with a periodic component in one direction:

1fespace Uh1(Th,P2);
2fespace Uh2(Th,P2,periodic=[[1,x],[3,x]]);
3fespace Ph(Th,P1);
4
5// definition of the composite space Xh = Uh1 X Uh2 X Ph
6// any of the three following lines works:
7fespace Xh(<Uh1,Uh2,Ph>);
8fespace Xh(Uh1*Uh2*Ph);
9fespace Xh=Uh1*Uh2*Ph;

• composite space with the first component defined on a triangular 2D mesh Th and the second component defined on a curve mesh ThL (here ThL is the boundary of Th). This can be useful for example for volume-surface coupling or FEM-BEM coupling:

 1load "msh3"
2mesh Th = square(50,50); // Th is a 2d volume mesh
3meshL ThL = extract(Th); // ThL is a 1d curve mesh
4fespace Uh(Th,P1);
5fespace UhL(ThL,P1);
6
7// definition of the composite space Xh = Uh X UhL
8// any of the three following lines works:
9fespace Xh(<Uh,UhL>);
10fespace Xh(Uh*UhL);
11fespace Xh=Uh*UhL;


## Stokes with P2-iso-P1 elements

Let us illustrate the use of composite spaces with the following 2D Stokes problem:

(50)$\begin{split}\left \{ \begin{array}{rcl} -\Delta \mathbf{u} + \nabla p &=& \mathbf{f} \quad \text{in} \quad \Omega, \\ \nabla \cdot \mathbf{u} &=& 0 \quad \text{in} \quad \Omega, \\ \mathbf{u} &=& g \quad \text{on} \quad \Gamma, \end{array} \right .\end{split}$

where $$\mathbf{u}=(u_1,u_2)$$ is the fluid velocity and $$p$$ the pressure.

In order to define the variational form, we multiply the first equation (resp. the second equation) of (50) by a test function $$\mathbf{v}$$ (resp. $$q$$) and integrate on $$\Omega$$:

\begin{split}\forall (\mathbf{v}, \: q ), \quad \left \{ \begin{aligned} \int_{\Omega} \nabla \mathbf{u} . \nabla \mathbf{v} \: d\boldsymbol{x} + \int_{\Omega} \nabla p \cdot \mathbf{v} d\boldsymbol{x} &= \int_{\Omega} \mathbf{f} \cdot \mathbf{v} \: d\boldsymbol{x}, \\ - \int_{\Omega} div(\mathbf{u}) q d\boldsymbol{x} &= 0. \end{aligned} \right .\end{split}

Using Green’s formula in the second integral, we obtain:

(51)\begin{split}\forall (\mathbf{v}, \: q ), \left \{ \begin{aligned} \int_{\Omega} \nabla \mathbf{u} . \nabla \mathbf{v} \: d\boldsymbol{x} - \int_{\Omega} p div(\mathbf{v}) d\boldsymbol{x} &= \int_{\Omega} \mathbf{f} \cdot \mathbf{v} \: d\boldsymbol{x}, \\ - \int_{\Omega} div(\mathbf{u}) q d\boldsymbol{x} \color{blue}{-\int_{\Omega} \epsilon p q d\boldsymbol{x}} &= 0. \end{aligned} \right .\end{split}

The stabilization term in blue is added to the variational form to fix the constant for the pressure (we take for example $$\epsilon=10^{-10}$$).

We choose the P2-iso-P1 finite element for the velocity and pressure. This element requires two different meshes: a coarser mesh for pressure and a finer mesh for velocity obtained by splitting each triangle of the pressure mesh into four triangles.

### using solve or problem

In order to solve this problem, we can make use of the new composite spaces as in the script below:

 1int nn = 30; // number of edges in each direction
2mesh ThP = square(nn,nn,[2*pi*x,2*pi*y],flags=3); // Pressure mesh
3mesh ThU = trunc(ThP,1,split=2);  // Velocity mesh
4
5fespace Uh(ThU,[P1,P1]); // Velocity space
6fespace Ph(ThP,P1);      // Pressure space
7
10macro div(u1,u2) (dx(u1)+dy(u2)) //
11
12// definition of the boundary condition
13func g1 = sin(x)*cos(y);
14func g2 = -cos(x)*sin(y);
15
16// definition of the right-hand side
17func f1 = 0;
18func f2 = -4*cos(x)*sin(y);
19
20Uh [u1,u2],[v1,v2];
21Ph p,q;
22
24+ int2d(ThU)(-div(u1,u2)*q -div(v1,v2)*p)
25+ int2d(ThP)(-1e-10*p*q)
26- int2d(ThU)([f1,f2]'*[v1,v2])
27+ on(1,2,3,4, u1=g1, u2=g2);
28
29plot([u1,u2], cmm="u");
30plot(p, cmm="p");


You can also find this example in the FreeFEM distribution here.

Note that with the problem or solve syntax, the composite nature of the FE space has to be indicated directly in the problem/solve instruction using angular brackets < and >:

1solve Stokes (<[u1,u2],[p]>, <[v1,v2],[q]>) = ...


The explicit definition of the composite fespace Xh=Uh*Ph is optional.

Remark that if you omit to indicate the composite nature of the problem with <, > and write

1solve Stokes ([u1,u2,p], [v1,v2,q]) = ...


FreeFEM falls back to the “standard” evaluation of the variational form and outputs the following error message: Exec error : all the finite element spaces must be defined on the same mesh in solve.

### using varf and matrix

Composite FE spaces can also be used with the varf/matrix syntax. We can replace the solve instruction

1solve Stokes (<[u1,u2],[p]>, <[v1,v2],[q]>) = int2d(ThU)((Grad(u1,u2):Grad(v1,v2)))
2+ int2d(ThU)(-div(u1,u2)*q -div(v1,v2)*p)
3+ int2d(ThP)(-1e-10*p*q)
4- int2d(ThU)([f1,f2]'*[v1,v2])
5+ on(1,2,3,4, u1=g1, u2=g2);


by

 1fespace Xh=Uh*Ph;
2
4+ int2d(ThU)(-div(u1,u2)*q -div(v1,v2)*p)
5+ int2d(ThP)(-1e-10*p*q)
6+ int2d(ThU)([f1,f2]'*[v1,v2])
7+ on(1,2,3,4, u1=g1, u2=g2);
8
9matrix M = Stokes(Xh,Xh);
10real[int] b = Stokes(0,Xh);
11real[int] sol = M^-1*b;
12
13[u1[],p[]] = sol; // dispatch the solution


Note

The sign of the linear form is flipped because with the varf/matrix syntax we are solving $$M x = b$$ whereas with problem or solve we are solving $$M x - b = 0$$.

Note that in this case the explicit definition of the composite fespace Xh=Uh*Ph is mandatory, as it is used in the instantiation of the variational form in Stokes(Xh,Xh). However, this time the <, > notation in the varf is optional: we can also write

1varf Stokes ([u1,u2,p], [v1,v2,q]) = ...


In the last instruction, the solution vector obtained when solving the linear system is dispatched to the velocity and pressure FE functions:

1[u1[],p[]] = sol; // dispatch the solution


Under the hood, the matrix and right-hand side of a composite problem are assembled in a contiguous way with respect to the different components. This means that the linear system we are solving has the expected structure

$\begin{split}\left( \begin{array}{cc} \mathbf{A}&-B^T\\ -B&-\epsilon I \end{array} \right) \left( \begin{array}{cc} \mathbf{u}\\ p \end{array} \right) = \left( \begin{array}{cc} \mathbf{f}\\ 0 \end{array} \right)\end{split}$

## Parallel assembly and solution

With the definition of composite problems, we also introduce a way to parallelize the assembly and solution of the linear system in a transparent way. There are two ways to make use of this functionality: with the parallel direct solver MUMPS and with PETSc.

### with MUMPS

With minimal changes in the script, we can parallelize the assembly of the linear system on multiple MPI processes and solve it in parallel using the MUMPS solver.

First, we need to load the MUMPS and bem plugins (this functionality is implemented in the bem plugin for now but this will be changed in future releases):

1load "MUMPS"


Now we just need to specify that we want to solve the problem with a sparse direct solver (MUMPS), with the linear system being distributed over all MPI processes (with master=-1):

1solve Stokes (<[u1,u2],[p]>, <[v1,v2],[q]>, solver=sparsesolver, master=-1) = ...


It works the same way when using the varf/matrix syntax:

1fespace Xh=Uh*Ph;
2
3varf Stokes (<[u1,u2],[p]>, <[v1,v2],[q]>) = ...
4
5matrix M = Stokes(Xh,Xh, solver=sparsesolver, master=-1);
6real[int] b = Stokes(0,Xh);
7real[int] sol = M^-1*b;
8
9[u1[],p[]] = sol; // dispatch the solution


At the end of the solution phase, all MPI processes hold the global solution of the problem in (u1,u2,p).

In order to use multiple MPI processes, we run the script in parallel on e.g. 4 computing cores with

1ff-mpirun -np 4 script.edp -wg


### with PETSc

Alternatively, we can use the PETSc library (see PETSc and SLEPc) to solve the linear system. In contrast to the usual fully distributed PETSc framework (see PETSc examples), with the composite syntax the parallel data distribution is hidden to the user and the solution output is given in the global space. This is part of the continuous efforts to hide the difficulties associated with the parallelization of a user script, making it more transparent to the user.

In order to use PETSc as a solver, we simply load the PETSc plugin and define a PETSc Mat instead of a matrix for the linear system matrix:

 1load "PETSc"
3
4fespace Xh=Uh*Ph;
5
7+ int2d(ThU)(-div(u1,u2)*q -div(v1,v2)*p)
8+ int2d(ThP)(-1e-10*p*q)
9+ int2d(ThU)([f1,f2]'*[v1,v2])
10+ on(1,2,3,4, u1=g1, u2=g2);
11
12Mat M = Stokes(Xh,Xh);
13real[int] b = Stokes(0,Xh);
14real[int] sol = M^-1*b;
15
16[u1[],p[]] = sol; // dispatch the solution


We can then indicate what type of PETSc solver we want to use with PETSc-specific command-line arguments. We can start by using a simple distributed LU solver:

1ff-mpirun -np 4 script.edp -wg -pc_type lu


We can also specify the solver by putting the corresponding PETSc flags directly in the script, with the set() instruction:

1Mat M = Stokes(Xh,Xh);
2set(M, sparams = "-pc_type lu");


An interesting feature of using PETSc to solve a composite problem is that we can easily define a fieldsplit preconditioner combining separate preconditioners defined for each variable of the composite space, as the underlying composite PETSc operator already carries the block structure corresponding to the different variables.

This means that we can reference each variable – or split – right out of the box in order to define our fieldsplit preconditioner. They come in the same order as in the definition of the composite space ; in our Stokes example, the velocity space is split 0 and the pressure space is split 1. As an illustration, we can use the Schur complement method to solve our Stokes problem, using a multigrid HYPRE preconditioner for the velocity block:

1Mat M = Stokes(Xh,Xh);
2set(M, sparams = "-pc_type fieldsplit -pc_fieldsplit_type schur -fieldsplit_0_pc_type hypre");