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Transmission problem

Consider an elastic plate whose displacement change vertically, which is made up of three plates of different materials, welded on each other.

Let \(\Omega_i\), \(i=1,2,3\) be the domain occupied by \(i\)-th material with tension \(\mu_i\) (see Soap film).

The computational domain \(\Omega\) is the interior of \(\overline{\Omega_1}\cup \overline{\Omega_2}\cup \overline{\Omega_3}\). The vertical displacement \(u(x,y)\) is obtained from:

(65)\[\begin{split}\begin{array}{rcll} -\mu_i\Delta u &=& f & \textrm{ in }\Omega_i\\ \mu_i\p_n u|_{\Gamma_{i}} &=& -\mu_j\p_n u|_{\Gamma_{j}} & \textrm{ on }\overline{\Omega_{i}}\cap\overline{\Omega_{j}} \textrm{ if }1\le i< j\le 3 \end{array}\end{split}\]

where \(\p_n u|_{\Gamma_{i}}\) denotes the value of the normal derivative \(\p_n u\) on the boundary \(\Gamma_i\) of the domain \(\Omega_i\).

By introducing the characteristic function \(\chi_i\) of \(\Omega_i\), that is:

\[\chi_i(x)=1\ \textrm{ if }x\in\Omega_i;\ \chi_i(x)=0\ \textrm{ if }x\not\in\Omega_i\]

we can easily rewrite (65) to the weak form. Here we assume that \(u=0\) on \(\Gamma=\p\Omega\).

Transmission problem: For a given function \(f\), find \(u\) such that:

\[a(u,v) = \ell(f,v) \textrm{ for all }v\in H^1_0(\Omega)\]
\[\begin{split}\begin{array}{rcl} a(u,v) &=& \int_{\Omega}\mu \nabla u\cdot \nabla v\nonumber\\ \ell(f,v) &=& \int_{\Omega}fv\nonumber \end{array}\end{split}\]

where \(\mu=\mu_1\chi_1+\mu_2\chi_2+\mu_3\chi_3\). Here we notice that \(\mu\) become the discontinuous function.

This example explains the definition and manipulation of region, i.e. sub-domains of the whole domain. Consider this L-shaped domain with 3 diagonals as internal boundaries, defining 4 sub-domains:

 1 // Mesh
 2 border a(t=0, 1){x=t; y=0;};
 3 border b(t=0, 0.5){x=1; y=t;};
 4 border c(t=0, 0.5){x=1-t; y=0.5;};
 5 border d(t=0.5, 1){x=0.5; y=t;};
 6 border e(t=0.5, 1){x=1-t; y=1;};
 7 border f(t=0, 1){x=0; y=1-t;};
 8 border i1(t=0, 0.5){x=t; y=1-t;};
 9 border i2(t=0, 0.5){x=t; y=t;};
10 border i3(t=0, 0.5){x=1-t; y=t;};
11 mesh th = buildmesh(a(6) + b(4) + c(4) +d(4) + e(4)
12     + f(6) + i1(6) + i2(6) + i3(6));
13 
14 // Fespace
15 fespace Ph(th, P0); //constant discontinuous functions / element
16 Ph reg=region; //defined the P0 function associated to region number
17 
18 // Plot
19 plot(reg, fill=true, wait=true, value=true);
TransmissionProblem_Region

Fig. 174 The function reg

region is a keyword of FreeFEM which is in fact a variable depending of the current position (is not a function today, use Ph reg=region; to set a function). This variable value returned is the number of the sub-domain of the current position. This number is defined by buildmesh which scans while building the mesh all its connected component.

So to get the number of a region containing a particular point one does:

1 // Characteristic function
2 int nupper = reg(0.4, 0.9); //get the region number of point (0.4,0.9)
3 int nlower = reg(0.9, 0.1); //get the region number of point (0.4,0.1)
4 cout << "nlower = " <<  nlower << ", nupper = " << nupper<< endl;
5 Ph nu = 1 + 5*(region==nlower) + 10*(region==nupper);
6 
7 // Plot
8 plot(nu, fill=true,wait=true);
TransmissionProblem_Nu

Fig. 175 The function nu

This is particularly useful to define discontinuous functions such as might occur when one part of the domain is copper and the other one is iron, for example.

We this in mind we proceed to solve a Laplace equation with discontinuous coefficients (\(\nu\) is 1, 6 and 11 below).

 1 // Problem
 2 solve lap (u, v)
 3     = int2d(th)(
 4           nu*(dx(u)*dx(v) + dy(u)*dy(v))
 5     )
 6     + int2d(th)(
 7         - 1*v
 8     )
 9     + on(a, b, c, d, e, f, u=0)
10     ;
11 
12 // Plot
13 plot(u);
TransmissionProblem_U

Fig. 176 The isovalue of the solution \(u\)

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