lap_mat.edp

FreeFem++可以生成高质量的有限元网格。可以用于流体力学

//  laplace with matrix  
verbosity=10;
mesh Th=square(10,10);
fespace Vh(Th,P1);
Vh u1,u2;
                    
varf  a(u1,u2)= int2d(Th)(  dx(u1)*dx(u2) + dy(u1)*dy(u2) )
                    +  on(1,2,3,4,u1=1);
                   
varf b([u1],[u2]) = int2d(Th)(  u1*u2 );

matrix A= a(Vh,Vh,solver=CG); 
matrix B= b(Vh,Vh,solver=CG,eps=1e-20); 

Vh bb ,bc,rhs;
bc[]= a(0,Vh);       // to save the
u1=x;
bb[] = bc[] .* u1[];
u1[] = A^-1*bb[];
//plot(u1,cmm="solution = x ",wait=1,value=1);
u1=x*y;
bb[] = bc[] .* u1[];
u1[] = A^-1*bb[];
//plot(u1,cmm="solution = x*y ",wait=1,value=1);
u1=-4;  //  $-\Delta (x^2 + y^2) $
bb[] = B*u1[];
u1= x^2 + y^2 ;
bb[] += bc[] .* u1[];
u1[] = A^-1*bb[];
u2= x^2 + y^2;

cout << "  u1(1,2) =" <<  u1(.1,.2) << " ~= " << u2(.1,.2) 
     << " ==  " << .1^2+.2^2 <<  endl;
plot(u1,cmm="solution = $x^2 + y^2$ ",wait=1,value=1);