Interpolation matrix between two meshes

General Discussion
1

Hi,

Can some explain the following piece of code (two questions below), which is from Distributed Lagrange Multiplier for Fluid-Structure Interactions | SpringerLink

fespace Vh(Thf, P1b); // velocity space
fespace Qh(Thf, P1); // pressure space
fespace Wh(Ths, P1b); // Lagrangian coordinates X,Y space
fespace Lh(Ths, P1); // Lagrangian multiplier space
fespace Zh(Thf, [P1b,P1b,P1]); // fluid space
fespace Rh(Ths, [P1,P1,P1b,P1b]);// solid space

Vh u,v,uh,vh;
Qh p,ph;
Wh Xoo,Yoo;
Rh [lamx,lamy,X,Y], [lamxo,lamyo,Xo,Yo]=[0,0,x,y];
Zh [uo,vo,po] = [0,0,0];

…

varf ba( [lamx,lamy,X,Y], [uh,vh,ph] ) =
int2d(Ths, qft=qf9pT, mapt=[Xo,Yo]) (
c1 * trace(Grad(lamx,lamy)’ * (Grad(Xo,Yo) * Grad(uh,vh)))
+ c2 * (lamx * uh+lamy * vh) );

Question 1: ‘lamx’ and ‘uh’ are in different FEM spaces using two different meshes, can we compute “lamx * uh+lamy * vh” ? Notice that uh is defined in Vh using Thf, and lamx is defined in Rh using Ths, and Thf and Ths are two different meshes for fluid and solid respectively.

Question 2: Is the FEM interpolation matrix hidden inside FreeFEM++ to transfer information between two meshes?

Thanks in advance!
Yongxing.