I want to introduce a flat plate without thickness into a grid, essentially means inserting a line or surface that fluid cannot cross.This is akin to defining a boundary condition. The sketch is below.

The red line is the baffle. How can I create this baffle?

Thank you for your answer. I ran your program successfully. But something wrong happened with my code.

real xinlet = -40;
real xoutlet = 80;
real yside = 40;
real length=5;
real width=1;
real n=.5;
border cylinder1(t=0,1){ x=-2.5+length*t;y=-0.5;label=2;}
border cylinder2(t=0,1){ x=-2.5+length*t;y=0.5;label=2;}
border cylinder3(t=0,1){ x=-2.5;y=-0.5+width*t;label=2;}
border cylinder4(t=0,1){ x=2.5;y=-0.5+width*t;label=2;}
border inlet(t=-1,1){ x=xinlet;y=yside*t;label=1;}
border baffle(t=0,1){ x=-2.5;y=0.1*t;label=2;}
border outlet(t=-1,1){ x=xoutlet;y=yside*t;label=3;}
border latsup(t=0,1){ x=xoutlet-(xoutlet-xinlet)*t;y=yside;label=3;}
border latbot(t=0,1){ x=xoutlet-(xoutlet-xinlet)*t;y=-yside;label=3;}
mesh th=buildmesh(inlet(-2*yside*n)+outlet(2*yside*n)+latsup((xoutlet-xinlet)*n)+latbot(-(xoutlet-xinlet)*n)+cylinder1(-50)+cylinder2(50)+cylinder3(10)+cylinder4(-10)+baffle(10));
plot(th,wait=true);

The error message is below

Missing edge 24 v0 = 0[-2.5,0] v1= 1[-2.5,0.1] 0 0x0
There are 1 lost edges
The boundary is crossing maybe!
Fatal error in the mesh generator 10
current line = 11
Meshing error: Bamg
number : 10,
catch Err bamg
Meshing error: Bamg
number : 10,
err code 5 , mpirank 0