[Gmsh] Mesh refinement in the wake of a point

[David Nolte]

Dear all,

I want to refine a unstructured mesh in the "wake" of a given point,
i.e. from that point onward in a given direction. Ideally fanning out
with a small angle.
Example: fluid simulation of a "backward-facing step", or any case where
shear layers appear downstream of a separation point, like in the wake
of a bluff body, etc.
I looked into the "Field" options gmsh offers. I think some kind of
anisotropic Threshold/Attractor would be perfect, where I could specify
distances for x, y, z separately. Can this be achieved somehow?

example mesh "backward-facing step"

h = 0.5;
s = 0.5;
H = h + s;
Lu = 2*h;
Ld = 20*h;

lc = 0.1;
lcp = lc;

Point(1) = {-Lu, h, 0, lc};
Point(2) = {-Lu, 0, 0, lc};
Point(3) = {0, 0, 0, lcp};
Point(4) = {0, -s, 0, lc};
Point(5) = {Ld, -s, 0, lc};
Point(6) = {Ld, h, 0, lc};

Line(1) = {1, 2};   // inlet
Line(2) = {2, 3};   // upstream channel bottom
Line(3) = {3, 4};   // step vertical boundary
Line(4) = {4, 5};   // downstream channel bottom
Line(5) = {5, 6};   // outlet
Line(6) = {6, 1};   // channel top

Line Loop(7) = {1, 2, 3, 4, 5, 6};

Plane Surface(1) = {7};

// Refinement
// Field[1] = Attractor;
// Field[1].NodesList = {3};  // corner point

// Field[2] = Threshold;
// Field[2].IField = 1;
// Field[2].LcMin = lc/10.;
// Field[2].LcMax = lc;
// Field[2].DistMin = 0.1;
// Field[2].DistMax = 0.5;
// Background Field = 2;

Physical Line(1) = {1};   // inlet
Physical Line(2) = {5};   // outlet
Physical Line(3) = {2, 3, 4, 6};   // walls

Physical Surface(0) = {1};


Thanks a lot in advance, and thanks for this great tool!
Regards,
David