Quadrangular mesh generation

Hello everybody;

I have an issue trying to work on a Mike Zero mesh generation file.

I work on a large floodplain and would like to get better mesh resolution along river branch and lower resolution on floodplain. This is to find the best compromise between computing time and results quality.

I managed to apply quadrangular mesh on a test file and have been convinced by the method. Unfortunately, I would like to apply it to the entire area and get some problems to do so.

The river system is quite complex. Attach is a schema of the network and loops (not all clockwise direction …) I try to digitalize with land/water boundaries to separate quadrangular from triangular mesh and open boundaries for upstream/downstream conditions. There is a total of 6 portions to be mesh with quadrangular cells.

When polygon is selected; Start and End arcs defined; I basically get this message: “The polygon subdivision fail due to the unequal points on both side”; then “ValidatePolygon() return FALSE”

Does somebody could help me to solve my problem and to remesh the DEM? Any other advice for quadrangular mesh generation?

Thanks

J.R

Hi,

if you get the message  “The polygon subdivision fail due to the unequal points on both side”, you should try to set the same number of vertices along the polylines on the left and on the right side of the quadrangular area.

Alternatively, you may also try to use the "Transfinite interpolation" method to generate the quadrangular mesh for this polygon: this method is a bit more flexible, and could allow a different number of vertices on both sides.

Regards,
Mathieu

Thanks Mathieu =)

Okey; this worked indeed great but I suppose there is other constrains for the quadrangular mesh generation.

Actually, the “ValidatePolygon() return FALSE” error message appear to be frequent even if the symmetry is respected regarding to the amount of points. Indeed, I managed to draw large polygons as a first test (high distance between river banks and sides of the polygon) but when I try to refine polygons (by adding points and reduce the distance between banks and polygons sides) for quadrangular mesh, the message appear and mesh generation is not anymore possible. I don’t have any idea where the message is from; any links with meanders (which are numerous) ?

The “Transfinite interpolation" option seems to not be accessible in my version (not possible to select it…).

Thanks again. Best

Jérémie

Hi Jérémie,

Indeed there are many constraints which have to be fulfilled when generating a mesh, and especially when dealing with quadrangular meshes.

When using the default 'Algebraic box method', you are not allowed to have more than 4 nodes along the arcs, each of them defining the 4 sides of the quadangular area. So if you have intermediate nodes (instead of vertices), that could be the problem.

Another problem can raise when drawing meanders with high curvature: in that case it may result in some quadrangular elements attempting to cross the meander, outside the quadrangular area, which is not accepted. So you can try to reduce the curvature and see it's better. Alternatively you may try to split a long quandarangular area into smaller areas (by adding arcs to cut the polygon into multiple parts). That may help to avoid odd mesh geometry in meanders.

The "Transfinite interpolation" method wasn't available in older versions of the Mesh Generator, so I suppose you are using one of these versions!

Best regards
Mathieu

Hi Mathieu;

Thanks again =)
Splitting meanders into smaller polygons was indeed the best solution.
Actually I am using the 2008 version so, once again, you are right !
Wish you the best.

Jérémie