---
name: "mkgeo_contraction"
section: "1rheolef"
platform: "linux"
package: "rheolef"
version: "7.2-4"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/mkgeo_contraction"
---

# mkgeo\_contraction(1rheolef)

mesh of an abrupt contraction (rheolef-7.2)

## NAME

mkgeo\_contraction - mesh of an abrupt contraction (rheolef-7.2)

## SYNOPSIS

```
mkgeo_contraction [options] [nx [ny]]
```

## EXAMPLE

The following command build a triangle-based 2d unstructured mesh of the \[-10,0\]x\[0,4\] u \[0,10\]x\[0,1\] mesh

```
    mkgeo_contraction
    geo contraction.geo
```

## DESCRIPTION

This command is convenient for building a mesh for the abrupt contraction, as it is a very classical benchmark in complex fluid flow problems. It calls `bamg` unstructured mesh generator with anisotropy feature. The mesh files goes on name.geo where *name* is the basename for the output (see option `-name` below). The three auxiliary files required for automatic mesh generation with Rheolef combined with `bamg` are also provided:

```
        name.bamg
        name.bamgcad
        name.dmn
```

## THE GEOMETRY

The geometry is \[-Lu,0\]x\[0,d\] u \[0,Ld\]x\[0,1\]. By default c=4 is the contraction ratio and Lu=Ld=10 are the upstream and downstream pipe lengths.

`-c` *float* \
 `-Lu` *float* \
 `-Ld` *float*

 These options control the geometry parameters.

`-cartesian` \
 `-zr` \
 `-rz`

 These options control the geometry coordinate system. Default is Cartesian.

## THE DISCRETIZATION

The optional *nx* and *ny* arguments are integers that specifies the subdivision in each direction. By default nx=1 and ny=nx. Changing the density applies the factor 1/nx to all the mesh edges lengths.

`-hmin` *float*

 Controls the edge length at the re-entrant corner of the contraction. Default is `hmin`=0.1. Changing the density by the previous options applies the factor 1/n to `hmin` also.

## BOUNDARY DOMAINS

The boundary sides are represented by domains: `axis`, `wall`, `upstream` and `downstream`.

## OTHERS OPTIONS

- `-name` *string*

   Set the basename for the output files. By default, the basename is `contraction`.

`-[no]split`

 Split each triangle in three subtriangles by inserting the barycenter as an additional node. This is useful for using the Scott and Vogelius incompressible mixed finite element P2-P1d for velocity-pressure approximation. Default is no split.

`-[no]clean`

 Clear temporary files (this is the default).

`-[no]verbose`

 In verbose mode, print to stderr all commands and logs.

## IMPLEMENTATION

This documentation has been generated from file main/sbin/mkgeo\_contraction.sh

## AUTHOR

Pierre  Saramito  \<Pierre.Saramito@imag.fr\>

## COPYRIGHT

Copyright   (C)  2000-2018  Pierre  Saramito  \<Pierre.Saramito@imag.fr\> GPLv3+: GNU GPL version 3 or later  \<http://gnu.org/licenses/gpl.html\>. This  is  free  software:  you  are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.