periodicBladeMesh

(py)gmsh based tool for the simplest turbomachinery mesh

All parameters of the geometry and mesh can be defined in json file, theoretically, there is no need to touch the python code. Practically, the code is definitely missing something, but it might be a good starting point.

Internal references can be used in json files. E.g.:

 "scaled by"  : {"$ref": "#/geometry/true chord"},
 "TE"         : {"$ref": "#/mesh/LE"},

json files cannot handle mathematics. Following will not work:

 "length of inlet"  : 3*{"$ref": "#/geometry/true chord"}, 

.msh files are widely accepted for mesh definition but quite often only ASCII version of file in 2.2 can be read. If this is your case, use:

 "version"    : 2.2,

Inputs that can be set to false (such as version, save , ...), can be omitted from the setup.

gmsh is very well documented in official documentation, but info useful for setup of this tool is pointed out in file in this repository.

Refinements

• baseline size is applied on the surface of the blade
• mesh size from curvature (number of nodes on 2 $\pi$ arc) is quite a tricky parameter. It could take care of the edges and there would be no need for tuning of parameters of their refinement. But this parameter also refines the periodicities, which can lead to lower quality of mesh. Use it carefully.
• if more refinements are affecting a region (e.g. on trailing edge refinement of itself, blade surface and wake take place), minimum required cell size is taken into account

Predefined refinements

• if allowed, leading and trailing edges are (independently) refined to (L/T)E : size with a given (L/T)E : radius
• if allowed, wake region - rectangle aligned with outlet flow with given wake : length and wake : thickness - is refined to wake : size
• all the predefined refinements have a diffuse parameter, it sets a distance in which the mesh is not affected by given refinement

Example of refinement setup in a json file:

"refine TE": true,   | region around trailing edge will be refined 
"TE" : {
  "size"   : 1e-4,   | cells in the closest region will have size 1e-4 m
  "radius" : 1e-3,   | the closest region is a circle with radius 1e-3 m
  "diffuse": 2       | the transition between given size and rest of the 
                     | mesh will happen in a circle with radius 
                     | (1+diffuse)*radius = 3e-3 m
},

Boundary layer meshing

Inputs for boundary layer mesh on the blade surface are:

• size of first layer,
• growth ratio and
• total thickness.

In specific cases a periodic mesh created with this tool can be used for 3D calculations. In such cases it might be useful to refine the mesh in the vicinity of the side walls.

Parameters for 3D setup are:

"n layers in z"             : 100,
"side walls boundary layer" : true,
"SW BL properties"          :  {
  "ratio"     : 1.1,
  "size last" : 3e-3,
  "size first": 1e-5
}

For 2D cases set "n layers in z" : 1, other parameters are then not taken into account.

It should be noted, that if in current implementation n layers in z is ignored when creating boundary layer mesh on side walls. Number of cells in z direction is reported.

To obtain purely planar mesh run with flag -planar.

Test cases:

SPLEEN turbine blade

run with

  python3 mesh.py data/spleen.json

schreiber84 compressor blade

run with

  python3 mesh.py data/schreiber84.json