Volume Meshing

Introduction

The objective of this section is to create a volume mesh using pyHyp. The surface mesh serves as the seed for hyperbolically marching the mesh to the farfield. Generating the volume mesh in this way is fast, repeatable, and results in a high-quality mesh. More details on pyHyp can be found in the pyHyp documentation or in the code itself.

Files

Navigate to the directory aero/meshing/volume in your tutorial folder. Copy the following files from the surface meshing directory:

$ cp ../surface/wing.cgns .

Create the following empty runscript in the current directory:

  • run_pyhyp.py

Dissecting the pyHyp runscript

Open the file run_pyhyp.py with your favorite text editor. Then copy the code from each of the following sections into this file.

Import libraries

from pyhyp import pyHyp

This is the standard way of importing the pyHyp library.

Options

For each module in MACH, we generally pass in options using a dictionary. A complete list of definitions of the pyHyp options can be found in the pyHyp documentation. Here we will point a few of the more basic options. For pyHyp, the options can be organized like so:

options = {
    # ---------------------------
    #   General options
    # ---------------------------
    "inputFile": "wing.cgns",
    "fileType": "CGNS",
    "unattachedEdgesAreSymmetry": True,
    "outerFaceBC": "farfield",
    "autoConnect": True,
    "BC": {},
    "families": "wall",

General options:

inputFile

Name of the surface mesh file.

fileType

Filetype of the surface mesh file. Either cgns or plot3d.

unattachedEdgesAreSymmetry

Tells pyHyp to automatically apply symmetry boundary conditions to any unattached edges (those that do not interface with another block).

outerFaceBC

Tells pyHyp to which boundary condition to apply to the outermost face of the extruded mesh. Either farfield or overset.

families

Name given to wall surfaces. If a dictionary is submitted, each wall patch can have a different name. This can help the user to apply certain operations to specific wall patches in ADflow.

    # ---------------------------
    #   Grid Parameters
    # ---------------------------
    "N": 49,
    "s0": 1e-5,
    "marchDist": 23.2 * 14,

Grid parameters:

N

Number of nodes in off-wall direction. If multigrid will be used this number should be 2m-1 (n+1), where m is the number of multigrid levels and n is the number of layers on the coarsest mesh.

s0

Thickness of first off-wall cell layer.

marchDist

Distance of the far-field.

The following options are related to the algorithms that are used to generate the mesh and usually these default values do not need to be modified. More information can be found in the pyHyp documentation. For example, epsE may be of interest when dealing with concave corners.

    # ---------------------------
    #   Pseudo Grid Parameters
    # ---------------------------
    "ps0": -1.0,
    "pGridRatio": -1.0,
    "cMax": 1.0,
    # ---------------------------
    #   Smoothing parameters
    # ---------------------------
    "epsE": 1.0,
    "epsI": 2.0,
    "theta": 3.0,
    "volCoef": 0.2,
    "volBlend": 0.0005,
    "volSmoothIter": 20,
    # ---------------------------
    #   Solution Parameters
    # ---------------------------
    "kspRelTol": 1e-10,
    "kspMaxIts": 1500,
    "kspSubspaceSize": 50,
}

Running pyHyp is quite simple, as shown below. After the mesh extrusion is done, we can write the volume mesh with the writeCGNS function.

hyp = pyHyp(options=options)
hyp.run()
hyp.writeCGNS("wing_vol.cgns")

Run it yourself!

You can now run the python file with the command:

$ python run_pyhyp.py

For larger meshes, you will want to run pyHyp as a parallel process. This can be done with the command:

$ mpirun -np 4 python run_pyhyp.py

where the number of processors is given after -np. You can open wing_vol.cgns in Tecplot to view the volume mesh.