Helix Mesh

This project builds a structured hexahedral mesh for a twisted pipe with three trapezoidal ribs.

The implementation currently covers:

• Milestone 1: explicit corrected 2D cross-section topology
• Milestone 2: structured 2D quadrilateral meshing on the 18-patch topology
• Milestone 3: axial extrusion to a twisted 3D hexahedral mesh
• Milestone 4: Gmsh *.msh export
• Milestone 5: AVL-oriented CGNS export through Gmsh conversion

Topology Overview

The corrected cross-section uses 18 quadrilateral patches:

• 6 inner fluid patches
• 6 outer fluid gap patches
• 6 outer rib_solid patches

Each rib is trapezoidal and split into two outer rib_solid patches along its centerline. The six inner quadrilateral patches meet directly at the center point, so there is no separate small core polygon in the current corrected topology.

The inner gap midpoints remain Cartesian chord midpoints between neighboring rib-tip corners and do not lie on r_tip.

Twist Definition

The mesh twist is not entered directly as total_twist_deg. Instead, it is derived from pitch_angle_deg:

total_twist_deg = length / (pi * diameter * tan(pitch_angle_deg * pi / 180)) * 360

with:

• diameter = 2 * r_pipe
• phi(z) = total_twist * z / length

The rib widths are specified as direct chord distances in meters:

• rib_width_outer
• rib_width_inner

They are converted internally to angular widths through the chord relation:

• rib_width_outer_angle = 2 * asin(rib_width_outer / (2 * r_pipe))
• rib_width_inner_angle = 2 * asin(rib_width_inner / (2 * r_tip))

Output Modes

The script supports six output modes:

• plot_2d
• plot_2d_mesh
• plot_3d_preview
• export_gmsh
• export_gmsh_full
• export_avl_cgns

You can choose the mode either:

• in parameters.json via output_mode
• or via CLI flags, which take precedence

Current Export Behavior

export_gmsh

Writes a fluid-only discrete Gmsh *.msh mesh. Internal rib_solid cells are not exported as volume cells in this mode. Fluid-facing surfaces next to omitted rib solids are exported as BND_W_RibWall.

Typical groups:

• Volume: DOM_GasVolume
• Surfaces: BND_I_Inlet_Gas, BND_O_Outlet_Gas, BND_W_ReactorWall, BND_W_RibWall, axis_core

export_gmsh_full

Writes the full discrete Gmsh mesh with both domains:

• DOM_GasVolume
• DOM_RibVolume

export_avl_cgns

Writes a CGNS file for AVL import experiments through the path:

internal mesh -> Gmsh .msh -> Gmsh CGNS conversion

The implementation first tries the Python Gmsh API and falls back to a gmsh executable if needed.

Important distinction:

• --export-gmsh and --export-gmsh-full create Gmsh MSH files
• AVL's "MSH Gambit mesh (*.msh)" importer is not the same as Gmsh MSH support
• the supported AVL path is the CGNS export, not direct Gmsh-MSH import

Current stable AVL recommendation:

• use raw Gmsh CGNS naming
• keep strip_cgns_prefixes = false
• use hex_order = reverse

The currently reliable AVL-compatible raw naming looks like:

• S_1_BND_I_Inlet_Gas
• V_1_DOM_GasVolume

The project keeps the intended engineering names embedded in that scheme, but does not strip the S_ / V_ prefixes by default because that post-processing made some importers reject the CGNS file.

Parameters File

The default parameter file is parameters.json inside the helix_mesh folder.

The script now uses parameters.json not only for geometry and meshing, but also for optional export settings such as:

• output mode
• output file name
• CGNS export settings
• AVL hex-order selection

CLI arguments can still override individual values.

Full JSON Parameter List

Output and Export Settings

• output_mode Supported values: plot_2d, plot_2d_mesh, plot_3d_preview, export_gmsh, export_gmsh_full, export_avl_cgns
• outfile Generic output target path for the selected mode
• fluid_only AVL export only; if true, omit the rib solid volume
• gmsh_exe Path or command name for the Gmsh executable fallback
• keep_intermediate AVL export only; keep the intermediate Gmsh .msh
• intermediate_msh Explicit path for the intermediate .msh
• save_all Pass -save_all to Gmsh CGNS conversion
• hex_order Supported values: reverse, flip_i, flip_k, default
• strip_cgns_prefixes Experimental CGNS post-processing flag; currently not recommended for AVL

Geometry

• r_pipe
• length
• n_ribs
• rib_height
• rib_width_outer
• rib_width_inner
• pitch_angle_deg

Axial and Patch Resolution

• n_z
• n_radial_outer
• n_radial_inner
• n_tangential_rib_half
• n_tangential_gap_half
• n_tangential_inner

Core Topology

• core_mode Supported values: center_point, annulus_tube
• core_radius_fraction
• r_inner_tube

Miscellaneous

• msh_version

Example parameters.json

{
  "output_mode": "export_avl_cgns",
  "outfile": "helix_pipe_multidomain.cgns",
  "fluid_only": false,
  "gmsh_exe": "gmsh",
  "keep_intermediate": false,
  "intermediate_msh": null,
  "save_all": false,
  "hex_order": "reverse",
  "strip_cgns_prefixes": false,
  "r_pipe": 0.007,
  "length": 0.391,
  "n_ribs": 3,
  "rib_height": 0.002,
  "rib_width_outer": 0.002,
  "rib_width_inner": 0.001,
  "pitch_angle_deg": 60,
  "n_z": 200,
  "core_mode": "center_point",
  "core_radius_fraction": 0.15,
  "r_inner_tube": 0.0015,
  "n_radial_outer": 4,
  "n_radial_inner": 8,
  "n_tangential_rib_half": 2,
  "n_tangential_gap_half": 8,
  "n_tangential_inner": 8,
  "msh_version": 2.2
}

CLI Options

The CLI supports both output-mode flags and value overrides.

Output Selection

• --plot-2d
• --plot-2d-mesh
• --plot-3d-preview
• --export-gmsh OUTFILE
• --export-gmsh-full OUTFILE
• --export-avl-cgns OUTFILE

If none of these is passed, the script uses output_mode from parameters.json.

Generic Output Override

• --outfile PATH

This overrides the JSON outfile for the currently selected mode.

AVL / CGNS Export Options

• --fluid-only
• --gmsh-exe PATH
• --keep-intermediate
• --intermediate-msh PATH
• --save-all
• --hex-order reverse|flip_i|flip_k|default
• --strip-cgns-prefixes

Geometry and Mesh Overrides

• --length FLOAT
• --nz INT
• --pitch-angle-deg FLOAT
• --core-mode center_point|annulus_tube
• --r-inner-tube FLOAT
• --r-pipe FLOAT
• --rib-height FLOAT
• --rib-width-outer FLOAT
• --rib-width-inner FLOAT

Typical Commands

Use only parameters.json

.\.venv\Scripts\python.exe .\helix_mesh\helix_mesh.py

Write a fluid-only Gmsh mesh

.\.venv\Scripts\python.exe .\helix_mesh\helix_mesh.py --export-gmsh helix_pipe_structured.msh

Write the full multi-domain Gmsh mesh

.\.venv\Scripts\python.exe .\helix_mesh\helix_mesh.py --export-gmsh-full helix_pipe_structured_full.msh

Write the stable AVL CGNS export

.\.venv\Scripts\python.exe .\helix_mesh\helix_mesh.py --export-avl-cgns helix_pipe_multidomain.cgns

Keep the intermediate MSH for inspection

.\.venv\Scripts\python.exe .\helix_mesh\helix_mesh.py --export-avl-cgns helix_pipe_multidomain.cgns --keep-intermediate

Run an AVL orientation variant

.\.venv\Scripts\python.exe .\helix_mesh\helix_mesh.py --export-avl-cgns helix_flip_i.cgns --hex-order flip_i --keep-intermediate

Physical Names and Domains

The intended engineering names are:

• Fluid volume: DOM_GasVolume
• Rib solid volume: DOM_RibVolume
• Fluid inlet: BND_I_Inlet_Gas
• Rib inlet: BND_I_Inlet_Rib
• Fluid outlet: BND_O_Outlet_Gas
• Rib outlet: BND_O_Outlet_Rib
• Reactor wall: BND_W_ReactorWall
• Rib wall: BND_W_RibWall

For Gmsh exports, these names are assigned as both physical-group names and entity names.

For AVL CGNS export, the stable raw Gmsh CGNS path currently keeps prefixed family names such as V_1_DOM_GasVolume.

Core Topology Note

In center_point mode, axis_core is intentionally empty because the inner region closes all the way to the center point.

In annulus_tube mode, axis_core exists as an actual open inner boundary around the free inner tube / probe region.

Requirements

Use a local .venv. Before running tests or exports, make sure the environment contains at least:

• numpy
• pytest
• matplotlib
• gmsh

For AVL CGNS export, the Python Gmsh module is the primary path. A callable gmsh executable is used only as fallback.

Windows PowerShell example:

.\.venv\Scripts\python.exe -c "import numpy, pytest, matplotlib, gmsh; print('requirements available')"
gmsh -version

Manual Checks

Inspect a Gmsh mesh:

gmsh helix_pipe_structured.msh

OpenFOAM-side check, if available:

gmshToFoam helix_pipe_structured.msh
checkMesh

AVL-side check:

1. Import the CGNS file through the CGNS importer, not through the Gambit-MSH importer.
2. Confirm that the gas and rib domains are present.
3. Confirm that the inlet, outlet, reactor wall, and rib wall groups are present.

AVL Troubleshooting

• If AVL rejects a direct *.msh, check whether it expects Gambit/Fluent MSH instead of Gmsh MSH.
• Prefer the CGNS export path for AVL experiments.
• Start with strip_cgns_prefixes = false.
• Use --keep-intermediate to inspect the intermediate .msh in Gmsh.
• Use --save-all only as an experiment if zones appear to be missing.
• If needed, test the fallback conversion manually: gmsh intermediate.msh -format cgns -o test.cgns