classy_blocks

Python classes for easier creation of OpenFOAM's blockMesh dictionaries.

About

blockMesh is a very powerful mesher but the amount of manual labour it requires to make even the simplest meshes makes it mostly useless. Even attempts to simplify or parametrize blockMeshDicts with #calc or even the dreadful m4 quickly become unmanageable and cryptic.

classy_blocks' aim is to minimize the amount of meticulous work by providing a more intuitive workflow, off-the-shelf parts and some automatic helpers for building and optimization of block-structured hexahedral meshes. Still it is not an automatic mesher and therefore some kinds of geometry are more suited than others.

Tutorial

Check out the classy_blocks tutorial on damogranlabs.com!

Useful For

Fields

• Turbomachinery (impellers, propellers)
• Microfluidics
• Flow around buildings
• Heat transfer (PCB models, heatsinks)
• Airfoils (2D)
• Solids (heat transfer, mechanical stresses)

Cases

• Simpler rotational geometry (immersed rotors, mixers, cyclones)
• Pipes/channels
• Tanks/plenums/containers
• External aerodynamics of blunt bodies
• Modeling thin geometry (seals, labyrinths)
• Parametric studies
• Background meshes for snappy (cylindrical, custom)
• 2D and axisymmetric cases
• Overset meshes

Not Good For

• External aerodynamics of vehicles (too complex to mesh manually, without refinement generates too many cells)
• Complex geometry in general
• One-off simulations (use automatic meshers)

How To Use It

• To install the current stable version from pypi, use pip install classy_blocks
• To download the cutting-edge development version, install the development branch from github: pip install git+https://github.com/damogranlabs/classy_blocks.git@development
• If you want to run examples, follow instructions in Examples
• If you want to contribute, follow instructions in CONTRIBUTING.rst

Features

Workflow

As opposed to blockMesh, where the user is expected to manually enter pre-calculated vertices, edges, blocks and whatnot, classy_blocks tries to mimic procedural modeling of modern 3D CAD programs. Here, a Python script contains steps that describe geometry of blocks, their cell count, grading, patches and so on. At the end, the procedure is translated directly to blockMeshDict and no manual editing of the latter should be required.

Building Elements

Unchecked items are not implemented yet but are on a TODO list

• Manual definition of a Block with Vertices, Edges and Faces
• Operations (Loft, Extrude, Revolve)
  • Loft
  • Extrude
  • Revolve
  • Wedge (a shortcut to Revolve for 2D axisymmetric cases)
  • Connector (A Loft between two existing Operations)
• Sketches of common cross-sections
  • Quarter and Semi circle
  • Circle
  • Boxed circle
  • Oval with straight sides
  • Ellipse (and ovals in various configurations)
  • Cartesian grid
• Simple way of creating custom Sketches
• Easy shape creation from Sketches
• Predefined Shapes
  • Box (shortcut to Block aligned with coordinate system)
  • Elbow (bent pipe of various diameters/cross-sections)
  • Cone Frustum (truncated cone)
  • Cylinder
  • Ring (annulus)
  • Hemisphere
• Stacks (collections of similar Shapes stacked on top of each other)
• Predefined parametric Objects
  • T-joint (round pipes)
  • X-joint
  • N-joint (multiple pipes)
  • Collector (a barrel with multiple radial outlets)
• Other building tools
  • Use existing Operation's Face to generate a new Operation
  • Chain Shape's start/end surface to generate a new Shape
  • Expand Shape's outer surface to generate a new Shape (Cylinder/Annulus > Annulus)
  • Contract Shape's inner surface into a new Shape (Annulus > Cylinder/Annulus)
  • Join two Operations by extending their Edges
  • Offset Operation's faces to form new operations

Modifiers

After blocks have been placed, it is possible to create new geometry based on placed blocks or to modify them.

• Move Vertex/Edge/Face
• Delete a Block created by a Shape or Object
• Project Vertex/Edge/Face
• Optimize point position of a Sketch or mesh vertices

Meshing Specification

• Simple definition of all supported kinds of edges with a dedicated class (Arc/Origin/Angle/Spline/PolyLine/Project)
• Automatic sorting/reorienting of block vertices based on specified front and top points
• Automatic calculation of cell count and grading by specifying any of a number of parameters (cell-to-cell expansion ratio, start cell width, end cell width, total expansion ratio)
• Edge grading (separate specification for each edge)
• Automatic propagation of grading and cell count from a single block to all connected blocks as required by blockMesh
• Projections of vertices, edges and block faces to geometry (triangulated and searchable surfaces)
• Face merging as described by blockMesh user guide. Breaks the pure-hexahedral-mesh rule but can often save the day for trickier geometries. Automatic duplication of points on merged block faces
• Auto grading for Low-Re meshes: boundary layer with specified cell-to-cell expansion, transition with 2:1 expansion, and specified 'bulk' cell size

Examples

How to run:

• Install classy_blocks as described above
• cd to directory of the chosen example
• Run python <example.py>; that will write blockMeshDict to examples/case
• Run blockMesh on the case
• Open examples/case/case.foam in ParaView to view the result

For instance:

cd examples/chaining
python tank.py
blockMesh -case ../case

Operations

Analogous to a sketch in 3D CAD software, a Face is a set of 4 vertices and 4 edges. An Operation is a 3D shape obtained by swiping a Face into 3rd dimension by a specified rule. Here is a Revolve as an example:

# a quadrangle with one curved side
base = cb.Face(
    [ # quad vertices
        [0, 0, 0],
        [1, 0, 0],
        [1, 1, 0],
        [0, 1, 0]
    ],
    [ # edges: None specifies a straight edge
        cb.Arc([0.5, -0.2, 0]),
        None,
        None,
        None
  ]
)

revolve = cb.Revolve(
    base, # face to revolve
    f.deg2rad(45), # revolve angle
    [0, -1, 0], # axis
    [-2, 0, 0]  # origin
)

revolve.chop(0, count=15) # first edge
revolve.chop(1, count=15) # second edge
revolve.chop(2, start_size=0.05) # revolve direction
mesh.add(revolve)

See examples/operations for an example of each operation.

Shapes

Some basic shapes are ready-made so that there's no need for workout with Operations.

A simple Cylinder:

inlet = cb.Cylinder([x_start, 0, 0], [x_end, 0, 0], [0, 0, radius])
inlet.chop_radial(count=n_cells_radial, end_size=boundary_layer_thickness)
inlet.chop_axial(start_size=axial_cell_size, end_size=2*axial_cell_size)
inlet.chop_tangential(count=n_cells_tangential)

inlet.set_start_patch('inlet')
inlet.set_outer_patch('wall')
inlet.set_end_patch('outlet')
mesh.add(inlet)

See examples/shape for use of each shape

3D pipes with twists and turns (chained Elbow and Cylinder Shapes)

Chaining and Expanding/Contracting

Useful for Shapes, mostly for piping and rotational geometry; An existing Shape's start or end sketch can be reused as a starting sketch for a new Shape, as long as they are compatible. For instance, an Elbow can be chained to a Cylinder just like joining pipes in plumbing.

Moreover, most shapes* can be expanded to form a wall version of the same shape. For instance, expanding a Cylinder creates an ExtrudedRing or an ExtrudedRing can be filled to obtain a Cylinder that fills it.

A simple tank with rounded edges

A flywheel in a case. Construction starts with a Cylinder which is then expanded and chained from left towards right. VTK Blocking output for debug is shown in the middle

Venturi tube

See examples/chaining for an example of each operation.

Custom Sketches and Shapes

A Sketch is a collection of faces - essentially a 2D geometric object, split into quadrangles. Each Face in a Sketch is transformed into 3D space, creating a Shape.

A number of predefined Sketches is available to choose from but it's easy to create a custom one.

disk_in_square = cb.WrappedDisk(start_point, corner_point, disk_diameter/2, normal)
shape = cb.ExtrudedShape(disk_in_square, length)

Sketch Smoothing and Optimization

Points that define a custom sketch can only be placed approximately. Their positions can then be defined by Laplacian smoothing or optimization to obtain best face quality.

See examples/shape/custom for an example with a custom sketch.

Stacks

A collection of similar Shapes; a Stack is created by starting with a Sketch, then transforming it a number of times, obtaining Shapes, stacked on top of each other.

An Oval sketch, translated and rotated to obtain a Shape from which a Stack is made.

A Grid sketch, consisting of 3x3 faces is Extruded 2 times to obtain a Stack. The bottom-middle box is removed from the mesh so that flow around a cube can be studied:

base = Grid(point_1, point_2, 3, 3)

stack = ExtrudedStack(base, side * 2, 2)

# ...
mesh.delete(stack.grid[0][1][1])

See examples/stack/cube.py for the full script.

An electric heater in water, a mesh with two cellZones. The heater zone with surrounding fuild of square cross-section is an extruded WrappedDisk followed by a RevolvedStack of the same cross-sections. The center is then filled with a SemiCylinder.

See examples/complex/heater for the full script.

Assemblies

A collection of pre-assembled parametric Shapes, ready to be used for further construction.

Three pipes, joined in a single point.

Projection To Geometry

Any geometry that snappyHexMesh understands is also supported by blockMesh. That includes searchable surfaces such as spheres and cylinders and triangulated surfaces.

Projecting a block side to a geometry is straightforward; edges, however, can be projected to a single geometry (will 'snap' to the closest point) or to an intersection of two surfaces, which will define it exactly.

Geometry is specified as a simple dictionary of strings and is thrown in blockMeshDict exactly as provided by the user.

geometry = {
    'terrain': [
        'type triSurfaceMesh',
        'name terrain',
        'file "terrain.stl"',
    ],
    'left_wall': [
        'type       searchablePlane',
        'planeType  pointAndNormal',
        'point      (-1 0 0)',
        'normal     (1  0  0)',
    ]
}

box = cb.Box([-1., -1., -1.], [1., 1., 1.])
box.project_side('bottom', 'terrain')
box.project_edge(0, 1, 'terrain')
box.project_edge(3, 0, ['terrain', 'left_wall'])

Edges and faces, projected to an STL surface

Mesh for studying flow around a sphere. Edges and faces of inner ('wall') and outer ('prismatic layers') cells are projected to a searchableSphere, adding no additional requirements for STL geometry.

Face Merging

Simply provide names of patches to be merged and call mesh.merge_patches(<master>, <slave>). classy_blocks will take care of point duplication and whatnot.

box = cb.Box([-0.5, -0.5, 0], [0.5, 0.5, 1])
for i in range(3):
    box.chop(i, count=25)
box.set_patch('top', 'box_top')
mesh.add(box)

cylinder = cb.Cylinder(
    [0, 0, 1],
    [0, 0, 2],
    [0.25, 0, 1]
)
cylinder.chop_axial(count=10)
cylinder.chop_radial(count=10)
cylinder.chop_tangential(count=20)

cylinder.set_bottom_patch('cylinder_bottom')
mesh.add(cylinder)

mesh.merge_patches('box_top', 'cylinder_bottom')

Offsetting Faces

It is possible to create new blocks by offsetting existing blocks' faces. As an example, a sphere can be created by offsetting all six faces of a simple box, then projected to a searchableSphere.

See examples/shapes/shell.py for the sphere tutorial.

Automatic Blocking Optimization

Once an approximate blocking is established, one can fetch specific vertices and specifies certain degrees of freedom along which those vertices will be moved to get blocks of better quality.

Block is treated as a single cell for which OpenFOAM's cell quality criteria are calculated and optimized per user's instructions.

Points can move freely (3 degrees of freedom), along a specified line/curve (1 DoF) or surface (2 DoF).

# [...] A simple setup with two cylinders of different radii,
# connected by a short conical frustum that has bad cells
# [...]

mesh.assemble()

# Find inside vertices at connecting frustum
finder = cb.RoundSolidFinder(mesh, frustum)
inner_vertices = finder.find_core(True).union(finder.find_core(False))

optimizer = cb.Optimizer(mesh)

# Move chosen vertices along a line, parallel to x-axis
for vertex in inner_vertices:
    clamp = cb.LineClamp(vertex, vertex.position, vertex.position + f.vector(1, 0, 0))
    optimizer.add_clamp(clamp)

optimizer.optimize()

mesh.write(os.path.join("..", "case", "system", "blockMeshDict"), debug_path="debug.vtk")

The result (basic blocking > optimized):

See examples/optimization for the diffuser example.

Airfoil core with blunt trailing edge (imported points from NACA generator) and adjustable angle of attack. Exact blocking is determined by in-situ optimization (see examples/complex/airfoil.py). A simulation-ready mesh needs additional blocks to expand domain further away from the airfoil.

Automatic Edge Grading

When setting cell counts and expansion ratios, it is possible to specify which value to keep constant. Mostly this will be used for keeping thickness of the first cell at the wall consistent to maintain desired y+ throughout the mesh. This is done by simple specifying a preserve="..." keyword.

Example: a block chopped in a classic way where cell sizes will increase when block size increases: The same case but with a specified preserve="start_size" keyword for the bottom and preserve="end_size" for the top edge:

Debugging

By default, a debug.vtk file is created where each block represents a hexahedral cell. By showing block_ids with a proper color scale the blocking can be visualized. This is useful when blockMesh fails with errors reporting invalid/inside-out blocks but VTK will happily show anything.

Also!

2D mesh for studying Karman Vortex Street

A parametric, Low-Re mesh of a real-life impeller (not included in examples)

A gear, made from a curve of a single tooth, calculated by py_gear_gen

A complex example: parametric, Low-Re mesh of a cyclone

See examples/complex/cyclone for a full example of a complex building workflow.

Prerequisites

Package (python) dependencies can be found in pyproject.toml file. Other dependencies that must be installed:

• python3.8 and higher
• OpenFoam: .org or .com version is supported, foam-extend's blockMesh doesn't support multigrading but is otherwise also compatible. BlockMesh is not required to run Python scripts. There is an ongoing effort to create VTK meshes from within classy_blocks. See the wip_mesher branch for the latest updates.

Technical Information

There's no official documentation yet so here are some tips for easier navigation through source.

The Process, Roughly

1. User writes a script that defines operations/shapes/objects, their edges, projections, cell counts, whatever is needed.
2. All the stuff is added to mesh.
3. Mesh converts user entered data into vertices, blocks, edges and whatnot.
4. The mesh can be written at that point; or,
5. Modification of placed geometry, either by manually moving vertices or by utilizing some sort of optimization algorithm.
6. Output of optimized/modified mesh.

Support?

If you are stuck, try reading the classy_blocks tutorial on damogranlabs.com.

You are free to join the OpenFOAM Discord channel where classy_blocks users and developers hang out.

If you have collosal plans for meshing but no resources, write an email to Nejc Jurkovic and we'll discuss your options.