4.2. Geometry

The Geometry pane is used to define the model geometry. This includes whether the model is 2D or 3D, and the overall domain extents (xmin, xmax, ymin, ymax, zmin, zmax). If there is complex geometry, the “Auto-size” button can automatically set the extents to surround the geometry.

The geometry section provides tools for adding geometry objects (from STL files, or primitive elements), filters to transform geometry objects, and wizards to automate creating common complex geometries. Geometry objects can be copied, removed, or combined using Boolean operations. All the geometry operations are done using the Visualization Toolkit (VTK)’s library.

Geometry toolbar icons:

Icon

Description

Add

Add geometry to model

Filter

Modify selected geometry with filter

Wizard

Add geometry from wizard

Remove

Remove the selected geometry

Copy

Add duplicate of the selected geometry (copy/paste)

Union

Add union of two or more selected geometries

Intersect

Add intersection of two or more selected geometries

Difference

Add difference of two or more selected geometries

In the geometry tree, the geometry object is displayed with the following icons:

Icon

Geometry Type

Geometry

polydata

Function

implicit

Filter

filter

Union

union

Intersect

intersect

Difference

difference

4.2.1. Adding Geometry

To add a Geometry object, click on the Add (add) icon and select the geometry to add from the drop-down menu.

There are two types of geometric objects supported in the GUI: implicit functions (quadrics), and objects defined by polydata (everything else: STL files, procedural shapes, primitives, parametrics, wizard geometry).

The following geometric objects can be added:

  • STL files (select a *.stl file from a file dialog)

  • Procedural shapes (cylinder, bend, circle to rectangle, body of revolution)

  • Implicit (Quadric) functions (sphere, box, cylinder, cone, quadric, superquadric)

  • Primitives (sphere, box, cylinder, cone)

  • Parametrics (torus, boy, conic spiral, etc.)

4.2.2. Procedural shapes

Procedural shapes were introduced in the 21.1 release. These are triangulated polydata objects that are defined by a series of parameters. They offer better control on the triangles quality than implicits or primitives.

4.2.2.1. Cylinder

Procedural cylinder
Parameters:

  • Radius (R)

  • Height (H)

  • Number of divisions along the height

  • Starting angle in the circumferential direction

  • Ending angle in the circumferential direction

  • Number of divisions along the circumferential direction

  • Bottom cap

  • Number of divisions in the bottom cap’s radial direction

  • Top cap

  • Number of divisions in the top cap’s radial direction

The top and bottom caps are shaped like fans, i.e., they converge radially towards the center of the cap. Caps are not needed if the cylinder extends past the MFiX domain in the axial direction, when inlet/outlet boundary conditions are defined along the MFiX box (say at y=ymin and y=ymax for a vertical cylinder).

It is recommended to include caps when combining shapes (union, intersection, difference) since the boolean operations are more robust with closed shapes.

4.2.2.2. Bend

Procedural cylinder
Front section:

  • Length (L f)

  • Radius (R f)

  • Number of divisions along the front section length

Back section:

  • Length (L b)

  • Radius (R b)

  • Number of divisions along the back section length

Bend section:

  • Major radius (R M)

  • Minor radius (R m)

  • Starting angle (\({\theta}\) f)

  • Ending angle (\({\theta}\) b)

  • Number of divisions in the angular direction

Circumference:

  • Starting angle in the circumferential direction

  • Ending angle in the circumferential direction

  • Number of divisions along the circumferential direction

Caps:

  • Bottom cap

  • Number of divisions in the bottom cap’s radial direction

  • Top cap

  • Number of divisions in the top cap’s radial direction

4.2.2.3. Cylinder to rectangle

Procedural circle to rectangle shape

Distance: distance between circle and rectangle in z-direction (D)

Resolution: number of divisions in the z-direction

Circle section:

  • Radius (R)

  • Cap: option to fill the circle

  • Resolution: Number of divisions along the cap radial direction

Rectangle section:

  • Width in x-direction (W)

  • Resolution: number of divisions along the rectangle’s width

  • Height in y-direction (H)

  • Resolution: number of divisions along the rectangle’s height

  • X Offset, Y Offset: location of the rectangle’s center (x0,y0)

  • Cap: option to fill the rectangle

Notes:

  1. The cylinder’s center is located at (x=0,y=0,z=0).

  2. The number of divisions along the circle’s circumference is the sum of the number of divisions of the rectangle’s width and height.

4.2.2.4. Body of revolution

Procedural body of revolution

The body of revolution shape revolves a profile (xy plane) around the z-axis. A series of line or arc segments is defined in a table. Click the Add (plus sign) to add a segment and select either line or arc.

Segments:

Line:

  • (X1, Y1): coordinates of the first point

  • (X2, Y2): coordinates of the second point

  • Div: Number of divisions along the segment

Arc

  • (X1, Y1): coordinates of the arc’s center

  • R: radius

  • Theta1: starting angle (degrees)

  • Theta2: ending angle (degrees)

  • Div: Number of divisions along the segment

Revolution:

  • Starting angle (degrees)

  • Ending angle (degrees)

  • Resolution: Number of divisions along the circumferential direction

  • Offset: offset in y-direction (per revolution)

Notes:

  1. See figure above for an example. The first and last segments are used to close the shape (bottom and top caps). Having a closed volume is preferred if the shape is used in boolean operation with another shape.

  2. For complex shapes, it may be useful to align the view in the xy plane, set the ending angle to a small value (say 5 degrees) and set the style to wire with a black color. This allows to get a better view of the profile (see figure below). Once the profile is properly defined, set the ending angle to 360 degrees to generate the full 3D shape.

Procedural body of revolution

4.2.3. Applying Filters

To apply a filter to the selected geometry, select a filter from the Filter menu. The filter options can be edited in the parameter section. The following filters are included:

Filter

Description

vtk class

sample implicit

converts an implicit function to polydata

vtkSampleFunction

transform

rotate, scale, translate polydata

vtkTransformPolyDataFilter

clean

merge duplicate points and remove unused points and degenerate cells

vtkCleanPolyData

fill holes

fill holes

vtkFillHolesFilter

triangle

make sure all polys are triangles

vtkTriangleFilter

decimate

reduce the number of triangles

vtkDecimatePro

quadric decimation

reduce the number of triangles

vtkQuadricDecimation

quadric clustering

reduce the number of triangles

vtkQuadricClustering

linear subdivision

subdivide based on a linear scheme

vtkLinearSubdivisionFilter

loop subdivision

subdivide based on the Loop scheme

vtkLoopSubdivisionFilter

butterfly subdivision

subdivide based on 8-point butterfly scheme

vtkButterflySubdivisionFilter

smooth

move points based on Laplacian smoothing

vtkSmoothPolyDataFilter

windowed sinc

move points based on a windowed sinc function interpolation kernel

vtkWindowedSincPolyDataFilter

reverse sense

reverse order and/or normals of triangles

vtkReverseSense

Refer to the VTK website for details.

4.2.4. Wizards

Three wizards are available to more easily create common multiphase flow geometries: cyclones, reactors, and hoppers. A special “distributed” wizard is used to distribute one geometry inside another geometry with random, cubic, or body centered cubic positions. Random rotations can also be applied with the wizard.

Cyclone Hopper Reactor

4.2.5. Remove & Copy

To remove a selected geometry, click the Remove (Remove) button. If a geometry is grouped as part of a Boolean Operation, removing it is disabled until that Boolean Operation is removed first.

To duplicate the selected geometry, click the Copy (Duplicate) button. If multiple geometry objects are selected, duplication is disabled.

4.2.6. Boolean Operation

There are two types of geometric objects supported in the GUI: implicit functions (quadrics) and objects defined by polydata (everything else: STL files, primitives, parametrics, wizard geometry). Boolean operations cannot be performed between polydata and implicit geometry objects; the implicit function needs to be converted to polydata by using the sample implicit filter. Converting the implicit function also needs to be done in order for the GUI to export a STL file that the mfixsolver can use.

Boolean operations can only be performed with geometry objects of the same type (implicit, polydata). Boolean operations can not be performed between polydata and implicit geometry objects. If an implicit and a polydata must be managed together, the implicit object must be first converted to a polydata object using the sample implicit filter.

Note

Boolean operation between two polydata objects is supported by MFiX, but for complex objects the VTK library may crash.