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Background mesh

First step of the meshing process is a definition of a background mesh.
Background mesh is an initial mesh from which snappyHexHesh starts generating the final mesh following an input geometry.
The background mesh must include hexahedras only.
Boundaries of the background mesh can fully (or partially) define the outer shape of the final mesh (typically for external aero simulations), or can disappear (typically for internal aero simulations). Therefore, background mesh position and dimensions are crucial.
The simplest way for defining the background mesh is to set and run blockMesh utility.
In this example, the blockMesh mesh is defined so that the input geometry sits inside the backgroundm mesh, see Figure .

Figure: Background mesh position.

Go to the case and run:
# blockMesh > log.blockMesh 2>&1 &
The background mesh together with the input geometry can be visualized in ParaView. In the test case directory run:
# paraFoam &
- visualize internalMesh (from Mesh Parts)
- set Wireframe representation (default is Surface)
- open *.stl files from costant/triSurface directory
Whenever the background mesh is properly defined the snappyHexMesh-ing can be performed.

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The open code

The OpenFOAM source code is open
Compiled data can be distributed only with its source code
Each user of OpenFOAM can add/remove/modify the source code and freely use it
The OpenFOAM project is rapidly developing, due to its open-source nature. The OpenFOAM is quickly becoming popular and its user community is growing.

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Previous: Example: Meridional average Up: Example: Meridional average Next: Example: Blade-to-blade view
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Step by step guide

Step 1 — Load the OpenFOAM case into ParaView and make sure that you included the rotating zone. Depending on the way how you loaded the OpenFOAM case you can use either the “Include Zones” check-box above the mesh/field selection frames (figure

left), or “Read Zones” check-box below to read the rotating zone (figure

right). The former way, more frequent in Linux, will add available zones at the end of the “Mesh Parts” selection list (and you need to select them manually). The latter way, more frequent in Windows, will automatically read all available zones after clicking on the “Apply” button. Choose some non-zero simulation time, select appropriate components (“Mesh parts”) and fields (“Volume fields”) and press “Apply”. This will load the mesh.

Step 2 — Meridional average can be calculated by an application of the filter Meridional Average, which is part of Turbo Blade Post. You should see the icon of the filter in the toolbar. Note that the filter is available (i.e. coloured and clickable) only when the data selected in “Pipeline browser” are of the type “Unstructured grid” (see panel “Information”, section “Statistics”). If we loaded several blocks (mesh parts), we would first need to extract the appropriate mesh zone using the filter Extract block, see figure

.

Either use this button, or select the filter Meridional Average in the Filters > Turbomachinery (or Filters > Alphabetical) menu, or use the search box from Filters > Search. This will add the filter into the “Pipeline browser”.

Step 3 — The properties of the filter Meridional Average are shown in the figure

. Some of the options are advanced and can be displayed using the “Toggle advanced properties” button (wheel symbol). The setup consists of a just a few numbers: (a) rotation axis, (b) axis origin, (c) clip out radius and (d) resolution. The “resolution” is the number of points of the resulting projection in radial or axial direction, whichever is larger. A non-zero “Inner radius” is necessary if the rotation axis pierces through the computational mesh, i.e. if there is no hole along the axis. In this tutorial the rotation axis is the axis

, the origin coincides with the coordinate system origin and we choose the clip out radius to be 0.01 m, as shown in the figure. Confirm the settings by pressing the “Apply” button. A non-zero “Outer radius” can be used to clip out some unwanted parts.

Either use this button, or select the filter Meridional Average in the Filters > Turbomachinery (or Filters > Alphabetical) menu, or use the search box from Filters > Search. This will add the filter into the “Pipeline browser”.

Figure: Properties of the filter Meridional Average, part of the Turbo Blade Post postprocessing toolbox. The parameters have default values, except for the non-zero “Clip out radius”.

Step 4 — Once the filter completes, it will produce a projection as in the figure

. By default, it shows the distance of individual points to the nearest surface (hub, shroud, inlet or outlet). However, all scalar fields have been averaged by the filter and are available in the field selection drop-down list in the main toolbar. The figures

and

show the averaged results for static pressure and relative velocity, respectively.

Figure: Resulting projection of the geometry constructed by the Turbo Blade Post toolset (Meridional Average filter) showing an auxiliary field, together with the original geometry and the cutting cylinder that corres

Figure: Meridional average of the static pressure for an incompressible flow in a pump constructed by Turbo Blade Post / Meridional Average. Results are from a calculation by TurbomachineryCFD using OpenFOAM.

Figure: Meridional average of relative velocity restricted to the slice for an incompressible flow in a pump. Contructed by Turbo Blade Post / Meridional Average. Results are from a calculation by TurbomachineryCFD using OpenFOAM.

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