In this week’s blog post we will take a closer look at surface and volume extruders. This feature is very powerful for extending inlets and outlets. Extending open boundaries is a recommendation since we don’t want them to affect the solution of the simulation in terms of numerical instabilities. For example, in pump simulations it is recommended to use 5-7 times the diameter of the pipe/volute as extended distance for the open boundaries, otherwise the boundaries may affect the solution within the pump. The extruders will be attached to the geometry with an interface, on which you can measure physical entities, so you will not lose the possibility to evaluate the simulation at the open boundaries.
Note: there are many ways of extruding inlets and outlets. The approach described in this blog post is just one of these approaches and it is chosen since the workflow is robust and you will get an accurate solution meanwhile the increased number of cells in the computational domain is kept low.
How to create and implement extruders
In the Operations node (within the Geometry node) in your simulation, you find Surface extruder at Surface preparation. The surface extruder operation creates a new part, connected to the surface you want to extrude, and this part is what you later should use as input to the Volume extruder. The volume extruder is the operation which will generate the extruder mesh. In the picture below you see the inputs needed for the surface extruder operation, where you also define the extrusion distance. You can create the operation for the volume extruder automatically by enabling the Upon creation option already in this window, or you can find this operation as a mesh operation.
In the volume extruder operation you always use the surface extruder as input part. Here you also have the possibility to activate Auto-retraction, which runs a check for intersections and retracts the volume if necessary. Hopefully you will not have any issues with your surfaces, but this option might help you if your geometry has underlying issues.
Before executing the extruder operations you need to assign the surface extruder part to the region of interest, which often is an Existing region (Body 2 in this example) connected to your geometry which needs extruding.
An interface between the previous open boundary (in this example an outlet is extended) and the surface extruder is automatically created and overwrites the boundary condition at the previous open boundary. You then have to manually change the boundary condition for the surface extruder’s open boundary. In this example we change the new outlet boundary to a pressure outlet.
By executing the extruder operations you will now mesh the extrusion. In the picture below you see an extrusion using One sided hyperbolic stretching method (extruder part visualized with in purple). This type of stretching method is often a good choice to use in order to get a good growth rate of the cells in downstream direction. In the next section of this blog post there will be a more detailed discussion regarding different settings for volume extruders.
Settings for the volume extruder
In the video below there is a step-by-step description of how to set up this example simulation. At around two minutes into the video there is a visualization of the different stretching methods available (visualized by the extruder mesh that is generated). The settings available to control the mesh are
- Number of layers: How many layers to use from the previous outlet to the new outlet.
- Stretching method: Define how the relation between layers should grow.
- Constant – all layers use the same thickness.
- Geometric – ratio in thickness between the layer of interest divided by the previous layer.
- Hyperbolic (one sided) – uses a hyperbolic stretching law, growing from small to larger cells towards the upstream direction.
- Hyperbolic (two sided) – uses a hyperbolic stretching law where both the inlet and the outlet of the extrusion get thinner layers towards the boundaries and have the maximum stretching in the middle of the extrusion.
- Stretching mode: Choosing with what relation you want to define the value for stretching.
- Stretching – ratio in thickness between the layer of interest divided by the previous layer.
- Initial wall thickness – thickness of first layer.
- Thickness ratio – ratio between the first and last layer.
- Stretch value: Value for the stretching.
Multi-extruders and types of grid
In one of our previous blog posts at the Volupe blog, the new features from version 2022.1 within marine applications, multi-extruders were presented. This feature allow you to use multiple surfaces within the same operation, which both makes the operation more robust and your simulation tree will be more organized. Please re-visit the blog post via the link below for more details about how to use this feature.
Simcenter STAR-CCM+ version 2022.1 news part 3 – Marine application
In the blog post there is also discussions about the more advanced setting of chosing O-grid or H-grid for your extruders. This setting is set in the surface extruder. The picture below visualizes the difference between the two grid types.
We at Volupe hope that this blog post will help you with your simulations in Simcenter STAR-CCM+. If there are any questions you are always welcome to write to us at firstname.lastname@example.org.
Christoffer Johansson, M.Sc.