Hello and Welcome!
In this lesson, we will learn about the Apply Share Topology subtask of the Describe Geometry
task in the Ansys Fluent Meshing Watertight Geometry Workflow.
This lesson is part-1 of the two-part series and will focus on the join-intersect method
to performing share topology operation.
We will dig into the finer details of this method and understand how and when to use it.
Let’s get started.
For multi-body geometries where different bodies may be in contact with each other,
it is a recommended practice to perform the ‘Share Topology’ operation at the CAD
creation phase.
This ensures that all the overlapping areas are handled appropriately.
Quite often, this means that instead of each of the bodies having separate faces which
leads to non-conformal mesh at the interface, the two bodies share a single face at the
interface resulting in a conformal mesh.
Though the Fluent solver is capable of handling both kinds of meshes, conformal meshes are
generally preferred due to their relatively higher numerical efficiency and smaller simulation times.
In certain cases, however, when it is only possible to perform partial share topology
or in fact, not possible to perform share topology at all at the CAD creation phase,
this operation can be performed during the meshing phase when using the watertight geometry workflow.
Let us now use a demo problem to understand how to setup and use the share topology operation
in the watertight geometry workflow.
Launch Ansys Fluent in meshing mode and select Watertight Geometry workflow.
In the ‘Import Geometry’ task, leave all other options to default and read the provided geometry file.
We will not be adding any local sizing and will create the surface mesh with default
settings for this demo.
As soon as the surface meshing is created, we are prompted with a message ‘The Share
Topology task should be added in order to proceed’, and an ‘Apply Share Topology’
task is added as a subtask to the ‘Describe Geometry’ task.
Ansys Fluent automatically adds this task to the workflow after the surface mesh is
created, if it detects that a multi-body part has been imported without any shared topology.
Let’s quickly look at our CAD model.
Here we have two pipes connected to each other using multiple nuts and bolts, which make
up the solid domain of model, and the internal fluid domain which is the fluid flow region.
Let us now look at how to setup the share topology task in the workflow.
Before moving to the ‘Apply Share Topology’ task, the Describe Geometry task needs to
be executed.
Here, since we have both the solid and the fluid regions, we will select the third option
for the Geometry Type.
We will leave all other options to default and click on Update.
In the Apply Share Topology task, we have two basic user inputs.
The Max Gap Distance is the maximum gap that is expected between two bodies.
Surfaces from different bodies that are separated by a distance greater than this will be ignored
for the shared topology operation.
To highlight the surfaces that are marked for sharing, click on Mark Gaps.
It is mandatory for the Max Gap Distance to be less than or equal to half the Min Size value that was
used to generate the surface mesh.
Furthermore, it should never exceed the thickness of the smallest of the solid or fluid bodies
in the model, which otherwise may lead to collapse of such bodies.
For e.g., at the current value of 0.517, Fluent can identify two distinct shared regions : 1
between the bolt and the washer and the other between the washer and the pipe.
If we instead use a value of 2 mm, notice that the entire washer surface is highlighted.
Since the thickness of the washer is 2mm, when we specify the Max Gap Distance to be
2mm, Fluent is assuming that the pipe is in contact with the top surface of the washer,
instead of the bottom, in effect collapsing the washer.
On the other hand, it is possible that the default value is not sufficient for a few
surfaces, and hence needs to be increased.
Therefore, precaution needs to be taken while defining the Max Gap Distance to ensure that
all the surfaces that need to be shared are selected, all the while ensuring that no bodies
are collapsed.
The next input is the Share Topology Method, which is by default set to Join-Intersect.
The other option for this method is Interface Connect, which will be discussed in part-2
of this lesson.
Let’s look at the Join-Intersect method now.
This name basically represents two different ways of connecting surfaces – Join and Intersect.
The default is Join only and is picked as the option for Join and Intersect Method setting
under Advanced Options.
In this method, the overlapping areas of the surfaces are first identified and separated
from their parent surface, based on the specified parameters in the Advanced Options section,
and then are merged into a single surface.
As you can see here , the previously single surface for the fluid region has been split
into two, to join with the inner surfaces of the pipes.
The new surface is also remeshed to create a conformal mesh between the different objects.
The settings under the Advanced Options section can be left at their default values for most
of the cases.
For more information on each of these settings, please refer to the User Guide.
Another option for the Join and Intersect Method is Intersect only.
This method is primarily used for connecting bodies at the boundaries rather than fixing
overlapping faces.
This method is especially helpful when there are baffles or zero-thickness surfaces in
the model that intersect other surfaces.
In this method, mesh faces that intersect each other along the boundaries of surfaces
are selected and connected.
Once the connection is established, the area is locally remeshed to generate a conformal mesh.
The last option for Join-Intersect method is the Join and Intersect, where both the
previously discussed methods are employed to share topology between different bodies.
Once the appropriate max gap distance and the share topology method, with its associated
options are selected, click update to perform the share topology operation.
Once done, the overlapping and intersecting faces are now fixed, and a conformal mesh
is created.
Let’s summarize what we learnt in this lesson.
In cases where share topology cannot be performed at the CAD creation phase, it is possible
to do this operation at the meshing phase when using the watertight geometry workflow
using the Apply Share Topology task.
We looked at the default method i.e., join-intersect method to share topology between different bodies.
We learnt about the functioning of the join and the intersect methods and the care that
needs to be taken while defining the max gap distance.
With that, let’s wrap up this lesson.