Hello and Welcome! In this video, which is part-2 of the two-part series on the various Fill With methods available in
Generate the Volume Mesh task, we will specifically discuss about the hexcore and poly-hexcore fill with methods,
and their required user inputs.
Let’s get started.
We will first discuss the hexcore Fill With method.
The hexcore volume mesh approach is a hybrid meshing scheme where axis-aligned Cartesian cells
are generated in the core region of the domain and tetrahedral cells are generated near the boundaries.
The transition between the hexahedral elements and the tetrahedral elements is created by
triangulating the external surface of the last hexcore layer and then generating the
tetrahedral elements that are connected to the surface mesh or the boundary layer mesh.
No pyramid elements are used to facilitate the hexahedral-tetrahedral transition.
The hexcore approach is useful mainly for computational models with large internal regions and few internal boundaries such as
intrusions or holes.
Let us now use a demo problem to see this approach in action.
Launch Ansys Fluent in meshing mode. Go to File, Read, and select Mesh, and then pick the provided mesh file.
Once Fluent finishes reading the file, you will notice that the watertight geometry workflow has been automatically setup
and all the tasks till the Add Boundary Layers task have already been successfully completed.
The model we have here is that of a generic two-element rear wing found on a formula series type race car,
enclosed within a virtual wind tunnel.
Except for the inlet and the outlet, all the other sides of the wind tunnel
and the wing geometry are considered as walls.
The model consists of only the fluid domain with the wing considered as a void.
Now, go to the Generate the Volume Mesh task.
Here, under Fill With, select hexcore.
Based on this selection, new user inputs appear in the task which are Buffer Layers, Peel Layers,
Min Cell Length and Max Cell Length.
Let us now understand each of these inputs and how they influence the hexcore volume fill approach.
As the names indicate, the Min Cell Length and Max Cell Length parameters control the minimum and maximum sizes of the cells
that are created during the volume mesh generation.
By default, Fluent automatically calculates these values depending on existing surface mesh of the computational model.
However, the user can change these values as required.
The hexcore and the poly-hexcore methods follow the Octree meshing scheme.
In this scheme, discrete values of cell sizes are used which results in multiple levels of isotropic Cartesian hexahedral mesh.
The cell sizes in each adjacent level of the mesh differ by a factor of 2. Let’s understand this in detail.
In our demo problem, based on the existing surface mesh, Fluent estimates a min and max cell length of 2 mm and 128 mm, respectively.
If we now change the minimum to a different value, say 3 mm, notice that max cell length is automatically updated,
such that it is some power of two times the min cell length.
In this case, it is 2^6 times min cell length.
On the other hand, if we change the maximum value, to say 280, it is automatically modified such that
it is some power of two times the min cell length.
Here, the maximum value is set to 256 mm since it is 2^7 times the min cell length.
Buffer layers are additional layers of Cartesian cells that are created at the transition location, before stepping to the next size.
By default, the number of buffer layers is set to 2.
It is quite easy to see the impact of this parameter when looking at these images.
The buffer layers are set to 1 for the image on the left and 3 for the image on the right.
In the first case, there is a relatively rapid transition between two levels of the Cartesian mesh,
whereas it is much smoother in the 2nd case. The default value of 2 is generally sufficient for most cases.
Let us now look at the peel layers parameter.
These layers control the gap between the Cartesian mesh and the geometry or last layer of boundary layer mesh
when they are included in the computational model.
The smaller the number of peel layers, the closer is the Cartesian mesh to the boundary surface
or the boundary layer mesh and vice-versa.
The gap is filled with tetrahedral elements which act as transition cells between the triangulated surface mesh
or the boundary layer prism mesh and the core Cartesian mesh.
The default value for Peel Layers is 1, which corresponds to a gap that is equal to the height of an ideal tetrahedral cell on the boundary face.
If Peel Layers is set to 0, the gap size can be smaller than the ideal height which results in a mesh
that contains the maximum number of Cartesian cells possible for the chosen parameters.
Here is a comparison of two meshes, where the one on the left was created with peel layers set to 1
and the one on the right with peel layers set to 2.
The larger thickness of the tetrahedral cell layer for the mesh on the right can be easily noticed.
In almost all aspects, the Poly-hexcore fill with method is identical to the hex-core method,
except that the polyhedral elements are created instead of tetrahedral elements.
As discussed in part-1, polyhedral meshes are known to reduce the overall cell count of the mesh.
As a result, solution convergence is accelerated.
Furthermore, each poly cell contains at minimum of 7 faces and is therefore surrounded by many adjacent neighbor cells,
resulting in better approximation of the gradients, and lower numerical diffusion effects,
which are caused when the flow is not normal to the cell faces.
The primary inputs for the poly-hexcore fill method are identical to hexcore method.
Here is a side-by-side comparison of the two meshes for our demo geometry.
Notice that in addition to the difference in cell type i.e., tetrahedral for hexcore and polyhedral for poly-hexcore,
there is also a difference in the total cell count and the minimum quality with the poly-hexcore mesh containing fewer, higher quality cells.
The core Cartesian mesh looks nearly similar for both cases.
Based on originally estimated min and max size of 2mm and 128 mm,
considering the Octree meshing scheme, there are 7 distinct layers of hexahedral elements with different cell sizes.
At the transition location between levels, the larger cell is split into 8 smaller cells,
which is generally referred to as the 1/8th Octree transition.
This leads to the creation of hanging nodes for the finer cell size layer.
As a result, all hexcore and poly-hexcore meshes created in Ansys Fluent Meshing are by default non-conformal type of meshes,
and hence require the use of an appropriate solver such as Ansys Fluent which can handle such meshes.
Let us summarize what we learnt.
In this lesson, which is part-2 of the two-part series on discussing the various fill with methods in the Generate the Volume Mesh
task of the Ansys Fluent Meshing Geometry Workflow, we focused our attention on understanding the hexcore and poly-hexcore
volume fill methods. We also discussed the required user inputs and how they influence the overall volume mesh generation in detail.
With that let’s wrap up the lesson.