[CellZone]

Hi,

1) everybody is talking about the time effort to have a good mesh. I am wondering, why it is so time consuming, to build up a good mesh? Because the meshing itselt is normally done automatically?

So is the main challenge in meshing basically to get a waterproofed geometry?

So for CFD you need to create a surface mesh of the geometry. This is for example done in Ansa. Here the challenge is, that if I get my geometry of for example Catia, then when I load it in Ansa, there will be a huge mess because there are intersecting surfaces and holes, so that you first have to fix this, in order to create the waterproofed surface mesh?

2) The first step in a CFD Simulation is to create a surface mesh, right? So if I have my geometry I can use Ansa or ICEM to create this mesh. If I have made my mesh I can afterwards use it both in CCM+ , OpenFOAM, PowerFlow, ... , is this right? I only have to watch out, that it is exported the right Format?

Thank you for your help

[lcarasik]

Originally Posted by CellZone

Hi,

1) everybody is talking about the time effort to have a good mesh. I am wondering, why it is so time consuming, to build up a good mesh? Because the meshing itselt is normally done automatically?

So is the main challenge in meshing basically to get a waterproofed geometry?

No, honestly the easiest part is getting a waterproof geometry. Designing a good mesh is critical to capturing the physics of the problem correctly.

Originally Posted by CellZone

So for CFD you need to create a surface mesh of the geometry. This is for example done in Ansa. Here the challenge is, that if I get my geometry of for example Catia, then when I load it in Ansa, there will be a huge mess because there are intersecting surfaces and holes, so that you first have to fix this, in order to create the waterproofed surface mesh?

2) The first step in a CFD Simulation is to create a surface mesh, right? So if I have my geometry I can use Ansa or ICEM to create this mesh. If I have made my mesh I can afterwards use it both in CCM+ , OpenFOAM, PowerFlow, ... , is this right? I only have to watch out, that it is exported the right Format?

Not exactly, you can generate a surface mesh in some cases (or software). Mesh for 3-D have to be volumetric and not surface meshes. Ideally, ICEM and other GENERAL meshing software can export to the standard solvers assuming they are either a FVM or FEM method.

[CellZone]

Thanks for your quick response

Designing a good mesh is critical to capturing the physics of the problem correctly.

But don't the commerical packages already have strong automatic meshers?

Mesh for 3-D have to be volumetric and not surface meshes.

Sure, the computing area has to be 3D volumetric mesh. But when I for example use Ansa, then I try to make a waterproofed geometry. Afterwards I make a mesh over my geometry in Ansa (so this is the surface mesh?). Then I Export it to example CCM+. And then I start making my volumetric mesh. But to recognize the computing area, where my volumetric mesh has to be placed, I need to Export a mesh out of Ansa?

[BlnPhoenix]

Originally Posted by CellZone

But don't the commerical packages already have strong automatic meshers?

They say meshing is an art form. Which you will understand, once you try to mesh complexer problems. Automatic meshers do not magically create good quality meshes, with a minimum cell count and all physics well resolved. They need someone who makes them do stuff that in the end makes sense for CFD.

[vangelis]

I would like to agree that the challenging part of creating a good quality CFD mesh for a complicated geometry is creating first a watertight closed surface.
Intersecting parts, closing gaps and holes, finding leaks, proximities etc.

Once this is done, then surface and volume meshing is automated.

In ANSA there are many tools to create watertight geometries and indeed this is the most time consuming part of the process.
There are of course alternative approaches, such as surface wrapping to create a watertight surface mesh, but you may lose some accuracy as this is a more draft approach.

Once you have the watertight geometry in ANSA you should use Batch Mesh to create surface mesh, layers and volume mesh in an automated manner.
Surface meshing in ANSA automatically resolves all the features (curvature, sharp edges, proximities etc) and you have complete control of the process.

Of course for solvers like Powerflow, Xflow and other non NS codes, you only need a surface mesh.

[CellZone]

Originally Posted by vangelis

Surface meshing in ANSA automatically resolves all the features (curvature, sharp edges, proximities etc) and you have complete control of the process.

What I do not understand: why do I have to make a surface mesh in Ansa, export it to e.g. to CCM+ in order to create a volume mesh for my computing area? Why can I not just import my geometry and CCM+ recognizes the surface of the geometry and starts creating a volume mesh. I do not get why I need a surface mesh?

Originally Posted by vangelis

Of course for solvers like Powerflow, Xflow and other non NS codes, you only need a surface mesh.

I only need a surface mesh? But in Powerflow I also have to create voxels (volumetric elements) ?

Thanks to all of you for your participation in this Topic - it's very important for my understanding

[vangelis]

The are two different volume meshing approaches:
One is creating the volume mesh by growing it from the surface mesh,
the traditional ANSA approach
The other approach is octree based meshing where the volume mesh
does not concide with the surface mesh. The surface mesh there is only
used as a description of the geometry. Octree volume meshing creates
a new surface mesh.

In both cases however you need a surface mesh, whether this is good
quality CFD mesh for growing volume mesh from it, or a simple STL mesh
as a description of the geometry.

You can read geometry in StarCCM but even star would have to create a surface mesh before generating the volume mesh. The surface must first be discretized in shell elements in order to be volume meshed by any of the two approaches.

Finally if you create a surface mesh in ANSA you can also create a volume mesh
in it and then output to StarCCM just for the solution.