Learning Forum

[Ivosp]

The problem I am simulating is, simplified, a box inside a fluid flow. I want to analyze how different AOAs (angles of attack) affect some things of the box. For that, I created a spherical fluid body (pink in pic) which can be rotated by changing some parameters in workbench/spaceclaim. This is so that the mesh does not have to be recalculated each time, but so that only the transition between the two fluid bodies has to be calculated. For this, I use a non-conformal mesh. Just for clarification, the box is handled as a dead area.

I set both boundaries, the spherical surface of the outer and inner fluid, which represent the transition, to internal.

When it comes to "Update Regions" of course the 2 fluid bodies are set to fluid. Then there are 2 more regions created by fluent, fluid:1, fluid:3, due to the Non-conformal mesh setting. The only region type working for both of them is dead. But then Fluent changes one of the boundaries from internal to wall. The resulting mesh is shown in the picture below. Due to that, no flow goes through the fluid-fluid transition.

I already found a similar problem with the solution to change the question Change all fluid-fluid boundary types from 'wall' to 'internal'? from No to Yes. If I do this, fluent changes the boundary between the inner spherical fluid body and the dead box to internal, because at that point fluent doesn't know that is is a dead body.

Can anyone help me?

 

[Rob]

If the boundary is internal/interior it's not nonconformal, the latter is for meshes that don't match.  The outer and inner fluids make sense, and the volume inside the object would be dead (void) or solid. Other than the inflation between the zones it looks OK. 

[Ivosp]

Okay, but what kind of boundaries should I use then (for the transition between the fluid bodies)? How do I make sure, that the fluid goes through? I need to make it nonconformal due to the parameters.

[Rob]

Are you intending to alter the mesh angle in Fluent Solver, or at the geometry stage & then remesh?  

For a conformal mesh you want an interior/internal boundary between the zones. They're much the same once you're in Fluent, but Fluent Meshing may pick one over the other. 

[Ivosp]

It's my first time dealing with parameters. I created them in Spaceclaim and after that a "Parametric Set"-Box was created in Workbench, with an arrow to the geometry stage. If I click on that box, a new window tab inside Workbench where I can see and change the parameters. Since the arrow of the box only goes into the geometry stage, I guess I am altering the mesh angle at the geometry stage. Nevertheless, fluent doesn't want a conformal mesh and therefore a shared topology, I only got errors. How is it possible to alter the mesh angle in Fluent Solver?

 

[Rob]

Fluent should want conformal mesh, unless you've set something else. 

In Fluent have a look at Cell Zone > Mesh Motion. If you copy settings from Frame Motion the internal should be slit and a nonconformal mesh set up. Depending on how many runs you want you may be better off just building a few models and running manually. 

[Ivosp]

But Fluent Meshing displays an error if I try to do a conformal mesh (-> Fluent Meshing switches Share Topology? automatically to yes because of the 2 touching areas) saying sth about self-intersecting triangles. I used the Join-Intersect-Setting. How do I solve that problem?

So, just to be sure I understood it correctly:

• If I need a lot of runs, the best way is to simulate it with Mesh Motion inside Fluent Solution. I don't need parameters in Workbench for that and I don't need a remesh.
• If I only need a few runs, the best way is to change the angles inside the Geometry stage. For that, I need to remesh it individually for every single angle.

If so, I have the following question: Should I use a conformal mesh in both cases or what's best/needed? 

[Ivosp]

Addition to the last post:

If I remesh every time, is it even necessary/does it even make sense to have 2 fluid bodies? I mean, it's only one fluid in reality.  

[Rob]

If you're remeshing every time then a single body makes more sense, and that also eliminates the poly layers in the middle of the domain. 

I'd use a conformal mesh (share topology at geometry level) and then slit the interior to get a nonconformal mesh, the Frame to Mesh Motion will do that for you: I've been using the code a while so use the "old" command approach out of habit. You can then alter the AoA using the flow solver to cheat. 

[Ivosp]

Okay, I tried using a conformal mesh with shared topology at the geometry level. After importing the geometry to fluent meshing, I managed to generate the volume mesh with a conformal mesh (see pic below). It consists of 1 object and 2 cell zones (And 9 face zones). When clicking on the button for switching to Fluent Solution it gets stuck at some point and it says that 2 zones are missing ("WARNING: Current Mesh is incompatible with mesh operations defined. Following zones are missing: ..."). Those are the 2 spherical surfaces touching each other which are now connected via shared topology at the geometry level. Since it's my first time dealing with shared topology, especially at the geometry level, I am a little lost about what went wrong...

[Ivosp]

So the settings I used for describing the geometry are (the ones I changed is written in bold)

• Geometry Type -> The geometry consists of both fluid and solid regions and/or voids
• Will you cap openings and extract fluid regions? -> No
• Change all fluid-fluid boundary types from 'wall' to 'internal'? -> Yes
• Do you want to apply Share Topology? --> No
• Enable Multizone Meshing? -> No

The boundary types I used for the touching spherical surfaces are internal. 

I didn't change the rest of the settings. 

 

[Ivosp]

I changed the following setting to Yes:

• Do you want to apply Share Topology? --> Yes

The problem is still the same.

[Rob]

If you've changed the topology the Solver may complain about missing zones but that shouldn't prevent you from then revising the settings.