Floating point error in Fluent for heat transfer problem of complicated fiber-fluid model. Help?

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  • Last Post 23 April 2019
stewartlnelson posted this 17 April 2019

Hello, 

I'm running into the Floating Point Error in ANSYS Fluent. I've imported a model of a fiber structure that I have encased in "fluid". To do this, I extruded a volume over the space of the fiber model on a different plane and used a Boolean operation to differentiate the two. I'm just changing the element size for meshing. I've also created named sections by selecting the face of a fiber strand and applying it to all surfaces at that x, y, or z coordinate, so I'm fairly certain the fibers and fluid surfaces make contact. In Fluent, I'm running at steady state while using the energy equation and laminar-viscous model. The fluid (air) in this analysis is to be stagnant, so I'm treating that volume as a solid with air properties. I've applied a temperature of approximately 660 K to the top surface and 400 K to the bottom surface. The other four side wall surfaces are at a constant heat flux of 0. All other walls/contacts are set to coupling. I'm using hybrid initialization and when I run the calculation, my residuals (only energy) shoot way up after maybe 80 iterations. If I allow the calculation to continue, I am prompted with the floating point error. I've even relaxed energy to 0.3. Any ideas on what my issue could be and how to fix it? I'll attach a few images to give a better idea of what I'm doing.

 

Mesh with Surrounding Fluid

Mesh with the surrounding fluid

Fiber Mesh

Mesh with the fluid hidden

static temp of interfaces

This is a static temperature contour plot of the interfaces. You can see there is some slightly warmer areas on the top face, but heat isn't moving like it should. This was only after 13 iterations.

This is the same plot, but of the fluid walls.

 

I'd appreciate any help, If you need more information just let me know. Thank you!

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rwoolhou posted this 17 April 2019

I don't think you've got a connected mesh. The mesh looks like the fluid & solid both exist in the same space: the Boolean may not have worked. Can you plot a plane through the middle & colour by velocity contours? 

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stewartlnelson posted this 17 April 2019

I did, but only a blue plane was produced because there is zero velocity in the system due to the air being treated as a solid. Plotting static temperature on this plane mimics the last picture I posted of the temperature contour of the walls, just on a single plane. How would you suggest I go about fixing this issue if the mesh is not connected? I can provide more information as necessary. The fiber model started as an stl file that I converted to a solid in SpaceClaim before importing it to Design Modeler. 

stewartlnelson posted this 17 April 2019

Sorry I meant to reply to you directly, see my other post.

Thanks

rwoolhou posted this 17 April 2019

Thanks.

DM isn't overly good with stl so stick with SpaceClaim. 

In SpaceClaim you should have had a few (3-4 at a guess from the top image) volumes. Create a bounding box that's slightly smaller than the extent of the fibres (they need to stick out of the bounding box). All being well you can copy the fibres (you need to have a duplicate) and then subtract the fibre from the box.  

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stewartlnelson posted this 17 April 2019

I'll attempt this, but I happen to much more familiar with Design Modeler. Essentially all I used SpaceClaim for was to convert the stl file to a solid just so I could use it in Design Modeler. Just so I understand, I should be able to draw a boundary and subtract out my fibers, then place them back in the space?

rwoolhou posted this 18 April 2019

Yes, pretty much. There are several YouTube tutorials on SC and it's fairly easy to use if you have any CAD training. The reason for duplicating the solid bits is in SC there isn't a "retain" when you do the Boolean so you may lose the solid when carrying out the operation. 

stewartlnelson posted this 18 April 2019

I'm unfamiliar with SC, but it doesn't seem too hard. However, I'm not sure how to create a bounding box. I've watch a few videos and it seems to be used fill depressions. I'm not sure how to use it for my case, especially with the complexity of my geometry. I've also tried pulling a box around the fibers and using the combine tool to perform the boolean operation with the fiber as the cutter. So far it has only failed and said it was not able to intersect bodies. Any advice?

rwoolhou posted this 23 April 2019

Make sure the bounding box is slightly smaller than the box you'd actually need to enclose the fibres: the issue is probably due to the surface intersection at the bounding surface. Alternatively you may need to use the wrapper in Fluent Meshing, and there should be basic tutorials on that online: otherwise we do provide training via the ANSYS Learning Hub. 

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