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Extremely slow Lagrangian Particle Tracking

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Old   April 11, 2020, 14:50
Default Extremely slow Lagrangian Particle Tracking
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Robert Clark
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Hello all,

I'm relatively new to CFD, although my colleagues in my lab group have been extremely helpful getting me up to speed, but unfortunately they've been unable to help me with this issue.

I'm running into a weird issue where Lagrangian Particle tracking in a model where I'm using conjugate heat transfer is an order of magnitude or two slower than an almost identical model that doesn't have conjugate heat transfer. The actual fluid domains are almost identical (24" long, 0.75" ID pipes), and have extremely similar meshes, I just added in a solid domain surrounding the pipe to better simulate heat loss.

I tried to go through the two models to make sure every setting is identical, (minus the conjugate parts), so I can't understand why the particle tracking would be so slow in the model that has conjugate heat transfer between two domains. I have the coupled implicit solver frozen in both of them for what it's worth.

Is there some (hidden) setting that I'm just not seeing that could be causing my conjugate model to slow down? I built that model from the ground up on my own, where the other model I re-purposed from one that a colleague gave me that had different flow conditions.

Any help is greatly appreciated!!
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Old   April 16, 2020, 01:30
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i doubt it has anything to do with your heat transfer method and you say you have the the solver frozen too so that is a good indicator of this.
if you have two licenses available you can open both sims in the one gui and use the compare tool and this will show you the differences in settings in each relevant feature node.
when you say slow do you mean the flow is slow and this should be checked of course using some vector section scenes, or the production of injected particles and you can set some reporting up for this.
also i wonder if you really need a solid region for a simple pipe heat transfer which probably has uniform wall thickness and so can be well modeled by choosing the appropriate wall boundary condition settings
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Old   April 16, 2020, 13:49
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The reason I wanted to do a conjugate model was so that I could acquire solid surface temperatures at the fluid-wall interface.

Is there a way for me to get wall temperatures without doing conjugate? My ultimate goal is to be able to extract the surface wall temperature in field functions so I can use it for some additional modeling. Even so, I'm not sure what build-in field function would allow me to extract the wall temperature.

What I want to do is change a particle coefficient of restitution based on the wall temperature and the particle temperature.
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Old   April 17, 2020, 00:25
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using the environment setting for the wall thermal condition you then set the thermal resistance of the wall which is where you calc and input the wall thickness and conductivity as well as the external air temp and htc.
then in post.p you get the internal 'solid' wall temperature on that wall bc via the standard temperature field function which can then be used in other user functions easily.
the big disadvantage of using solids is slow convergence but this can be sped up quite a bit by setting the solid energy under relaxation factor to .99999 in most case without stability issues - in case you decide to use a solid wall but ensure you have probes monitoring and plotting the solid temp is a couple of places to ensure convergence.
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