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Fluids

DPM results changing with CFD mesh resolution-2nd

    • reza121
      Subscriber

      Hello. I hope you are doing well. It's been 2 weeks that I haven't received any update on the original question, "DPM results changing with CFD mesh resolution" despite my follow-ups. I think it is because of the new forum website so I am creating this new post and hope that I get some help here.

      Problem description: 

      In the following problem, I have a geometry consisting of multiple layers of screens that are supposed to filter out the particles from the incoming contaminated flow.
      A steady K-Omega SST turbulent model is used to solve the flow. The duct dimensions are 0.4*0.4*15 mm and the inlet velocity is changing between 1 to 15 for different cases. There is a one-way interaction between the particles and the flow. Therefore, the flow is solved first. After convergence (errors down to 1e-10 or lower), the particles are injected at the inlet using either the group injection feature (3 groups of 20 parcels) or surface injection with the same velocity as the flow at the inlet. the particle size is 1 micron and their mass flow rate is 1e-20 (infinitesimal). 

      I have cylindrical walls (representing screen wires) that are subtracted from the bulk air duct. The wire diameter is 100 microns and the screen opening is 200 microns. My domain is purely made of air with the walls subtracted. The walls are solid and not porous and a "trap" DPM boundary condition is applied to them. Therefore, neither the flow nor the particles pass through them. I am trying to count the number of the particles that will hit the walls and then will be eliminated from the domain and will be counted as captured in the report.

      In the following, you can see a screenshot of the CFD mesh. 7 layers of inflation are added. The cell size close to the walls zone is 4e-6m and away from that (I call it downstream and upstream of the domain) is 8e-6 in one case and 16e-6m in the other. The problem is that the number of trapped particles on the walls is dramatically different in these two cases while the CFD mesh is different only away from the walls. I have checked the Z-velocity (the velocity along the duct) as a grid study parameter and the graphs align very well. So, the grid resolution seems to be sufficient. However, DPM results are different and we know that for Lagrangian solutions, as long as the flow is converged and consistent (for the drag forces), DPM results should not vary with the grid size since it just calculates the particles' trajectories by solving F=ma for each particle along its path.

      Again, the issue is that even though the flow parameters show an identical trend with all the different mesh resolutions (scatter plot of flow z-velocity on a line according to one of the uploaded photos or skin friction factor on the walls), I am getting different numbers of trapped particles at the walls with different mesh resolutions in either vicinity of the walls or away from the walls. As I explained earlier in the previous posts, using the "body of influence" meshing feature, I give fluent meshing two mesh element sizes: one in the vicinity of the walls (called vicinity region) and one away from that (which I call up/downstream)

      Here are some of the particle trap results for different mesh resolutions:

      1- up/downstream 16e-6 , vicinity 4e-6.   # trapped= 50/60

      2- up/downstream 8e-6 , vicinity 4e-6.   # trapped= 35/60

      3- up/downstream 4e-6 , vicinity 4e-6.   # trapped= 26/60

      In the studies above, I kept a constant mesh size for the vicinity region and changed the mesh size in the up/downstream region only. Also, in another study that I carried out earlier, I kept the up/downstream mesh size constant and changed the vicinity mesh size and the DPM results still change drastically. In the photo below, you can see better what I mean by vicinity and up/downstream regions.

      additional information: BCs: velocity inlet (1.7 - 3 m/s) pressure outlet (0 gauge) Symmetry walls on 4 side walls of the duct,  no-slip walls for the screen walls.

      ***********************************************************************

      Robert, in the previous post, you replied as follows: "with 90 particles you're also subject to statistical issues. Assuming it's turbulent switch on turbulent dispersion and number of tries to 10-100. You need a lot more particles. 

      The difference may be mesh/flow related, but could just be injection position and luck, hence putting in many more parcels."

       

      While Random walk model was off, I used the group injection feature to create a lot of parcels but the results are still inconsistent for different mesh sizes. I am sure that I have enough number of particles because I get the exact same number of trapped particles if I inject half of that number into the domain while studying the same mesh size. Therefore, I don't think that the problem is with the insufficient number of particles and statistical issue.

       

      I also tried with the Random walk model activated. the answers are similar no matter what the "number of tries" option is. However, I would like to get it working without the Random walk model.

       


      I really appreciate any help you can provide.

      In the following posts, I will upload the photos one by one

    • reza121
      Subscriber

      Unfortunately, I can't upload the photos again. I sent an email to Ansys support and I hope that they give me access to upload the photos. Meanwhile, you can refer to the previous post titled as "DPM results changing with CFD mesh resolution" and see the photos there.

      Thanks

    • Rob
      Ansys Employee

      Found the OP  https://forum.ansys.com/forums/topic/dpm-results-changing-with-cfd-mesh-resolution/   Apologies, we're still getting the gremlins out of the forum back end. 

      • reza121
        Subscriber

        Thanks for your reply. So I'll keep our discussion in the other post since there are more threads there. By the way, I sent a reply, would you please check it at your convenience? 😊

        Thanks

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