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Fluids

Fluids

Topics related to Fluent, CFX, Turbogrid and more.

Why don’t the residuals converge?

    • Ivosp
      Subscriber

      I'd like to simulate the inflow/flow around a box. The fluid (air) itself is separated into 2 fluid bodies due to simulations planned for later. The inner body is a sphere, the outer is a cuboid.
       

      My setup:

      Meshing

      • Box as dead
      • Inverse Quality: Max = 0.78 (only a few), Average = 0.02
      • round about 2000000 cells
      • I've already connected the congruent spherical surfaces of the 2 fluid bodies in the geometry (in SpaceClaim) and it's therefore treated as one (in simulation as interface). Therefore I used a conformal mesh.
      • There is one velocity inlet (left side of the outer fluid body) and one pressure outlet (right side of the outer fluid body). All the other zones are walls.
      • I answered the following questions as:
        • Geometry Type -> The geometry consists of both fluid and solid regions and/or voids
        • Change all fluid-fluid boundary types from 'wall' to 'internal'? -> Yes
        • Do you need to apply Share Topology? -> No (Since the 2 spherical surfaces are already treated as one due to the settings in SpaceClaim)
        • Enable Multizone Meshing? -> No
        • Will you cap openings and extract fluid regions? -> No
      • I used a poly-hexcore mesh

      Solution

      • velocity inlet = 13 m/s
      • I left the rest of the settings as default 
        • The congruent spherical surfaces, let's say I name them zone1 (from outer fluid body body1) and zone2 (from inner fluid body body2), are listed as Internal under Boundary Conditions as follows:
          • zone1-zone2-body1-body2
          • interior--body1
          • interior--body2
      • The hybrid initialization was done after 10 iterations (with 7.3e-09)
      • For my first attempt, I set the number of iterations to 1000.
      • I clicked on Check Case... and it said No recommendations to make at this time.

      But after 1000 iterations the scaled residuals are still too high . The whole problem seems not that complex to me, but I am still a beginner and I have absolutely no idea what could be the reason for this. Can anyone help me?

      The scaled residuals stayed like this even after 1000 iterations...

       

    • Ivosp
      Subscriber

       

       

      For a better understanding, this is the model in Fluent Solver:

       

       

       

    • Rob
      Forum Moderator

      Plot contours of velocity around every 5 iterations for 50 iterations and look carefully at the output. Or put a couple of point monitors for velocity in the wake region. I suspect you've found a transient in the flow field from the way the residuals look, the monitors and contours will provide evidence of that. 

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