Fluids

Fluids

results seem to get worse as mesh becomes finer

    • Melanie Butts
      Subscriber

      I have a frame motion simulation of a fan, and the results seemed to get less realistic as I decreased my mesh size. Is there such a thing as overmeshing in CFD?

      These results are from my finest mesh (0.001583 m on the face of the fan blades, 0.01583 for the rest of the domain)

      image

      image

       

      Here are results from a medium grid (elements twice as big)

       
      Here are results from a simulation with elements 4 times as big
       
      More about the settings:
      Transient simulation
      Viscosity model: Laminar flow
      Zone conditions: 1200 rpm frame motion
      Boundary Conditions : 0 guage pressure inlet and outlet, with zero shear walls for the sides
      Convergence conditions: created report definitions for torque on the fan walls and mass flow rate applied to the outlet, set to converge each time step ignore first 20 itterations and use last 20 itterations
      Calculation activities: ran 300 time steps of size 0.05 s, with maximum of 50 itterations per time step
       
      Also, here are some stats on the quality of my mesh
      max skewness of finest mesh was 0.83517
      min orthogonal quality : 0.14128
       

       

    • Rob
      Ansys Employee

      What tends to happen is the solution stops changing at a certain mesh resolution. As you further refine beyond a certain point you can start to pick up additional flow features. 

      The fine mesh looks to be resolving the fan core: ie a region in the centre with little/no flow as the hub blocks the flow. Adjust the scale and also review the velocity vectors. 

      • Melanie Butts
        Subscriber

        Thank you Rob! 

        When you say adjust the scale, do you mean continue to decrease the mesh size? And when you say review the velocity vectors, do you mean look for vortecies and/or transient behavior in the flow?

         

        Thanks again

    • Rob
      Ansys Employee

      Display scale, you're looking at a (mostly) pale blue contour. If you tighten the scale and make sure it's the same for each case it'll make comparisons easier. 

      • Melanie Butts
        Subscriber

        Gottcha. Thanks!

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