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Mesh Independence study

    • Aras karimi
      Subscriber

      Hi to all,

      I am working on a 3D airfoil. Currently, I am doing a mesh independence study to assess the dependence of the simulation results.
      For this purpose, I plot the two parameters CL and CD when calculating. My solver is Fluent and I use K-W SST turbulence model. My mesh is the structure that I produced in icem-cfd. 
      I first started with a 552K cell mesh with  Y+ <1 , and the residuals converged very well. which is according to the following figure:

       

      Now, I increased the number of grid cells to 1,263,000 cells, but unfortunately, this time, the residuals fluctuate strongly and do not intend to converge, and I also receive the message (Stabilizing pressure coupled to enhance linear solver robustness.) repeatedly when solving, which is as follows:

      I really wonder why this is happening. To create a new mesh, I did not change the growth rate of the block lines at all, and only increased the number of nodes of the block lines, and at the end, I checked the mesh quality parameters, which were very good.

      I was very tired and disappointed.

      what is happening? Can anyone help me to solve the problem?

      If someone helps me, I will thank you a thousand times and I wish him health and well-being from the bottom of my heart.

      Regards

    • Rob
      Forum Moderator

      Have a look at the results you currently have. What is going on? If you run on does it improve? How does the mesh look - ie aspect ratio and growth rates as opposed to just cell quality? 

    • Aras karimi
      Subscriber

      Due to the slow solving speed and non-convergence of the residuals, I stopped solving it and did not see the results.

                                                Mesh 1:  (has very good convergence)

      Aspect Ratio: 1-5700

      Determinant : 0.87-1

      Orthogonal Quality: 0.82-1

      Quality: 0.87-1

      Skew: 0.69-1

       

                                                  Mesh 2:  (has very bad convergence)

      Aspect Ratio: 1-5660

      Determinant : 0.895-1

      Orthogonal Quality: 0.837-1

      Quality: 0.87-1

      Skew: 0.674-1

       

      If you pay attention, you can see that even the quality of the second mesh is a little better than the first mesh, but its convergence is much worse and full of (Stabilizing pressure coupled to enhance linear solver robustness.) messages.

      This is very strange behavior. Can the reason be discovered?

    • Rob
      Forum Moderator

      That's the cell quality. Mesh quality includes cell resolution and things like growth rate. I'd also question any mesh with a reported aspect ratio over 200-300 : you may be OK but it's not something I'd assume without reviewing the flow. 

    • Aras karimi
      Subscriber

      I expected you to say that the aspect ratio is a large number. Well, consider that  mesh1 and 2 have the same aspect ratio, with the difference that  mesh1 has very good convergence and  mesh2 with a message on the console and fluctuating resolutions and practically no convergence.

      My mesh is as follows:

       

      In the figure below, as you can see, the aspect ratio very large is related to the back of the airfoil:

       

      To reduce the aspect ratio, the mesh behind the airfoil should be very small ?  If this is the case, the number of mesh cells will increase and it is worrying.

      What do you mean by mesh quality? Do you mean the growth rate of block lines?

    • Rob
      Forum Moderator

      No, mesh quality also considers the sudden jump in cell size. If that coincides with something in the solution you may have a problem.

      Look at what's happening in the flow: what is different in the result plots between the two models? 

      • Aras karimi
        Subscriber

        Again, I did not understand what you mean about mesh quality. I know that the mesh jump causes instability in the solution, but I produced both meshes with exactly the same Ratios for the block lines, and also the quality of the second mesh is slightly better than the first mesh, but the second mesh does not converge.

        Well, this is very strange

         

         

         

        I said

        The second mesh does not converge and it progresses slowly due to an error appearing in the solution console. That's why I stopped the solution and I don't know what the result will be.

    • Rob
      Forum Moderator

      Except this time with a much finer mesh you may be picking up flow features you didn't get in the other mesh. 

      The screen grab showed solver warnings, but not any errors: if you continue does it diverge or just fail to converge based on residuals? 

      • Aras karimi
        Subscriber

        I continued the solution a little more, the residuals fluctuate in numbers between 1e-02 and 1e-03 and repeatedly after each iteration the message (Stabilizing pressure coupled to enhance linear solver robustness) appears on the console and it is quite clear that the time between both iterations with the message Coupled Pressure, compared to Two iterations without the coupled pusher message is more.

        I want to make the grid so small that the CL and CD values are almost constant. Does the increase in the number of cells in the grid lead to the fluctuation of the residuals?

        According to similar studies, I think my geometry should have different results up to about 3.5 million cells.

        I must do the mesh independence to ensure the correctness of the results.

    • Rob
      Forum Moderator

      The reduction in cell size may mean you're also picking up a flow feature you'd previously missed. It that's transient then, yes, it's potentially caused by refining the mesh. 

      Cell count isn't always a good indicator for studies. The authors may have used different meshing strategies making direct comparisons difficult. 

      • Aras karimi
        Subscriber

        If the residuals fluctuate by increasing the number of grid cells, and the solution does not converge, it is not a good thing. For me, after every repetition, the message "Pressur Coupled" appears and the solution is very slow.

        I feel exactly that somewhere the residuals want to diverge, but they barely fluctuate around 1e-03.

        Rob, are you saying that it is normal for the "Pressur Coupled" message to appear after each iteration?

        What is the meaning of this message and why does it appear for me?

        What is your suggestion?  what do I do ?

        Thank you for helping me more

    • Rob
      Forum Moderator

      Continues here /forum/forums/topic/mesh-independence-study-for-airfoil/

       

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