Learning Forum

[Aras karimi]

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.

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.

The aspect ratio of the network is as follows, which shows the elements behind the airfoil:

 

 

Now, I increased the network behind the airfoil and found that my non-convergence problem was solved:

 

 

See, with the increase in the number of cells behind the airfoil, my network has increased from 1,263,000 cells to 2,000,000 cells, which takes time to solve. If this amount of cell increase is applied on the surface of the airfoil, it gives much more accurate results than applying  at the back of the airfoil.

In many papers, I have seen that the mesh behind the airfoil is gradually enlarged up to the outlet pressure boundary condition, which is as follows:  but it gives me a message of non-convergence.

 

Why do I have to make the grid behind the airfoil so small to solve the problem of its non-convergence? Do you have a method or an idea to solve the problem of its high aspect ratio without increasing the cells behind the airfoil?

 

Thanks in advance

Regards

[Aras karimi]

Are the ratios appropriate and correct?

 

[mjmiddle]

3D Design is the wrong channel for this post. You should create the post under the Fluids channel.

[Aras karimi]

very well

So please delete this post.

Thankyou