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CFX or Fluent for Turbo machinery ?

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Old   May 7, 2011, 17:17
Unhappy CFX or Fluent for Turbo machinery ?
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Sijal Ahmed Memon (turboenginner@gmail.com)
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Dear Frends

I have simulated the NASA rotor 37 at design speed.

The turbulence model used is Spalart Allmaras (both in Fluent and CFX, beta in CFX R 12.0)

I have created two meshes
1) high quality mesh
2) bad quality mesh (only from cfx point of view as the minimum orthogonality angle is less than 20, on the other hand it satisfied all requirements of Fluent solver e.g. max skewness, cell squish index)

In short I am using two solvers (CFX and Fluent) and two meshes (one bad from CFX point of view only)

I have made the mesh Independence study for both meshes and found that the 0.7 million size produces the grid independent solution.

Now from results (see the attached Figure.) I have found interesting facts.

Results from both meshes on the Fluent almost overlap each other while the good mesh produces higher performance and bad mesh predicted the lowest performance for CFX.

Fluent results seem logical as they should be after grid Independence for both meshes, but I am confused with CFX results.

Any suggestion, advise or comment shall be highly appreciated and shall shed light on philosophy of these two widely used solvers

CFX mesh = good quality
Fluent mesh = bad quality
Attached Files
File Type: pdf two solver and two meshes with SA model.pdf (52.6 KB, 49 views)
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Old   May 11, 2011, 02:21
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any response please

also see the discussion on same topic in CFX forum

http://http://www.cfd-online.com/For...machinery.html
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Old   May 25, 2011, 14:06
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Hi far,

I will be involved in simulating flow in axial/centrifugal pumps. I have been using Fluent for a while on heat transfer simulations. The debate is going over whether I should stick with Fluent or switching to CFX, any suggestions will be very much appreciated.

Thx
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Old   May 27, 2011, 03:02
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First I would like to say that both flow solvers tend to provide the similar results if the mesh is of good quality and has appropriate no.of nodes and yplus values.

Therfore the first and the most important rule is to make, in any simulation of turbo machinery in particular and external flows in general, high quality mesh with all appropriate parameters e.g. yplus.

Now lets come to the difference

1. CFX has good turbulence models, although after merger with ANSYS all model seems to be incorporated in Fluent as well. Therefore this point does not make any difference any more.

2. Solution time : yes this is big factor where fluent is lagging behind CFX. In my estimate Fluent takes at least 3 days and CFX takes 12-18 hrs for same case (1 million nodes with 4 GB RAM).

3. Scaling : This means with increasing no. of nodes iteration time should not increase. CFX does provide this feature.
For example if you r running a case with 0.5 million mesh size and CFX is taking 12 hrs and fluent is taking 36 hrs. Now you double the mesh size from 0.5 million to 1.0 million. In this case CFX again takes 12 hrs but fluent may take 48 or more hrs. I am assuming you have enough computational resources.

4. Memory management. With CFX you can run 50% higher no of nodes on the same computer. In other words with fluent you can handle 1.0 million and CFX will go up to 1.5 million. Assuming 1.0 million nodes is the limit of your computer for fluent.


I would like to mention again: Fluent and CFX have very little difference in results, the most important thing is the mesh.Therefore instead of solver you should put more emphasis on acquiring the good skill on high end meshing sofwares (GRID PRO is my first choice and then comes ICEM CFD and GRIDGEN)

Best Regards
Far
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