CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > CFX

CFX cylinder or sphere benchmark results

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Search this Thread Display Modes
Old   August 8, 2005, 09:58
Default CFX cylinder or sphere benchmark results
Posts: n/a
Hi there, I try to validate CFX results of a sphere for Re = 200k and Re=410k. I benchmark my results on some graphs of Schlichting of a smooth sphere, but my results are far off: for Re=200k the drag is 37% too high and for Re=410k there is no dip in the drag at all, so this result is about 800% off!

The sphere has a diameter D=150mm. I made two meshes: one for Re=200k and one for Re=410k. Both meshes use an inflation layer creating prisms. For Re=200k it has a first prism height of 0.0313mm and for Re=410k the height is 0.0164mm (calculated using some expressions in CFX manual). I used 15 layers with an expansion factor of 1.1. Post-processing shows that for both the y+ < 1.65 so it should be fine enough. For the mesh faces I used an angular resolution of 1 deg, and a minimal edge length of 2.5mm. Both meshes have about 930.000 tets and 1.230.000 prisms.

For both models I used the SST model with auto wall treatment. For Re=200k I set the solver to auto timescale with a specified length scale of 50mm and a max dt of 0.02sec. The steady state solution was 37% too high. The 410k case was even worse. After that I ran a transient case, but the results barely changed.

Are there any CFX testcases or benchmarks available on sphere or cylinders? Has anybody a clue why these results are that far off?

Thanks in advance! Mel
  Reply With Quote

Old   August 8, 2005, 18:47
Default Re: CFX cylinder or sphere benchmark results
Glenn Horrocks
Posts: n/a

Some points:

1) The y+ value alone does not indicate an adequate mesh. Have you done a mesh independance check of the prism layer size and thickness, and circumferential mesh?

2) What upstream turbulence levels are you using? Again you need to check these

3) What blockage factor are you using? Have you established that your modelling domain is wide and long enough?

4) Are you using symmetry? Flow around a cylinder is symmetrical for some Re and asymmetric for others.

5) If you are in the steady laminar Re range it should be realtively easy to get accurate answers. Also in the high Re region the turbulence models should work well and again you should be able to get accurate answers. The intermediate ranges are a little harder, you may have to consider DES or LES for some regimes.

Regards, Glenn
  Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
CFX results postprocess in Tecplot seaharrier CFX 5 April 18, 2019 01:01
CFX vs Flow-3D Benchmark ck5285 CFX 22 June 3, 2010 04:46
Calibration of cylinder and sphere data. cobain Main CFD Forum 0 September 22, 2009 04:43
meshing F1 front wing Steve FLUENT 0 April 17, 2003 12:37
benchmark results stefan Siemens 3 September 10, 2001 09:48

All times are GMT -4. The time now is 14:08.