CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Drag comparison upwind vs. scheme

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 27, 2009, 08:22
Default Drag comparison upwind vs. scheme
  #1
Member
 
Lukasz
Join Date: Mar 2009
Posts: 67
Rep Power: 17
Luk_Fiz is on a distinguished road
Dear All,
Testing different computer configurations I noticed following problem.
Say that I simulate flow around the truck body on a given mesh. I start with the upwind differentation scheme having a drag of X. Afterwards I changed to hires and the drag reduces by approximately 25% and became not dependend on convergence level. I did not perform the mesh independence studies however I must say that the same situation occures in different simulaitons I perform of "abruptly" ended bodies, with sharp separation in the end of the body. For more "aerodynamical", "streamline" shaped bodies the difference between hires and upwind is much lower (for given mesh!!).

I would like to explain this. In generall the upwind and central schemes should give the same results on infinitely dense mesh. For finite density mesh there can be discrepancies but the higher order scheme should faster approach "real" solution on coarser mesh (am I right??).
- Does in mean, that in described case the drag predictions from hires scheme are closer to real ones? I mean, that if I dense the mesh the drag with upwind should be reduced?
- Does it mean that for "slim" bodies for given mesh, if the drag from upwing and hires is the same the solution is grid-independent???

I would be very grateful for help because it is related to rather big problems (many elements) and the calculation takes a lot of time, so it would be great to minimize number of grid/other studies.

Lukasz
Luk_Fiz is offline   Reply With Quote

Old   April 27, 2009, 12:31
Default
  #2
Member
 
Join Date: Apr 2009
Posts: 36
Rep Power: 17
minger is on a distinguished road
In general, you are correct in saying that the higher-order scheme will approach the 'real' answer as grid definition increases. This is referred to as order of accuracy. However, this is independed from the actual magnitude of accuracy.

For low grid-resolutions, a lower order scheme can produce more accurate results. For example, if you double the grid density, then the first-order backwards scheme should give an improvement of 2¹ = 2, whereas the second order will result in 2² = 4. You can see that for high-accuracy calculations, you can easily reduce the number of gridpoints by using higher-order schemes.
minger is offline   Reply With Quote

Old   April 27, 2009, 17:31
Default
  #3
Member
 
Lukasz
Join Date: Mar 2009
Posts: 67
Rep Power: 17
Luk_Fiz is on a distinguished road
thanks for Your answer, but can You clarify the statement:
"
For low grid-resolutions, a lower order scheme can produce more accurate results. For example, if you double the grid density, then the first-order backwards scheme should give an improvement of 2¹ = 2, whereas the second order will result in 2² = 4.
"

I did a mesh check, decreasing the size of elements in the wake by 3.
- in coarse mesh the drag force was 440N for upwind vs. 260N for hires
- in finer mesh the drag force was 340N for upwind vs. 250N for hires

Does in mean that hires shows "good" value on coarse mesh but performed coarening is still not enough for upwind?

Thanks
Luk
Luk_Fiz is offline   Reply With Quote

Old   April 27, 2009, 22:42
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,862
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Hi,

Another aspect of this issue is that first order upwinding schemes have lots of numerical dissipation. This means they artificially increase the viscosity of the fluid. This increased viscosity means the simulation is running at a lower effective Reynolds number and therefore separation points will move, wakes will be different etc etc.

Your mesh sensitivity study is not surprising - the hires scheme is changing by 4% but the upwinding scheme is changing by 29%. This means hires is likely to be closer to mesh independance.

Have a look at Computational Fluid Dynamics by Roache for lots of detail on this area or http://journaltool.asme.org/Template...umAccuracy.pdf for how Journal of Fluids Engineering have taken as far as an editorial policy.

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Reply

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
2nd order upwind and QUICK scheme Wiroon Main CFD Forum 19 September 17, 2019 04:47
Upwind flux scheme Biga FLUENT 0 January 12, 2006 13:41
fifth upwind biased scheme K S Chang Main CFD Forum 0 December 5, 2003 04:46
Help!explicit multistage scheme with upwind scheme D .T. Main CFD Forum 1 June 4, 2003 04:42
Genuinely Multidimensional Upwind Scheme Praveen Chandrashekar Main CFD Forum 9 October 25, 2001 07:47


All times are GMT -4. The time now is 13:12.