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

coefficient of lift and drag forces

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 28, 2008, 11:40
Default coefficient of lift and drag forces
  #1
nkiru
Guest
 
Posts: n/a
hello, i am new to cfd and fluent. i am simulating unsteady flow past a stationary cylinder for differnt reynolds numbers. but i am having difficulty getting an accurate value of Cd and Cl. i am presently using fluent 6.3. i started by using the laminar model and my fluid is water. what role does the residual plot have in the convergence of simulation? any suggestions as to why my Cd and Cl values are 100% off the ones in the papers? when does one use the grid reorder caption option in fluent?

thank you.
  Reply With Quote

Old   July 28, 2008, 19:41
Default Re: coefficient of lift and drag forces
  #2
CDE
Guest
 
Posts: n/a
have you set the reference values correctly?
  Reply With Quote

Old   July 29, 2008, 05:57
Default Re: coefficient of lift and drag forces
  #3
nkiru
Guest
 
Posts: n/a
hello, thank u for responding to my message i think i have set the reference values correctly. i set the area= diameter, depth=1, length= diameter. or is this not correct? because i still have Cl and Cd values different from that expected. can u please suggest anything i should try.i need a reply urgently.

thank u
  Reply With Quote

Old   July 29, 2008, 11:49
Default Re: coefficient of lift and drag forces
  #4
Jack
Guest
 
Posts: n/a
Hi Nkiru!

I think referent values are inportant in here, not only area, depth and length, but fluid properties also.

Jack
  Reply With Quote

Old   July 29, 2008, 12:01
Default Re: coefficient of lift and drag forces
  #5
pawan
Guest
 
Posts: n/a
Hi, Can I get some more information about the bc you are using? As well as the turbulence model + Re value ? One more important thing you are using "laminar model" and expecting the software to give you Cl Cd values? I think your missing something !
  Reply With Quote

Old   July 29, 2008, 12:29
Default Re: coefficient of lift and drag forces
  #6
nkiru
Guest
 
Posts: n/a
thank you for your reply.

i am currently simulating an unsteady flow past a stationary cylinder in which the fluid is water and the reynolds number is 200(although i am to simulate for different reynolds numbers). therefore i am using laminar model. the fluid (water) characteristics i specified are the density=1000, viscosity=0.001, and velocity=0.002(all in SI units). i have noticed that my answer varies with the time step choosen, such that the higher the time step, the smaller the Cd and Cl values are becoming. i have even tried for Reynolds numbers 500,100 and 185 (all using the same specifications except velocity which is dependant on the reynolds number selected) and it is still the same story.

someone told me that the variations in values are due to the fact that my solution is not converged and that i should try to run more simulations with decreasing values of time step until there is no/very little difference between the values obtained. and this value obtained for Cl and Cd would be the right one because the solution has finally converged. is this true? should i consider doing this? this would mean that each reynolds number will have its own time step size. is that correct?

please i require assistance urgently as i have a deadline.

any suggestions would be appreciated.

thank you.
  Reply With Quote

Old   July 31, 2008, 05:34
Default Re: coefficient of lift and drag forces
  #7
Dale
Guest
 
Posts: n/a
On this issue how can you get a plot of Cd anf Cl over time for an unsteady flow? I have run the simulation writing this data as wel las writing the time data but cannot find where to generate a plot.

  Reply With Quote

Old   July 31, 2008, 05:36
Default Re: coefficient of lift and drag forces
  #8
Dale
Guest
 
Posts: n/a
Also, check your solution for mesh convergance, adapt the mesh for areas of high velocity or pressure gradient by adapt-gradient. Choose a refine threshold that refines a sensible amount of cells.
  Reply With Quote

Old   July 31, 2008, 10:27
Default Re: coefficient of lift and drag forces
  #9
nkiru
Guest
 
Posts: n/a
thank you dale for your comments. i will try them and see what happens. fingers crossed.

cheers
  Reply With Quote

Old   August 14, 2008, 02:00
Default Re: coefficient of lift and drag forces
  #10
Rudresh
Guest
 
Posts: n/a
Setting residual values to 1e-4 or 1e-6 by experience is the one way of checking for the convergence.

But for some case monitoring some of the flow parameters with respect to the number of iterations ( Time step for Transient) is the best way of judging the convergence.

For your case you may monitor the wall forces / shear forces on the cylinder wall with reference to the time step/Iterations. Once the oscillations are periodic or constant , then the solution is converged.

At the inlet you have to impose the proper values for the Turbulence parameters.Calculate the Turbulent kinetic for the given inlet boundary condition and Impose the same. And dissipation rate should be imposed as zero at the inlet.

This gives a better CFD prediction for Drag.This has given drag values close to theoritical values for ship hull hydrodynamics.
  Reply With Quote

Reply


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
Fluent Good Lift coefficient BAD drag coefficient Rif Main CFD Forum 4 March 9, 2010 11:52
Lift & Drag forces from Transient Analysis Richard CFX 1 December 4, 2008 18:40
Drag and Lift forces extremely low João Lourenço FLUENT 4 January 28, 2008 14:25
Lift and Drag Forces around an Aerofoil studentcfd CFX 1 October 29, 2007 18:36
Fluent forces -lift/drag interpretation P Smith Main CFD Forum 2 October 26, 1999 16:00


All times are GMT -4. The time now is 17:13.