CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Lift fluctuations in a steady simulation?

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 16, 2011, 15:00
Default Lift fluctuations in a steady simulation?
  #1
Member
 
Kisorthman Vimalakanthan
Join Date: Apr 2011
Posts: 46
Rep Power: 14
k.vimalakanthan is on a distinguished road
Hi guys,

I'm modelling an airfoil problem at high angle of attack (14 deg) using Ansys CFX (quasi 2D). Currently I'm facing fluctuations in the coefficient of lift value with iterations (please see attached pic). I have tired the simulation with higher physical time scale (even up to 50s), still it does not seems to converge with platoa convergence.

After addressing this issue to some of my friends, they suggest that the simulation is exhibiting a unsteady behaviour in a steady simulation, which i'm not able to comprehend. Is this even possible?

If this is the case shouldn't my increased physical time scale even out the unsteadiness of the flow?

Any help is greatly appreciated,
Attached Images
File Type: jpg Unsteady_in_Steady.jpg (46.1 KB, 26 views)
__________________
Kisorthman Vimalakanthan
Dept. of Power and Propulsion
Cranfield University
Email: k.vimalakanthan@gmail.com
k.vimalakanthan is offline   Reply With Quote

Old   October 16, 2011, 15:22
Default
  #2
Senior Member
 
truffaldino's Avatar
 
Join Date: Jan 2011
Posts: 249
Blog Entries: 5
Rep Power: 17
truffaldino is on a distinguished road
The steady solution at such aoa is not stable, at this aoa you will have a separation bubble which sheds vortices, just as in case of von-Karman votices around cylinder. Steady solution does not make sence here. You should run transient and then average forces over vortex shedding time period, if you need a mean force.
truffaldino is offline   Reply With Quote

Old   October 16, 2011, 15:29
Default
  #3
Member
 
Kisorthman Vimalakanthan
Join Date: Apr 2011
Posts: 46
Rep Power: 14
k.vimalakanthan is on a distinguished road
Quote:
Originally Posted by truffaldino View Post
The steady solution at such aoa is not stable, at this aoa you will have a separation bubble which sheds vortices, just as in case of von-Karman votices around cylinder. Steady solution does not make sence here. You should run transient and then average forces over vortex shedding time period, if you need a mean force.
Many thanks truffaldino.

Do you have any idea why changing the local and physical time scales does not effect these 'bouncy' results?
also is it reasonable to average the forces over this periodic steady simulation?

Could you please also suggest any good reference for explaining these 'time marching' CFD techniques?
__________________
Kisorthman Vimalakanthan
Dept. of Power and Propulsion
Cranfield University
Email: k.vimalakanthan@gmail.com
k.vimalakanthan is offline   Reply With Quote

Old   October 17, 2011, 07:12
Default
  #4
Senior Member
 
truffaldino's Avatar
 
Join Date: Jan 2011
Posts: 249
Blog Entries: 5
Rep Power: 17
truffaldino is on a distinguished road
I think averaging oscillations of steady solver does not make much sence. You should swithch to unsteady. In this situation I usually use implicit scheme with time stepping with courant number somewhere between .1 and 1. Courant number is the ratio of flow travel distance during a timestep to grid spacing C=v*dt/dl, where v-flow velocity, dt-time step, dl-grid step.
truffaldino is offline   Reply With Quote

Reply

Tags
airfoil, fluctuations, lift, unsteadyness

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
Laminar gas flow under slight suction - transient works but not steady audrey CFX 1 September 8, 2011 20:43
divergence of scaled residuals (steady simulation) hanka FLUENT 6 December 16, 2010 05:45
How to speed up the simulation of a large amount of similar steady state cases tss Main CFD Forum 4 November 5, 2010 15:36
Thin foil analsis (sail) - Lift Coeff Problem Kelvin CFX 3 December 22, 2008 17:22
steady or unsteady simulation philippe FLUENT 3 May 7, 2003 07:01


All times are GMT -4. The time now is 17:44.