
[Sponsors] 
April 6, 2011, 23:55 
When to switch from steadystate to transient

#1 
Senior Member
Nick
Join Date: Nov 2010
Posts: 121
Rep Power: 7 
Hi all,
In a transitional simulation for a thick airfoil at a low Reynolds at a 4deg AOA number I find that residuals start to oscillate past a certain point. My guess is transient effects are coming into effect. My question is : Should the transient solver be initiated "before" the oscillations start or at some arbitrary point after? My second question is with regard to the Gamma Theta model and the grid requirement, is the 75 nodes advised by the manual upstream of the transition point or downstream ? Thanks for your attention. Nick 

April 7, 2011, 07:34 

#2 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,587
Rep Power: 90 
If all you care about is the steady state (or time averaged if transient) result then it usually does not matter how you get there. Starting with a unconverged steady state run is a lot better than a crude initial guess and will mean your transient run settles down much faster.
I was not aware of the 75 node recommendation. I would generalise and say do a mesh sensitivity check anyway and find out for yourself. 

April 7, 2011, 11:55 

#3 
Senior Member
Nick
Join Date: Nov 2010
Posts: 121
Rep Power: 7 
Thanks for your comments. Also I have a question about what to expect with the transient model. Let's say the mass residual starts to oscilate when it's at 10^3 in the steady solver. Are the residuals supposed to converge during each time step in the transient solver? I mean are they supposed to go from 10^3 to say 10^5 at each smalll time step?


April 7, 2011, 18:10 

#4 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,587
Rep Power: 90 
Residuals are calculated quite differently between steady and transient runs. The tolerances are not necessarily transferrable.
And yes, you need every transient time step to converge for a good solution. But if all you care about is the final solution (sorry Hitler) then a few dud timesteps is OK as long as the final result is well converged. 

April 7, 2011, 21:13 

#5 
Senior Member
Nick
Join Date: Nov 2010
Posts: 121
Rep Power: 7 
I see. I'll give it a shot. Thanks.


December 15, 2011, 00:52 

#6 
Member
iswadi
Join Date: Feb 2010
Posts: 44
Rep Power: 7 
Dear Nick,
i am doing the almost the same analysis as u. simulating a wing at low reynolds number condition. i`im using the sst turbulence modelling but my results cannot converge in my steady state. i suspect maybe there is a transient phenomenon in my case here. but i don`t have any proper guidence on the transient state. can u give me some advice on timestep selection? FYI information my analysis is 3D wing. Anybody who have experience on this transient analysis are also appreciated to share and discuss. 

December 15, 2011, 17:17 

#7 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,587
Rep Power: 90 
There is some advice on time step selection here:
http://www.cfdonline.com/Wiki/Ansys...gence_criteria 

December 15, 2011, 20:41 

#8 
Senior Member
Nick
Join Date: Nov 2010
Posts: 121
Rep Power: 7 
I would suggest starting with a very small timestep and watching the convergence behavior for a few time steps. If convergence target is achieved then try increasing the time step; otherwise you need to switch to even a smaller timestep.


December 15, 2011, 20:45 

#9 
Member
iswadi
Join Date: Feb 2010
Posts: 44
Rep Power: 7 
thanx mr ghorrocks,
i have already implemented the method listed in the link. my monitor points are not showing any significant amaunt of fluctuation value, but my momentum and mass residual is still flunctuated and not well below 1e4. what is the acceptable percentage difference of this monitor point flunctuation value? i suspect my low reynold number region (1e5) might introduce the transition region. therefore, i used the SST with theta gamma model to solve it. but unconverged solution keep on bothering me especially at higher AoA (>15).FYI, this is a FSI study on low reynolds number wing. 

December 16, 2011, 16:12 

#10 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,587
Rep Power: 90 
If you are in a transitional turbulence region then it is highly likely you have laminar separation bubbles and the like, they are frequently transient. In my experience I often have to run the transitional turbulence model transient because no steady solution exists.
Nick  In these cases you increase the timestep size, not decrease it. Once the initial convergence is done you increase time step size so the turbulence model can better capture small transient features. 

December 16, 2011, 20:57 

#11 
Senior Member
Nick
Join Date: Nov 2010
Posts: 121
Rep Power: 7 
Ghorrocks I wrote once convergence is achieved try increasing the timestep otherwise (if not achieved) decrease it. I think this is accurate in a transient solution.


December 17, 2011, 06:36 

#12 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 11,587
Rep Power: 90 
For a steady state simulation the usual approach is to use some small timesteps initially to get it started, then increase the timesteps as convergence starts to occur. You can run quite large time steps to accelerate convergence in many cases.
For transient runs the usual approach is to adjust the time step size to get 35 coeff loops per time step. This is easiest done using adaptive time stepping. In general (either steady or transient) if the simulation is diverging then one of the first things to try is decreasing the time step to recover it. But you only do this if there are signs of divergence. 

December 18, 2011, 17:05 

#13  
Super Moderator

Quote:
Here I am reproducing the exact words of that report by Langraty The solution differed significantly from the grid independent one only for the case of 25 streamwise nodes where there was only one cell in the transitional region. Nevertheless the grid independent solution appears to occur when there is approximately 75 – 100 streamwise grid points from the leading edge to the trailing edge (i.e. one chord length). As well, separation induced transition occurs over a very short length and for cases where this is important a fine grid is necessary. In short make sensitivity analysis as suggested by Glenn again and again. Few more lines from that report: One point to note is that for sharp leading edges often transition can occur at the leading edge due to a small leading edge separation bubble. If the grid is too course, the rapid transition caused by the separation bubble is not captured. A good example of this is the Zierke compressor. Contours of velocity (top) and turbulence intensity (Tu, bottom) for the Zierke compressor are shown in Figure A.5. On the suction side transition occurs at the leading edge due to a small leading edge separation bubble. On the pressure side transition occurs at about midchord. The effect of streamwise grid resolution on resolving the leading edge laminar separation and subsequent transition on the suction side is shown in Figure A.6. Clearly, if there are not a large enough number of streamwise nodes, the model cannot resolve the rapid transition and a laminar boundary layer on the suction side is the result. Note that if separation induced transition is present additional grid points in the streamwise direction are most likely needed. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Constant velocity of the material  Sas  CFX  15  July 13, 2010 08:56 
mass flow in is not equal to mass flow out  saii  CFX  2  September 18, 2009 08:07 
Mass Diffusion: Transient and Steady State BC  rval  CFX  3  November 19, 2008 01:52 
Steady State Vs Transient answers  Kushagra  CFX  6  July 6, 2008 22:41 
Transient vs Steady State  Adam  CFX  1  April 12, 2007 11:34 