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pimpleFoam large time step solver?!!

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Old   February 25, 2019, 16:11
Default pimpleFoam large time step solver?!!
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Hello guys,
i ve been looking around and i found that pimpleFoam is a Large time-step solver .


Can anyone please explain to me
What do we mean by large time step ?

and what is the minimum time step that we can use with pimpleFoam ?

and does this mean that we cannot use it for a simulation with a CFL <1 ?
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Old   February 26, 2019, 03:08
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pimpleFoam can work even if CFL > 1. I found that it works well until CFL = 4. Tobias Holzmann describes the usage of pimpleFoam in his Book Mathematics, Numerics, Derivations and OpenFOAM®. According to his record pimpleFoam can work until CFL ~ 20 before it starts producing unrealistic results.
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Old   February 26, 2019, 16:08
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Quote:
Originally Posted by RobertHB View Post
pimpleFoam can work even if CFL > 1. I found that it works well until CFL = 4. Tobias Holzmann describes the usage of pimpleFoam in his Book Mathematics, Numerics, Derivations and OpenFOAM®. According to his record pimpleFoam can work until CFL ~ 20 before it starts producing unrealistic results.

Thank you for the Answer ,
does this mean even if my courant number is > 1 the results am getting are accurate?


2/-What happens is i use vert small timesteps (adaptive timesteps) that may even get to 1e-5 .Will this affect the final results or no?


3/-Also the last thing is , I ve been trying to make this 2D airfoil simulation for the last month and i keep getting values of Cd very far from the experimental Data , I ve tried to change mesh with multiple wall distance and refinement ,the turbulence model, the solver and i even used fluent to see if there is a problem with my OpenFoam settings But i still get the same result:

Cd_simu =0.01342 While Cd_experimental=0.00883
Cl_simu=0.3424 while Cl_experimental=0.2954


At some point while i was desperately trying to find the problem and find Where the issue is . I used a case and solved that case with pimpleFoam and i made maxCo =0.9 in the system/controDict file


Than i used the same case to run the simulation once again with maxCo=0.2 surprisingly i found that the final results for Cd were very different because for the last case i Got Cd=0.050 and for the first one i got Cd=0.0135



Do you have any idea About what is the problem for this situation ?
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Old   February 27, 2019, 02:58
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Quote:
Originally Posted by losiola View Post
Thank you for the Answer ,
does this mean even if my courant number is > 1 the results am getting are accurate?
As far as i understand, yes. And it worked for my for laminar and turbelent flows in the past.


Quote:
What happens is i use vert small timesteps (adaptive timesteps) that may even get to 1e-5 .Will this affect the final results or no?
The results should be the same. Using pimpleFoam should circumvent the need for a very small timestep.


Quote:
Also the last thing is , I ve been trying to make this 2D airfoil simulation [...]
I have no experience with airfoil simulations.
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Old   March 19, 2019, 09:10
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Hello

Quote:
Originally Posted by losiola View Post
does this mean even if my courant number is > 1 the results am getting are accurate?
It depends on the variable you are looking for. In your case when looking for Cd or Cl , it is better to use CFL<1.

Quote:
Originally Posted by losiola View Post
What happens is i use vert small timesteps (adaptive timesteps) that may even get to 1e-5 .Will this affect the final results or no?
Well, It is not just about time step, your boundary layer mesh could be fine enough (Y+<1) to have better results

Quote:
Originally Posted by losiola View Post
3/-Also the last thing is , I ve been trying to make this 2D airfoil simulation for the last month and i keep getting values of Cd very far from the experimental Data , I ve tried to change mesh with multiple wall distance and refinement ,the turbulence model, the solver and i even used fluent to see if there is a problem with my OpenFoam settings But i still get the same result:


Cd_simu =0.01342 While Cd_experimental=0.00883
Cl_simu=0.3424 while Cl_experimental=0.2954


At some point while i was desperately trying to find the problem and find Where the issue is . I used a case and solved that case with pimpleFoam and i made maxCo =0.9 in the system/controDict file


Than i used the same case to run the simulation once again with maxCo=0.2 surprisingly i found that the final results for Cd were very different because for the last case i Got Cd=0.050 and for the first one i got Cd=0.0135
For this kind of simulations, it is better to use K-w SST as the turbulence model and very fine mesh close to the airfoil wall without using any wall functions. keep your CFL around 0.5 and let pimple solver select the relative time step. Also it is better to use nOuterCorrectors = 1 in fvSolution.
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