LES: Turbulent Channel Flow without initial solution (BC)
In this post, the kinetic energy is monitered and the turbulentInlet condition is explained. We cannot use Reynolds averaged profile as inlet, because it is only seen by time average.
While monitering kinetic energy of all flow (not just the turbulent part), when this value tends to some stable state the mean and the turbulent part is then established.
While monitering kinetic energy of all flow (not just the turbulent part), when this value tends to some stable state the mean and the turbulent part is then established.
Quote:
Originally Posted by Lieven
Hi Danesh,
Averaged profiles are not instantaneous profiles. You will never see them in real life as in 'hey look, here I have the log-profile'. It doesn't work like that. You obtain them after time averaging the instantaneous profiles.
What you do with LES is solving instantaneous flow fields. At not a single instance should your result look like the time-averaged profile. Only after time-averaging should you obtain these profiles. Imposing RANS-profiles at the inlet conflicts with this fact and is physically pure nonsense (in more scientific terms, you are messing up the time scales in your simulation).
With the ChannelFoam solver you should get the right profiles. Even when you start from a uniform velocity field. If not, there still is something incorrect in your case setup and my first guess would be that the solution hasn't converged yet. You can check this for example by monitoring the (mean) kinetic energy in the system:
If this parameter stabilizes you reach the point where you want to start averaging the velocity fields (you can use the fieldAverage functionObject) for this. Take at least 30 cycles as averaging time...
Cheers,
Lieven
Averaged profiles are not instantaneous profiles. You will never see them in real life as in 'hey look, here I have the log-profile'. It doesn't work like that. You obtain them after time averaging the instantaneous profiles.
What you do with LES is solving instantaneous flow fields. At not a single instance should your result look like the time-averaged profile. Only after time-averaging should you obtain these profiles. Imposing RANS-profiles at the inlet conflicts with this fact and is physically pure nonsense (in more scientific terms, you are messing up the time scales in your simulation).
With the ChannelFoam solver you should get the right profiles. Even when you start from a uniform velocity field. If not, there still is something incorrect in your case setup and my first guess would be that the solution hasn't converged yet. You can check this for example by monitoring the (mean) kinetic energy in the system:
Code:
volScalarField Ek("Ek",0.5*(U & U));
Info<<"mean Ek: " << gAverage(Ek) << nl << endl;
Cheers,
Lieven
Total Comments 0
