(Accoustic) Time stepping in high resolved LES

[twis91]

Hi all,

we are planning to do a very high resolved (dx is 3m) run in WRF-LES mode. Due to WRFs recommendation, dt should be roughly 0.018s which is very small and will cost us a lot of time. However, as far I know there are three theoretical solutions of this problem...

a) We want to simulate only the ABL and therefore, the wind speeds will be quite low allowing higher dt. To my knowledge WRF can be only run in compressible mode - is it right or is there a possibility to do incompressible calculations (even if we have to do slight modifications)?

b) Is it justifiable to increase dt, for example dt=0.5, and then to increase the amount of acoustic time steps accordingly (here to roughly 30)? I am not sure about the dynamic behavior in this case but this would at least save a bit of time.

c) Using a compressible pure LES or working with this fine time step - this is the option we want to avoid.

Kind regards!

[moulin]

The problem you encountered is a built-in symptom of using a compressible solver for low mach number problem, which is quite inefficient. I think you could try to couple an external incompressible LES solver at the inner domain, which enable you to take a larger time step.

[twis91]

Thanks for the answer - we are aware about that problem and that's why the question came up if WRF somehow supports incompressible solving algorithms.

Thanks so far for the answer