Time Step decreases after a long time of running, why?

Nidalsb · October 14, 2019, 06:48

Hi,

My time step starts out at ~1e-9 which is acceptable since my model is basically a 2mm block. After many time steps, almost 1500, my time step decreases to ~1e-15. Anyone knows the reason why this might happen?

Thanks a lot

Krao · October 14, 2019, 07:33

Which simulation you are using? What kind of solver?
Please go through the following link.
How to give enough info to get help

Assuming that you are using a transient solver, check your CFL(courant number), also, the solver settings and quality of the mesh you are using. If something is not ok please fix it

Nidalsb · October 14, 2019, 07:40

I am using rhoPimpleFoam, When I did the simulation using first order divergence schemes it worked just fine. now that I am solving using second order the time step reduces a lot after some time steps. basically my simulation is a pressure difference across porous media. I have a geometry of a porous structure and I am simulating flow across it. Please let me know if you need more info. My courant number is set to a max of 1, even if I increase it to 5 the time step remains ~1e-15

Krao · October 15, 2019, 03:40

I have not carried out simulations in your application but during dynamic simulation of external flow over a propeller, I had similar issues. I have overcome that by improving the quality of mesh and calculating the possible time step using CFL.

MaySea · October 26, 2020, 06:33

Hi Kmeti,

May I ask you how exactly did you improve the mesh quality? Did you decrease non-orthogonality?

Cheers,
Jan

October 14, 2019, 06:48	Time Step decreases after a long time of running, why?	#1
Nidalsb New Member Nidal Join Date: Nov 2018 Posts: 17 Rep Power: 8	Hi, My time step starts out at ~1e-9 which is acceptable since my model is basically a 2mm block. After many time steps, almost 1500, my time step decreases to ~1e-15. Anyone knows the reason why this might happen? Thanks a lot

October 15, 2019, 03:40		#4
Krao Senior Member Kmeti Rao Join Date: May 2019 Posts: 145 Rep Power: 8	I have not carried out simulations in your application but during dynamic simulation of external flow over a propeller, I had similar issues. I have overcome that by improving the quality of mesh and calculating the possible time step using CFL. MaySea likes this.

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October 14, 2019, 07:33		#2
Krao Senior Member Kmeti Rao Join Date: May 2019 Posts: 145 Rep Power: 8	Which simulation you are using? What kind of solver? Please go through the following link. How to give enough info to get help Assuming that you are using a transient solver, check your CFL(courant number), also, the solver settings and quality of the mesh you are using. If something is not ok please fix it

October 14, 2019, 07:40		#3
Nidalsb New Member Nidal Join Date: Nov 2018 Posts: 17 Rep Power: 8	I am using rhoPimpleFoam, When I did the simulation using first order divergence schemes it worked just fine. now that I am solving using second order the time step reduces a lot after some time steps. basically my simulation is a pressure difference across porous media. I have a geometry of a porous structure and I am simulating flow across it. Please let me know if you need more info. My courant number is set to a max of 1, even if I increase it to 5 the time step remains ~1e-15

October 26, 2020, 06:33		#5
MaySea Member Jan Majcher Join Date: Nov 2018 Posts: 39 Rep Power: 8	Hi Kmeti, May I ask you how exactly did you improve the mesh quality? Did you decrease non-orthogonality? Cheers, Jan