divergence at low U_LB

Hello everybody,

I have been trying to simulate some high speed flows u=50m/s using the external flow (obstacle) example… I have kept the u_LB low = 0.2 but even then after some time the average kinetic energy and forces reach “nan” … I have tried many things but eventually the simulation always reaches nan. It would be very helpful if someone can help me understand what i am doing wrong. Thanks a lot.

Kind Regards

What is your value of omega or tau?


Thank you for your reply david. Sorry, could’nt mail u back earlier.

The tau was initially 0.5. I increased it to 1. and also changed the BGK dynamics to regularisedBGK. Now theres no divergence. The simulation converges peacefully. :slight_smile:

Thank you for extending your help

i still have two doubts.

  1. Most of the flow gets diverted when it hits the geometry (as expected) but there is some some flow which passes through the solid geometry. i am unable to understand why this is happening. Is it because of insufficient resolution or due to some thing else?!!
  2. there are nozzles on the geometry through which the flow should happen but not as expected. Normally, In fluent i would setup a mass flow rate at the nozzle inlet perpendicular to onlet plane but i dont know how to do it here. So to start with i have put a velocity inlet on the entire plane.

I use a STL file and it is not a pure surface. It has thickness.


hello ananthakrishnan ,
i have the same probleme when i try to increase the speed at 3.9 m/s in the examples of external flow around obstacle, and i have “nan” for forces and avreage kinetic energy after some iterations, i tried the solution that you propose by increasing tau from 0.5 to 1 but this give me a verry small dx= 0.0003077, my referce lenght has 15 m. this need a big informatique resources to do this simulation
please can you give me more details about the manner that you had resolved your problem