Weird results behavior upon changing time step size


I have been running some simulations regarding flow over a cylinder with a downstream plate using LES and I have found some very different results (strange even) with just a change of the time step size (everything else stayed the same). The simulation runs where I found this problem are Test1 8e-4 and Test1 5e-4. I am focusing on the results of lift and drag coefficient, specially on the fact that the lift is going almost flat towards the end of the run.

Link to my project: Project

Why does the time step size affect the results so much, or is it another thing that is causing the problem?

I would appreciate some help,

Thank you!

Hi, thanks for using the forum!

Could you please detail on how do the results look so different? I quickly compared the force plots for each simulation and they look very close to me.

The finer time step better captures the evolution of the fields, thus the curves look smoother. You can perform a sensitivity analysis against the time step size.

Hi, thanks for answering.

When comparing the ‘force and moment coefficients 1’ plots on both simulations, it is possible to see that for some reason the amplitude of the lift coefficient on the 5e-4s run has a decreasing behavior (reaching almost 0) that is not found on the 8e-4s run.

I am including both plots here for better clarity.

8e-4s run

5e-4s run

This different behavior can also be seen on the Z vorticity fields that I am also including

8e-4s run (last timestep)

5e-4s run (last timestep)

Thanks for the detailed explanation!

I will analyze and come back with my thoughts, or someone else from the community can also contribute.