[Batou]

Hello Guys.

I am currently running a transient Calculation (URANS, k-omega-SST) and have a Convergence issue.

My Residual Limit is set to 10e-4 for all values. According to my case, i need to set the timestep to around 3e-6 s. However, if my timestep is higher than 2.5e-7 s, say 5e-7s , I have convergence Problems with the turbulence kinetic energy.

for timestep 2.5 e-7 s, i get all (scaled) residual values to around 10e-5 or lower, except the turbulence kinetic energy residual, which is around 6e-5. With the timestep of 2.5e-7, the Solution is always converged after just one iteration, so i would like to rise the time step size.

When time step is at 5e-7 (still quite low, max CFL would be around 0.3 for this case), the value of all residuals rises a bit and the residual for turbulence kinetic energy is then above 10e-4 (around 3*10e-4) and does not decrease below 10e-4 anymore.

Does someone Have any ideas for this Problem? I do not think that manipulating Underrelaxation factors is useful since underrelaxation only might accelerate convergence decrease, but it does not decrease at all, it is at its minimum value above 10e-4.

Do you have any idea how to get my solution converge at higher timesteps?

I hope you understand my case, thanks in advance.