Convergence issues

[Symphony Chakraborty]

Dear Everyone,

I'm trying to achieve grid independence. For the first test, I ran my existing simulation with maxlevel = 8. For the second test, I set maxlevel = 7, which was running fine. For the third test, I set maxlevel = 9.  

Then the problem appears, and warnings are shown: src/poisson.h:219: warning: convergence for u.x not reached after 200 iterations

  res: 0.00616655 sum: -130.135 nrelax: 100 tolerance: 0.0001
src/poisson.h:219: warning: convergence for u.x not reached after 200 iterations
  res: 0.00175725 sum: 71.4775 nrelax: 100 tolerance: 0.0001

I have increased NITERMAX from 100 to 200 and mgp.nrelax from 200 to 300, but the warnings still appear at the same simulation time. 

Could you please suggest something that will help me remove these warnings so the simulation smoothly converges? Please have a look at my simulation code attached to this email.

With best regards,

Symphony

[Shyam Sunder Yadav]

Dear Symphony

Try reducing the time step...

Does the code runs at time step of 1e-7?

Are you getting divergence when the time step starts increasing after the time instant of 1e-4?

Best wishes

Dr. Shyam Sunder Yadav
Associate Professor
Mechanical Engineering
BITS Pilani
09902346342
http://www.bits-pilani.ac.in/pilani/ssyadav/Profile

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[Symphony Chakraborty]

Dear Shyam,

The code runs with a time step of 1e-6 and diverges when the time step reaches t = 1.01e-2.

Best regards,

Symphony

--

Dr Symphony Chakraborty, 

Post Doctoral Fellow,

Advanced Thermal Science and Flow Control Lab,

Institute of Applied Mechanics,

National Taiwan University, 

Taipei, Taiwan.

O: +886 (02)33665067

M: +886 (0)965414615

[Shyam Sunder Yadav]

Symphony the high density ratio, high surface tension combined with a small Delta at level 9 may be violating stability criterion.

The drop may be touching the embedded boundary at the mentioned time instant of 1.01e-2. High velocities may be generating at that instant leading to convergence failure...

Try with a small time step of 1.0e-7 (or even smaller) when the simulation time t is between, say, 1.0e-2 and 1.02e-2.

Another option is to use higher viscosity in cells next to the embedded boundary...

You can also try reducing the surface tension and density ratio...

Best wishes 

Dr. Shyam Sunder Yadav
Associate Professor
Mechanical Engineering
BITS Pilani
09902346342
http://www.bits-pilani.ac.in/pilani/ssyadav/Profile

On Tue, 17 Mar, 2026, 3:01 pm Symphony Chakraborty, <symphony.c...@gmail.com> wrote:

Dear Shyam,

The code runs with a time step of 1e-6 and diverges when the time step reaches t = 1.01e-2.

Best regards,

Symphony

On Tue, Mar 17, 2026 at 5:03 PM Shyam Sunder Yadav <ss.y...@pilani.bits-pilani.ac.in> wrote:

--

Dr Symphony Chakraborty, 

Post Doctoral Fellow,

Advanced Thermal Science and Flow Control Lab,

Institute of Applied Mechanics,

National Taiwan University, 

Taipei, Taiwan.

O: +886 (02)33665067

M: +886 (0)965414615

[Symphony Chakraborty]

Thank you, Shyam!

I will check it out.

Best regards,

Symphony