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

[YuShun Yang]

Dear Symphony,

I hope this email finds you well.

I am currently working on a droplet impact simulation using the embedded boundary + VOF method, and I am facing the exact same issue you described in this thread. When I increase the grid resolution to maxlevel = 9, the Poisson solver fails to converge for u.x, which subsequently leads to divergence at the exact moment of impact.

I carefully read through your previous discussion, including the suggestions to reduce the time step (dt) to 1e-7 during the critical impact phase, as well as trying to adjust the viscosity near the embedded boundary.

I was wondering if you were ultimately able to successfully resolve this divergence issue in your simulation? Did adjusting the time step or viscosity work for you?

If you could share any insights, updates, or suggestions, it would be immensely helpful to me, as I have been stuck on this exact same bottleneck for quite some time.

Thank you so much for your time, and thank you for sharing the initial discussion!

Best regards,

Yushun Yang

[Symphony Chakraborty]

Dear YuShun,

I'm still facing the same issue after reducing the time step (dt) to 1e-7 during the critical impact phase. Another issue in 3D is that the number of cells becomes too large at Maxlevel 9.

Whether in serial simulation, this issue is kind of resolved, whereas in parallel simulation, it is not, but I can not say right now, as my simulation has not converged yet.

However, this is cost-effective. 

Best regards,

Symphony

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