Dynamic mesh adaption & VOF
 

Hello everyone,


I'm pretty new to fluent, and I've been trying to learn dynamic grid adaption, but I have run into a problem I haven't been able to solve.



The simulation is simply a small waterfall (height 10cm, v_in = 0.2 m/s). I'm using the sharp VOF model and implicit body forces, laminar flow. It works alright with a static mesh, but if I try to adapt it dynamically I get strange air pockets appearing and disappearing inside the water phase, as shown in the video (watch at 0.25 speed for clarity).



https://youtu.be/5VEsqzZ7hk8


I have tried a number of different settings for the grid adaptation, starting with the values used in the recent Ansys webinar on their new VOF-to-DPM model (refinement threshold 1e-08, coarsening 1e-10, based on unscaled water VOF curvature, updating every time-step) and changing pretty much everything I could think of (thresholds, timesteps between 0.01 s and 0.0001 s, curvature/gradient, unscaled/scaled/normalized) up and down, but it hasn't worked. Initial grid sizes were 2.5 mm and 1.25 mm.


One thing I noticed is that these air pockets always appear close to an air-water interface. Another thing I noticed is that there is a kind of clash between the grid refinement and coarsening functions, where the cells next to the interface get refined, and in the next time-step the coarsening function wants to coarsen them, so there is a kind of oscillation between fine and coarse mesh that I suspect is causing weird problems.


Has anyone experienced similar problems? Does dynamic mesh adaption work in a case like this, and if so, what would be the right way to set it up?


Thanks in advance

Hi Markus,

Do you have the tutorial of this vof-to-dpm model. Could you share it with me. Actually, I have the similar problem as well.


Best Regards,
Jonny