TAGGED: cfl-time-step, fluent, surface-tension, timestep-size
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April 16, 2025 at 5:23 am
jose.simongreminger
SubscriberDear all,Â
I am running a transient simulation of a partially-filled rotating cylinder with a explicit VOF formulation. Therefore I a monitoring both the global CFL and the maximum local CFL. My global CFL is around 0.15 but my maximum local CFL is around 0.5. I wanted to ask you if I can increase my maximum local CFL, as long as I keep my global CFL below 0.5 (to keep a stable simulation). Moreover, I also trying to imlpement surface tension effect. However, when I introduce surface tension between the water and the air, I have a large numerical diffusion at the interface when I start rotating. This numerical diffusion at the interface only starts when I start rotating the cylinder, and seems to be independent of the timestep that I take. I am using a constant value for the surface of 0.072N/m, CSF model, Wall adhesion.
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April 16, 2025 at 8:34 am
Rob
Forum ModeratorHow are you rotating the cylinder? Time step will be limited by cell rotation, in that you can't pass cells in a step. How well resolved is the free surface region, and how big (roughly) is the domain?Â
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April 16, 2025 at 9:29 am
jose.simongreminger
SubscriberHello Rob,Â
Thank you for your answer.Â
I am rotating the cylinder by applying a circular acceleration on the XY plane, and tilted gravity force on the Z axis. My domain is a cylinder with a 10mm diameter and a 15mm height. The resolution of the free surface is 0.1mm. If I understand you correctly I should then always keep the maximal local CFL below 0.5, even if the global CFL is at 0.15 and could be increased, right?
I needed to go this resolution to avoid numerical diffusion (without surface tension). But then when I introduce again surface tension, I have numerical diffusion at the interface again.Â
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April 16, 2025 at 10:40 am
Rob
Forum ModeratorThe resolution may need to be increased around the free surface but surface tension forces are going to dominate. Are you using geo-reconstruct for interface tracking? Compressive is good, but you may be on the limits for it's accuracy.Â
CFL is a useful measure, but also check the time the rotation takes to cross 0.1mm (cell size) at the (mesh) interface as that's not checked by VOF.Â
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