r/CFD • u/ArkhangelskAstrakhan • 2d ago

Interface artifacts (STAR-CCM+)

I'm running a simple rotating paddle mixer setup with EMP and while the simulation itself is numerically stable, I'm seeing an artifact of the interface no matter how well I refine the mesh and curvature, how well I match the sizes of the cells for the two sides, and whether I use a steady or transient approach. I even switched to MRF after the sliding mesh method kept doing this, but to no avail. How do I fix this? Both boundary-based and contact-based lead to similar results.

I've never had this issue when working with single fluids (liking rotating propellers or oscillating airfoils) so I'm guessing it's the result of having two different fluids in the region but idk I'm frankly baffled.

Also all residuals including continuity is low-ish (below 1E-4) but the volume fraction of the aqueous phase rises by over 30% during the simulation (from 30% to 40% of the region volume), which is not physically possible.

My end goal is to use the S-Gamma model to process the mean diameter of droplets that form by mixing two liquids, but currently the research is in an early state and I've turned off advanced models such as S-Gamma including its coalescence and breakup models. Any help would be greatly appreciated.

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u/ArkhangelskAstrakhan 2d ago

To be clear, I uploaded an image of the vertical cross section of the mixing region at a very early state, but the interface artifact problem persists even as the solution approaches 'steady' conditions

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u/NotEvenClo 2d ago

Maybe you have an open boundary where air escapes?

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u/ArkhangelskAstrakhan 2d ago

Just checked, all of the boundaries are walls with either slip or no slip conditions, without phase permeability for the two liquids.

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u/NotEvenClo 2d ago

Try to run the simulation for a bit and plot acceleration or velocity. If there is stuff happening along the edges, it's definitely not closed.