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Fluid in 2D channel dambreak extension

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Old   August 9, 2019, 13:46
Default Fluid in 2D channel dambreak extension
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Josh McCraney
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Hi all!

I'm running the tutorial dambreak, and have changed the geometry to a rectangle (no bump on the bottom) and the initial condition to a circular arc (see video here https://imgur.com/a/nsTBIO8#32DCvNL).

I've turned gravity and viscosity off for both fluids. I've also enforced a static contact angle of 71 degrees. After a very long time the surface starts to become static, getting closer and closer to equilibrium: why (perhaps not shown in this short video, but if I run for longer times this happens)?

Seems odd to me since there is no gravity or viscosity acting to stabilize the interface.
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Old   August 12, 2019, 21:45
Default Numerical Dissipation
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Peter Brady
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Hi Josh,


My first guess would be numerical dissipation. Even with a higher order schemes there will be some small truncation errors, which can be interpreted as "numerical turbulence". This is actually the basis of some implicit turbulence models and will act to extract energy from the flow field. Hence, if you are removing energy, the flow field will eventually damp towards an equilibrium solution.



Could this be a mechanism to explain what you are seeing?


Cheers,
-pete
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Old   August 13, 2019, 07:55
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Josh McCraney
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Quote:
Originally Posted by pbrady2013 View Post
Hi Josh,


My first guess would be numerical dissipation. Even with a higher order schemes there will be some small truncation errors, which can be interpreted as "numerical turbulence". This is actually the basis of some implicit turbulence models and will act to extract energy from the flow field. Hence, if you are removing energy, the flow field will eventually damp towards an equilibrium solution.



Could this be a mechanism to explain what you are seeing?


Cheers,
-pete
Hi Pete

Yea, this is something I was worried about. I reduced the time step, and it looks like the issue is going away, which would imply numerical dissipation could be the issue.

However, when I reduce the time step sufficiently small I actually get some instabilities in the interface, and even topological breakup! I can post a video, but have you ever seen this? (CFL number is restricted less than 0.5)
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