Additional scalar in Free-surface flows

-__-!!! · March 2, 2011, 12:37

Hi all,

I have a free surface flow plunging into a tank. This is modelled as a multiphase (MP) free surface flow - tried both the FS and the inhomogeneous. The region above the water surface is air and a small opening at the top of the tank is modelled as a vent.

I put a non-dispersive tracer (additional variable) on the water phase and was hoping to track it in the tank. This is solved with the flow and MP equations turned off in expert parameters. I noticed that most of the tracer just moves from the plunging flow into the gas phase (region above the water level), with only very little carried forward into the water in the tank.

I am guessing this is a model limitation here. So two questions:

1) If there was no plunging flow, and it's a proper open channel flow, would the tracer escape from water to the air above it? I am guessing yes, but what's to stop a large amount of tracer from spreading into the small volume fraction of water in the predominant air phase?

2) So if tracer does escape to the water in the air phase because of the plunge, is there work-around?

Any comments would be appreciated!

ghorrocks · March 2, 2011, 17:46

In an inhomogenous model the tracer will not necesarily follow the water phase as the air phase has a different velocity field. Using a homogenous model will help, if this model is appropriate.

Getting the tracer to follow the flow will be simply (not that simple actually) a matter of reducing numerical dissipation in your model. Use second order advection discretisation for momentum and the AV, and make sure your grid is fine enough to resolve the flow properly and check you are adequately converged.

March 2, 2011, 12:37	Additional scalar in Free-surface flows	#1
-__-!!! New Member Join Date: Mar 2011 Posts: 1 Rep Power: 0	Hi all, I have a free surface flow plunging into a tank. This is modelled as a multiphase (MP) free surface flow - tried both the FS and the inhomogeneous. The region above the water surface is air and a small opening at the top of the tank is modelled as a vent. I put a non-dispersive tracer (additional variable) on the water phase and was hoping to track it in the tank. This is solved with the flow and MP equations turned off in expert parameters. I noticed that most of the tracer just moves from the plunging flow into the gas phase (region above the water level), with only very little carried forward into the water in the tank. I am guessing this is a model limitation here. So two questions: 1) If there was no plunging flow, and it's a proper open channel flow, would the tracer escape from water to the air above it? I am guessing yes, but what's to stop a large amount of tracer from spreading into the small volume fraction of water in the predominant air phase? 2) So if tracer does escape to the water in the air phase because of the plunge, is there work-around? Any comments would be appreciated!

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March 2, 2011, 17:46		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	In an inhomogenous model the tracer will not necesarily follow the water phase as the air phase has a different velocity field. Using a homogenous model will help, if this model is appropriate. Getting the tracer to follow the flow will be simply (not that simple actually) a matter of reducing numerical dissipation in your model. Use second order advection discretisation for momentum and the AV, and make sure your grid is fine enough to resolve the flow properly and check you are adequately converged.