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-   -   Which features to model hydraulic structures? (https://www.cfd-online.com/Forums/flow-3d/98406-features-model-hydraulic-structures.html)

 bbayat March 10, 2012 02:34

Which features to model hydraulic structures?

Dear users
I am going to model some hydraulic structures such as spillway, bottom outlet, flip bucket, stilling basin, flow near sluice gates, ... which contain water and air mixtures in some flow regions; or maybe we have some aerator from an opening.
It would be so appreciable that someone tells me if I should consider any of the following features in my models:
1- air entrainment model,
2- two phase fluid (water+air)
3- bubble and phase change model
4- cavitation model
5- Density evaluation

Thanks a lot

 JBurnham March 13, 2012 15:55

if the air is entrained at a free surface or can escape, then

1) air entrainment
2) density evaluation
3) drift-flux (use water viscosity for 'two-fluid mixture' viscosity, and ~0.95 for drag coefficient).

don't use cavitation model for your case.

 bbayat March 14, 2012 09:35

Dear Jeff

Now that I found which models to activate, I have some more detailed questions and I would be happy if you could help:

- Air entrainment model
Does "entrainment rate coefficient" mean Qair/Qwater ?
What should we consider for "surface tension coef."?

- Density evaluation
which option to use is better?

Should I change: Fluids> water at 20> edit> phase change> solidifaction fluid1 properties> density value to density of air? [Floscience: FSI-03-TN61]

If I use RNG, can I let "Turbulent mixing length" value empty? If not what is a rule of thumb for its value?

If we know the Q(air) from an aerator from a specific location, how can we model it?

I do apologize, making so many questions.

 JBurnham March 14, 2012 11:37

Most of those questions can be answered from the User Manual or a quick Google search. As to how to add air at a known flow rate from an aerator, use a mass source or mass/momentum source and specify the density at the source as that of air (see the User Manual for descriptions of these options).

The entrainment rate coefficient is a scaling coefficient and is described in the Turbulent Air Entrainment at Free Surfaces tech note. Do not use the air entrainment model unless you are seeking to model air entrainment at free surfaces (i.e., only use density eval and drift-flux if you are only adding air at a known flow rate from an aerator). Good luck.

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