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Old   July 1, 2009, 10:44
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Johannes Gérson Janzen
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Dear friends,

I have no experience with CFX. I need to simulate the hydrodynamics of stabilization ponds. I am encountering difficulties in defining the water depth in the pond. At the beginning of the simulation the pond must already have some depth of water. How do I do that?

Thanks
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Old   July 1, 2009, 11:42
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second, have a look at the "free-surface flow over a bump" tutorial, and see how they define the water heigth.
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Old   July 2, 2009, 03:31
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you should make some expressions to define volume of water and volume of air through initialisation.
1- define an expression for initial water level, say: InitWatLevel = 2[m]
2- make a step function defining water volume: WatVol = step((InitWatLevel - z)/1[m])
3- make a step function defining air volume: AirVol = 1 - WatVol
4- use WatVol and AirVol expressions in Domain/Initialisation to define volume fraction of each fluid

good luck

ps. check Tutorial 7: Free Surface Flow Over a Bump
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Old   July 2, 2009, 03:48
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5- don't forget the hydrostatic pressure \rho\cdot g\cdot h
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Old   July 3, 2009, 12:21
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The height of the pond should be controlled by outlet weir. Try to find the outlet weir elevation and do a simple math.
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Old   July 6, 2009, 16:13
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Thanks for the reply.

The pond consists of one inlet pipe and one outlet pipe. The output may be in a position below the inlet. As inlet condition, I'm defining the normal speed. In the pond the conditions are similar as in the manual:

h = 2 [m]
initialVOFAir step = ((z - h) / 1 [m])
initialVOFWater = 1 - initialVOFAir
initialP = 998 [kg m ^ -3] * g * (h-z) * initialVOFWater

As outlet condition, we adopted the normal speed (which equals the speed of entry). What is the error that is being committed?
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Old   July 6, 2009, 16:38
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Quote:
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... What is the error that is being committed?
you tell us ...

btw the equation initialVOFAir = step((z - h) / 1 [m])

you forgot to define the air pressure in the pressure initialisation

PS if you have multiphase leave one phase volume fraction initialisation as automatic.
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Old   July 7, 2009, 11:02
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Originally Posted by jgersonj View Post
Thanks for the reply.

The pond consists of one inlet pipe and one outlet pipe. The output may be in a position below the inlet. As inlet condition, I'm defining the normal speed. In the pond the conditions are similar as in the manual:

h = 2 [m]
initialVOFAir step = ((z - h) / 1 [m])
initialVOFWater = 1 - initialVOFAir
initialP = 998 [kg m ^ -3] * g * (h-z) * initialVOFWater

As outlet condition, we adopted the normal speed (which equals the speed of entry). What is the error that is being committed?
The level of the lagoon can be controlled by the operators by adjusting the outlet exit level depending on the seasonal condition. So depending on what you really want to model, it usually should good enough to determine the water level based on traditional hydraulic calculations or based on outlet pipe elevation. And since the lagoon has very large area, it is also acceptable to simplify the inlet/outlet structure for the modeling purpose.

For designing optimization purpose, you may also consider a worst case scenario water level and use it as your modeling base
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