Defining appropriate model and BC for simulation

oringo · August 25, 2013, 19:56

Hi everyone,
I want to simulate situation in pumping station. The goemetry is shown in link below. The pumping station is connected with river, where of course level of water is fixed. There is one vertical pump in building. When pump is off the lvl of water in pumping station is the same as lvl of the river. When pump works with fixed capacity, the lvl of water near intake is lover than lvl in river, depending on capacity, building geometry etc.
I must get delta of the level of water in river and near pump intake.
I thought of usage VOF Implicit model with open channel option, but I don't know if it's good approach?
The next think are BC. When I thing about physical situation in pumping station I should use fixed lvl of water at inlet and mass flow at the outlet. After calculation I want to get free surface distribution in building. The problem is there is not mass flow outlet BC in Fluent. I could use eg. mass flow inlet and outflow BC, but I think I wouldn't get good free surface distribution in this case.
I would be very appreciate for every advice in this case, choosing of model, BC. Maybe I should use CFX, it has other BC which would better in this situation?
BR,
Mark

https://docs.google.com/file/d/0B_M5...it?usp=sharing

vig · August 30, 2013, 08:04

You could use open channel flow for this case.
At the outlet, mass flow bc could also be used with the direction vector pointing outward.

oringo · September 5, 2013, 10:02

Thanks a lot for advice. I've tried to do it like you wrote, but I have another problem. I've attached picture of situation below.
I've turned on option open channel flow in Models > Multiphase > VOF
On my inlet I've choosen BC pressure inlet and turned on option open channel to determine level of free surface, bottom level and total height (depending on capacity).
On my outlet I've choosen BC mass flow-inlet (with vector pointing outward) to determine capacity. I don't know what is level of free surface within the outlet, I want to get it from simulation, so I didn't turned on option open channel there. When I look at Volume fraction contour in my domain I see that whole outlet is filled by water and this is not physical of course...
Should I turn on open channel option in BC outlet (mass flow- inlet)? But after that I must enter free surface level which is unknown. I assume that it's lower than at inlet but don't know how much.
I would be very grateful for every advice in this case.
Kind regards,
Mark

https://docs.google.com/file/d/0B_M5...it?usp=sharing

vig · September 5, 2013, 21:24

I am not sure whether I understood your objective completely.
Are you just interested to know the free surface level and velocity at the outlet which is a outcome of solution.
If that is the case, you could use pressure outlet by specifying the pressure from the neighboring cells.

Else could you make the problem specification more clear.

oringo · September 6, 2013, 02:23

I want to get free surface level within the outlet to calculate delta of the level of water in inlet (river) and near oultet. I have fixed capacity (mass flow) at outlet according to I must calculate it. Because of that I need BC mass flow outlet. Of course real geometry is 3D and it's more complicated but now I work on 2D to reduce costs and learn as much as I can.
I suppose that I'd have to determine free surface lvl at the outlet which will be the same like at the inlet. I would also extend my domain at the outlet to keep the plane I'm interested in "independent from" free surface at the outlet (which is unknown before simulation).
I hope this would be more clear for you.
Mark

vig · September 6, 2013, 05:26

So, you have pump somewhere near the outlet which is sucking the water with some capacity.

What about putting the mass sink term near the neighboring cells of outlet. As you are doing 2D, number of marked cells could be the height of the pump where it is located. Due to this sink, there would be drop in the water level near the outlet. Pressure at outlet bc could be extrapolated from interior.

May or may not work.

August 25, 2013, 19:56	Defining appropriate model and BC for simulation	#1
oringo New Member Marek Join Date: Feb 2013 Posts: 6 Rep Power: 13	Hi everyone, I want to simulate situation in pumping station. The goemetry is shown in link below. The pumping station is connected with river, where of course level of water is fixed. There is one vertical pump in building. When pump is off the lvl of water in pumping station is the same as lvl of the river. When pump works with fixed capacity, the lvl of water near intake is lover than lvl in river, depending on capacity, building geometry etc. I must get delta of the level of water in river and near pump intake. I thought of usage VOF Implicit model with open channel option, but I don't know if it's good approach? The next think are BC. When I thing about physical situation in pumping station I should use fixed lvl of water at inlet and mass flow at the outlet. After calculation I want to get free surface distribution in building. The problem is there is not mass flow outlet BC in Fluent. I could use eg. mass flow inlet and outflow BC, but I think I wouldn't get good free surface distribution in this case. I would be very appreciate for every advice in this case, choosing of model, BC. Maybe I should use CFX, it has other BC which would better in this situation? BR, Mark https://docs.google.com/file/d/0B_M5...it?usp=sharing

August 30, 2013, 08:04		#2
vig New Member anonymous Join Date: Jan 2011 Posts: 23 Rep Power: 15	You could use open channel flow for this case. At the outlet, mass flow bc could also be used with the direction vector pointing outward. oringo likes this.

September 5, 2013, 10:02		#3
oringo New Member Marek Join Date: Feb 2013 Posts: 6 Rep Power: 13	Thanks a lot for advice. I've tried to do it like you wrote, but I have another problem. I've attached picture of situation below. I've turned on option open channel flow in Models > Multiphase > VOF On my inlet I've choosen BC pressure inlet and turned on option open channel to determine level of free surface, bottom level and total height (depending on capacity). On my outlet I've choosen BC mass flow-inlet (with vector pointing outward) to determine capacity. I don't know what is level of free surface within the outlet, I want to get it from simulation, so I didn't turned on option open channel there. When I look at Volume fraction contour in my domain I see that whole outlet is filled by water and this is not physical of course... Should I turn on open channel option in BC outlet (mass flow- inlet)? But after that I must enter free surface level which is unknown. I assume that it's lower than at inlet but don't know how much. I would be very grateful for every advice in this case. Kind regards, Mark https://docs.google.com/file/d/0B_M5...it?usp=sharing

September 5, 2013, 21:24		#4
vig New Member anonymous Join Date: Jan 2011 Posts: 23 Rep Power: 15	I am not sure whether I understood your objective completely. Are you just interested to know the free surface level and velocity at the outlet which is a outcome of solution. If that is the case, you could use pressure outlet by specifying the pressure from the neighboring cells. Else could you make the problem specification more clear.

September 6, 2013, 02:23		#5
oringo New Member Marek Join Date: Feb 2013 Posts: 6 Rep Power: 13	I want to get free surface level within the outlet to calculate delta of the level of water in inlet (river) and near oultet. I have fixed capacity (mass flow) at outlet according to I must calculate it. Because of that I need BC mass flow outlet. Of course real geometry is 3D and it's more complicated but now I work on 2D to reduce costs and learn as much as I can. I suppose that I'd have to determine free surface lvl at the outlet which will be the same like at the inlet. I would also extend my domain at the outlet to keep the plane I'm interested in "independent from" free surface at the outlet (which is unknown before simulation). I hope this would be more clear for you. Mark

September 6, 2013, 05:26		#6
vig New Member anonymous Join Date: Jan 2011 Posts: 23 Rep Power: 15	So, you have pump somewhere near the outlet which is sucking the water with some capacity. What about putting the mass sink term near the neighboring cells of outlet. As you are doing 2D, number of marked cells could be the height of the pump where it is located. Due to this sink, there would be drop in the water level near the outlet. Pressure at outlet bc could be extrapolated from interior. May or may not work.