Questions about modeling of a reservoir

hwangpo · October 29, 2012, 01:53

Hi everyone,
I modeled a reservoir a few days ago, and I was locked at several questions.
Information of this 3-D model:
the reservoir shape is like box (length*width*height-360*243*87)
9 outlets in a line on the intake wall

questions:
1. Boundary conditions: the water surface uses free slip wall; channel banks and bottom use no slip wall; outlets use mass flow rate(100 kg/s); I dont know which B.C. is better for 'inlet', mass rate flow, static presure or opening? If I want to use static pressure, how can I specify the value of the pressure? the value can be given as 87m ? or 0 ?
2. CFD-post: how can I get the acceleration pattern? Use CEL?

THANK YOU very much for helping me.
online1.jpg

sakurabogoda · October 29, 2012, 04:02

Hi, I think static pressure is the most suitable BC for inlet. Bz at the inlet the velocity/mass flow rate is nearly zero. Isn't it? (I don't have a full idea) For value of static pressure, you should think about reference pressure too.

Do you have any idea of the outlet BC? Static pressure or mass flow rate?

ghorrocks · October 29, 2012, 05:03

Sakura - it is too early to jump to conclusions about boundary conditions. He has not described what he is trying to model yet.

So what are you trying to model? What do you know about the inlets? Are they always underwater? Does the free surface move much?

hwangpo · October 30, 2012, 06:45

Quote:

Originally Posted by sakurabogoda

Hi, I think static pressure is the most suitable BC for inlet. Bz at the inlet the velocity/mass flow rate is nearly zero. Isn't it? (I don't have a full idea) For value of static pressure, you should think about reference pressure too.

Do you have any idea of the outlet BC? Static pressure or mass flow rate?

thank u for your reply
the outlet BC use mass flow rate
my reference pressure is set as 1 atm, what value of static pressure will be right at inlet BC? the water depth is about 87m.
thank u so much.

ghorrocks · October 30, 2012, 06:49

What are you trying to model? What results are you trying to get? What do you know about the flow conditions - you have already mentioned the outlet flow rate, but what about the upstream flow?

hwangpo · October 30, 2012, 06:53

Quote:

Originally Posted by ghorrocks

Sakura - it is too early to jump to conclusions about boundary conditions. He has not described what he is trying to model yet.

So what are you trying to model? What do you know about the inlets? Are they always underwater? Does the free surface move much?

sorry for that.
i am going to model the flow pattern upstream the intakes.
i want to use rigid lid to simulate the water surface. outlet BC uses the mass flow rate, then I wonder if I can use the static pressure at the inlet.
I think the pressure at the inlet is a function of depth. Am I right?
So I really do not know how to set the inlet BC.
I appreciate your reply very much.

ghorrocks · October 30, 2012, 17:54

If you can assume the free surface level then the hydrostatic pressure plays no role and can be ignored. Then you can define your inlet as a constant pressure boundary. If the free surface moves or something else requires you to include the hydrostatic pressure then you might have to include the hydrostatic component.

hwangpo · October 30, 2012, 22:46

thank u very much

October 29, 2012, 01:53	Questions about modeling of a reservoir	#1
hwangpo Member Ben B. Huang Join Date: Oct 2012 Posts: 54 Rep Power: 14	Hi everyone, I modeled a reservoir a few days ago, and I was locked at several questions. Information of this 3-D model: the reservoir shape is like box (lengthwidthheight-36024387) 9 outlets in a line on the intake wall questions: 1. Boundary conditions: the water surface uses free slip wall; channel banks and bottom use no slip wall; outlets use mass flow rate(100 kg/s); I dont know which B.C. is better for 'inlet', mass rate flow, static presure or opening? If I want to use static pressure, how can I specify the value of the pressure? the value can be given as 87m ? or 0 ? 2. CFD-post: how can I get the acceleration pattern? Use CEL? THANK YOU very much for helping me. online1.jpg

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October 29, 2012, 04:02		#2
sakurabogoda Senior Member S.Bogoda Join Date: Jul 2012 Posts: 133 Rep Power: 14	Hi, I think static pressure is the most suitable BC for inlet. Bz at the inlet the velocity/mass flow rate is nearly zero. Isn't it? (I don't have a full idea) For value of static pressure, you should think about reference pressure too. Do you have any idea of the outlet BC? Static pressure or mass flow rate?

October 29, 2012, 05:03		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Sakura - it is too early to jump to conclusions about boundary conditions. He has not described what he is trying to model yet. So what are you trying to model? What do you know about the inlets? Are they always underwater? Does the free surface move much?

October 30, 2012, 06:49		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	What are you trying to model? What results are you trying to get? What do you know about the flow conditions - you have already mentioned the outlet flow rate, but what about the upstream flow?

October 30, 2012, 17:54		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	If you can assume the free surface level then the hydrostatic pressure plays no role and can be ignored. Then you can define your inlet as a constant pressure boundary. If the free surface moves or something else requires you to include the hydrostatic pressure then you might have to include the hydrostatic component.

October 30, 2012, 22:46		#8
hwangpo Member Ben B. Huang Join Date: Oct 2012 Posts: 54 Rep Power: 14	thank u very much