modeling of turbidity flow in CFX

sadjad · December 8, 2015, 22:48

Hi guys

We have a flume that want to run turbidity flow. This flume has a sluice gate or valve. Behind the sluice gate there is salty water and in front of it there is water. With suddenly removal of sluice gate the salty water runs into the water. (Turbidity flow)

Length of flume: 500 cm

Width of flume: 35 cm

Height of flume: 70 cm

Height of free surface: 66 cm

Height of air in the flume: 4 cm

Attention: flow is free surface.

Now there is the main question in CFX that if we should define two fluid (Water & salty water) with section 66*35 or three fluid with section 70*35 (Water & salty water & air)?

And in the case of tree fluid with section 70*35 (Water & salty water & air) the expression of the air how could be defined or written? I cannot write it.

Please help me.

ghorrocks · December 9, 2015, 05:36

I do not understand your question - but you appear to be asking how to model a water/air model where the water has variable salinity and turbidity. The simple approach is to define a 2 phase fluid and add additional variables for the salinity and turbidity. A more accurate approach is to use a multicomponent mixture for the water. This can also model variable salinity and turbidity, but can include the effects of variable properties (such as the effects on density and viscosity).

sadjad · December 9, 2015, 07:14

you know....! this question is simple !
this is a turbidity flow or density flow

I have two assumptions in my model :

1 : i draw the flume with this dimentions L=500 cm , W=35 cm , H=66 cm
and just define water & salty water

2 : i draw the flume with this dimentions L=500 cm , W=35 cm , H=70 cm
and define water & salty water & air (H of water & salty water = 66 , H of air=4 cm above the free surface)

what is true? 1 or 2 ?

attention : above the flume is open and flow is free surface

Do you understand?

ghorrocks · December 9, 2015, 17:12

The question may be simple but a clear explanation of your question is still required. I am still guessing here. Use an image to show more clearly what you are talking about.

Are you saying you have 66cm depth of water/saline with 4cm of air above it. And your question is whether it is OK to model it as a 66cm deep single phase model or do you need to model the 70cm depth with an air layer and use a free surface model?

If that is your question - it depends on the free surface. If the free surface is flat, or at least known, then yes you can model this as a single phase simulation. If variation of the free surface causes significant flow features then you will need to model the free surface.

sadjad · December 14, 2015, 16:52

https://www.dropbox.com/s/w6verub7wq4kh3e/Drawing1-Model.jpg?dl=0

https://www.dropbox.com/s/d7fib1hsznnvc68/Drawing2-Model.jpg?dl=0

please open links and watch images.
then answer me 1 or 2 ?

which one is better to model?

ghorrocks · December 14, 2015, 17:49

I answered your question in my last post.

Quote:

If that is your question - it depends on the free surface. If the free surface is flat, or at least known, then yes you can model this as a single phase simulation. If variation of the free surface causes significant flow features then you will need to model the free surface.

Please do not PM with CFD questions. Post CFD question on the forum.

sadjad · December 15, 2015, 01:17

OK. Thanks.
Free surface is flat and no special turbulence.
So, according to you : don't need to model air in my work. But i have 2 phases. water & salty water. That is not single phase.
Does CFX give me a logical answer?

ghorrocks · December 15, 2015, 05:06

No, water and salty water are not two phases (for most applications). The salt diffuses into the water at the molecular level so it is a multicomponent mixture. Multiphase models are for micro scale mixing.

So you model the salty water as a multicomponent mixture.

sadjad · December 15, 2015, 05:56

OK
But in my model the term of advection and motion of salty water into the water is important. not diffusion
this is simulation of river flow into dam reservoir.
river flow : depth of flow and temperature are small and has some mud
water of dam reservoir : depth of flow and temperature are higher than river flow .then density of water of river is bigger than water of dam reservoir . so flow moves or transfers in bed of dam and finally deposits.so this is not diffusion.

ghorrocks · December 15, 2015, 06:24

Multicomponent flows have advection/motion as well..... Please read into this option in the CFX documentation because your flow is not a multiphase flow, it is a multicomponent flow.

sadjad · December 15, 2015, 16:42

OK. Ghorrocks
Thanks a lot.
You helped me very very good.
Good luck.

sadjad · January 10, 2016, 12:30

which one of cfx tutorials are about multicomponent flow ?

ghorrocks · January 10, 2016, 16:57

Any one with a mixture of gasses.

December 8, 2015, 22:48	modeling of turbidity flow in CFX	#1
sadjad New Member sadjad Join Date: Oct 2015 Posts: 15 Rep Power: 10	Hi guys We have a flume that want to run turbidity flow. This flume has a sluice gate or valve. Behind the sluice gate there is salty water and in front of it there is water. With suddenly removal of sluice gate the salty water runs into the water. (Turbidity flow) Length of flume: 500 cm Width of flume: 35 cm Height of flume: 70 cm Height of free surface: 66 cm Height of air in the flume: 4 cm *Attention: flow is free surface.* Now there is the main question in CFX that if we should define two fluid (Water & salty water) with section 6635 or three fluid with section 7035 (Water & salty water & air)? And in the case of tree fluid with section 7035 (Water & salty water & air) the expression of the air how could be defined or written? I cannot write it.* Please help me.

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December 9, 2015, 05:36		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,718 Rep Power: 143	I do not understand your question - but you appear to be asking how to model a water/air model where the water has variable salinity and turbidity. The simple approach is to define a 2 phase fluid and add additional variables for the salinity and turbidity. A more accurate approach is to use a multicomponent mixture for the water. This can also model variable salinity and turbidity, but can include the effects of variable properties (such as the effects on density and viscosity).

December 9, 2015, 07:14		#3
sadjad New Member sadjad Join Date: Oct 2015 Posts: 15 Rep Power: 10	you know....! this question is simple ! this is a turbidity flow or density flow I have two assumptions in my model : 1 : i draw the flume with this dimentions L=500 cm , W=35 cm , H=66 cm and just define water & salty water 2 : i draw the flume with this dimentions L=500 cm , W=35 cm , H=70 cm and define water & salty water & air (H of water & salty water = 66 , H of air=4 cm above the free surface) what is true? 1 or 2 ? attention : above the flume is open and flow is free surface Do you understand?

December 9, 2015, 17:12		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,718 Rep Power: 143	The question may be simple but a clear explanation of your question is still required. I am still guessing here. Use an image to show more clearly what you are talking about. Are you saying you have 66cm depth of water/saline with 4cm of air above it. And your question is whether it is OK to model it as a 66cm deep single phase model or do you need to model the 70cm depth with an air layer and use a free surface model? If that is your question - it depends on the free surface. If the free surface is flat, or at least known, then yes you can model this as a single phase simulation. If variation of the free surface causes significant flow features then you will need to model the free surface.

December 14, 2015, 16:52		#5
sadjad New Member sadjad Join Date: Oct 2015 Posts: 15 Rep Power: 10	https://www.dropbox.com/s/w6verub7wq4kh3e/Drawing1-Model.jpg?dl=0 https://www.dropbox.com/s/d7fib1hsznnvc68/Drawing2-Model.jpg?dl=0 please open links and watch images. then answer me 1 or 2 ? which one is better to model?

December 15, 2015, 01:17		#7
sadjad New Member sadjad Join Date: Oct 2015 Posts: 15 Rep Power: 10	OK. Thanks. Free surface is flat and no special turbulence. So, according to you : don't need to model air in my work. But i have 2 phases. water & salty water. That is not single phase. Does CFX give me a logical answer?

December 15, 2015, 05:06		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,718 Rep Power: 143	No, water and salty water are not two phases (for most applications). The salt diffuses into the water at the molecular level so it is a multicomponent mixture. Multiphase models are for micro scale mixing. So you model the salty water as a multicomponent mixture.

December 15, 2015, 05:56		#9
sadjad New Member sadjad Join Date: Oct 2015 Posts: 15 Rep Power: 10	OK But in my model the term of advection and motion of salty water into the water is important. not diffusion this is simulation of river flow into dam reservoir. river flow : depth of flow and temperature are small and has some mud water of dam reservoir : depth of flow and temperature are higher than river flow .then density of water of river is bigger than water of dam reservoir . so flow moves or transfers in bed of dam and finally deposits.so this is not diffusion.

December 15, 2015, 06:24		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,718 Rep Power: 143	Multicomponent flows have advection/motion as well..... Please read into this option in the CFX documentation because your flow is not a multiphase flow, it is a multicomponent flow.

December 15, 2015, 16:42		#11
sadjad New Member sadjad Join Date: Oct 2015 Posts: 15 Rep Power: 10	OK. Ghorrocks Thanks a lot. You helped me very very good. Good luck.

January 10, 2016, 12:30		#12
sadjad New Member sadjad Join Date: Oct 2015 Posts: 15 Rep Power: 10	which one of cfx tutorials are about multicomponent flow ?

January 10, 2016, 16:57		#13
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,718 Rep Power: 143	Any one with a mixture of gasses.