How to set phases with different velocity

kbaker · May 2, 2012, 04:42

Hello I am want to implement stratified two-phase flow by CFX. The phases are (Oil and Water) the oil flowing at the lower part of the pipe and water at the upper part. The two phases flows at different superficial velocities I need to ask how I can set the velocity of each phase in this case in CFX-Pre?
In Bump tutorial I saw they set only one velocity for both phases? Furthermore reference to my case which it better the segregated solved or the coupled solver?

Thanks a lot
Kbaker

juliom · May 2, 2012, 14:01

Dear colleague,
You do this, jsut defining to phases, and using the Euler-Euler model, specifically particle model.
At the inlet you define each velocity..
Good luck!

kbaker · May 2, 2012, 17:11

Hi thanks for the answer. Is Euler-Euler model is same as homogeneous model with free surface model available in Multiphase menu? How can I select particle model I think it not appropriate to my case cause I have continuous fluids (not have particles anymore within the domain of solution)?

ghorrocks · May 2, 2012, 19:09

You cannot see the phase velocities in the bump tutorial as it is a homongenous model - both phases share a single velocity field and this is appropriate for free surface models where you have one phase or the other and they do not mix. If you run an inhomogenous model then velocities will be available for both phases.

mariam.sara · May 3, 2012, 07:44

?????????????????????

kbaker · May 3, 2012, 07:52

Thanks ghorrocks for the reply. In my case the phases not mixed (I have stratified flow case) and there is a clear interface between them. If I use in-homogeneous model I think CFX will force the phases to be mixed and I not want this occurs? I want the phases flows within the pipe with different velocities separately? so is the in-homogeneous model ensure this thing?

ghorrocks · May 3, 2012, 08:22

Looks like I have confused mariam. At least I have achieved something today

If the phases never mix then use the homogenous model. You only use inhomogenous when interphase slip happens. In your case you define a boundary with the velocity defined by its position, but still using the homogenous model.

kbaker · May 3, 2012, 10:04

may you clear to me pls how I can apply what mentioned in your phrase (you define a boundary with the velocity defined by its position) in CFX-Pre?

ghorrocks · May 3, 2012, 10:08

If the surface is at z=0, then for z<0 you define velocity=0 [m/s], and where z>0 you define velocity=1[m/s].

kbaker · May 3, 2012, 13:08

You mean I need write CEL expression at inlet for the velocity? if it a CEL expression may you write it for the same conditions you mentioned me above in your example with surface at z=0 ?

ghorrocks · May 3, 2012, 18:23

Yes, a CEL expression. Have a look inthe documentation and tutorials on CEL, it is quite a simple language.

kbaker · May 4, 2012, 04:17

Well in Bump tutorial at inlet boundary they set the turbulent length scale as UpH is this the value for the height above the interface or the full channel height?
Another thing in Bump tutorial too at outlet boundary they define static pressure as equation represented by DownPres ? should I set the same relation to my case? As I know from the tutorial guide this relation depends on the expected height of free surface at outlet in my simulation the height of the free surface unknown at outlet (at inlet it known and I tried CEL expression for it) and I want it computed from the downstream flow reference to the boundary at inlet?

ghorrocks · May 4, 2012, 07:36

You should choose your boundary conditions to match what you are modelling.

Also boundaries which only see a single phase are much simpler. If you can make use of this it will be simpler.

kbaker · May 4, 2012, 07:56

1) You not answer my question about UpH at inlet boundary in Bump tutorial is it represent the height above the interface (air level above the interface)?

2) Another thing what you advice me to use at the outlet boundary? pressure outlet or opening boundary?

ghorrocks · May 4, 2012, 09:23

I have not looked at thtat tutorial in detail for a while. But if you understand the tutorial you will be able to work it out. So you better look at it a bit closer.

Use an outlet when you do not expect back flow as it is a more numerically stable boundary. Use openings when back flow is possible, but be aware that it will be less numerically stable. If you are putting a boundary near a separation it is probably better the move the boundary down stream to avoid it.

kbaker · May 4, 2012, 15:27

Thanks a lot ghorrocks for the valuable information. I have some doubts hope you explain it for me:
1) Reference to my case which it is better to use the segregated solver or coupled solver?
2) When I select homogeneous model>free surface model>standard should I select interface compression level or leave it clear? If yes how many interface level should I select?

Thanks
kbaker

ghorrocks · May 5, 2012, 08:25

1. Depends which side of bed you got out of this morning. They both have sterngths and weaknesses and I don't have time to describe them all.
2) The default settings are usually good. But if you have some time I recommend trying all the options and seeing what ones work well for your simulation.

May 2, 2012, 04:42	How to set phases with different velocity	#1
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	Hello I am want to implement stratified two-phase flow by CFX. The phases are (Oil and Water) the oil flowing at the lower part of the pipe and water at the upper part. The two phases flows at different superficial velocities I need to ask how I can set the velocity of each phase in this case in CFX-Pre? In Bump tutorial I saw they set only one velocity for both phases? Furthermore reference to my case which it better the segregated solved or the coupled solver? Thanks a lot Kbaker

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
why shroud velocity can't be set	myleader	CFX	6	October 18, 2011 05:12
can i set the velocity and pressure at the inlet at the same time by UDF	minyang.cau	FLUENT	0	July 15, 2009 00:14
How to set up the velocity in the let boudary changing with time.	flying	OpenFOAM Running, Solving & CFD	7	June 5, 2009 10:26
udf inlet velocity is not set correctly, why ?	GaoGe	FLUENT	0	June 17, 2008 09:33
How to set transient ang velocity?	edi ghirardi	FLUENT	0	April 12, 2005 10:34

May 2, 2012, 14:01		#2
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	Dear colleague, You do this, jsut defining to phases, and using the Euler-Euler model, specifically particle model. At the inlet you define each velocity.. Good luck!

May 2, 2012, 17:11		#3
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	Hi thanks for the answer. Is Euler-Euler model is same as homogeneous model with free surface model available in Multiphase menu? How can I select particle model I think it not appropriate to my case cause I have continuous fluids (not have particles anymore within the domain of solution)?

May 2, 2012, 19:09		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You cannot see the phase velocities in the bump tutorial as it is a homongenous model - both phases share a single velocity field and this is appropriate for free surface models where you have one phase or the other and they do not mix. If you run an inhomogenous model then velocities will be available for both phases.

May 3, 2012, 07:44		#5
mariam.sara Senior Member Join Date: Jan 2011 Posts: 339 Rep Power: 16	?????????????????????

May 3, 2012, 07:52		#6
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	Thanks ghorrocks for the reply. In my case the phases not mixed (I have stratified flow case) and there is a clear interface between them. If I use in-homogeneous model I think CFX will force the phases to be mixed and I not want this occurs? I want the phases flows within the pipe with different velocities separately? so is the in-homogeneous model ensure this thing?

May 3, 2012, 08:22		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Looks like I have confused mariam. At least I have achieved something today If the phases never mix then use the homogenous model. You only use inhomogenous when interphase slip happens. In your case you define a boundary with the velocity defined by its position, but still using the homogenous model.

May 3, 2012, 10:04		#8
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	may you clear to me pls how I can apply what mentioned in your phrase (you define a boundary with the velocity defined by its position) in CFX-Pre?

May 3, 2012, 10:08		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	If the surface is at z=0, then for z<0 you define velocity=0 [m/s], and where z>0 you define velocity=1[m/s].

May 3, 2012, 13:08		#10
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	You mean I need write CEL expression at inlet for the velocity? if it a CEL expression may you write it for the same conditions you mentioned me above in your example with surface at z=0 ?

May 3, 2012, 18:23		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Yes, a CEL expression. Have a look inthe documentation and tutorials on CEL, it is quite a simple language.

May 4, 2012, 04:17		#12
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	Well in Bump tutorial at inlet boundary they set the turbulent length scale as UpH is this the value for the height above the interface or the full channel height? Another thing in Bump tutorial too at outlet boundary they define static pressure as equation represented by DownPres ? should I set the same relation to my case? As I know from the tutorial guide this relation depends on the expected height of free surface at outlet in my simulation the height of the free surface unknown at outlet (at inlet it known and I tried CEL expression for it) and I want it computed from the downstream flow reference to the boundary at inlet?

May 4, 2012, 07:36		#13
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You should choose your boundary conditions to match what you are modelling. Also boundaries which only see a single phase are much simpler. If you can make use of this it will be simpler.

May 4, 2012, 07:56		#14
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	1) You not answer my question about UpH at inlet boundary in Bump tutorial is it represent the height above the interface (air level above the interface)? 2) Another thing what you advice me to use at the outlet boundary? pressure outlet or opening boundary?

May 4, 2012, 09:23		#15
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	I have not looked at thtat tutorial in detail for a while. But if you understand the tutorial you will be able to work it out. So you better look at it a bit closer. Use an outlet when you do not expect back flow as it is a more numerically stable boundary. Use openings when back flow is possible, but be aware that it will be less numerically stable. If you are putting a boundary near a separation it is probably better the move the boundary down stream to avoid it.

May 4, 2012, 15:27		#16
kbaker Senior Member Khalid Baker Join Date: Mar 2009 Location: IRAQ Posts: 168 Rep Power: 17	Thanks a lot ghorrocks for the valuable information. I have some doubts hope you explain it for me: 1) Reference to my case which it is better to use the segregated solver or coupled solver? 2) When I select homogeneous model>free surface model>standard should I select interface compression level or leave it clear? If yes how many interface level should I select? Thanks kbaker

May 5, 2012, 08:25		#17
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	1. Depends which side of bed you got out of this morning. They both have sterngths and weaknesses and I don't have time to describe them all. 2) The default settings are usually good. But if you have some time I recommend trying all the options and seeing what ones work well for your simulation.