how to calculate water average velocity in free surface models

hmasenger · January 17, 2015, 11:23

Hi dear friends

i am modeling water flow over an spillway by a free surface model.the problem is that i don't know how to calculate average water velocity just in water phase .i thing CFX calculates a wrong value while using "average water velocity@outlet"
i think cfx considers velocity in whole outlet surface but i need it to be calculated on only water phase.
one more question:
does any one knows how can i calculate water depth in free surface model? i mean i know how to use an iso surface at VOF=0.5 and then making Poly line by intersections an then plotting Y at that poly line , but can you suggest me an expression to subtract bottom elevation from water surface elevation?

best regards

JuPa · January 17, 2015, 13:33

You want water superficial velocity.

Regarding your other question, try:
1) Create a plane in the x-y position.
2) Create an Isoclip on the plane, and select air volume fraction > 0. It will then show a plane of air without water, which you can then use to do contours etc.

hmasenger · January 17, 2015, 13:36

Yup.
I also used superfacial velocity but still it turns a wrong value for avarage velocity.
I think the problem is with water section area.cfx usex whole area to calculate avg but it should only considers water depth

hmasenger · January 17, 2015, 13:37

How can I force that to cfx?

JuPa · January 17, 2015, 14:05

Quote:

Originally Posted by hmasenger

Yup.
I also used superfacial velocity but still it turns a wrong value for avarage velocity.
I think the problem is with water section area.cfx usex whole area to calculate avg but it should only considers water depth

How are you working out the average superficial velocity? And where do you want to calculate the average?

hmasenger · January 17, 2015, 14:07

I call the calculator and use predefined expression" avg(water superfacial velocity )@outlet

hmasenger · January 17, 2015, 14:09

As u see the outlet contains about 30% water and remain is for air

JuPa · January 17, 2015, 14:17

Quote:

Originally Posted by hmasenger

I call the calculator and use predefined expression" avg(water superfacial velocity )@outlet

Don't use the arithmatic average. Use the area weighted average, i.e.:

areaAve(Water.superficial velocity)@outlet

hmasenger · January 17, 2015, 14:36

I have already tried that. Nopp the problem still exists

.

JuPa · January 17, 2015, 14:44

Quote:

Originally Posted by hmasenger

I have already tried that. Nopp the problem still exists

.

What is the problem when you do areaAve(Water.superficial velocity)@outlet?

hmasenger · January 17, 2015, 14:48

Still The average is less than the value that should be! because it considers the whole area of outlet.

JuPa · January 17, 2015, 15:32

I don't agree. Area average is a weighted average. If there is no water superficial velocity in some part of an area then its weighted average in that area is zero.

Try this, and if you still don't get the required answer then your results are wrong.
1) Create a new plane on the outlet.
2) Create a new iso clip, and set its location on the plane you just created.
3) In the iso clip, set the water volume fraction to be visable when it is >= 0.1.
4) Go to calculators and determine what the areaAve(Water.superficialvelocity)@iso clip is

hmasenger · January 17, 2015, 15:35

Thanx for the clue.I will inform you about the results

hmasenger · January 17, 2015, 15:42

That worked ricochet.problem solved.!! .that was exactly what I need.tnx alot

JuPa · January 17, 2015, 18:58

Good!! :-)

subhankarnits · September 20, 2018, 03:09

Hi @hmasenger what is size of elements you had taken for capturing sharp interface between water and air?

ghorrocks · September 20, 2018, 03:18

You will find that for most free surface applications CFX smears the free surface over about 4 elements. To get this it needs to be well converged and with good time accuracy. This means that the sharpness of the free surface is simply proportional to the mesh size - so finer mesh equals sharper resolution.

subhankarnits · September 20, 2018, 03:31

In my project the main aim is to capture water surface height and outlet discharge. There is no need of boundary layer velocity profile etc. But I heard that for some turbulence model results are largely depends on y+ value. So should I check for y+ value or just finer mesh near the free surface will do my job?

Opaque · September 20, 2018, 10:42

Careful. The free surface is not a wall.

You may be sharpening the free surface resolution, but there is no influence on the yplus since it is not a wall.

subhankarnits · September 20, 2018, 10:45

Yes I know that. But channel bed is wall so there should I check y+ ?

September 20, 2018, 03:18		#17
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	You will find that for most free surface applications CFX smears the free surface over about 4 elements. To get this it needs to be well converged and with good time accuracy. This means that the sharpness of the free surface is simply proportional to the mesh size - so finer mesh equals sharper resolution. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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January 17, 2015, 13:33		#2
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	You want water superficial velocity. Regarding your other question, try: 1) Create a plane in the x-y position. 2) Create an Isoclip on the plane, and select air volume fraction > 0. It will then show a plane of air without water, which you can then use to do contours etc.

January 17, 2015, 13:36		#3
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	Yup. I also used superfacial velocity but still it turns a wrong value for avarage velocity. I think the problem is with water section area.cfx usex whole area to calculate avg but it should only considers water depth

January 17, 2015, 13:37		#4
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	How can I force that to cfx?

January 17, 2015, 14:07		#6
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	I call the calculator and use predefined expression" avg(water superfacial velocity )@outlet

January 17, 2015, 14:09		#7
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	As u see the outlet contains about 30% water and remain is for air

January 17, 2015, 14:36		#9
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	I have already tried that. Nopp the problem still exists .

January 17, 2015, 14:48		#11
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	Still The average is less than the value that should be! because it considers the whole area of outlet.

January 17, 2015, 15:32		#12
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	I don't agree. Area average is a weighted average. If there is no water superficial velocity in some part of an area then its weighted average in that area is zero. Try this, and if you still don't get the required answer then your results are wrong. 1) Create a new plane on the outlet. 2) Create a new iso clip, and set its location on the plane you just created. 3) In the iso clip, set the water volume fraction to be visable when it is >= 0.1. 4) Go to calculators and determine what the areaAve(Water.superficialvelocity)@iso clip is

January 17, 2015, 15:35		#13
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	Thanx for the clue.I will inform you about the results

January 17, 2015, 15:42		#14
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	That worked ricochet.problem solved.!! .that was exactly what I need.tnx alot

January 17, 2015, 18:58		#15
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Good!! :-)

September 20, 2018, 03:09		#16
subhankarnits Member subhankar das Join Date: Jul 2018 Posts: 32 Rep Power: 8	Hi @hmasenger what is size of elements you had taken for capturing sharp interface between water and air?

September 20, 2018, 03:31		#18
subhankarnits Member subhankar das Join Date: Jul 2018 Posts: 32 Rep Power: 8	In my project the main aim is to capture water surface height and outlet discharge. There is no need of boundary layer velocity profile etc. But I heard that for some turbulence model results are largely depends on y+ value. So should I check for y+ value or just finer mesh near the free surface will do my job?

September 20, 2018, 10:42		#19
Opaque Senior Member Join Date: Jun 2009 Posts: 1,873 Rep Power: 33	Careful. The free surface is not a wall. You may be sharpening the free surface resolution, but there is no influence on the yplus since it is not a wall.

September 20, 2018, 10:45		#20
subhankarnits Member subhankar das Join Date: Jul 2018 Posts: 32 Rep Power: 8	Yes I know that. But channel bed is wall so there should I check y+ ?