# Defining Incompressible fluid

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 November 29, 2013, 18:17 Defining Incompressible fluid #1 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 161 Rep Power: 4 Hello, I have facing problem with defining the compressible/ incompressible fluid. I chose "constant" when I selected the material. I assumed that it means density will be constant in spatial and temporal. But after completing the calculation, from fluent I found two different value for area weighted average velocity and mass weighted average velocity. But it should be the same, since I never defined compressible fluid. Is there any way so that I can assign "incompressible" fluid? Any suggestion will be highly appreciated. Regards, Tanjina.

 November 30, 2013, 08:25 #2 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 You already assigned an incompressible fluid by selecting a constant density material property. There is nothing you can do to make the fluid even "more incompressible" If we want to find out why the two expressions dont yield the exact same result, we need more information.

 November 30, 2013, 08:55 #3 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 8 Your fluid is incompressible after choosing constant density.. we have to understand the meaning of the reporting: area-average and mass-average or massflow average.. some mathematics behind and phyical understanding..

November 30, 2013, 19:59
#4
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Tanjina Afrin
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Quote:
 Originally Posted by flotus1 You already assigned an incompressible fluid by selecting a constant density material property. There is nothing you can do to make the fluid even "more incompressible" If we want to find out why the two expressions dont yield the exact same result, we need more information.
Thanks Flotus and Zaktatir. Yes, I also thought that the fluid is incompressible after defining constant density. but it seems it is not. Is there any way to know how Fluent calculate area averaged velocity, mass averaged velocity and mass flow rate?

And I tried to check the density after simulation. So I clicked "density" from "report-->surface integral". It gives me different density for different portion, which shouldn't be.

Regards,
Tanjina

 December 1, 2013, 04:32 #5 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 8 One again: constant density leads to incompressbile handling of the Navier-Stokes or the (U)RANS equations (it does depend on the model). Check Volume Reports with max/min value of density with any weighting average and let us know

December 1, 2013, 07:00
#6
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Alex
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Quote:
 Originally Posted by Tanjina Thanks Flotus and Zaktatir. Yes, I also thought that the fluid is incompressible after defining constant density. but it seems it is not. Is there any way to know how Fluent calculate area averaged velocity, mass averaged velocity and mass flow rate?
-> fluent manual

Quote:
 Originally Posted by Tanjina And I tried to check the density after simulation. So I clicked "density" from "report-->surface integral". It gives me different density for different portion, which shouldn't be.
A surface integral of any constant quantity still depends on the surface area. That is why you get different values.
It would be much easier to have a look at a contout plot of the density to see that it really is constant.

 December 1, 2013, 13:39 #7 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 161 Rep Power: 4 Dear Zaktatir and Flotus, I checked with volume integral, max and minimum density is okay, ie. max density is 998.2 and min is 1.225. And tried with volume weighted and mass weighted velocity ( since there is no area weighted velocity in volume integral option), I found different velocity and density also. I checked the density with contour. Density is different near the outlet from the other portion. But initially density was same for all the part. What does make it different ? Please help me with some suggestion How can I solve these issues. And please suggest me some book or paper so that I can understand these part of numerical modeling. Thanks in advance. Regards, Tanjina

December 1, 2013, 14:25
#8
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Alex
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Quote:
 I checked with volume integral, max and minimum density is okay, ie. max density is 998.2 and min is 1.225. And tried with volume weighted and mass weighted velocity ( since there is no area weighted velocity in volume integral option), I found different velocity and density also.
Please try to be a little clearer. I dont really get what you are trying to tell us.
WHERE are you measuring WHICH quantity?

Quote:
 I checked the density with contour. Density is different near the outlet from the other portion. But initially density was same for all the part. What does make it different ?
Would you mind posting this picture?
Are you sure that you are actually using the fluid you defined as incompressible? I suspect that your simulation uses a different fluid than the one you think it does.

Edit...
Wait a second. Densities of 998.2 and 1.225 sound like the standard values fluent uses for water and air. Do you have both phases in your simulation?

December 1, 2013, 15:52
#9
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Tanjina Afrin
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Dear Flotus,

Yes, I am using VOF model. I am posting the picture of pseudo model,not the actual model from where you can see that after simulation, density is not same all through the domain.But from the actual model also, I got the same things. For few hours I don't have the access to actual model. I will post later the actual model image.

Zaktatir asked me to check the minimum and maximum density of model. So I checked that from "volume integral--->maximum--->density". And from volume integral, I also checked the mass-average and volume average velocity and density. I found two different velocity from mass average and volume average. If it's not clear still, please let me know.

I selected the reference pressure location outside of the inlet. Is there any possibility that air enters into the domain and makes the density change?

Quote:
 Originally Posted by flotus1 Please try to be a little clearer. I dont really get what you are trying to tell us. WHERE are you measuring WHICH quantity? Would you mind posting this picture? Are you sure that you are actually using the fluid you defined as incompressible? I suspect that your simulation uses a different fluid than the one you think it does. Edit... Wait a second. Densities of 998.2 and 1.225 sound like the standard values fluent uses for water and air. Do you have both phases in your simulation?
Attached Images
 Density_after simulation.jpg (57.2 KB, 9 views) initial_density.jpg (75.8 KB, 8 views)

 December 1, 2013, 18:22 #10 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 Take a minute to think about what happens when you have two different fluids and perform a VOF simulation. The legend in the pictures you posted might give you a hint. They show the density of the MIXTURE of both fluids... This density might be constant after initialization if you initialized with a constant fraction of each fluid. The information that you are performing a VOF simulation might have been helpful at the beginning of the thread

December 1, 2013, 18:35
#11
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Tanjina Afrin
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Hi Flotus,

I am sorry that I didn't mention about VOF model at first. I initialized with a constant fraction of each fluid. It's full of water from the beginning and only water will enter through the inlet.

So, in my understanding, at any time, water is the only fluid. So density should be constant. Please correct me if I am wrong since I am almost a new user of these type of model.

Regards,
Tanjina
Quote:
 Originally Posted by flotus1 Take a minute to think about what happens when you have two different fluids and perform a VOF simulation. The legend in the pictures you posted might give you a hint. They show the density of the MIXTURE of both fluids... This density might be constant after initialization if you initialized with a constant fraction of each fluid. The information that you are performing a VOF simulation might have been helpful at the beginning of the thread

 December 2, 2013, 04:42 #12 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 If you know that you only want to simulate water, then why do you run a VOF-simulation? My first idea would be to run a "normal" single-phase simulation if you dont want a second phase to be involved. Anyway, instead of judging the result of such a simulation from the density, it would be better to post-process the volume fraction of each phase. This way we would have found out that this is not an issue with compressibility of the medium (doubtable anyway at a density ratio of 1/1000) but that the second phase is involved. How does the second phase enter the domain? The only way I can think of is that this species is specified at the outlet and we have a backflow at this boundary. You should get warning messages about reversed flow during the simulation if my assumption is correct.

December 2, 2013, 10:49
#13
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Tanjina Afrin
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Hi Flotus,

Thank you very much for your reply. I used VOF because we have aspect. One plan is model will be half filled with water and other half will be air, and with time air will be replaced by water.Second plan is model will be filled fully by water.

For first plan, definitely we needed VOF model. But I am not sure about the second plan. Since for second plan, model is needed to full by water all the time, so I used VOF so that at inlet I can assign volume fraction 1 for water, and model will be filled with water. I checked with single phase, but couldn' find a way to enter water constantly from inlet. I don't have velocity at inlet, so I used pressure inlet.

And about backflow, at first there was some message that there is reverse flow, but after some run, it was disappear.

If you know any way that I can allow enter water through the inlet without using VOF, please let me know.

Regards,
Tanjina

Quote:
 Originally Posted by flotus1 If you know that you only want to simulate water, then why do you run a VOF-simulation? My first idea would be to run a "normal" single-phase simulation if you dont want a second phase to be involved. Anyway, instead of judging the result of such a simulation from the density, it would be better to post-process the volume fraction of each phase. This way we would have found out that this is not an issue with compressibility of the medium (doubtable anyway at a density ratio of 1/1000) but that the second phase is involved. How does the second phase enter the domain? The only way I can think of is that this species is specified at the outlet and we have a backflow at this boundary. You should get warning messages about reversed flow during the simulation if my assumption is correct.

December 2, 2013, 12:48
#14
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Alex
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Quote:
 Originally Posted by Tanjina If you know any way that I can allow enter water through the inlet without using VOF, please let me know.
You mean that instead of the standard material used by fluent (air) you want to run a single-phase simulation that uses water as a fluid?
Why not change the material under "cell zone conditions" to water? This way, all boundary conditions that might eventually produce an inflow will use water by default.
If I got you wrong on this point, just let me know.

December 2, 2013, 13:51
#15
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Tanjina Afrin
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Hi Flotus,

Yup, you got me right. I didn't find "material" under cell zone condition. I use ansys 14.0. There is another Tab called "material" in solution setup. I have added "water" there and air is default material there. So I was confused how would Fluent understand that water will flow?

Now I have used pressure inlet in single phase model. Is it okay? In this way will fluent be able to understand that water will flow through the model? Model is running now. After sometime of running,I hope I will understand what is happening with single phase.I will let you know the result.

Thank you very much for your time and patience.

Regards,
Tanjinathat
Quote:
 Originally Posted by flotus1 You mean that instead of the standard material used by fluent (air) you want to run a single-phase simulation that uses water as a fluid? Why not change the material under "cell zone conditions" to water? This way, all boundary conditions that might eventually produce an inflow will use water by default. If I got you wrong on this point, just let me know.

 December 2, 2013, 16:42 #16 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 8 What do you want to do now? Are u trying to simulate two-phase flow of air and water or only one-fluid-flow of water? Under Cell zones Conditions go to the cell zone name click on edit then you can give the fluid that you need What you have posted is the mixing density and that has nothing to do with compressibility. You need basic training Tajina

December 2, 2013, 17:16
#17
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Tanjina Afrin
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Hi Flotus,

Please find the attached image. Model is running well. Thanks a lot. Attached image is the density contour which shows that density is equal everywhere. But still get two different value for area weighted and mass weighted outlet velocity. I am not sure where I am done wrong.

Mass-weighted velocity=1.6218m/s
area weighted velocity=1.445 m/s

Dear Zaktatir, I am really sorry for bothering you too much.

Regards,
Tanjina
Attached Images
 density.jpg (68.1 KB, 6 views)

Last edited by Tanjina; December 2, 2013 at 17:24. Reason: wrong sentence

 December 3, 2013, 06:52 #18 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,104 Rep Power: 19 Since the misconceptions that led us here are fundamental, I also recommend that you get some basic training on how to operate Fluent before proceeding to more advanced simulations. You should at least work through the tutorials "Fluid Flow and Heat Transfer in a Mixing Elbow" and "Using the VOF Model" before trying to interpret any of the results you obtained.

December 3, 2013, 11:17
#19
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Tanjina Afrin
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Quote:
 Originally Posted by flotus1 Since the misconceptions that led us here are fundamental, I also recommend that you get some basic training on how to operate Fluent before proceeding to more advanced simulations. You should at least work through the tutorials "Fluid Flow and Heat Transfer in a Mixing Elbow" and "Using the VOF Model" before trying to interpret any of the results you obtained.
Hello,

Thanks for your kind recommendation. Yes, I've gone through these tutorials. Since I am very new to fluid mechanics and fluent, I didn't understand many things there. With time I am learning and I am continuously trying to learn through this forum and from internet also.

Thank you all for your generous help. I really appreciate it.

Regards,
Tanjina

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