Learning Forum

[rumth]

I need to clear the concepts about some simple questions:

1. What does back flow mean in Fluent?

2. What is Back flow volume fraction for VOF model?

3. In my simulation, more mass is going out through outlet than inlet. I have set velocity inlet = 553 m/s. At pressure outlet, Gauge pressure = 0 atm. Operating pressure = 1 atm.

If I set Gauge pressure = 2atm (greater than operating pressure) then mass flow through outlet will decrease, right?

4. For two phase, I am selecting water by marking region. How can I know the volume fraction of water in the domain?

 

regards,

Raju.

[jmccasli]

Hi Raju,

 

I have put answers to your questions below. I hope this helps.

 

Regards,

Jeremy

 

1. What does back flow mean in Fluent?

When you have recirculation near an outlet, flow will enter the domain from the outlet. This reverse flow into the domain from the outlet is called "back flow". Back flow must have prescribed values for the field variables (e.g., pressure and temperature), so it is necessary to specify this.

2. What is Back flow volume fraction for VOF model?

This specifies the phase volume fraction of any occurrence of back flow at an outlet. For example, let's say you have a VOF model with primary phase as air, and secondary phase as water. You will need to specify the secondary phase back flow volume fraction at an outlet. If you set it equal to 0, this will mean that all back flow is air. If you set it equal to 1, this will mean all back flow is water.

3. In my simulation, more mass is going out through outlet than inlet. I have set velocity inlet = 553 m/s. At pressure outlet, Gauge pressure = 0 atm. Operating pressure = 1 atm.

 

If I set Gauge pressure = 2atm (greater than operating pressure) then mass flow through outlet will decrease, right?

What drives the fluid through the outlet is the mean pressure gradient (pressure drop over length of pipe). Increasing the outlet pressure will decrease the pressure drop (in magnitude), so yes you should see the flow rate through the outlet decrease.

4. For two phase, I am selecting water by marking region. How can I know the volume fraction of water in the domain?

Perhaps you can clarify what you mean? The volume fraction for each phase is available as a post-processing quantity, so you can post-process it in any way you like. E.g., you can create contours, perform surface and volume reports, etc.

[rumth]

Hi Jeremy,

Thank you so much for your nice explanations. It helps me a lot.

 

Regards,

Raju.

[Ani94]

Hello

My question is regarding backflow total temperature that need to be set to avoid backflow. I am simulating flow through heat exchanger, where at process outlet I have given pressure outlet, hot fluid is entering shell at 493k and utility fluid at 338K through tubes. Solver is giving me the reverse flow is happening. My question here is what should be the backflow temperature when I don't know temperature at outlet.

It should be near atmospheric or it should be higher to avoid backflow.

(Physically there is no backflow at outlet)

[Chinmaya Kumar Patra]

Hi Jeremy

I understood the backflow concept.

I am simulating for two phase flow in fluent, during the iteration i got the message like "Reversed flow on 35 faces of pressure-outlet 5". So i couldnot able to figure out what does the line mean? If you can clarify, it will be helpful.

[NickFL]

It means that the computed flow field wants to have flow re-enter the domain at an outlet. When we specify the outlet boundary condition there are fields for what the backflow conditions are. If the flow is only leaving at the exit, these values do not show up in the computation. Typically we do not want to have flow coming in at the outlet, so we will, if possible, extend the domain to a place where no backflow occurs. If backflow only occurs in the first few iterations, then it really is not a problem and just means that we had a bad initial "guess".

[Chinmaya Kumar Patra]

 

Thanks Nick for the input. In my case the reverse flow message i am getting after 60-70 time steps and it persist upto the end i.e. around 20000th time step but for each time step the solution is converged. So should i place my oultlet beyond the point currently i am considering to prevent reverse flow. A typical set of message for two time step is given below

Flow time = 0.02499868241670707s, time step = 20643

Global Courant Number [Variable Time Step Criteria] : 0.441283

Updating solution at time level N...

Global Courant Number [Explicit VOF Criteria] : 0.153555
 done.
 physical-dt  1.0000e-06

  iter  continuity  x-velocity  y-velocity  z-velocity      energy     time/iter
 370599  7.6244e-05  6.7564e-09  6.6296e-09  9.4738e-09  1.8337e-11  0:00:00   20

 Reversed flow on 19 faces of pressure-outlet 5.
 370600  2.3498e-01  1.2303e-05  1.1941e-05  1.3743e-05  3.6681e-05  0:00:01   19

 Reversed flow on 19 faces of pressure-outlet 5.
 370601  2.7168e-02  5.7825e-06  5.7242e-06  6.5056e-06  6.2679e-07  0:00:00   18

 Reversed flow on 19 faces of pressure-outlet 5.
 370602  1.7321e-02  1.6978e-06  1.6518e-06  1.8528e-06  8.3496e-08  0:00:00   17

 Reversed flow on 19 faces of pressure-outlet 5.
 370603  1.2406e-02  8.9093e-07  8.9053e-07  1.0275e-06  2.3910e-08  0:00:00   16

 Reversed flow on 19 faces of pressure-outlet 5.
 370604  9.0921e-03  7.4967e-07  7.6008e-07  8.8222e-07  7.6721e-09  0:00:00   15

 Reversed flow on 19 faces of pressure-outlet 5.
 370605  6.6246e-03  6.3146e-07  6.3900e-07  7.4103e-07  2.7183e-09  0:00:00   14

 Reversed flow on 19 faces of pressure-outlet 5.
 370606  4.7882e-03  5.0056e-07  5.0368e-07  5.8592e-07  1.1237e-09  0:00:00   13

 Reversed flow on 19 faces of pressure-outlet 5.
 370607  3.4446e-03  3.8009e-07  3.8047e-07  4.4329e-07  5.6069e-10  0:00:00   12

 Reversed flow on 19 faces of pressure-outlet 5.
 370608  2.4715e-03  2.8101e-07  2.8003e-07  3.2685e-07  3.2886e-10  0:00:00   11

 Reversed flow on 19 faces of pressure-outlet 5.
 370609  1.7704e-03  2.0421e-07  2.0288e-07  2.3757e-07  2.1352e-10  0:00:00   10

 Reversed flow on 19 faces of pressure-outlet 5.

  iter  continuity  x-velocity  y-velocity  z-velocity      energy     time/iter
 370610  1.2677e-03  1.4680e-07  1.4557e-07  1.7131e-07  1.4667e-10  0:00:00    9

 Reversed flow on 19 faces of pressure-outlet 5.
 370611  9.0807e-04  1.0479e-07  1.0380e-07  1.2298e-07  1.0215e-10  0:00:00    8

 Reversed flow on 19 faces of pressure-outlet 5.
 370612  6.5202e-04  7.4474e-08  7.3717e-08  8.8076e-08  7.2170e-11  0:00:00    7

 Reversed flow on 19 faces of pressure-outlet 5.
 370613  4.6888e-04  5.2779e-08  5.2212e-08  6.3012e-08  5.0949e-11  0:00:00    6

 Reversed flow on 19 faces of pressure-outlet 5.
 370614  3.3765e-04  3.7336e-08  3.6917e-08  4.5067e-08  3.6300e-11  0:00:00    5

 Reversed flow on 19 faces of pressure-outlet 5.
 370615  2.4338e-04  2.6383e-08  2.6072e-08  3.2238e-08  2.6007e-11  0:00:00    4

 Reversed flow on 19 faces of pressure-outlet 5.
 370616  1.7570e-04  1.8632e-08  1.8400e-08  2.3076e-08  1.8769e-11  0:00:00    3

 Reversed flow on 19 faces of pressure-outlet 5.
 370617  1.2695e-04  1.3154e-08  1.2980e-08  1.6535e-08  1.3627e-11  0:00:00    2

 Reversed flow on 19 faces of pressure-outlet 5.
 370618  9.1844e-05  9.2854e-09  9.1540e-09  1.1867e-08  9.9896e-12  0:00:00    1
!370618 solution is converged
Flow time = 0.02499968241670707s, time step = 20644

Global Courant Number [Variable Time Step Criteria] : 0.145451

Updating solution at time level N...

Global Courant Number [Explicit VOF Criteria] : 0.0483624
 done.
 physical-dt  3.1758e-07

  iter  continuity  x-velocity  y-velocity  z-velocity      energy     time/iter
 370618  9.1844e-05  9.2854e-09  9.1540e-09  1.1867e-08  9.9896e-12  0:00:00   20

 Reversed flow on 19 faces of pressure-outlet 5.
 370619  3.2095e-02  4.0304e-06  3.9061e-06  4.5219e-06  1.1961e-05  0:00:00   19

 Reversed flow on 19 faces of pressure-outlet 5.
 370620  8.1917e-03  3.4091e-06  3.4359e-06  3.6267e-06  7.1104e-08  0:00:00   18

 Reversed flow on 19 faces of pressure-outlet 5.
 370621  6.3102e-03  3.4113e-06  3.4373e-06  3.6405e-06  7.0692e-09  0:00:00   17

 Reversed flow on 19 faces of pressure-outlet 5.
 370622  4.8154e-03  2.9165e-06  2.9366e-06  3.1148e-06  1.8227e-09  0:00:00   16

 Reversed flow on 19 faces of pressure-outlet 5.
 370623  3.6150e-03  2.2834e-06  2.2971e-06  2.4394e-06  5.4397e-10  0:00:00   15

 Reversed flow on 19 faces of pressure-outlet 5.
 370624  2.6823e-03  1.7079e-06  1.7169e-06  1.8239e-06  1.9852e-10  0:00:00   14

 Reversed flow on 19 faces of pressure-outlet 5.
 370625  1.9749e-03  1.2454e-06  1.2511e-06  1.3294e-06  1.0016e-10  0:00:00   13

 Reversed flow on 19 faces of pressure-outlet 5.
 370626  1.4469e-03  8.9475e-07  8.9831e-07  9.5482e-07  6.4478e-11  0:00:00   12

 Reversed flow on 19 faces of pressure-outlet 5.
 370627  1.0573e-03  6.3702e-07  6.3922e-07  6.7990e-07  4.4939e-11  0:00:00   11

 Reversed flow on 19 faces of pressure-outlet 5.
 370628  7.7122e-04  4.5096e-07  4.5231e-07  4.8161e-07  3.1496e-11  0:00:00   10

 Reversed flow on 19 faces of pressure-outlet 5.

  iter  continuity  x-velocity  y-velocity  z-velocity      energy     time/iter
 370629  5.6189e-04  3.1810e-07  3.1891e-07  3.4002e-07  2.2032e-11  0:00:00    9

 Reversed flow on 19 faces of pressure-outlet 5.
 370630  4.0911e-04  2.2386e-07  2.2433e-07  2.3955e-07  1.5472e-11  0:00:00    8

 Reversed flow on 19 faces of pressure-outlet 5.
 370631  2.9767e-04  1.5729e-07  1.5756e-07  1.6852e-07  1.0914e-11  0:00:00    7

 Reversed flow on 19 faces of pressure-outlet 5.
 370632  2.1699e-04  1.1041e-07  1.1055e-07  1.1845e-07  7.7326e-12  0:00:00    6

 Reversed flow on 19 faces of pressure-outlet 5.
 370633  1.5822e-04  7.7444e-08  7.7516e-08  8.3213e-08  5.5071e-12  0:00:00    5

 Reversed flow on 19 faces of pressure-outlet 5.
 370634  1.1547e-04  5.4297e-08  5.4326e-08  5.8435e-08  3.9415e-12  0:00:00    4

 Reversed flow on 19 faces of pressure-outlet 5.
 370635  8.4327e-05  3.8056e-08  3.8061e-08  4.1026e-08  2.8273e-12  0:00:00    3
!370635 solution is converged
Flow time = 0.025s, time step = 20645
Specified flow time reached    flow time = 0.025, total time = 0.025

 

 

[NickFL]

Here you need to make some engineerining judgement. Is 19 faces a significant portion of your outlet? How strong is the backflow? Fluent will report the message when the velocity has even a tiny component opposite the outward pointing normal. For example, if there are 20k faces at the outlet and most have an outflow of 1 [m/s] and 19 have a minor backflow of -1e-3 [m/s], than your solution may be good. Also the area of interest should be far away from the exit.

And because you do have backflow, you need to go back and double check what you are specifing as the backflow conditions make sense in regards to what the rest of the solution is telling you.

[Chinmaya Kumar Patra]

In my case i have 200 faces at the outlet and in 19 faces i got reverse flow. As you have mentioned this '' if there are 20k faces at the outlet and most have an outflow of 1 [m/s] and 19 have a minor backflow of -1e-3 [m/s]" how to check the velocities for backflow and oultflow at the oultet in ansys fluent ?

Regarding the area of interest: i am trying to calculate the pressure drop across a channel by substracting the pressure at inlet and outlet and i think my outlet pressure is computed at the exit so is it reason for creating the backflow?

[NickFL]

Given that about 10% of the faces have backflow, we should check what that is in terms of area percentage.

Think about what backflow is. Based upon that, you could use several techniques to find what are the velocities coming in at the exit. You could use contours at the exit. Or velocity vectors. Or there are several other ways. What I would use might be different that someone else. Modeling and result post-processing is an art. Do what you are comfortable with.

Regarding the area of interest: i am trying to calculate the pressure drop across a channel by substracting the pressure at inlet and outlet and i think my outlet pressure is computed at the exit so is it reason for creating the backflow?

You likely have the solver applies an average velocity of 0 [Pa] at the exit and a given velocity or mass-flow-rate at the inlet. This means at the outlet certain parts might be slightly higher or lower, but on average, zero. If the upstream values are lower, then the flow will tend to move upstream at the exit, causing backflow.

I would recommend that you set this complex model aside for a few days and work through some simple examples. If you do not understand the basics of the flow modeling, it will be very difficult to debug a complex model.