[ghorrocks]

If you are defining the mass flow then the translational periodic boundaries I suggested might not be the best approach. In that case I would use an inlet boundary with the known fluid height and velocity profile, and an outlet at the bottom. No need for periodic boundaries at all.

Also: make sure you do not get confused with the term mass flow rate. Your device appears to have constant mass flow rate along its entire length as there is just one inlet and outlet. But the velocity profile will change. Do not get mass flow and velocity confused.

[zhihuawan]

Originally Posted by ghorrocks

If you are defining the mass flow then the translational periodic boundaries I suggested might not be the best approach. In that case I would use an inlet boundary with the known fluid height and velocity profile, and an outlet at the bottom. No need for periodic boundaries at all.

Also: make sure you do not get confused with the term mass flow rate. Your device appears to have constant mass flow rate along its entire length as there is just one inlet and outlet. But the velocity profile will change. Do not get mass flow and velocity confused.

You may not see that I say the purpose to set the periodic condition is to decrease the computer length of the channel ,or else it will take a long time to calculate,which I cannot bear,do you konw my meaning?The method is reading from a article publised in the jouranal of International heat and transfer,Thank you very much,

[ghorrocks]

If I understand correctly, you are only interested in the fully developed flow. You have previously stated that you know the fully developed velocity profile and fluid depth. So don't you know everything about the fully developed flow then? What is the point of the CFD model?

[zhihuawan]

Originally Posted by ghorrocks

If I understand correctly, you are only interested in the fully developed flow. You have previously stated that you know the fully developed velocity profile and fluid depth. So don't you know everything about the fully developed flow then? What is the point of the CFD model?

Yes ,you are right,I want to study the fully developed flow,and my purpose is to verify the model in the article(in the image),in the article ,the author has use this method to simulate the falling film progress, I think this method is very useful for my research in the future,because my research is familar to this model in the article,only to solve the periodic condition can I simulate the film flowing character and heat transfer character.Thank you.

[zhihuawan]

Originally Posted by ghorrocks

If I understand correctly, you are only interested in the fully developed flow. You have previously stated that you know the fully developed velocity profile and fluid depth. So don't you know everything about the fully developed flow then? What is the point of the CFD model?

Yes ,you are right,I want to study the fully developed flow,and my purpose is to verify the model in the article(in the image),in the article ,the author has use this method to simulate the falling film progress, I think this method is very useful for my research in the future,because my research is familar to this model in the article,only to solve the periodic condition can I simulate the film flowing character and heat transfer character.Thank you.Attachment 60265

[zhihuawan]

this is the article ,can you see that,if you want ,I can send to you,but I do not know how to send it to you.

[ghorrocks]

I don't have time to read your article, you will have to do that.

You need to be very clear what is known in advance in this simulation - that is the boundary conditions and initial conditions, and what are the outputs you wish your simulation to give you.

It is not clear to me what are the inputs and outputs for this simulation. Can you please explain this?

[zhihuawan]

Originally Posted by ghorrocks

I don't have time to read your article, you will have to do that.

You need to be very clear what is known in advance in this simulation - that is the boundary conditions and initial conditions, and what are the outputs you wish your simulation to give you.

It is not clear to me what are the inputs and outputs for this simulation. Can you please explain this?

I just want to simlulate the film flow on the plate,and study the film thickness ,wave characterist,wave frequency,wave velocity characterist of the film with different situlation,such as different mass flow,different tilted,different medium,after that ,I will study the heat transfer characterist in this falling film.But I now am distrubed by this question.
The boundary for this simulation is like this,A,B,C,D is the periodic condition,E is no-slip wall,the upside of the channel is opening,in the same time ,A has a film flow in .The initial condition is that all the region is full of air without water,and then the water flow in ,we simulate transient condition .Thank you for your reply.

[ghorrocks]

If you expect this flow to develop waves of varying shape and size then how can you model this with periodic interfaces? That would suggest you need to model the whole flow.

[zhihuawan]

Originally Posted by ghorrocks

If you expect this flow to develop waves of varying shape and size then how can you model this with periodic interfaces? That would suggest you need to model the whole flow.

If you have read some articles about the wave film ,you may know that the wave film is periodic,in this article the author has said that the length of the model include one wave length,therefore, we need not to simulate the whole flow which will take too much calculation source,this the key of this method,do you have any question? I think you should not focus on this reanson why I choose this method，I think we should find way to solve my problem,it is important,do you think so ?

[ghorrocks]

I just did a little demo version of your simulation and it seemed to work fine. I used periodic translational interfaces to connect the top and bottom faces, and defined a mass flow rate to specify the flow rate. To get the air and water volume fractions I used an initial condition of a previous simulation with normal inlet/outlet pair. Maybe this is what you need to get the volume fraction defined properly - an initial condition which will define how much of each phase is present.

I realise I have answered a different question to your actual question on this thread, but in my experience on the forum about 75% of questions asked are not actually addressing the problem the person starting the thread is facing. On deeper questioning the problem is a more fundamental problem and the question asked is either irrelevant or a bad way of approaching it. See http://xyproblem.info/ for a discussion on this.

[zhihuawan]

Originally Posted by ghorrocks

I just did a little demo version of your simulation and it seemed to work fine. I used periodic translational interfaces to connect the top and bottom faces, and defined a mass flow rate to specify the flow rate. To get the air and water volume fractions I used an initial condition of a previous simulation with normal inlet/outlet pair. Maybe this is what you need to get the volume fraction defined properly - an initial condition which will define how much of each phase is present.

I realise I have answered a different question to your actual question on this thread, but in my experience on the forum about 75% of questions asked are not actually addressing the problem the person starting the thread is facing. On deeper questioning the problem is a more fundamental problem and the question asked is either irrelevant or a bad way of approaching it. See http://xyproblem.info/ for a discussion on this.

Yes,this is what I want,you are great,does your mass flow rate is uniform?how can you set the normal inlet/outlet pair for the air and water volume fraction?can you send me some setting images or cases for me,I sincerely thank you very much.

[ghorrocks]

I set the total mass flow rate.

When you set a translational periodic boundary then all variables are transferred from one side to the other. You do not need to do anything special for volume fraction (unless you wish to).

[zhihuawan]

Originally Posted by ghorrocks

I set the total mass flow rate.

When you set a translational periodic boundary then all variables are transferred from one side to the other. You do not need to do anything special for volume fraction (unless you wish to).

You may misunderstand my question, I mean that in the inlet ,there are two phase in,if you give a total mass flow rate,all the phase will be water liquid ,but with no air,this is not what I want,please see the image .I want the liquid flow under the channel ,and the air flow upside,so we cannot give a total mass flow rate,Thank you for your reply.flow.jpg

[ghorrocks]

I specified the mass flow rate and used the initial condition to define the air/water interface. The initial condition had the air/water interface defined in it. As that was the initial condition it then continued that into the solution.

[zhihuawan]

Originally Posted by ghorrocks

I specified the mass flow rate and used the initial condition to define the air/water interface. The initial condition had the air/water interface defined in it. As that was the initial condition it then continued that into the solution.

I have two questions,
(1)I know you give the initial condition to define the air/water interface,but after it simulates,the mass flow will fully flow of the inlet in the future(because the air inlet and water inlet is in the face),you cannot assure the water flow under the channel,the air flow upon the channel,I think we should give a ccl. files to define the mass flow.
(2)second,I do not know how to give a initial condition to define the air/water interface,I cannot find the button to define it ,I use the CFX 18.0 in my computer ,can you tell me stepply ,thank you very much.

[ghorrocks]

1) It works fine for me.
2) I defined an initial condition by doing a simulation with a normal inlet at the top with a prescribed mass flow rate and the initial water height defined. I used an outlet at the bottom. I then competed this simulation and used this result file as the initial condition of a simulation with the inlet/outlet replaced with translational periodic interfaces.

[zhihuawan]

Originally Posted by ghorrocks

1) It works fine for me.
2) I defined an initial condition by doing a simulation with a normal inlet at the top with a prescribed mass flow rate and the initial water height defined. I used an outlet at the bottom. I then competed this simulation and used this result file as the initial condition of a simulation with the inlet/outlet replaced with translational periodic interfaces.

I am terribly sorry，if you are convinent,you can send your case to me,I will be very grateful, for I cannot understand how to realize it ,thank you very much.

[ghorrocks]

I have PM you with a google drive location.

[zhihuawan]

Originally Posted by ghorrocks

I have PM you with a google drive location.

I tried for a lot of time,but it just does not work,I wonder if it is my error operation.how large is it ?My e-mail is enough to receive it,my emai is hitzhwan@163.com, you can have a try.is there any way to solve it.Thank you very much.