[zhihuawan]

Recently I use the CFX software to simulation a rectangle channel ，I need to use a periodic condition，I see that we can use a periodic inteface ，but when I use that condition, I find I cannot define the same position to a velocity inlet, I ant to ask you how can I deal with it ?Thank you very much.

[evcelica]

Define the same position to a velocity inlet?
Just define total mass flow instead. it will calculate the non uniform velocity at the interface. If you are specifying velocity, it isn't periodic, you would just be specifying it at the inlet and outlet.

[zhihuawan]

Originally Posted by evcelica

Define the same position to a velocity inlet?
Just define total mass flow instead. it will calculate the non uniform velocity at the interface. If you are specifying velocity, it isn't periodic, you would just be specifying it at the inlet and outlet.

Think you for your reply ,but in my situation ,the university is not uniform,how to solve it ,I know we can use a expression for the mass flow,but I do not how to make CEL expression, can you help me?

[ghorrocks]

There are many examples of CEL expressions in the CFX tutorials. You can get them from the ANSYS Customer webpage.

[zhihuawan]

Originally Posted by ghorrocks

There are many examples of CEL expressions in the CFX tutorials. You can get them from the ANSYS Customer webpage.

do you know how to build a segmentation cel expression？

[ghorrocks]

What is a segmentation CEL expression?

[zhihuawan]

Originally Posted by ghorrocks

What is a segmentation CEL expression?

I mean I do not know how to build a cel expression ，in the fluent ，we can use the text to build a udf ,and then change the suffix to" .c", when I try to do this way ,and choose to change the suffix to ".cll"， I cannot to import this document into the CFX pre ,can you tell me how can I do it ?Thank you for your pleasure.

[zhihuawan]

how to build a cel expression ，in the fluent ，we can use the text to build a udf ,and then change the suffix to" .c", when I try to do this way ,and choose to change the suffix to ".cll"， I cannot to import this document into the CFX pre ,can you tell me how can I do it ?Thank you for your pleasure.

[ghorrocks]

In CFX you do not need a separate file to define expressions. You can define them directly in CFX-Pre using the CEL (CFX Expression Language), and they are part of the CCL.

Many of the CFX examples use CEL expressions. I REALLY recommend you look at the CFX tutorials, as I recommended in post #4.

[zhihuawan]

Originally Posted by ghorrocks

In CFX you do not need a separate file to define expressions. You can define them directly in CFX-Pre using the CEL (CFX Expression Language), and they are part of the CCL.

Many of the CFX examples use CEL expressions. I REALLY recommend you look at the CFX tutorials, as I recommended in post #4.

Thank you very much for your reply,I have tried to do as you say ,but the condition expression is not a constant value, in my situlation, I use a translational periodocity interface model, and I choose a mass flow rate model to describe the boundary,but my mass flow m=if(y<0.0009,y/0.0009[m]*0.09[kg s^-1],0[kg s^-1]),when I choose this expression to the cfx-pre,it will give a error as this: The parameter 'Mass flow rate 'in object 'flow :flow analysis 1/domain interface:domain interface 1/interface models /mass and momentum/momentum interface model' is defined to be 'single valued 'but it depends on the following field valued variables:y.Because I cannot put the image into the forum,if you are convinent ,I can send to your e-mail or we-chat.
I think we should use a separate file to define the .ccl file,now I am reading the CFX tutorials,but it is difficult to understand some terminology,such as what is "single valued".Thanks for your reply.

[ghorrocks]

Single valued means that the mass flow rate is a single value, such as 1 kg/s. You have defined a function where the mass flow rate is a function of position. This does not make sense as the mass flow rate is the integral of the velocity and density over the inlet - how can the integral be one value above y=0.0009 and another below?

What you should do is put a wall at your inlet and outlet to cover the section which has no flow, and use a normal translational periodic boundary with a defined flow rate for the remainder.

[zhihuawan]

Originally Posted by ghorrocks

Single valued means that the mass flow rate is a single value, such as 1 kg/s. You have defined a function where the mass flow rate is a function of position. This does not make sense as the mass flow rate is the integral of the velocity and density over the inlet - how can the integral be one value above y=0.0009 and another below?

What you should do is put a wall at your inlet and outlet to cover the section which has no flow, and use a normal translational periodic boundary with a defined flow rate for the remainder.

You may not know my research object,my object is a verticle rectangle ,and two fluid such as water and air come in one side,and the water is falling film on the wall,the water film thickness is 0.9mm,the wide of this channel is 3.6mm,so there exists only water below the 0.9mm.and because of that the water velocity is parabolic,so the mass flow varies with the y position .So I should use the ccl. to define the mass flow for the normal translational periodic boundary.I am looking for your reply.

[zhihuawan]

Originally Posted by evcelica

Define the same position to a velocity inlet?
Just define total mass flow instead. it will calculate the non uniform velocity at the interface. If you are specifying velocity, it isn't periodic, you would just be specifying it at the inlet and outlet.

In the inlet ,I have two phase, one is water film ,other is air ,how can I give this mass inlet,thank you for your reply.

[ghorrocks]

Can you post an image of what you are modelling? Please show the geometry and the expected fluid flow.

[zhihuawan]

Originally Posted by ghorrocks

Can you post an image of what you are modelling? Please show the geometry and the expected fluid flow.

I have submit the image to the internet ,can you see it ?if you can not ,I can send to your e-mail.

[ghorrocks]

How do I post an image on the forum? FAQ: https://www.cfd-online.com/Wiki/Ansy...n_the_forum.3F

[zhihuawan]

Originally Posted by ghorrocks

How do I post an image on the forum? FAQ: https://www.cfd-online.com/Wiki/Ansy...n_the_forum.3F

region.jpg

If three-dimensional simulation takes the whole flow region as the computational domain, it will lead to a considerable computational cost. This study applies periodic boundary conditions tsimulate the fully developed film flow in a relatively short domain(150δ*4δ*150δ) which is shown in the figure.In my simulation,the water film thickness is 0.9mm(δ=0.9mm,the blue region represents the water film ,the white region represents the air),the wide of this channel is 3.6mm,so there exists only water below the 0.9mm.The domain contains thin film on a flat plate with air on the top of it. The calculation domain is long enough for one wave length based on the experimental measurement. Boundary A, B and C, D are set as periodic boundary conditions. Boundary E is set as no slip wall. Boundary F is set as opening pressure boundary. The initial conditions of the film thickness and velocity are evaluated by Nusselt theory (whose velocity is parabolic). It is worth mentioning that the choices of the size of the computational domain and the boundary conditions are both based on the film flow characteristics and adequate verification work.Maybe you understand my object ,now I can not solve how to give the variable mass flow condition and periodic conditon ,I am looking for your reply.
.I am looking for your reply.

[zhihuawan]

Originally Posted by zhihuawan

Attachment 60254

If three-dimensional simulation takes the whole flow region as the computational domain, it will lead to a considerable computational cost. This study applies periodic boundary conditions tsimulate the fully developed film flow in a relatively short domain(150δ*4δ*150δ) which is shown in the figure.In my simulation,the water film thickness is 0.9mm(δ=0.9mm,the blue region represents the water film ,the white region represents the air),the wide of this channel is 3.6mm,so there exists only water below the 0.9mm.The domain contains thin film on a flat plate with air on the top of it. The calculation domain is long enough for one wave length based on the experimental measurement. Boundary A, B and C, D are set as periodic boundary conditions. Boundary E is set as no slip wall. Boundary F is set as opening pressure boundary. The initial conditions of the film thickness and velocity are evaluated by Nusselt theory (whose velocity is parabolic). It is worth mentioning that the choices of the size of the computational domain and the boundary conditions are both based on the film flow characteristics and adequate verification work.Maybe you understand my object ,now I can not solve how to give the variable mass flow condition and periodic conditon ,I am looking for your reply.
.I am looking for your reply.

velocity.jpg
This is the Nusselt theory velocity for the liquid film.

[ghorrocks]

I see no reason why the normal, default translational periodic boundaries would not work. For a translational periodic boundary you do not need to define anything - the flow out the exit side just becomes the flow in the inlet side. You do not need to modify anything.

So just use default translational periodic boundaries. The solver will work out the velocity profile and free surface height which will result in fully developed flow.

[zhihuawan]

Originally Posted by ghorrocks

I see no reason why the normal, default translational periodic boundaries would not work. For a translational periodic boundary you do not need to define anything - the flow out the exit side just becomes the flow in the inlet side. You do not need to modify anything.

So just use default translational periodic boundaries. The solver will work out the velocity profile and free surface height which will result in fully developed flow.

I know it can work,but the translational periodic boundary default give a constant mss flow condition,but in this matter,I need a mass flow varying with the position,but I do not how to solve it .I am trying to give a expression,but it will give a error as I last said.