[Cmaroline]

Hello!
I started Fluent, my simualtion is a microchip with a flow focusing. 2 inlets and 1 outlet. The 2 inlets are both water, but different velocity.
This is the microchip : 3.PNG

Which model should I use? I want this kind of result.
exemple.jpg

Thank you so much for your help!!

[Tigrgrou]

Flow in this kind of device are often laminar, so you should start with the default configuration of Fluent.

[Cmaroline]

Thank you for your answer.
How can I differenciate the two phases even if it's the same fluid?
Do I put VOF model?

[Tigrgrou]

It depends on the type of flow. If it is for surface tracking, VOF is appropriate.

[Cmaroline]

The flow is laminar. But for observe flow focusing I need a primary phase -> sample and a secondary phase -> sheath. But my problem here is on the sheath channel (after input sheath) I don't have any velocity or any pressure increase. I don't know how I can solve this issue.

[Tigrgrou]

VOF should work for this kind of flow. What do you mean by 'I don't have any velocity or any pressure increase.'

[Cmaroline]

I got a velocity different of 0 only on the region where the channel is pinched, somewhere esle the velocity is equal to 0.
But I don't understand because I differenciate the two velocities. And for the volume factor of sheath and sample I put 0 to sheath on the sample inlet and 1 to sheath on the sheath inlet.
I don't found good results....

[Cmaroline]

I tried a lot of parameters combinaison but I don't find the good result..

[Tigrgrou]

If you switch off the vof model and use only a single fluid with the same properties, do you observe a velocity field different from 0?

[Cmaroline]

No! The entire microchip is blue (velocity equal to 0).

[Tigrgrou]

The problem is not the vof in this case. could you give more details (screenshots) of your setup? What are your boundary conditions for both inlets and outlet? How many iterations do you run?

[Cmaroline]

I choose VOF with primary phase -> sample secondary phase -> sheath in bondary conditions I put in sheath and insample on velocity inlet, with both velocity and after for other bondary conditions, screenshots. Is it enough?
Iterations equal to 100.22-031.PNG22-03.PNG

[Tigrgrou]

Could you test with a much larger velocity for instance 1m/s? Do you a transient solver or a steadt solver?

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[LuckyTran]

This isn't a VOF problem.

A much simpler way is to use a passive scalar and use it as a tracer to see which water is which.

[Cmaroline]

Thank you Tigrgrou but it is a kind of exercize and I must apply theses velocities.


LuckyTran, I will try your method and I'm comming back towards you

[Tigrgrou]

Tracer is a good idea if both fluids have the same properties. Pathline could also be checked to evaluate there is a problem.

[Cmaroline]

Unfortunately when I program pathlines the software crash.... I got the student version for the moment (free version)

[Cmaroline]

LuckyTran I try to found how create a passive scalar but I don't know how I can do! Can you help me please?

[LuckyTran]

It would help if I use the correct verbiage. It's actually a user-defined scalar in Fluent. You define the scalar in parameters & customization. I forget the details but one thing to be careful with is the diffusivity, you need to make sure that the scalars have the same properties as each other and as the actual fluid (water), not only the laminar/molecular diffusivity but also the same turbulent Schmidt number so that you have the same effective diffusivity.

In the user's guide, it's in in 8.1 User-Defined Scalar (UDS) Transport Equations

[Cmaroline]

My issue is that I just have both velocities intlet and not much. I don't know how I can found the laminar/ viscosity properties and turbulent Scmidt number.