[asal]

Hello all!

I need to simulate a case that I need to specify volume flow out (l/s). I tried pressure outlet and set a negative pressure outlet both with and without target mass flow, but it is difficult to obtain desired volume flow and it needs several try and errors.
This is the same for volume flow inlet.
Any solution to this?

Thanks.

[rsaurabh]

Try with some pressure value and keep on going with valuable changes in previous value. I have also done the same.

[LuckyTran]

Use a velocity inlet. Velocity x Area = volumetric flow rate

There's no velocity outlet because it generally results in an ill-posed problem. But you can use a velocity inlet with a negative velocity to force an outlet flowrate.

[asal]

This might work for volume flow inlet. What about volume flow outlet?

[LuckyTran]

You can use a velocity inlet as an outlet

[asal]

Hello and thanks for your reply.
I guess you mean "velocity inlet" with negative sign for the velocity. If so, then no, you cannot!
What temperature you will assign then?
How do you specify the out flow direction? You have to impose a direction for the velocity (for example perpendicular) which might not be true!

[LuckyTran]

That's exactly why there is no velocity outlet option. It's non-physical to begin with. How the hell do you impose a volumetric flowrate at the outlet in the first place? The N-S equations are inherently upwind biased because stuff flows from upstream to downstream.

There are an infinite number of solutions that will all satisfy a volumetric outflow BC. Either you accept that you have an ill-posed problem and give up or you live with some assumptions such as assuming an outflow direction and temperature. Otherwise, the problem is stupidly posed.

[asal]

Hello and thanks again for your comment.
I totally agree with you.
However, what do you suggest in the following case:

How to model an exhaust with certation flow rate through? For instance a range hood? You only know the flow rate through the exhaust, but you don't know the temperature and you don't want to impose any direction.

[LuckyTran]

I suggest you try solving a real problem. If you do not make an assumption about the exit properties, then there are infinite number of solutions to your problem. For a given volumetric flow rate at the outlet, I can always find an infinite number of velocity and density profiles that satisfy that constraint. There's a doubly infinite set of possible solutions.

A range hood has a fan in it. A fan does not fix the volumetric flow rate, it provides mechanical work. The volumetric flow rate depends on the losses in the system (i.e. the pressure upstream and downstream of the fan). Just because you turn on range hood does not mean you get a certain flow rate, the flowrate is whatever it needs to be to satisfy the pressure drop of the system. That is, the system adjusts to the new conditions. That is why, fan curves are specified in volumetric flow rate at a given pressure and temperature, because you need the pressure & temperature to figure out the density and mass-flow.

Unless you stuck a probe in the exhaust to measure the velocity profile, you cannot know the flow through the exhaust.

[asal]

thanks again for your reply.

I have one more question related to this question.
Why we cannot fix this with "Target Mass Flow Rate"? If I calculate the mass flow correspondent to the volume flow and set to the bounday, then it should work, but it don't!!
What is the problem?