Problem with mass flow inlet
 

Hi,

I need help on a ventilation case in a road tunnel.
The setup is as follows:

Attachment 20463

Attachment 20464

It's only a part of the tunnel and there are two outlets and one inlet in the ceiling.

The solver is compressible and it's basically a multi-species gas dispersion case, but the problem already occurs with air only.

My problem: I get a pressure incontinuity under the inlet. The pressure directly on the inlet patch is on the same level as in the rest of the tunnel, but several centimeters under the inlet i get a pressure drop and high velocity (40 m/s compared to 2 m/s inlet velocity!). I attach my boundary files and I really hope you have a hint why the problem occurs.

Attachment 20465

Attachment 20466

Attachment 20467

Best regards, Robert

Hello Robert,
I am not sure how good I understood your problem, however I will try to help you a bit.

I feel your problem is in boundary condition, the thing is you have specified inlet is flowRate 5.5, I am wondering how can you expect 5.5 in both outlets or this is the condition you need to specify. I am bit confused about this.
your maximum input is 5.5 but you are expecting 11 kg/s output.
If I am wrong please explain your problem clearly, with some velocity distribution picture.

Thanks,
Sivakumar

Hi Sivakumar,

thanks for your reply. I share your feeling that this problem is about boundary conditions, but I intended to set up a higher outlet then inlet flow, because this case should be a part of a tunnel with equidistantly placed ventilation slots. The "missing" air should get in at the side patches.

My biggest problem is setting up the BC for T, U and p at the two passive inlets, the two active outlets and the active inlet, taking into account buoyancy effects and the resulting pressure difference by height.

Do you have an idea of how to set this up?

Thanks,

Robert

Hello Robert,
I am not able to help you directly, if you like just give me your email id, I will give you my friend email id. He is expert in heat transfer problems, as well he is doing consultancy for OpenFoam.

I hope he can help you nicely.

Thanks,
Sivakumar
sivamscomput@gmail.com

Hello Sivakumar,

it seems I was able to fix it by myself, finally.
The PressureInletOutletVelocity BC works for the side patches and prevents the internal pressure loss. The flow field looks good, too.

But thank you very much for the offer, I might come back for it if my current try should fail, too. ;)

Thanks,

Robert