# Negative pressures

Hi!

I would like to assess the head losses in a pipe with two elbows.
I fixed the pressure P=0 Pa at the end of it and, also, I put a flow rate as boundary condition in the inlet.

When i am going to check the pressure at the beginning of the pipe, this is less than zero, which it is impossible since the head losses have to be positive.

Could someone help me ?

My project is the following:

Simulation: TAILRACE. RUN 1

Thanks for all,

Cheers.

For the case you have mentioned the pressure is not at 0 Pascal as described. Also did you do some calculations on what pressure to expect for this case? @CFD-SQUAD, can you see if you find something in the simulation that causes this result?

Cheers,

Jousef

Hi All,

If Iâ€™m not mistaken, the pressure can be negative because it is relative. So let say your pressure is at atm (101325 pa) and you see a -10000 pa then the actual pressure is 100325 pa instead so technically still positive.

Not too sure as I mostly worked with velocity and TKE as my main parameters so if further insight can be gained that would be great.

Cheers.

Regards,
Barry

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Hi @andrea_perez,
Have a look at the third simulation of this similar project as of your project. I have applied absolute pressure at the outlet and obtained pressure less then atmospheric pressure at the inlet.
https://www.simscale.com/projects/HamzaBaig/draft_tube_cfd_simulation-/

Regards.

2 Likes

Yes, I know head losses have to be 80 mmwc.

Although the value of head losses werenâ€™t the same, the inlet pressure would have to be greater than the oultet, otherwise, the energy wouldnt preserve which it is impossible.

P_inlet >= P_outlet + HeadLoss

What is it the explanation?

@HamzaBaig, you have the same problem in your third simulation:

https://www.simscale.com/projects/HamzaBaig/draft_tube_cfd_simulation-/

Thanks and sorry for the inconvenience.

Regards,

Andrea

Your simulation looks good to me! The only thing I would change is that under simulation control I would reduce the end time to 10 seconds (or even less, perhaps 1 second is enough) and divide the delta t by 100 since the time that it will take your phenomenon to reach steady state is a lot less than 1000 seconds! In this case your large delta t does not seem to have caused you any issues, but it can sometimes.

Hi,

Thanks for your suggestion. I also tried but the result was the same: Inlet pressure less than outlet.

Do you have another idea ?