# Negative pressure

**URL:** https://community.freefem.org/t/negative-pressure/3114
**Category:** General Discussion
**Created:** [April 19, 2024, 4:52pm UTC](https://community.freefem.org/t/negative-pressure/3114 "2024-04-19T16:52:23Z")
**Posts on this page:** 5
**Page:** 1

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### Author: ![Aubium](https://avatars.discourse-cdn.com/v4/letter/a/41988e/32.png) [@Aubium](https://community.freefem.org/u/Aubium)
#### Post date: [April 19, 2024, 4:52pm UTC](https://community.freefem.org/t/negative-pressure/3114/1 "2024-04-19T16:52:23Z")

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Greetings fellow members,

I’m implementing a DG scheme for a Poiseuille flow in Freefem. While I get very satisfying results on the velocity, I’m surprised with negative values for the pressure. Anyone had a similar issue ?

Best regards.

**Expected pressure :**  
 ![Expected Pressure](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/2X/4/4a5a423c708a58dcd9a3f3b8f0c871b2803544aa.png)

**My result :**  
 ![My_Result](https://canada1.discourse-cdn.com/flex030/uploads/freefem/original/2X/8/815d251505ade3079abca08a3e51a2c221eba0a8.png)

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### Author: ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [April 25, 2024, 4:08pm UTC](https://community.freefem.org/t/negative-pressure/3114/2 "2024-04-25T16:08:55Z")

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Because generally the pressure is defined through a constant, so the solve this problem we impose the nulle mean value of the pressure .

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<div class="post-metadata">

### Author: ![Aubium](https://avatars.discourse-cdn.com/v4/letter/a/41988e/32.png) [@Aubium](https://community.freefem.org/u/Aubium)
#### Post date: [April 25, 2024, 4:52pm UTC](https://community.freefem.org/t/negative-pressure/3114/3 "2024-04-25T16:52:28Z")

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Thank you sir for the reply, it makes perfect sense.

Any idea how to retrieve the ‘physically acceptable’ values ? I tried adding a penalization term of the form intalledges of alpha\*jump(p)\*jump(q), but no success…

Best regards !

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### Author: ![clthu](https://avatars.discourse-cdn.com/v4/letter/c/f04885/32.png) [@clthu](https://community.freefem.org/u/clthu)
#### Post date: [April 29, 2024, 5:02am UTC](https://community.freefem.org/t/negative-pressure/3114/4 "2024-04-29T05:02:23Z")

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Is your problem an incompressible NS equation? then the pressure is the hydrodynamic pressure and it could be negative (it is only the relative pressure to a constant), and I think you could just add any constant to make sense

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### Author: ![Aubium](https://avatars.discourse-cdn.com/v4/letter/a/41988e/32.png) [@Aubium](https://community.freefem.org/u/Aubium)
#### Post date: [April 29, 2024, 10:36am UTC](https://community.freefem.org/t/negative-pressure/3114/5 "2024-04-29T10:36:39Z")

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I see now.

I just wanted for it to make more sense on its own, without forcing a constant to add or by stabilizing the scheme with a term…

Thank you sir !