Operating Pressure and Gauge pressure

Gretchen · September 30, 2019, 07:53

Hi guys, I am trying to simulate nozzle flow conditions. I have a doubt regarding boundary conditions setup. If I want my far side domain outlet as pressure outlet set at atmospheric pressure. Then should I set my gauge pressure at 101325 while my operating pressure is set to 0? Or should I try with operating pressure set as 101325 and gauge pressure set to zero? Which one is correct and what is the difference between the two?

LuckyTran · September 30, 2019, 10:14

In general, both will work.

Having a non-zero operating pressure helps for machine precision (which isn't usually any issue nowadays). Some models specifically require a specific value for the operating pressure: some combustion models, some natural convection models, etc...

Gretchen · September 30, 2019, 10:31

I am using K-omega with SST model. For this model, are there any specific requirements?

LuckyTran · September 30, 2019, 12:55

Nothing special. Just use whatever combination is most aesthetically pleasing to you or helps you interpret the results more reasily.

sbaffini · October 1, 2019, 08:04

It depends from case to case, but doesn't depend from the turbulence model.

If most of your domain is around 101325 but you use 0 as reference, you are wasting 6 orders of magnitude in the pressure precision, i.e., you can see pressure differences only of order 1e-9/1e-10 instead of 1e-15/1e-16.

If this matters or not depends from case to case. Of course, it is mandatory to get it right in single precision

September 30, 2019, 07:53	Operating Pressure and Gauge pressure	#1
Gretchen New Member Yup Join Date: Sep 2019 Posts: 2 Rep Power: 0	Hi guys, I am trying to simulate nozzle flow conditions. I have a doubt regarding boundary conditions setup. If I want my far side domain outlet as pressure outlet set at atmospheric pressure. Then should I set my gauge pressure at 101325 while my operating pressure is set to 0? Or should I try with operating pressure set as 101325 and gauge pressure set to zero? Which one is correct and what is the difference between the two?

September 30, 2019, 10:14		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	In general, both will work. Having a non-zero operating pressure helps for machine precision (which isn't usually any issue nowadays). Some models specifically require a specific value for the operating pressure: some combustion models, some natural convection models, etc... Last edited by LuckyTran; September 30, 2019 at 12:54.

September 30, 2019, 10:31	K-omega	#3
Gretchen New Member Yup Join Date: Sep 2019 Posts: 2 Rep Power: 0	I am using K-omega with SST model. For this model, are there any specific requirements?

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September 30, 2019, 12:55		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	Nothing special. Just use whatever combination is most aesthetically pleasing to you or helps you interpret the results more reasily.

October 1, 2019, 08:04		#5
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,151 Blog Entries: 29 Rep Power: 39	It depends from case to case, but doesn't depend from the turbulence model. If most of your domain is around 101325 but you use 0 as reference, you are wasting 6 orders of magnitude in the pressure precision, i.e., you can see pressure differences only of order 1e-9/1e-10 instead of 1e-15/1e-16. If this matters or not depends from case to case. Of course, it is mandatory to get it right in single precision