Reference pressure and buoyancy density of simple pipe flow

dotapro · March 12, 2015, 19:45

I'm trying to model 100m long pipe flow with a hole release of 0.1 m diameter whereby methane is flowed from inlet with mass flow rate of 6.178 kg/s and 26.58 Celsius. The outlets are the release from hole and pipe outlet with 0 Pa static pressure. May I know what value I should put for reference pressure and buoyancy density? Should I consider pgh in this case? Thanks for your help.

ghorrocks · March 12, 2015, 20:08

The reference pressure should be a typical pressure in the device. If the outlet is 0Pa (relative to atmosphere I presume) then 1 atmosphere should be OK for reference pressure.

Read the documentation and the examples about how to handle the hydrostatic head.

dotapro · March 12, 2015, 20:21

Thanks for your reply. What happens if my outlet is relative to water instead of air ? Again, what value for I should input for reference pressure and static pressure outlet?

ghorrocks · March 12, 2015, 22:37

I do not understand your question.

The reference pressure is a typical absolute pressure in your domain, so the simulation can use a variance from this reference pressure as the pressure variable. When the pressure variable is minimised you minimise round off errors.

dotapro · March 13, 2015, 01:47

My apology for not clarifying it clearly and lengthy message. I trying to model subsea gas release whereby my first simulation consists of simple pipe flow as stated above while my second simulation is methane releases from hole into the sea water.My inlet pressure,P1 is 28 bar. So, my question is reference pressure should be calculated using bernoulli's pressure drop in circular pipe equation where ∆p=fxL/D xρ/2xV^2, pipe outlet static pressure is P2 = P1 - pressure drop, and my hole release static pressure is assume to be 0 Pa . Is my assumption correct?

ghorrocks · March 13, 2015, 04:07

I still do not understand what you are saying. Where does the Bernoulli equation come into it?

It sounds like you simply need to set the reference pressure to the ambient water pressure at the depth you are modelling. Then all pressures will be relative to that reference pressure.

March 12, 2015, 19:45	Reference pressure and buoyancy density of simple pipe flow	#1
dotapro Member Join Date: Oct 2013 Location: Malaysia Posts: 39 Rep Power: 12	I'm trying to model 100m long pipe flow with a hole release of 0.1 m diameter whereby methane is flowed from inlet with mass flow rate of 6.178 kg/s and 26.58 Celsius. The outlets are the release from hole and pipe outlet with 0 Pa static pressure. May I know what value I should put for reference pressure and buoyancy density? Should I consider pgh in this case? Thanks for your help.

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March 12, 2015, 20:08		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	The reference pressure should be a typical pressure in the device. If the outlet is 0Pa (relative to atmosphere I presume) then 1 atmosphere should be OK for reference pressure. Read the documentation and the examples about how to handle the hydrostatic head.

March 12, 2015, 20:21		#3
dotapro Member Join Date: Oct 2013 Location: Malaysia Posts: 39 Rep Power: 12	Thanks for your reply. What happens if my outlet is relative to water instead of air ? Again, what value for I should input for reference pressure and static pressure outlet?

March 12, 2015, 22:37		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	I do not understand your question. The reference pressure is a typical absolute pressure in your domain, so the simulation can use a variance from this reference pressure as the pressure variable. When the pressure variable is minimised you minimise round off errors.

March 13, 2015, 01:47		#5
dotapro Member Join Date: Oct 2013 Location: Malaysia Posts: 39 Rep Power: 12	My apology for not clarifying it clearly and lengthy message. I trying to model subsea gas release whereby my first simulation consists of simple pipe flow as stated above while my second simulation is methane releases from hole into the sea water.My inlet pressure,P1 is 28 bar. So, my question is reference pressure should be calculated using bernoulli's pressure drop in circular pipe equation where ∆p=fxL/D xρ/2xV^2, pipe outlet static pressure is P2 = P1 - pressure drop, and my hole release static pressure is assume to be 0 Pa . Is my assumption correct?

March 13, 2015, 04:07		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	I still do not understand what you are saying. Where does the Bernoulli equation come into it? It sounds like you simply need to set the reference pressure to the ambient water pressure at the depth you are modelling. Then all pressures will be relative to that reference pressure.