Pressure inlet vs. Velocity inlet

hamed.majeed · October 31, 2018, 15:58

Consider an incompressible pipe flow,

How can I ensure that these combinations give the same results:
1. Pressure inlet + Pressure outlet.
2. Velocity inlet + Pressure outlet.

LuckyTran · October 31, 2018, 16:22

They are much pretty guaranteed not to.

With a pressure inlet you impose a uniform stagnation pressure. The velocity profile is more or less guaranteed to be non-uniform except for very exceptional cases.

With a velocity inlet you impose a uniform velocity, and here you are pretty much guaranteed to get a non-uniform pressure profile. Again, except for very very exceptional cases.

Time4Tea · November 2, 2018, 15:25

To add to what LuckyTran said:

A pressure-inlet is going to vary the velocity/mass flow to achieve the specified total pressure. A velocity-inlet is going to do the opposite and vary the total pressure to achieve the specified velocity.

If you do option #1; measure the average velocity on the inlet; and then apply that to the same geometry with option #2 (as a uniform velocity), you should get the same average total pressure on the inlet as in option #1, but, the distributions will be different (unless you save the velocity profile from case #1 and apply that in option #2, which would probably be the best way to achieve what you want).

Maddin · November 4, 2018, 09:28

It's possible (with other software) - you have to map the profile of the "other" boundary condition you want.
Run pressure inlet and write the velocity profile of the inlet and then map this profile to the velocity inlet condition to have the same pressure profile should work.

For quick simulations I prefer velocity inlet, for deep dive action the pressure inlet (to run a constant pressure difference).

November 2, 2018, 15:25		#3
Time4Tea Senior Member Lee Strobel Join Date: Jun 2016 Posts: 133 Rep Power: 10	To add to what LuckyTran said: A pressure-inlet is going to vary the velocity/mass flow to achieve the specified total pressure. A velocity-inlet is going to do the opposite and vary the total pressure to achieve the specified velocity. If you do option #1; measure the average velocity on the inlet; and then apply that to the same geometry with option #2 (as a uniform velocity), you should get the same average total pressure on the inlet as in option #1, but, the distributions will be different (unless you save the velocity profile from case #1 and apply that in option #2, which would probably be the best way to achieve what you want). hamed.majeed likes this.

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October 31, 2018, 15:58	Pressure inlet vs. Velocity inlet	#1
hamed.majeed Senior Member Hamed Abdul Majeed Join Date: May 2012 Location: New Orleans, LA, US Posts: 147 Rep Power: 14	Consider an incompressible pipe flow, How can I ensure that these combinations give the same results: 1. Pressure inlet + Pressure outlet. 2. Velocity inlet + Pressure outlet.

October 31, 2018, 16:22		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	They are much pretty guaranteed not to. With a pressure inlet you impose a uniform stagnation pressure. The velocity profile is more or less guaranteed to be non-uniform except for very exceptional cases. With a velocity inlet you impose a uniform velocity, and here you are pretty much guaranteed to get a non-uniform pressure profile. Again, except for very very exceptional cases. Time4Tea likes this.

November 4, 2018, 09:28		#4
Maddin Senior Member Join Date: Mar 2009 Posts: 260 Rep Power: 18	It's possible (with other software) - you have to map the profile of the "other" boundary condition you want. Run pressure inlet and write the velocity profile of the inlet and then map this profile to the velocity inlet condition to have the same pressure profile should work. For quick simulations I prefer velocity inlet, for deep dive action the pressure inlet (to run a constant pressure difference).