Average static pressure = 0

MissCFD · March 19, 2015, 10:57

Can anybody tell me why specifying average static pressure = 0 or 101325 or any other value has the same effect ?

Thank you for your help

Thomas MADELEINE · March 19, 2015, 11:24

Hi,
we will need a little more details to help you I think.
Where did you put average static pressure (Outlet I guess) ?
What was your other boundary conditions ?
What do you mean by same effect ? (all does not converge ? all results are the same ?)
what is the fluid you model ?

It is possible that pressure has no effect on your simulation. For example, water will not be really affected by pressure in outlet if you specify a velocity in inlet. The flow is incompressible, so the flow will be quite the same at 1 bar as at 2 or 3 bar (more or less)

Firstly, check the obvious solution :
Should it have some differences ?
Do you use the right units ?
Check the reference pressure ?

MissCFD · March 19, 2015, 11:41

I simulate a passage of stator and a passage of rotor for a hydraulic turbine. So I use water.
I use a mass flow rate condition for inlet and pressure condition for outlet. My reference pressure is 1 atm.

I have tried three options in outlet: average static pressure = 0, constant static pressure (P=0) and equilibrium Radial. I don’t really see differences between these cases apart from equilibrium radial condition where total pressure at inlet is a little more important and a little less at outlet that the two others cases.

It isn’t very clear for me why water will not be really affected by pressure in outlet if you specify a velocity in inlet? Does that mean that it’s the inlet velocity that defines the pressure in my domain … ?

Moreover, it’s maybe a silly question but why I should impose an average static pressure = 0 and not 1000 Pa or 101300 Pa ? I have the same question with the constant static pressure.

Thank you again

Quote:

Originally Posted by Thomas MADELEINE

Hi,
we will need a little more details to help you I think.
Where did you put average static pressure (Outlet I guess) ?
What was your other boundary conditions ?
What do you mean by same effect ? (all does not converge ? all results are the same ?)
what is the fluid you model ?

It is possible that pressure has no effect on your simulation. For example, water will not be really affected by pressure in outlet if you specify a velocity in inlet. The flow is incompressible, so the flow will be quite the same at 1 bar as at 2 or 3 bar (more or less)

Firstly, check the obvious solution :
Should it have some differences ?
Do you use the right units ?
Check the reference pressure ?

Thomas MADELEINE · March 19, 2015, 13:11

massflow and velocity at inlet are the same kind of condition

pressure on water will not change the density nor the temperature and the viscosity. so basically when you change the outlet pressure the only thing that change (more or less) is the pressure (by the difference you set.

example
Run 1
P outlet = 1 bar
Ptot inlet = 1.4 bar
Ptot outlet = 1.1 bar

Run 2
P outlet = 4 bar
Ptot inlet = 4.4 bar
Ptot outlet = 4.1 bar

and the same velocity, work and other stuff for both simulation.

for the average static pressure, radial equilibrium, constant static pressure, look at the help in ansys it is quite simple.

March 19, 2015, 10:57	Average static pressure = 0	#1
MissCFD Senior Member Join Date: Aug 2014 Posts: 160 Rep Power: 11	Can anybody tell me why specifying average static pressure = 0 or 101325 or any other value has the same effect ? Thank you for your help

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March 19, 2015, 11:24		#2
Thomas MADELEINE Senior Member Thomas MADELEINE Join Date: Oct 2014 Posts: 126 Rep Power: 11	Hi, we will need a little more details to help you I think. Where did you put average static pressure (Outlet I guess) ? What was your other boundary conditions ? What do you mean by same effect ? (all does not converge ? all results are the same ?) what is the fluid you model ? It is possible that pressure has no effect on your simulation. For example, water will not be really affected by pressure in outlet if you specify a velocity in inlet. The flow is incompressible, so the flow will be quite the same at 1 bar as at 2 or 3 bar (more or less) Firstly, check the obvious solution : Should it have some differences ? Do you use the right units ? Check the reference pressure ?

March 19, 2015, 13:11		#4
Thomas MADELEINE Senior Member Thomas MADELEINE Join Date: Oct 2014 Posts: 126 Rep Power: 11	massflow and velocity at inlet are the same kind of condition pressure on water will not change the density nor the temperature and the viscosity. so basically when you change the outlet pressure the only thing that change (more or less) is the pressure (by the difference you set. example Run 1 P outlet = 1 bar Ptot inlet = 1.4 bar Ptot outlet = 1.1 bar Run 2 P outlet = 4 bar Ptot inlet = 4.4 bar Ptot outlet = 4.1 bar and the same velocity, work and other stuff for both simulation. for the average static pressure, radial equilibrium, constant static pressure, look at the help in ansys it is quite simple.