ANSYS CFX-Pre settings

xyq102296 · May 9, 2011, 04:23

Hello everyone,

I have some questions about settings in the CFX-Pre,

I got some reference settings which I am really confused.

in Basic setting, Reference Pressure: 1 atm,

Basic setting under Buoyancy
Buoyancy reference density: average density;
reference location, automative
and I also set my outlet as 0 Pa reference, does anyone know how to explain all those reference values?

xyq102296 · May 9, 2011, 08:35

ok, i got the reference pressure is used to avoid problems with round-off errors. But still i am confused on other reference values.
Need help~~

ghorrocks · May 9, 2011, 19:49

As you say the reference states are there to reduce round off errors. You can set different reference states for some physical models but in general this is not a good idea.

xyq102296 · May 10, 2011, 08:10

Hello Ghorrocks,

could you tell me what are the meanings of others?
in Basic setting, Reference Pressure: 1 atm is for round-off problems----clear

Basic setting under Buoyancy
Buoyancy reference density: average density what's the function during solving the problem?
reference location, automatic where is the reference location?
and I also set my opening outlet as 0 Pa reference what's this pressure reference?

Thank you in advance!

ghorrocks · May 10, 2011, 19:49

The buoyancy reference is the reference density for buoyancy calculations. Buoyancy is driven by very small density differences so an appropriate reference density is important.

xyq102296 · May 13, 2011, 04:38

In the post-processing, on the contour, I can show the pressure on the plane i choose. So what's that pressure meaning? static or kinematic pressure?
From the help file, when buoyancy is activated, the pressure excludes the hydrostatic pressure gradient. But I still need to set the average static pressure=0Pa over the outlet. The kinematic is defined as 0.5*Rho*v^2
but here I have no idea what's this pressure. If it is not static pressure, why I need to set static pressure at the outlet?

ghorrocks · May 13, 2011, 08:06

The variable "pressure" is static pressure. If you are running buoyancy it has the hydrostatic component removed.

You define the boundary conditions to agree with what you know about the flow. If you don't know the exit pressure then move the boundary to somewhere you do know the pressure. Or use a velocity boundary if you know that.

xyq102296 · May 13, 2011, 08:16

Since "Pressure" is static pressure, what is "static pressure minus hydrostatic component", how to relate it with mean velocity inside the model?

If I have a vertical model and set average static pressure=0Pa at top, fluid goes inside from bottom, then what kind of pressure does it show to me? How can I find the dynamic pressure

ghorrocks · May 14, 2011, 07:11

The variable total pressure includes the velocity component. If you want the velocity component by itself you will need to define a variable for that.

xyq102296 · May 15, 2011, 03:56

Basic setting under Buoyancy
Buoyancy reference location, automatic
where is the reference location when solving?

May 9, 2011, 04:23	ANSYS CFX-Pre settings	#1
xyq102296 Member newansysuser Join Date: Oct 2010 Posts: 33 Rep Power: 16	Hello everyone, I have some questions about settings in the CFX-Pre, I got some reference settings which I am really confused. in Basic setting, Reference Pressure: 1 atm, Basic setting under Buoyancy Buoyancy reference density: average density; reference location, automative and I also set my outlet as 0 Pa reference, does anyone know how to explain all those reference values?

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May 9, 2011, 08:35		#2
xyq102296 Member newansysuser Join Date: Oct 2010 Posts: 33 Rep Power: 16	ok, i got the reference pressure is used to avoid problems with round-off errors. But still i am confused on other reference values. Need help~~

May 9, 2011, 19:49		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	As you say the reference states are there to reduce round off errors. You can set different reference states for some physical models but in general this is not a good idea.

May 10, 2011, 08:10		#4
xyq102296 Member newansysuser Join Date: Oct 2010 Posts: 33 Rep Power: 16	Hello Ghorrocks, could you tell me what are the meanings of others? in Basic setting, Reference Pressure: 1 atm is for round-off problems----clear Basic setting under Buoyancy Buoyancy reference density: average density what's the function during solving the problem? reference location, automatic where is the reference location? and I also set my opening outlet as 0 Pa reference what's this pressure reference? Thank you in advance!

May 10, 2011, 19:49		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	The buoyancy reference is the reference density for buoyancy calculations. Buoyancy is driven by very small density differences so an appropriate reference density is important.

May 13, 2011, 04:38		#6
xyq102296 Member newansysuser Join Date: Oct 2010 Posts: 33 Rep Power: 16	In the post-processing, on the contour, I can show the pressure on the plane i choose. So what's that pressure meaning? static or kinematic pressure? From the help file, when buoyancy is activated, the pressure excludes the hydrostatic pressure gradient. But I still need to set the average static pressure=0Pa over the outlet. The kinematic is defined as 0.5Rhov^2 but here I have no idea what's this pressure. If it is not static pressure, why I need to set static pressure at the outlet?

May 13, 2011, 08:06		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	The variable "pressure" is static pressure. If you are running buoyancy it has the hydrostatic component removed. You define the boundary conditions to agree with what you know about the flow. If you don't know the exit pressure then move the boundary to somewhere you do know the pressure. Or use a velocity boundary if you know that.

May 13, 2011, 08:16		#8
xyq102296 Member newansysuser Join Date: Oct 2010 Posts: 33 Rep Power: 16	Since "Pressure" is static pressure, what is "static pressure minus hydrostatic component", how to relate it with mean velocity inside the model? If I have a vertical model and set average static pressure=0Pa at top, fluid goes inside from bottom, then what kind of pressure does it show to me? How can I find the dynamic pressure

May 14, 2011, 07:11		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	The variable total pressure includes the velocity component. If you want the velocity component by itself you will need to define a variable for that.

May 15, 2011, 03:56		#10
xyq102296 Member newansysuser Join Date: Oct 2010 Posts: 33 Rep Power: 16	Basic setting under Buoyancy Buoyancy reference location, automatic where is the reference location when solving?