Force Calculation

nofrizal · January 5, 2016, 05:47

Hi guys,

My question is very basic.

I read through the Ansys CFX-Solver Theory Guide but it doesn't seem to answer my question so posting on here!

What is the equation used or the theory behind in CFX-post force calculation (Function calculator) for "ave(Force Z)@Plane 4"

My plane 4 is a circular shape with diameter 25mm.
Thanks in advance

ghorrocks · January 5, 2016, 18:48

The "Force Z" variable is not often used. It is the force acting on that element face and this is not usually very useful.

the ave() function does an arithmetic average over all the element faces on Plane 4. It does not take into account the different areas of the element faces.

The function you probably want is force_z()@plane 4. This avoids these problems.

nofrizal · January 6, 2016, 00:10

Glenn,
Thank for your response, but I think I misleading the question. What I want to know is how CFX calculated the normal force on the particular surface. What is the equation used?

Thanks in advance

ghorrocks · January 7, 2016, 18:24

The equation is the obvious one: The normal force is pressure * area and the shear force is wall shear * area.

-Maxim- · January 11, 2016, 04:55

I wish Ansys would just define their equations, expressions and variables properly in their documentation. Thanks ghorrocks for the clarification.

ghorrocks · January 11, 2016, 05:35

A clarification: the force field variable is evaluated at the control volume level. That means the pressure used is from the integration points and the area is the wall surface. This can then be integrated over the whole surface to give the force on the entire surface.

nofrizal · January 11, 2016, 21:26

Thanks Ghorrocks for the insight.

navidmkl2 · March 27, 2019, 15:22

Hi Ghorrocks,

i'm wondering if you have ever used the variable "wall pressure force x/y/z"?

I'm gonna give it a test to calculate force (for example in y direction) using:

myForceY = areaInt(Wall Shear Y)@mySurface + sum(Wall Pressure Force Y)@mySurface

I am going to check if the second term of the right hand side is equal to :

areaInt_y(Pressure)@mySurface

Or not. Do you have any idea in prehand?

ghorrocks · March 27, 2019, 15:40

You can do whatever you like, and exploring whether things which should be equal are in fact equal is a useful check to do. I think you will find the results are complex

navidmkl2 · March 28, 2019, 06:14

Some updates if you are interested:

having an expression defined as:
myWPFy = sum(Wall Pressure Force Y)@mySurface
or
myWPFyAve = ave(Wall Pressure Force Y)@mySurface

will cause the case crashes right away after the Solver part starts. With no useful error.

More strange is that for example:
myWPFyAreaAve = areaAve(Wall Pressure Force Y)@mySurface
or
myWPFyAreaInt = areaInt(Wall Pressure Force Y)@mySurface

lets the case to run (I know they are not correct, but just to be sure the error is not coming from the variable itself).

Even stranger, when I open the solved case in CFDPost then "sum" and "ave" work.

I am a bit confused now. Would be great if someone knows why having "sum" or "ave" does not let the solution to go forward.

January 5, 2016, 05:47	Force Calculation	#1
nofrizal New Member nofrizal Join Date: Dec 2011 Location: Malaysia Posts: 5 Rep Power: 14	Hi guys, My question is very basic. I read through the Ansys CFX-Solver Theory Guide but it doesn't seem to answer my question so posting on here! What is the equation used or the theory behind in CFX-post force calculation (Function calculator) for "ave(Force Z)@Plane 4" My plane 4 is a circular shape with diameter 25mm. Thanks in advance

March 27, 2019, 15:40		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,729 Rep Power: 143	You can do whatever you like, and exploring whether things which should be equal are in fact equal is a useful check to do. I think you will find the results are complex __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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January 5, 2016, 18:48		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,729 Rep Power: 143	The "Force Z" variable is not often used. It is the force acting on that element face and this is not usually very useful. the ave() function does an arithmetic average over all the element faces on Plane 4. It does not take into account the different areas of the element faces. The function you probably want is force_z()@plane 4. This avoids these problems.

January 6, 2016, 00:10		#3
nofrizal New Member nofrizal Join Date: Dec 2011 Location: Malaysia Posts: 5 Rep Power: 14	Glenn, Thank for your response, but I think I misleading the question. What I want to know is how CFX calculated the normal force on the particular surface. What is the equation used? Thanks in advance

January 7, 2016, 18:24		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,729 Rep Power: 143	The equation is the obvious one: The normal force is pressure * area and the shear force is wall shear * area.

January 11, 2016, 04:55		#5
-Maxim- Senior Member Maxim Join Date: Aug 2015 Location: Germany Posts: 415 Rep Power: 12	I wish Ansys would just define their equations, expressions and variables properly in their documentation. Thanks ghorrocks for the clarification.

January 11, 2016, 05:35		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,729 Rep Power: 143	A clarification: the force field variable is evaluated at the control volume level. That means the pressure used is from the integration points and the area is the wall surface. This can then be integrated over the whole surface to give the force on the entire surface.

January 11, 2016, 21:26		#7
nofrizal New Member nofrizal Join Date: Dec 2011 Location: Malaysia Posts: 5 Rep Power: 14	Thanks Ghorrocks for the insight.

March 27, 2019, 15:22		#8
navidmkl2 New Member Navid Mikaillian Join Date: Mar 2018 Posts: 7 Rep Power: 8	Hi Ghorrocks, i'm wondering if you have ever used the variable "wall pressure force x/y/z"? I'm gonna give it a test to calculate force (for example in y direction) using: myForceY = areaInt(Wall Shear Y)@mySurface + sum(Wall Pressure Force Y)@mySurface I am going to check if the second term of the right hand side is equal to : areaInt_y(Pressure)@mySurface Or not. Do you have any idea in prehand?

March 28, 2019, 06:14		#10
navidmkl2 New Member Navid Mikaillian Join Date: Mar 2018 Posts: 7 Rep Power: 8	Some updates if you are interested: having an expression defined as: myWPFy = sum(Wall Pressure Force Y)@mySurface or myWPFyAve = ave(Wall Pressure Force Y)@mySurface will cause the case crashes right away after the Solver part starts. With no useful error. More strange is that for example: myWPFyAreaAve = areaAve(Wall Pressure Force Y)@mySurface or myWPFyAreaInt = areaInt(Wall Pressure Force Y)@mySurface lets the case to run (I know they are not correct, but just to be sure the error is not coming from the variable itself). Even stranger, when I open the solved case in CFDPost then "sum" and "ave" work. I am a bit confused now. Would be great if someone knows why having "sum" or "ave" does not let the solution to go forward.