How to find center of pressure

cfdnewbie1 · December 22, 2010, 20:02

Hello,

I've done a simulation of a sample car body, and I would like to evaluate the distribution of aerodynamic forces between front and rear of the car.
To do so I need to know the position of the center of pressure.
I've read in this forum and it seems that with Fluent you can do this simply with a report, I couldn't find nothing similar in Star-ccm. Is it right?

In a previous post I've found that you can use the surface integral report to calculate the coordinates.

I've defined three user field functions like this:
$$Position[0]*$Pressure (for X direction)
surface integrated this and divided by the surface integral of Pressure.

I've found three coordinates that seems good, but if I calculate a moment defining this coordinates as origin of my axis I found non zero moments as expected.

What is wrong?
There's a simpler procedure to find the center of pressure location?

Thank you everybody for your help.

Regards.

C.

abdul099 · January 6, 2011, 17:59

Why don't you create a force report and a moment report? You can easily calculate it by hand.

Chonker · February 11, 2011, 12:21

I have a similar problem, how is this calculated by hand, I have a Moment about Y (horizontal) and a Force in X (drag longitudinal) and Z (lift vertical). This gives me an equation with two unknowns in (the distance x and z from the origin that the forces act).

Drag*z - Lift*x = Moment

Is this right? How can the two unknowns be solved?

and_ · February 16, 2011, 10:52

You create a Momentum coefficient report.
As the Axis Origin you use e.g. the front tire.
As the Reference Radius you use the distance between front and back tire.
You don't need to know the center of mass or pressure since you cut at the tires.
That is the momentum doesn't act around the the center of mass or pressure, but around the front tire's contact with the ground

December 22, 2010, 20:02	How to find center of pressure	#1
cfdnewbie1 New Member Join Date: Nov 2010 Posts: 5 Rep Power: 16	Hello, I've done a simulation of a sample car body, and I would like to evaluate the distribution of aerodynamic forces between front and rear of the car. To do so I need to know the position of the center of pressure. I've read in this forum and it seems that with Fluent you can do this simply with a report, I couldn't find nothing similar in Star-ccm. Is it right? In a previous post I've found that you can use the surface integral report to calculate the coordinates. I've defined three user field functions like this: $$Position[0]*$Pressure (for X direction) surface integrated this and divided by the surface integral of Pressure. I've found three coordinates that seems good, but if I calculate a moment defining this coordinates as origin of my axis I found non zero moments as expected. What is wrong? There's a simpler procedure to find the center of pressure location? Thank you everybody for your help. Regards. C.

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January 6, 2011, 17:59		#2
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 22	Why don't you create a force report and a moment report? You can easily calculate it by hand.

February 11, 2011, 12:21		#3
Chonker New Member Luke Join Date: Nov 2010 Posts: 2 Rep Power: 0	I have a similar problem, how is this calculated by hand, I have a Moment about Y (horizontal) and a Force in X (drag longitudinal) and Z (lift vertical). This gives me an equation with two unknowns in (the distance x and z from the origin that the forces act). Dragz - Liftx = Moment Is this right? How can the two unknowns be solved?

February 16, 2011, 10:52		#4
and_ New Member Join Date: Jul 2010 Posts: 11 Rep Power: 16	You create a Momentum coefficient report. As the Axis Origin you use e.g. the front tire. As the Reference Radius you use the distance between front and back tire. You don't need to know the center of mass or pressure since you cut at the tires. That is the momentum doesn't act around the the center of mass or pressure, but around the front tire's contact with the ground