Reference pressure for Drag/Cd reports?

jt94 · May 18, 2017, 15:27

Hi,

I'm running simulations on a vehicle and have generated reports for drag coefficient and drag. My question is in the report, what vales should you enter for reference; pressure, density and velocity.

Currently I have reference velocity as the inlet speed (70mph) density as 1.225kg/m^3 and pressure as 101325 Pa but I've been told by someone I work with to use 0Pa as the reference pressure for both reports but this doesn't seem right to me?

Thanks and advance for any help

Jake

me3840 · May 18, 2017, 20:36

These are reference values. There are no right or wrong answers on what you should enter, they're conventions. Most vehicle-based analyses will use the freestream velocity and density as references.

The reference pressure on the report only matters in certain situations (you should look at the user guide to find out which and why!).

The actual value is gauge pressure from the simulation's reference pressure, so 0 Pa is fine.

Jon Faried · January 25, 2018, 10:40

Out of curiosity: if I'd like to create a report based on the absolute pressure (which includes the reference pressure, which it turn takes into account the hydrostatic pressure induced by gravity) rather than the gauge pressure, how would I do so?

fluid23 · January 29, 2018, 18:15

You would have to create your own field function, I think. I am not sure why one would want to do so, though. Can you elaborate on what you are trying to get out of it?

I would look at how the reference pressure is used before I go down the rabbit hole to make sure you are getting what you want out of it.

User Guide > Star-CCM+ > Post-Processing > Reporting Results > Specific Reports > Force Coefficient > Eq. 1154.

May 18, 2017, 15:27	Reference pressure for Drag/Cd reports?	#1
jt94 New Member Jake Join Date: Apr 2017 Posts: 2 Rep Power: 0	Hi, I'm running simulations on a vehicle and have generated reports for drag coefficient and drag. My question is in the report, what vales should you enter for reference; pressure, density and velocity. Currently I have reference velocity as the inlet speed (70mph) density as 1.225kg/m^3 and pressure as 101325 Pa but I've been told by someone I work with to use 0Pa as the reference pressure for both reports but this doesn't seem right to me? Thanks and advance for any help Jake

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May 18, 2017, 20:36		#2
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	These are reference values. There are no right or wrong answers on what you should enter, they're conventions. Most vehicle-based analyses will use the freestream velocity and density as references. The reference pressure on the report only matters in certain situations (you should look at the user guide to find out which and why!). The actual value is gauge pressure from the simulation's reference pressure, so 0 Pa is fine.

January 25, 2018, 10:40		#3
Jon Faried New Member Join Date: Jan 2017 Posts: 13 Rep Power: 9	Out of curiosity: if I'd like to create a report based on the absolute pressure (which includes the reference pressure, which it turn takes into account the hydrostatic pressure induced by gravity) rather than the gauge pressure, how would I do so?

January 29, 2018, 18:15		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	You would have to create your own field function, I think. I am not sure why one would want to do so, though. Can you elaborate on what you are trying to get out of it? I would look at how the reference pressure is used before I go down the rabbit hole to make sure you are getting what you want out of it. User Guide > Star-CCM+ > Post-Processing > Reporting Results > Specific Reports > Force Coefficient > Eq. 1154.