Stagnation pressure

jackwabbit · March 23, 2013, 01:38

In my simulation of the flow around the Ahmed body to determine drag coefficient, at the front face, I am not getting the expected value of stagnation pressure. Any idea what could be the issue involved?

vangelis · March 23, 2013, 09:18

what value do you get?
higher or lower than expected?

http://www.cfd-online.com/Forums/ope...-than-one.html

jackwabbit · March 23, 2013, 12:49

The pressure works out to be 1000 Pa higher than the expected value. What would you recommend for this case specifically since the Ahmed body analysis has been done over so many times. And yes, I'll read on that.

vangelis · March 26, 2013, 02:41

Try expressing this value as static pressure coefficient.
This value should equal 1 at stagnation point.

jackwabbit · March 26, 2013, 06:00

I did. But I got a value close to 2 where it was supposed to 1.
My physical conditions with respect to pressure are

Initial conditions
Pressure - 101325 Pa
Reference values
Pressure - 101325 Pa
Force coefficient report
Reference pressure - 101325 Pa

My inlet and outlet conditions are Velocity Inlet and Flow split outlet, respectively.

I'm sorry but I'm still in the learning stages. Could you please help me out?

vangelis · March 26, 2013, 06:14

I am not familiar with StarCCM+ unfortunately.
My only guess would be to use another BC type for outlet
like Pressure Outlet instead of Flow Split

I am sure someone more familiar with Star can guide you

good luck

Vangelis

jackwabbit · March 26, 2013, 07:15

Ah, right. Still, thanks.

rwryne · March 27, 2013, 14:35

Quote:

Originally Posted by jackwabbit

I did. But I got a value close to 2 where it was supposed to 1.
My physical conditions with respect to pressure are

Initial conditions
Pressure - 101325 Pa
Reference values
Pressure - 101325 Pa
Force coefficient report
Reference pressure - 101325 Pa

My inlet and outlet conditions are Velocity Inlet and Flow split outlet, respectively.

I'm sorry but I'm still in the learning stages. Could you please help me out?

Just to be clear, you realize that Initial Conditions and Boundary conditions use the reference pressure right? i.e. they are gauge pressures.

So when you set 101325 as your initial condition, you are setting the absolute pressure to 202650. Any pressure other than a "reference pressure" is understood to be <typed in value> + Reference Pressure

jackwabbit · March 30, 2013, 01:09

Oh, yes. Can't believe I missed that. Thanks! Changed, and it's fine, now.

March 23, 2013, 01:38	Stagnation pressure	#1
jackwabbit New Member Anonymous 92 Join Date: Feb 2013 Posts: 18 Rep Power: 13	In my simulation of the flow around the Ahmed body to determine drag coefficient, at the front face, I am not getting the expected value of stagnation pressure. Any idea what could be the issue involved?

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March 23, 2013, 09:18		#2
vangelis Senior Member Vangelis Skaperdas Join Date: Mar 2009 Location: Thessaloniki, Greece Posts: 287 Rep Power: 21	what value do you get? higher or lower than expected? http://www.cfd-online.com/Forums/ope...-than-one.html

March 23, 2013, 12:49		#3
jackwabbit New Member Anonymous 92 Join Date: Feb 2013 Posts: 18 Rep Power: 13	The pressure works out to be 1000 Pa higher than the expected value. What would you recommend for this case specifically since the Ahmed body analysis has been done over so many times. And yes, I'll read on that.

March 26, 2013, 02:41		#4
vangelis Senior Member Vangelis Skaperdas Join Date: Mar 2009 Location: Thessaloniki, Greece Posts: 287 Rep Power: 21	Try expressing this value as static pressure coefficient. This value should equal 1 at stagnation point.

March 26, 2013, 06:00		#5
jackwabbit New Member Anonymous 92 Join Date: Feb 2013 Posts: 18 Rep Power: 13	I did. But I got a value close to 2 where it was supposed to 1. My physical conditions with respect to pressure are Initial conditions Pressure - 101325 Pa Reference values Pressure - 101325 Pa Force coefficient report Reference pressure - 101325 Pa My inlet and outlet conditions are Velocity Inlet and Flow split outlet, respectively. I'm sorry but I'm still in the learning stages. Could you please help me out?

March 26, 2013, 06:14		#6
vangelis Senior Member Vangelis Skaperdas Join Date: Mar 2009 Location: Thessaloniki, Greece Posts: 287 Rep Power: 21	I am not familiar with StarCCM+ unfortunately. My only guess would be to use another BC type for outlet like Pressure Outlet instead of Flow Split I am sure someone more familiar with Star can guide you good luck Vangelis

March 26, 2013, 07:15		#7
jackwabbit New Member Anonymous 92 Join Date: Feb 2013 Posts: 18 Rep Power: 13	Ah, right. Still, thanks.

March 30, 2013, 01:09		#9
jackwabbit New Member Anonymous 92 Join Date: Feb 2013 Posts: 18 Rep Power: 13	Oh, yes. Can't believe I missed that. Thanks! Changed, and it's fine, now.