Axisymmetric model

killian153 · July 23, 2018, 16:26

Hello all !

Just a simple and quick question : if I make a simulation of a rocket nozzle in an axisymmetric model, do i have to double the results I get ? For example if I get a thrust of 450 N, do I have to double this value as it is axisymmetric or can I consider this it is directly the "real" value ?

Thanks !

fluid23 · July 25, 2018, 16:05

There is no way to answer this question without knowing what software you are using or how you are obtaining thrust. Some packages will provide just the 2D analysis (integration of pressure and shear) along the body. Others will integrate around your annulus.

Either way, you would never multiply by 2 to get from a 2D to 3D value. If you integrate around the annulus you get a constant multiplyer of 2pi... not 2.

killian153 · July 25, 2018, 16:09

Quote:

Originally Posted by fluid23

There is no way to answer this question without knowing what software you are using or how you are obtaining thrust. Some packages will provide just the 2D analysis (integration of pressure and shear) along the body. Others will integrate around your annulus.

Either way, you would never multiply by 2 to get from a 2D to 3D value. If you integrate around the annulus you get a constant multiplyer of 2pi... not 2.

Hello!

Sorry, I thought I mentioned the software. It's Star CCM.

And again sorry, I meant 2pi, because yes, of course, you can't get from 2D to 3D with a factor 2

fluid23 · July 26, 2018, 09:29

In star-ccm you will want to use the 2pi factor. According to the user manual; 'Any volumetric and area quantities reported for the Axisymmetric model are assumed to be for a 1-radian sector. If you want the value of a quantity for the full 360 deg revolution, multiply reported quantities by 2pi.'

killian153 · July 26, 2018, 09:31

Quote:

Originally Posted by fluid23

In star-ccm you will want to use the 2pi factor. According to the user manual; 'Any volumetric and area quantities reported for the Axisymmetric model are assumed to be for a 1-radian sector. If you want the value of a quantity for the full 360 deg revolution, multiply reported quantities by 2pi.'

Thank you very much! Now I can work knowing this important detail

fluid23 · July 26, 2018, 09:37

You are very welcome, glad I could help. In case you are not aware, there are a lot of software-specific user forums here as well, including one for star-ccm+. That would be the best place to turn for most of your questions in the future. The main user forum is for more generalized/theoretical type discussions.

killian153 · August 1, 2018, 14:37

Thank you very much for your help.

Yes I should have posted it in the star ccm section, sorry.

But I have something weird with this 2Pi factor.. My simulation is based on an existing rocket engine from an amateur guy, which is designed to have 100 lbf of thrust (about 45kg). In my model, I got 520 N of thrust, which is approximately 53 kg. It's quite similar but if I multiply by 2Pi, it becomes 330 kg, which is totally inaccurate.

I set up the same chamber pressure, dimensions etc. so I really don't understand.

EDIT: I tested my model on an other rocket nozzle which is supposed to have 250lbf of thrust (113 kg) and I got 111 kg with my model!
So, I think that Star CCM converts automatically the axi value. I select my thrust report on the inlet so I guess it's automatic.

What do you think about that ?

July 23, 2018, 16:26	Axisymmetric model	#1
killian153 New Member Killian Join Date: Nov 2017 Posts: 26 Rep Power: 8	Hello all ! Just a simple and quick question : if I make a simulation of a rocket nozzle in an axisymmetric model, do i have to double the results I get ? For example if I get a thrust of 450 N, do I have to double this value as it is axisymmetric or can I consider this it is directly the "real" value ? Thanks !

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Axisymmetric model	sunilpatil	FLUENT	2	September 3, 2015 19:34
Superlinear speedup in OpenFOAM 13	msrinath80	OpenFOAM Running, Solving & CFD	18	March 3, 2015 05:36
2d axisymmetric simulation of supersonic jet on liquid surface using VOF model	DKanungo	Fluent Multiphase	0	June 20, 2014 02:15
axisymmetric model issue	prc	Siemens	4	February 7, 2007 06:42
how to develop axisymmetric model	manish kadam	Phoenics	0	April 16, 2006 23:53

July 25, 2018, 16:05		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	There is no way to answer this question without knowing what software you are using or how you are obtaining thrust. Some packages will provide just the 2D analysis (integration of pressure and shear) along the body. Others will integrate around your annulus. Either way, you would never multiply by 2 to get from a 2D to 3D value. If you integrate around the annulus you get a constant multiplyer of 2pi... not 2.

July 26, 2018, 09:29		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	In star-ccm you will want to use the 2pi factor. According to the user manual; 'Any volumetric and area quantities reported for the Axisymmetric model are assumed to be for a 1-radian sector. If you want the value of a quantity for the full 360 deg revolution, multiply reported quantities by 2pi.'

July 26, 2018, 09:37		#6
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	You are very welcome, glad I could help. In case you are not aware, there are a lot of software-specific user forums here as well, including one for star-ccm+. That would be the best place to turn for most of your questions in the future. The main user forum is for more generalized/theoretical type discussions.

August 1, 2018, 14:37		#7
killian153 New Member Killian Join Date: Nov 2017 Posts: 26 Rep Power: 8	Thank you very much for your help. Yes I should have posted it in the star ccm section, sorry. But I have something weird with this 2Pi factor.. My simulation is based on an existing rocket engine from an amateur guy, which is designed to have 100 lbf of thrust (about 45kg). In my model, I got 520 N of thrust, which is approximately 53 kg. It's quite similar but if I multiply by 2Pi, it becomes 330 kg, which is totally inaccurate. I set up the same chamber pressure, dimensions etc. so I really don't understand. EDIT: I tested my model on an other rocket nozzle which is supposed to have 250lbf of thrust (113 kg) and I got 111 kg with my model! So, I think that Star CCM converts automatically the axi value. I select my thrust report on the inlet so I guess it's automatic. What do you think about that ?