Axisymmetric model
 

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 !

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 :D

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 :)

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.

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 ?