# Boundary Conditions for Back Face (Nozzle Part) of a Missile

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 October 2, 2023, 08:41 Boundary Conditions for Back Face (Nozzle Part) of a Missile #1 New Member   David Turner Join Date: Feb 2023 Posts: 4 Rep Power: 3 Hi everyone I was thinking about a missile's back wall (Nozzle part). For a missile analysis, I use velocity-inlet or mass-flow-inlet boundary conditions for the nozzle exit of a missile or rocket geometry. But recently, I tried implementing wall boundary condition (BC) for the back face. I know, in real, there is a nozzle exit and it generates thrust at this face. When I turned it to wall BC, the drag coefficient decreased and that face generated thrust even with the implementation of no-slip wall BC. I had a conflict, so I decided to ask you guys. In my opinion, using the velocity-inlet or mass-flow-inlet boundary condition as a thrust at the nozzle exit reflects real-life conditions more. Are there any other correct BCs for back face or flying bodies? If not, what's your modeling method, and which one is better for modeling it right? Thank you, and have a nice run

 October 3, 2023, 04:54 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144 What the correct boundary condition for that face depends on what you are trying to do and what you are trying to learn from the model. Can you explain that to us? __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 October 3, 2023, 13:36 #3 New Member   David Turner Join Date: Feb 2023 Posts: 4 Rep Power: 3 of course. I'm trying to model and calculate the drag coefficient of a rocket for different angles of attack that have thrust at the end of its body.

 October 3, 2023, 17:57 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144 OK, thanks. In that case, I would expect the jet coming out of the thruster to entrain a lot of flow around it, and this entrained flow will affect the lift and drag. So I think you will find you will need to include the jet so this entrained flow is included in the model and makes the drag and lift coefficients realistic. monkeypuppets likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 Tags back, face, nozzle