# Boundary Conditions for Drag Analysis of Rocket Nose

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 November 4, 2019, 10:55 Boundary Conditions for Drag Analysis of Rocket Nose #1 New Member   Shashwat Join Date: Aug 2019 Posts: 16 Rep Power: 5 I am new to CFD and I am using ANSYS fluent to calculate drag coefficient of a rocket nose cone. I am using the pressure-based solver and viscous model I am using is Spalart-Allamaras, strain-vorticity based. Boundary conditions are as below: Inlet - Velocity Inlet: 312 m/s(~0.9 Mach) Outlet - pressure outlet - 0 Gauge pressure Temperature at inlet and outlet - 300 K The results I am getting are varying greatly so I think there is a mistake in the boundary condition. So can you suggest what is the problem in the boundary conditions?

November 4, 2019, 11:17
#2
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Lucky
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Quote:
 Originally Posted by Legolas_1204 The results I am getting are varying greatly so I think there is a mistake in the boundary condition. So can you suggest what is the problem in the boundary conditions?

What is varying?

Boundary conditions, even if they are wrong, will give results. It may not be the result that you want, but a result nonetheless.
For example, I can mistakenly set the inlet temperature to 200 K and simulate the wrong conditions. But I should still get results for 200 K. Any other issues that arise, is its own issue. What is the actual problem you are encountering?

 November 4, 2019, 16:42 #3 Member   Joshua Join Date: Aug 2014 Posts: 49 Rep Power: 10 Did you specify the reference values (length, velocity, area etc.) correctly? Fluent uses those values to calculate the lift and drag coefficients. If you left them as the default values, Fluent will report incorrect numbers.

November 5, 2019, 01:24
#4
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Shashwat
Join Date: Aug 2019
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Quote:
 Originally Posted by LuckyTran What is varying? Boundary conditions, even if they are wrong, will give results. It may not be the result that you want, but a result nonetheless. For example, I can mistakenly set the inlet temperature to 200 K and simulate the wrong conditions. But I should still get results for 200 K. Any other issues that arise, is its own issue. What is the actual problem you are encountering?
Example:

I have run the analysis of Von-karman nose cone and a Truncted Ogive nose cone. Theoretically Von-karman has lower drag than truncted ogive but I am getting opposite.
And second thing, when I go supersonic 'Turbulence viscosity ratio exceed' warning appeares and solution diverges eventually. So I think there is a problem in boundary condition

November 5, 2019, 01:25
#5
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Shashwat
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Quote:
 Originally Posted by jbo214 Did you specify the reference values (length, velocity, area etc.) correctly? Fluent uses those values to calculate the lift and drag coefficients. If you left them as the default values, Fluent will report incorrect numbers.
No, I have specified all the reference values and I have double-checked it so they are correct.

November 6, 2019, 06:15
#6
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Quote:
 Originally Posted by Legolas_1204 ... Inlet - Velocity Inlet: 312 m/s(~0.9 Mach) ...
Not a good choice. If I were you, I would use pressure-far-field boundary conditions. Also I don't know anything on your domain. Velocity-inlet is intended for incompressible flows, and its use in compressible flows will lead to a nonphysical result.

November 6, 2019, 15:52
#7
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Shashwat
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Quote:
 Originally Posted by Roh Not a good choice. If I were you, I would use pressure-far-field boundary conditions. Also I don't know anything on your domain. Velocity-inlet is intended for incompressible flows, and its use in compressible flows will lead to a nonphysical result.
Thank you. I have read about it somewhere so I'll try it.