[Legolas_1204]

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?

[LuckyTran]

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?

[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.

[Legolas_1204]

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

[Legolas_1204]

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.

[Roh]

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.

[Legolas_1204]

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.