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Old   January 29, 2006, 03:14
Default Axissymmetric "laminar flow?" -> Calculation of Cd
Rahul Sangole
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Dear All,

I am working on fluent for the first time. It isnt the prime area of my project, but I need the value of Cd for a shape.

I am designing an airship. Due to some constraints, the shape is as follows: a hemisphere followed by a cylinder, followed by another hemisphere. (just like our capsule medicines).

The flow velocity is around 2m/s.

I created the grid in gambit.

If I run the grid in fluent using the Segregated solver and Spalart-Allmaras condition for a flow velocity of 60m/s, the velocity vectors look ok. The separation and wake look fine.

But I cant seem to run it at a flow vel. of 2m/s. It gives a divergence error:temperature. I thought that at such low speeds, the flow can be considered laminar, so I selected that viscous model, but it didnt solve the problem.

Please help me. All I want to do is find out the values of Cd for a couple of shapes.

Warm Regards, Rahul Sangole.
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Old   January 29, 2006, 05:41
Default Re: Axissymmetric "laminar flow?" -> Calculation o
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Use "incompressible" for material density. At such low velocities, temperature and density variations are much to small to be taken into account. Using "ideal gas" for 2m/s can give you divergence.

Be careful when you say "I thought... the flow can be considered laminar"!! First calculate Re number. I bet it is more than 20000. So it is not a laminar flow.

Best whishes, Razvan
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Old   January 29, 2006, 10:56
Default Re: Axissymmetric "laminar flow?" -> Calculation o
Rahul Sangole
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Dear Razvan,

Thanks for the information and the heads-up.

I tried using incompressible, but now I get an error - "Error: pressure-far-field boundary conditions can only be used with ideal gases. Error Object: ()"

What should I do?

Warm regards, Rahul.
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Old   January 30, 2006, 03:31
Default Re: Axissymmetric "laminar flow?" -> Calculation o
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Pressure-far-field boundary condition is available only when using "ideal gas" formulation for density. You are now using "incompressible" formulation, so you must use a pair of velocity-inlet+presure-outlet, or velocity-inlet+outflow boundary conditions (prefferably the first one).

Somebody told me that "constructive criticism" is welcome. So I will not spare you this time. When I began learning about CFD I already had a very well consolidated base in fluid mechanics. What I'm trying to tell you is that you should have noticed immediately that pressure-far-field condition requires Mach number input, which is a characteristic of compressible fluids (for incompressible fluids Mach=infinnity). I don't want to suggest that you don't have that base, but you should be more careful next time. And also study Fluent's documentation manual, you'll find there almost anything you need to know as a beginner in this field.

I am telling you all this because next time I'm not so sure you will be so lucky and you might not get any answer at all for your "easy" questions!

Cheer up! It could have been worse.

Best whishes and long life in CFD, Razvan
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