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Old   January 10, 2003, 13:07
Default High speed flow problems
  #1
Sawa
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Hi CFD-experts,

I am still quite new at CFD and before modelling the more difficult (and more interesting) cases I thought I could start with a small 2D-case with similarities to what I'm going to model later in 3D. But now I'm facing some problems and hope you can help...

I sat up a case similar to one in the Fluent tutorial: "Modeling External Compressible Flow" (if you don't know it: flow around an airfoil, Mach 0.8, Solver coupled-implicit).

I don't have an airfoil, but a small oval-shape tubular body (I hope you understand what I mean). In difference to the tutorial case, the body and its interior fluid zone are also meshed as I want to see for example the temperature distribution not only around but also inside the body. Flow around this body is 0.6 Mach, temperature only 200K and pressure only 5000 Pa (I use pressure-far-field bc). Solver is coupled-implicit.

Problems: 1) I can't start the solving process as I always get a floating point exception.

I changed the solver settings in many ways but couldn't do anything about it.

Can anybody tell me what in general might be reasons for such floating point exceptions and what I can do about it?

2) When I changed the fluid zone inside the body to a solid zone, I could start the solving process, the results for the flow field around the body look fairly good and reasonable.

Then I changed the solid zone inside the body back to a fluid zone. Same result: floating point exceptions. I tried to use segregated solver and this works: I can start the solving process and get a result that might be not too far from reality.

Anyway: Can someone tell me if - such a procedure (changing solid zone into fluid zone) is okay? - segregated solver is suitable for high speed flows in this case?

For me as a newbie this case is difficult enough to handle as I am still trying to understand the backgrounds in CFD. I hope you can help with some hints...

Thanks, Sawa
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Old   January 10, 2003, 21:39
Default Re: High speed flow problems
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Alamgir
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Why did u change the zone? U should have either solid or liquid and u knew it before going to solving the problem. Changing zones is not a good idea.......there are many know-unknown reasons for that floating point exemption... first of all check the values...is there any 0, then go to under relaxation factor (URF) and decrease it by 1/2.

alamgir
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Old   January 11, 2003, 11:53
Default Re: High speed flow problems
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Sawa
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Hi Alamgir,

thanks for your answer.

Of course I know that there is a liquid. But as I wrote in my first posting: If I start the solving process with this zone defined as a fluid I get a floating point exception. If I define it as a solid I can start the solving process without any problems. If there weren't any problems I would have never seen any reason for changing zones...

You suggest checking "the values" if there is a 0. Can you specify which values you mean? I checked many values but maybe I missed some...

Greetings, Sawa
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Old   January 14, 2003, 02:10
Default Re: High speed flow problems
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Alamgir
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1. Now decrease the under relaxatio factor (solve>control) untill the floating point gone.

2. U can also set the initial values during initialization, there could be some zeros, so make them some small values.

Try for 1 then 2 then both for trial and error method.. good luck

Alamgir
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