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high speed flow over a rigid body @ AoA

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Old   May 25, 2012, 14:22
Default high speed flow over a rigid body @ AoA
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Andrew Forbes
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Hello CFX forum,

I have been lurking around here for a couple weeks, gathering information, as I worked on a project for an open-ended research class. I'm very new to this software, we had no training in it at all (research class, not a CFD software class), it is coming down to crunch time and I am still having a couple issues that I need to iron out before next week. I believe it has come time for me to ask a couple questions, I think.

The nature of our project is that we are modeling lattice style missile fins in order to examine flight characteristics within their standard operation regime. The flight conditions used are standard atmosphere at 8000m (P=35651 Pa, T=236.23K) at mach 4 between 0 and 15 degrees angle of attack.

The strategy used is to model 2 cells, one diamond shaped and one triangular and then combine the parts to form a result for the whole fin. I was interested in doing it a different way (essentially, modeling a flat plate and then combining the results) but our professor has insisted, on multiple occasions, that we do it his way. So be it.

The basic geometry setups are single cells situated in a rectangular control volume in which I used a boolean operation to subtract the physical object from said CV. The inlet is defined supersonic at the obvious velocity/temp/pressure conditions, outlet definied as supersonic, CV walls defined as free-slip walls, fin cell walls defined as no-slip walls with a smooth surface.

So far, our solutions are perfectly acceptable for angles of attack lower than 10 degrees. The solver runs nicely, everything converges relatively quickly, results look as they theoretically should. The problem starts coming in when I ramp up the angle to 10 or higher. The solver RMS values go all over the place, the mach number spikes (hard) and results in a stack overflow (from what I can tell by the error messages).

I guess my question is something along these lines: How can I tell CFX that the walls (mainly top and bottom walls) of the CV aren't actually walls and should not be treated as such by the flow? I believe part of the issue with the unstable calculations is the fact that a roughly 1200 m/s flow hitting a wall at 10+ degrees causes some pretty intense conditions. I have tried defining them as an opening, inlet, outlet with no in/out flow and a static pressure set to atmospheric with no better results.

I apologize if the information I have provided is not enough or if it seems oversimplified (it is), I tried to be as thorough as possible. I'm not well versed in the software and I'm self-learning as I go. Any insight a veteran might be able to give would be helpful. I know you guys do this in your spare time and I appreciate the help this resource has been so far. I can provide more information if needed.
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Old   May 25, 2012, 15:51
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Andrew Forbes
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not sure if I should edit or just reply to my own thread...

Anyway, I think I figured it out so this can be closed/deleted. Ended up calling the top and bottom sections of the CV an inlet and outlet with the same conditions as the flow and it solved just fine. It is really a pretty simple problem and I knew it would be a simple solution once I managed to stumble onto it. I spent the last week trying all sorts of stuff and it figures that I'd get it no more than an hour after I made the OP.
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Old   May 26, 2012, 06:05
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Glenn Horrocks
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Glad to see you sorted it out.
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