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Gas flow through a channel with sudden radius expansion

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Old   May 15, 2014, 23:21
Default Gas flow through a channel with sudden radius expansion
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(a brief introduction before the actual problem)

Hello fellow members!

I'm currently studying mechanical engineering, and one of my greatest areas of interest lies on fluid dynamics (and I'm still a complete rookie at it, mind you). My teacher asked me to try out/learn more about SU2 on his behalf, so here I am.

(the actual problem)

I went throught some of the tutorials and found them to be very good (only problem I found was that the configuration text files which describes the problems themselves weren't on a proper format on my notepad, I had to rearrange all the text myself, does anyone know how I could avoid such work?).

After this, I started trying to make a configuration file for my own problem, using one of the tutorials' configurations as a base file, but I'm finding some issues. The problem itself is one developed by Patankar, and goes as the title says: it's a gas flow inside a channel with radius r, and it suddenly expands to a radius R > r. It's also important to state that the tubes on first part of the flow (with radius r) are isolated, whereas the tubes on the second part, with radius R, are maintained on a Temperature T.

Before trying the mesh specific to this problem, I'm trying to make a configuration file to the tutorial on the channel with a bump and applying some changes (mostly, I'm trying to add the Temperature to the lower wall)
Link for the channel bump tutorial:
(http://adl-public.stanford.edu/docs/...p+in+a+Channel)

I tried applying the Navier Stokes equations to the physical problem (the tutorial uses Euler) and everything went OK. But when I try to change the wall conditions from MARKER_EULER to MARKER_ISOTHERMAL (in the proper format, with entity name and temperature), the solver crashes.

Does anyone have any idea what could be the problem?

I'll include the little changes I've made along with the original commands:

Original:

PHYSICAL_PROBLEM= EULER

FREESTREAM_PRESSURE= 101300.0

FREESTREAM_TEMPERATURE= 288.0

MARKER_EULER= ( upper_wall, lower_wall )

New:

PHYSICAL_PROBLEM= NAVIER_STOKES

KIND_TURB_MODEL= SA

FREESTREAM_TEMPERATURE= 300.0

REYNOLDS_NUMBER= 5000000.0

MARKER_ISOTHERMAL= ( upper_wall, 500.0 )

MARKER_ISOTHERMAL= ( lower_wall, 500.0 )

I'm inclined to believe that i didn't describe the boundary counditions properly, so if anyone could shed any light on the matter I'd be really thankful!
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Old   May 18, 2014, 21:43
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Francisco Palacios
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Quote:
Originally Posted by Blooper View Post
(a brief introduction before the actual problem)

Hello fellow members!

I'm currently studying mechanical engineering, and one of my greatest areas of interest lies on fluid dynamics (and I'm still a complete rookie at it, mind you). My teacher asked me to try out/learn more about SU2 on his behalf, so here I am.

(the actual problem)

I went throught some of the tutorials and found them to be very good (only problem I found was that the configuration text files which describes the problems themselves weren't on a proper format on my notepad, I had to rearrange all the text myself, does anyone know how I could avoid such work?).

After this, I started trying to make a configuration file for my own problem, using one of the tutorials' configurations as a base file, but I'm finding some issues. The problem itself is one developed by Patankar, and goes as the title says: it's a gas flow inside a channel with radius r, and it suddenly expands to a radius R > r. It's also important to state that the tubes on first part of the flow (with radius r) are isolated, whereas the tubes on the second part, with radius R, are maintained on a Temperature T.

Before trying the mesh specific to this problem, I'm trying to make a configuration file to the tutorial on the channel with a bump and applying some changes (mostly, I'm trying to add the Temperature to the lower wall)
Link for the channel bump tutorial:
(http://adl-public.stanford.edu/docs/...p+in+a+Channel)

I tried applying the Navier Stokes equations to the physical problem (the tutorial uses Euler) and everything went OK. But when I try to change the wall conditions from MARKER_EULER to MARKER_ISOTHERMAL (in the proper format, with entity name and temperature), the solver crashes.

Does anyone have any idea what could be the problem?

I'll include the little changes I've made along with the original commands:

Original:

PHYSICAL_PROBLEM= EULER

FREESTREAM_PRESSURE= 101300.0

FREESTREAM_TEMPERATURE= 288.0

MARKER_EULER= ( upper_wall, lower_wall )

New:

PHYSICAL_PROBLEM= NAVIER_STOKES

KIND_TURB_MODEL= SA

FREESTREAM_TEMPERATURE= 300.0

REYNOLDS_NUMBER= 5000000.0

MARKER_ISOTHERMAL= ( upper_wall, 500.0 )

MARKER_ISOTHERMAL= ( lower_wall, 500.0 )

I'm inclined to believe that i didn't describe the boundary counditions properly, so if anyone could shed any light on the matter I'd be really thankful!

Thanks a lot for your interest in SU2,

Instead of

MARKER_ISOTHERMAL= ( upper_wall, 500.0 )

MARKER_ISOTHERMAL= ( lower_wall, 500.0 )

could you please try with

MARKER_ISOTHERMAL= ( upper_wall, 500.0, lower_wall, 500.0 )

Best Regards,

Francisco

PS.- If you are a rookie in CFD, my recommendation is to look for a similar problem in the TestCases folder and play with it before proposing something new.
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Old   May 19, 2014, 17:42
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Hey Francisco, thanks for the reply!

Indeed, when correcting the command format as you recommended, the simulation starts, unlike before. The problem now is that the script crashes after a few iterations (very few, 48 iterations). The last message is the following:

CSysSolve:modGramSchmidt: w[i+1] = NaN

I'm currently trying to work around this, but if you know what it's about then it would be of great help.

Thanks again!

Last edited by Blooper; May 19, 2014 at 17:47. Reason: typo
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Old   June 5, 2014, 02:31
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hlk
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What you are describing is a symptom of the solution diverging.

Since you are using a config file which was originally for Euler it is likely that the CFL number and multigrid settings likely need to be changed. I recommend lowering the CFL number (try something a little less than one ramped up to 1 to start, if that works you can try increasing the CFL number to reduce the number of iterations) and turning of multigrid (MGLEVEL = 0).
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Old   June 5, 2014, 15:40
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Indeed hlk, I had already tried lowering the CFL number and it is now working again (I didn't even turn off multigrid - what would that change?). Anyway, all is running smoothly now, thanks a bunch for the reply.
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