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Gravity in STAR-CD

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Old   November 20, 2007, 12:42
Default Gravity in STAR-CD
  #1
Sachin Patil
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I would like to see hydrostatic pressure distribution in STAR-CD.

Also when I run simple vertical pipe case in STAR-CD, I define top as Inlet and bottom as outlet. I specify V velocity which is -2 m/s and gravrity in Y direction with -1 in gravity features of STAR-CD.

But I get the same results, if I run with gravity or without gravity in STAR-CD.

Does STAR-CD take into account gravity in single phase flow?

I know that STAR-CD take into account gravity for VOF means free surface flow with air and water.

Please kindly clarify my belief.

Thanking you in advance, Sachin
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Old   November 20, 2007, 13:26
Default Re: Gravity in STAR-CD
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Harry
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You need to turn density to something other than constant and activate buoyancy

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Old   November 20, 2007, 13:35
Default Re: Gravity in STAR-CD
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Sachin Patil
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Thanks for the reply. I will turn density to non constant option. And it may also helo me to actiave buoyancy, because before when I tried to actvate buoyancy it shows buoyancy can not be turned on for constant density.

I will keep you updated.

Regards, Sachin
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Old   November 21, 2007, 05:14
Default Re: Gravity in STAR-CD
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Anton Lyaskin
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Actually you can see the difference even without buoyancy and variable density - plot static pressure instead of piezo
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Old   November 21, 2007, 12:13
Default Re: Gravity in STAR-CD
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Sachin Patil
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About plotting static pressure and piezometric pressure, I got exactly same values uoto all decimal places. STAR-CD manual says that piezoetric pressure= static pressure - (density*gravity*height).

But I got the same graphs, similar post registres, if I compare piezo Vs. static pressure, which is surprising and illogical.

Doest STAR-CD take into account gravity for single phase incompressible flow?

Thanks .
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Old   November 21, 2007, 14:00
Default Re: Gravity in STAR-CD
  #6
murash-v
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yes, STAR-CD take into account gravity for single phase incompressible flow
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Old   November 21, 2007, 14:26
Default Re: Gravity in STAR-CD
  #7
Sachin Patil
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Hi Thanks for the reply.

How can I see effect of gravity in STAR-CD for single phase incompressible flow?

I tried to do it by yself. I turned on gravity and then turned off, I got same results, which looks pretty surprising.

I set gravity in Y direction with x=0, y=-1, and z=0 with g=9.8066 m/s2. But I can not turn on buoyancy for single phase incompresssible flow, as it says that Buoyancy can not be turned on for constatnt density.

Am I setting up problem correctly or I have to do anything else to see effect of gravity?

Thanks for reading my long probelm.

Sachin
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Old   November 21, 2007, 15:15
Default Re: Gravity in STAR-CD
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Anton Lyaskin
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Your question really puzzled me. I've checked it too, and you're right - no difference!
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Old   November 21, 2007, 15:40
Default Re: Gravity in STAR-CD
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Anton Lyaskin
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Well, according to the manuals gravity is taken to the account only for free-surface and buoyancy problems.

Though I can't imagine right now an incompressible problem where gravity induced pressure gradients will be important, this neglection of gravity will at least require additional steps in post processing if you want REAL static pressure.

If my memory serves me right, the behavior was different in v3.15, i.e. plotting pressure and static pressure was giving different pictures. But I think for incompressible flows it was merely a post-processing option.
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Old   January 31, 2008, 18:54
Default Re: Gravity in STAR-CD
  #10
borizzzzz
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Hello,

I have got the same problem while trying to sediment solid phase (5%, Eulerian treatment) in a closed volume of static fluid: there is no gravity there . Any comments on managing the problem? The only way I can try to do: source term gravity subroutine and turn-off built-in gravity. I am looking forward for your reply!

Best regards, Boris
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Old   January 31, 2008, 23:13
Default Re: Gravity in STAR-CD
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Sachin
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Hi To the best of my knowledge gravity is not taken into account in STAR-CD unless it is free surface problem. Any comments / suggestions are welcome

Sachin
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