# Gravity in STAR-CD

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 November 20, 2007, 12:42 Gravity in STAR-CD #1 Sachin Patil Guest   Posts: n/a 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

 November 20, 2007, 13:26 Re: Gravity in STAR-CD #2 Harry Guest   Posts: n/a You need to turn density to something other than constant and activate buoyancy

 November 20, 2007, 13:35 Re: Gravity in STAR-CD #3 Sachin Patil Guest   Posts: n/a 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

 November 21, 2007, 05:14 Re: Gravity in STAR-CD #4 Anton Lyaskin Guest   Posts: n/a Actually you can see the difference even without buoyancy and variable density - plot static pressure instead of piezo

 November 21, 2007, 12:13 Re: Gravity in STAR-CD #5 Sachin Patil Guest   Posts: n/a 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 .

 November 21, 2007, 14:00 Re: Gravity in STAR-CD #6 murash-v Guest   Posts: n/a yes, STAR-CD take into account gravity for single phase incompressible flow

 November 21, 2007, 14:26 Re: Gravity in STAR-CD #7 Sachin Patil Guest   Posts: n/a 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

 November 21, 2007, 15:15 Re: Gravity in STAR-CD #8 Anton Lyaskin Guest   Posts: n/a Your question really puzzled me. I've checked it too, and you're right - no difference!

 November 21, 2007, 15:40 Re: Gravity in STAR-CD #9 Anton Lyaskin Guest   Posts: n/a 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.

 January 31, 2008, 18:54 Re: Gravity in STAR-CD #10 borizzzzz Guest   Posts: n/a 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

 January 31, 2008, 23:13 Re: Gravity in STAR-CD #11 Sachin Guest   Posts: n/a 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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