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Sphere at Re=1e+7

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Old   June 13, 2018, 08:24
Smile Sphere at Re=1e+7
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Danny
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Hi,
i'm trying to simulate a 3D-Sphere in water at Reynolds Number 1e+7 (i.e. 10'000'000) in Star-CCM+, but at the moment i'm not achieving the correct CD result of about 0.19. My physics options are:

Velocity Inlet V = 10.014 m/s
Density rho= 998.6 kg/m3
Sphere Diameter D = 1m
Turbulence Model = K-w
Time = Steady
Fluid Type = Liquid.

The Mesh options on the body are:
Number of Prism Layer = 10
Stretching Factor = 1.5


Can someone please help me?
I really appreciate much.

-Danny-
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Old   June 13, 2018, 10:06
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hi..... The mesh near the sphere must be finely refined to capture the flow accurately..in order to that go to continnum..>mesh-->model-->click on prism layer mesher--->distribution mode-wall thickness(look at properties window)---> you can specify first layer thickness under reference value . First layer thickness can be calculated by online means with desired y+ value approx equal to 1
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Old   June 13, 2018, 10:19
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I'm running the analysis with these values:
Prism layer thickness = 2.5 cm
Number of prism layer = 40
Thickness of the near wall = 1e-6
And an y+ less than 1.

Could you tell me if this is a correct configuration?
Many thanks.
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Old   June 13, 2018, 13:30
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this seems right according to me
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Old   June 14, 2018, 06:54
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Hi, firstly many thanks. I've setted the grid in this way:

Number of prism layer = 22
Total prism layer lenght = 5 cm

Achieving an y+ maximum of 0.92 like in the attached picture.

Now my Cd is 0.12
What can i improve to rise the Cd value to the right 0.19?
Thanks so much.

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Old   June 14, 2018, 12:30
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what turbulence model and options are you using?

edit:
sorry, I see you are using k-w. still curious about your options.

make sure your wake is refined as well as your near wall mesh. the wake has an impact on drag.

also, try doing an unsteady simulation. your wake is definitely not steady and your cd will oscillate. the average of the oscillation is what you should report as your drag value.
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Old   June 15, 2018, 02:32
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Hi

Based on my experience, cd is the challenging one compared to cl.

1.Are your reference values (density, area and velocity) okay with respect to the literature?
2.Try to use Spallart-Almarus turbulence model. It is meant for external flows.
3.(Most important) improve the prism layer meshing. Go for a stretch factor of 1.02 with y+<1.

Hope these suggestions help.
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Old   June 15, 2018, 07:16
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i cannot reach y+ about 1...i don't know how to achieve that value..i tried all the ways. Can you help me please?
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Old   June 15, 2018, 23:33
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Try reducing the stretch factor with more number of prism layers. It should work.
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Old   June 16, 2018, 10:25
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Quote:
Originally Posted by HHK View Post
Try reducing the stretch factor with more number of prism layers. It should work.
ok i'm trying this way. hope works
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