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Old   June 13, 2018, 07:08
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   August 28, 2018, 14:28
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Hey.
Might be a bit 2 late, but anyway, here it goes:
To correctly derive any force coefficients in CFD you need to solve the boundary layer, which means that you need to use low Reynolds models and have y+< 5 (maybe 1) around the surface of the sphere, which is really computational expansive for high Reynolds. Say you have 10m/s of reference speed but the maximum speed near the surface is 3x that, so, for 30m/s you'd need the first cell to be approx. 1e-5<h<1e-6m tall. Considering the 1m diameter, we are talking about some really fine mesh.



A way around would be to try your best to have 30 <y+<100 for the first layer of elements around the sphere and use the Hi Re kw SST turbulence model and hope for the best. You could also try a transient solver and average the results of the last 20-50 values, after convergence.



Also, check for boundary interference in the results (narrow domains). The stretch value might be a bit high for hydrodynamics, try the default 1.2.



gl
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Old   August 29, 2018, 03:03
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Quote:
Originally Posted by calim_cfd View Post
Hey.
Might be a bit 2 late, but anyway, here it goes:
To correctly derive any force coefficients in CFD you need to solve the boundary layer, which means that you need to use low Reynolds models and have y+< 5 (maybe 1) around the surface of the sphere, which is really computational expansive for high Reynolds. Say you have 10m/s of reference speed but the maximum speed near the surface is 3x that, so, for 30m/s you'd need the first cell to be approx. 1e-5<h<1e-6m tall. Considering the 1m diameter, we are talking about some really fine mesh.



A way around would be to try your best to have 30 <y+<100 for the first layer of elements around the sphere and use the Hi Re kw SST turbulence model and hope for the best. You could also try a transient solver and average the results of the last 20-50 values, after convergence.



Also, check for boundary interference in the results (narrow domains). The stretch value might be a bit high for hydrodynamics, try the default 1.2.



gl
l8r
Hey,
it's not late at all. Thanks for your suggestions, much appreciated. I'm trying to do the mesh with your advices. Can i send you my results?
Really thanks again.
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Old   August 29, 2018, 07:34
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Hi.

I could try to have a look! Send as much info as you can. You could send the model file too without mesh and/or results so we can get a small file. If you could also send your reference with the problem description and the target CD value that's good too.

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