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-   -   Sphere at Re=1e+7 (https://www.cfd-online.com/Forums/siemens/202969-sphere-re-1e-7-a.html)

danny9019 June 13, 2018 08:08

Sphere at Re=1e+7
 
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-

calim_cfd August 28, 2018 15:28

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 :rolleyes:

danny9019 August 29, 2018 04:03

Quote:

Originally Posted by calim_cfd (Post 704362)
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 :rolleyes:

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.

calim_cfd August 29, 2018 08:34

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.

l8r


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