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#1 |
Member
Danny
Join Date: Jun 2018
Location: Rome, Italy
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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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#2 | |
Senior Member
Filippo Maria Denaro
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Quote:
Computing viscous drag at such Re number require a very fine grid. You cannot model the wall BC if your goal is computing the viscous forces |
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#3 |
Member
Danny
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So? What i can do?
Thanks very much. |
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#4 |
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Filippo Maria Denaro
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#5 |
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Danny
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The first element near to the body is 0.0004m lenght. The Total prism layer thickness is 0.05m.
Thanks again |
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#6 | |
Senior Member
Filippo Maria Denaro
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Quote:
you need to compare this height to the BL thickness, a fact expressed by the y+ value. To compute accurately the viscous stress at the wall you need at least 3-4 nodes at y+<1. |
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#7 |
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Danny
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So i have to obtain a y+ value less than 1?
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#8 |
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Filippo Maria Denaro
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#9 |
Member
Danny
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Hi FMDenaro, 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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#10 | |
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Filippo Maria Denaro
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Quote:
Still refining the grid close to the wall. Try also different model |
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#11 |
Member
Danny
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I'm running the simulation with the "All y+ treatment". Do you think it is better use "Low y+ model?
How can i refine the grid close to the wall? Increasing the number of layer? Because with 40 the Cd is still 0.12. Many thanks. |
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#12 | |
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Filippo Maria Denaro
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Quote:
More layers in the range y+<1 so that you can better describe the BL. Try to compare the results obtained with other models. |
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#13 |
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Danny
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In order to do this, i have to split the sphere in the regions where y+ is less than 1 and more than 1, right? In order to construct two different boundary layer?
Thanks again |
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#14 |
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Filippo Maria Denaro
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That depends on the grid generation code you are using. Using RANS you can exploit the symmetry of the geometry to solve only a half sphere, using structured grid in the BL and unstructured grid in the outer region.
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#15 |
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Danny
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I'm currently using RANSE and analyzing a quarter sphere exploiting the double symmetry. If i have get you i've to create two different boundary layer where: one more refined with more layer And the other less refined with less layers.
I have to obtain a value of y+ costant along the body right? Many thanks FMDenaro |
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#16 | |
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Filippo Maria Denaro
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Quote:
yes, you need to retain 3-4 grid points below y+=1 along the whole body |
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#17 |
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Danny
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#18 |
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Filippo Maria Denaro
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Unfortunately, I don't see any figure...
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#19 |
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Danny
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In The picture attached before this morning
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#20 |
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Danny
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This picture FMDenaro
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Tags |
drag coefficient, mesh 3d, starccm+ |
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