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Old   June 13, 2018, 08:06
Smile Sphere at Re=1e+7
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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, 08:25
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Quote:
Originally Posted by danny9019 View Post
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-



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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Old   June 13, 2018, 08:30
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So? What i can do?
Thanks very much.
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Old   June 13, 2018, 08:42
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So? What i can do?
Thanks very much.
What about your present grid resolution at the wall?
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Old   June 13, 2018, 09:03
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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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Old   June 13, 2018, 09:08
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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



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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Old   June 13, 2018, 09:11
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So i have to obtain a y+ value less than 1?
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Old   June 13, 2018, 09:15
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So i have to obtain a y+ value less than 1?

at least 3-4 nodes at y+<1
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Old   June 14, 2018, 06:52
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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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Old   June 14, 2018, 06:58
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Quote:
Originally Posted by danny9019 View Post
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.


Still refining the grid close to the wall. Try also different model
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Old   June 14, 2018, 07:02
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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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Old   June 14, 2018, 08:51
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Quote:
Originally Posted by danny9019 View Post
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.



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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Old   June 14, 2018, 09:20
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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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Old   June 14, 2018, 09:25
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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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Old   June 14, 2018, 09:48
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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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Old   June 14, 2018, 09:52
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Quote:
Originally Posted by danny9019 View Post
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



yes, you need to retain 3-4 grid points below y+=1 along the whole body
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Old   June 14, 2018, 10:01
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Quote:
Originally Posted by FMDenaro View Post
Still refining the grid close to the wall. Try also different model
So in the regions where is Yellow/ red i have to create a refined mesh and where is blue a coarse mesh?
Many thanks.
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Old   June 14, 2018, 10:04
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Unfortunately, I don't see any figure...
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Old   June 14, 2018, 10:07
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In The picture attached before this morning
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Old   June 14, 2018, 11:32
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Quote:
Originally Posted by danny9019 View Post
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.

This picture FMDenaro
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