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Simulation of convective heat loss from cylinder

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Old   April 27, 2015, 07:07
Default Simulation of convective heat loss from cylinder
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buidu
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Hi all,

I am running a simulation with flow passing a finite cylinder in order to get the overall covective coefficient of the cylinder.

Unstructural mesh was adopted for the calculation domain, and the elements near the cylinder is much denser than other parts. The distance of the inlet, top wall and downwall to the cylinder is 5 times of diameter, and the distance of the outlet to the cylinder is 10 times of diameter.

I ran the simulation with inlet air velocity of 0.2 and 3m/s using RSM model in steady condition.

Comparing with empirical euqation that calculating the Nu number, the result agree well in lower speed but much higher overestimated in case of 3m/s, around 20%.

Does anyone knows how should I improve the calculation result. Thank you very much!!!

Regards,
Hao
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Old   April 27, 2015, 12:20
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Did you refine your mesh with increasing speed/Reynolds number?
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Old   April 28, 2015, 03:57
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Quote:
Originally Posted by lcarasik View Post
Did you refine your mesh with increasing speed/Reynolds number?
Hi Lane,

I did not refine my mesh. But I checked the Y+, they are below 1 in all the speed on the surface of the cylinder.

Hao
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Old   April 28, 2015, 04:02
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What wall treatment are you using?
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Old   April 28, 2015, 04:53
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Quote:
Originally Posted by lcarasik View Post
What wall treatment are you using?
I used the enhaced wall treatment, the calculated Y+ were all below 1 at varied speeds.
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Old   April 28, 2015, 04:58
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I think you need to refine further downwind.

Make a cutplot of the velocity with visible mesh.
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Old   April 28, 2015, 05:10
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Which heat-flux model are you using?

And I agree with Christian, you need to refine your mesh downstream
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Old   April 28, 2015, 05:55
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I think you need to refine further downwind.

Make a cutplot of the velocity with visible mesh.
Yes, I will have a try. But why the wake will affect the convection on the cylinder?

I think the turbulence intensity will also affect the final result. When I ran with TI in 5%, it can convergence very well. However, when I reduce the TI to 0.1%, it showed that the turbulence viscousity ratio exceed 100000. You can see the contour in attachment. The high ration zone happened in front of and behind the cylinder. How do you think I should solve this problem?

Thank you.
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Old   April 28, 2015, 06:02
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Originally Posted by cymbourne View Post
Which heat-flux model are you using?

And I agree with Christian, you need to refine your mesh downstream
Thank you. I will do that. But do you think the turbulence intensity will also affect the results. I tried to run the case in low TI. However, the turbulence viscosity ratio reached its limitation in the zone near the cylinder. Do you think if I need to change other parameter of setting besides refine the mesh?
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Old   April 28, 2015, 08:11
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Check the visosity ratio after refining your mesh.

Regards
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