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-   -   Surface roughness (https://www.cfd-online.com/Forums/flow-3d/76559-surface-roughness.html)

 alefem May 28, 2010 11:45

Surface roughness

Dear all,

about surface roughness, there is a statement on the THEORY chapter of the manual 3.3.19,

"a surface roughness provided by the user can be used as a multiplicative factor on the geometric area to increase or decrease the effective heat transfer rate. Specifying a zero roughness ,for example, eliminates the heat-structure calculation entirely, which is useful for an adiabatic boundary".
which I think means that if the surface roughness=0, there is no heat transfer happened between the wall and the fluid, is that right?

But the result does not seem right, because the default surface roughness is 0, and I still can find the wall temperature is increaseing and the temperature of the melt is decreasing.

 ignatz May 29, 2010 03:32

Ale,
I see you have posted some other queries related to casting. Is this the industry you are in? If so, I don't think you need to worry about surface roughness at all.

 alefem May 30, 2010 09:54

Ignatz,
Yes, I am interestied in casting process.
In fact I am just testing all the manual and function of Flow3d in this stage to make sure that I will not make problem to simulate the real casting problem.

But I dont know why shoud I ignore the surface roughness, could you please give me more information?

Quote:
 Originally Posted by ignatz (Post 260786) Ale, I see you have posted some other queries related to casting. Is this the industry you are in? If so, I don't think you need to worry about surface roughness at all.

 melissa June 2, 2010 09:48

You can try using the viscous heating to model the friction that occurs in the gates due to the high speed causing sticking, if this is what you are trying to model. This just needs to be checked in the Physics tab>Heat Transfer>viscous heating.

 jld61 June 2, 2010 12:03

Possible confusion over roughness variables

It looks like there is some confusion due to the dual usage of the term "roughness" on components. The effects of roughness on the viscous forces on a component is introduced through the variable ROUGH() which has a default value of 0.0 but it is not used in heat transfer calculations. The variable ROBS(), which defaults to a value of 1.0, should be set to account for the effects of surface roughness on the heat transfer on a
surface.

 alefem June 3, 2010 12:48

Thank you melissa,
And I think if the viscous heating is actived, it should work on the whole fuild field, not only on the friction boundary.

Quote:
 Originally Posted by melissa (Post 261402) You can try using the viscous heating to model the friction that occurs in the gates due to the high speed causing sticking, if this is what you are trying to model. This just needs to be checked in the Physics tab>Heat Transfer>viscous heating.

 alefem June 3, 2010 12:49

I agree with you, there is some confusion.

Quote:
 Originally Posted by jld61 (Post 261426) It looks like there is some confusion due to the dual usage of the term "roughness" on components. The effects of roughness on the viscous forces on a component is introduced through the variable ROUGH() which has a default value of 0.0 but it is not used in heat transfer calculations. The variable ROBS(), which defaults to a value of 1.0, should be set to account for the effects of surface roughness on the heat transfer on a surface.

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