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Inconsistency in Fluent results with calculation

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Old   February 3, 2020, 18:03
Default Inconsistency in Fluent results with calculation
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Abhinand
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Hi all,

I do not know why an inconsistency in results from fluent and analytical calculation

For eg:
Skin friction coeff : From fluent (as field variable)

By calculation : tau = mu*(dx-velocity-dy)

I computed both and I found to be consistent

But when I check the heat flux calculation

Total surface heat flux (From fluent Field variable)

By hand calculation:
Kair*(gradient-dt-dy) is vastly different from the value given by fluent

From fluent manual:
For an isothermal wall BC, the heat flux is calculated by
Q = Hf*(Tw-Tf)
Hf = local fluid heat transfer coefficient
Tf = local fluid temperature

The Hf given by fluent is given by Q/(Tw - Tf) Tf specified in reference tables

But how does it calculate the Q??

I want to know how does fluent calculate the total surface heat flux.
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Old   February 4, 2020, 04:10
Default From the first principle
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Fluent solve energy equation and as a solution it has temperature field available. Using the gradient of this and thermal properties of the material, q is just diffusion heat flux. Do note that this also includes diffusion due to the turbulence, provided the flow is turbulent. And diffusion due to the turbulence could be significant, even close to the wall.
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Old   February 4, 2020, 16:43
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Thank you for your answer vinerm,

Can you please elaborate on how turbulent diffusivity (mu_t) comes into picture while calculating heat flux. Is my understanding correct?
Correct me if Im wrong

Q = -K(gradient dt-dy)
gradient is calculated by fluent after solving the temperature field

K = Klam + kturb
Kturb = cp*mu_t/Pr_t

I have an extremely turbulent boundary layer and my Kturb is like 200 times smaller than K.

Please advise on how the computation is done by fluent

Thanks

-Abhinand
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Old   February 5, 2020, 04:43
Default You are correct
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Yes, that's how Fluent computes the flux for every field. BL are so called because within them effects of turbulence more or less vanish and viscous forces take over. However, Fluent uses effective diffusivity since mesh close to wall may or may not have been resolved. Furthermore, the relation you mentioned is based on first-principle. Fluent does it slightly differently. Look at Theory Guide Section on Near-Wall Treatment for Energy within Turbulence chapter.
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