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Old   July 26, 2007, 10:37
Default HTC w/ adiabatic wall
  #1
Kali
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In my CFX case I have adiabatic walls with energy equation solved. When I ploted for wall heat transfer coefficients, it gave a heat transfer coeffient contour plot. I wonder what are those numbers..Does it calculate heat transfer coeffient using analogy from cf. Or its just a set of junk numbers. Please clarify.

Thanks in advance. Kali
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Old   July 26, 2007, 10:43
Default Re: HTC w/ adiabatic wall
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Robin
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It returns the heat transfer coefficient resulting from the turbulent wall function. This HTC is used by the solver to calculate the wall heat flux and/or temperature, depending on what boundary condition you have applied.

Regards, Robin
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Old   July 26, 2007, 11:00
Default Re: HTC w/ adiabatic wall
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Kali
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Robin, Is it independent of the type of BCs you give? Provided you near wall fluid temperature and fluid property remain the same.

I saw in the manual that T+ is function of qw and Tw-Tf. With adiabatic wall how does it calculate heat transfer coefficient from turbulent wall function. Would you please elaboare more with reference to the manual.

Thanks, Kali
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Old   July 26, 2007, 11:18
Default Re: HTC w/ adiabatic wall
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Robin
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Hi Kali,

T+ is defined as a function of qwall but it is modeled as eq. 183 in the v11 manual. The HTC is thus defined as:

HTC = rho*Cp*u_star/T+

Note that this HTC is grid dependant, since the fluid temperature is the conservative value of temperature at the wall node (the wall node stores values for the near wall control volume).

Keep this in mind if you intend to use this heat transfer coefficient for any other purposes, such as mapping to an FEA thermal analysis. If you are comparing to data, calculate an HTC the same way it was done for the experiment, don't use this one.

Regards, Robin
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Old   July 27, 2007, 11:47
Default Re: HTC w/ adiabatic wall
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Kali
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Robin, If you apply wall temperature, will the heat flux estimated will depend on the grid..paticularly the near wall Y+.

please comment.
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Old   July 27, 2007, 12:02
Default turbulence modeling for compressible flow
  #6
Kali
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What are the extra variables/factors need to be taken care in CFX for turbulence modeling for compressible flow.

Thanks, Kali
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Old   July 27, 2007, 16:49
Default Re: HTC w/ adiabatic wall
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Robin
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Yes, in the sense that it is a function of the boundary layer and your grid spacing will determine how accurately you resolve the boundary layer.

This is one reason why the SST model is preferred for heat transfer; it can give you a more accurate representation of the boundary layer (with sufficient mesh).

Regards, Robin
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Old   July 27, 2007, 16:52
Default Re: turbulence modeling for compressible flow
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Robin
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Use a compressible fluid material (Ideal Gas, Redlich-Kwong, etc.) and turn on the "Total Energy" equation. If your walls are moving relative to the frame (i.e. a spinning wall, counter-rotating wall, or wall with a specified velocity) turn on the viscous dissipation term. Everything else is handled automatically by the solver.

-Robin
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Old   July 29, 2007, 09:21
Default Re: turbulence modeling for compressible flow
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Kali
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Robin, I have taken care of those features. In particular to turbulenec meodeling, should I use some advanced option like production term modification etc..

Thanks, Kali
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Old   July 30, 2007, 09:08
Default Re: turbulence modeling for compressible flow
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Robin
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No, the code takes care of the rest.
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Old   August 7, 2007, 06:45
Default Re: HTC w/ adiabatic wall
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kali Charan Nayak
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Robin, When I ran the same model with KE turbulenec mode, my wall heat transfer coefficients quite low by 4-5 times? why?

Thanks in advance.

Quick reply will be helpful.

Kali
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Old   August 7, 2007, 10:20
Default Re: HTC w/ adiabatic wall
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Robin
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Hi Kali,

The wall treatment of the k-e and SST models differ significantly, especially where the near wall resolution is fine. For low Y+ values, the SST model will use a linear wall function, resolving the viscous sub-layer directly, and blend to a logarithmic wall function as Y+ increases. The k-e model will always use a wall function and therefore overpredict turbulent mixing near the wall if the Y+ is small.

This is probably the reason for the discrepancy but there are also the possibility that upstream b.l. and/or differences in the free-stream treatment have changed the local flowfield. I would suggest comparing the two results to see how and where they differ.

Finally, the SST model is much better for heat transfer. In the absence of any experimental data to definitively determine which is correct, I would give greater credence to the SST results.

Regards, Robin

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