Fire flame boundary HRRPUV cutoff value guide line

[xiaoyun Qin]

Smokeview stated that "Fire by default is colored a dark shade of
orange wherever the computed heat release rate per unit volume exceeds
a user-defined cutoff value".
I usually do not define the cutoff value, instead direct run smoke
view. most time for different cases, smoke view will give a different
cutoff value. the questions is:
is there any guideline to pick the cutoff value? what is the mechenism
Smokeview give different value for different cases?

[Kevin]

By default, the value is 20/dx kW/m3, where dx is the cell size. There
are additional limits in Smokeview. This parameter is mainly for
visualization. It does not have much physical meaning. The orange
color just gives you an idea where the combustion takes place.

[xiaoyun Qin]

Great, thank you very much for the quick help!

for many other CFD program, we run into the problem of how to visuliz the combustion zone.

Although you said it does not have much physical meaning, does it represent (at least more or less) a visuliable flame?

is there any publications talk about the issue?

 

Thanks,

 

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[xiaoyun Qin]

While I am waiting for your help, I have another question.

dx is the cell size, if I use a none uniform mesh, let's say 2 different size in x direction, 2 diffent size in y direction and 1 different size in z direction (all 5 sizes are different),what should I calculate the cell size?

 

Thanks in advance!

On Fri, May 20, 2011 at 8:20 AM, Kevin <mcgr...@gmail.com> wrote:

[Kevin]

Yes -- it represents where the fuel and oxygen mix and burn. However,
the color is just orange -- we do not have a way to color based on
detailed chemistry.

> >http://groups.google.com/group/fds-smv?hl=en.- Hide quoted text -
>
> - Show quoted text -

[Kevin]

(dx*dy*dz)^1/3

[Bryan Klein]

How was the value of 20 determined as the default?

The question comes from the desire to 'justify' the visualization to someone who is questioning why 20 and not 15, 30 or X.  I looked in the Smokeview technical reference and user guide to help answer this question but did not see information on this value.

This is not really a chalenge of the value, as if there is something better, but more of a request to understand the selection of it as the default.

[Bryan Klein]

For non-uniform meshes with transformations applied, is the cutoff value recalculated for cells of different sizes (varying dx) in the same mesh?

[Kevin]

Mainly trial and error with fires on fine and coarse grids. There is
one cutoff value per mesh, based on the smallest cell.