Converting optical density to visibility

[adamglew]

Hi, I am trying to convert CFAST's optical density output to visibility.

Visibility, S
Optical Density, D
Light Extinction Coefficient, K
A non-dimensional constant characteristic of the type of object being viewed through the smoke, C

In a previous thread on the FDS Google Group, Jason clarified that FDS uses K rather than D. See below:
https://groups.google.com/g/fds-smv/c/feHHTKYyJOY

This would imply that S = C/K (Eq. 22.22 from the FDS User Guide) could simply be replaced with S = C/D.

However, the FDS Guide states, "The gas phase output quantity ’EXTINCTION COEFFICIENT’ is K. A similar quantity is the ’OPTICAL DENSITY’, D = K/2.3, the result of using log10 in the definition", i.e. K = 2.3D.

This would imply that S = C/K should be replaced with S = C/2.3D.

Has anyone noticed this, looked into the topic before, or have thoughts on which is correct?

Thanks in advance

[drjfloyd]

In that post I was referring to the nomenclature as used in specific paper being referenced by the original poster. That paper had:

I(L) = I(0) exp(-D_L L)

D_L as used in that paper was equivalent to K in the FDS documentation; i.e., D_L was not the optical density.

The end of your post is correct.
Opitical density, D, is 8700 rho_soot / ln(10)
Excintction coefficient, K, is 8700 rho_soot.
Visibility, S, is C/K or C/(2.3 D)

[drjfloyd]

Do not post material that you do not own the copyright to. If you want to reference copyrighted material by others provide a link, DOI, or other reference but not the actual publication. I have deleted your post.