BACKING=’EXPOSED’ when OBS thickness is very high than SURF thickness

[Ranhill]

Hi,

What does this exactly means (taken from FDS user guide): If a SURF with BACKING=’EXPOSED’ is applied to a solid with a dimension that is much greater than the specified THICKNESS, FDS will then ignore BACKING=’EXPOSED’ and apply BACKING=’VOID’ instead? I guess the thickness here refers to the solid obstruction on which the surface is applied. So, if I have a solid obstruction of thickness of say 0.1m (my mesh is also 0.1m in size) and I apply the surface of thickenss say 0.01m on all the sides of the obstruction with BACKING=’EXPOSED’, then will FDS do the calculattion as void on the other side? Thanks!

[mcgratta]

Yes. BACKING='EXPOSED' is the default boundary condition, but in your example, it does not make sense.

[rmcdermo]

Now I'm confused too. I thought that with all the HT3D machinery in place, we now are doing EXPOSED whenever it is asked for without the one cell restriction?

[mcgratta]

When the user says VARIABLE_THICKNESS=T, FDS automatically regrids the interior of the solid appropriately. But in the case above, the user has specified a THICKNESS.

[Ranhill]

Thanks, all! I will just simplify my question. If my OBS is 0.1m thick and I apply some surface of say 0.001 m thick on the both sides of the surface (-x and +x direction), then fill FDS treat the heat conduction in x-direction with exposed backing? When will it automatically changed into void backing based on the thickness of the obstruction?

[mcgratta]

I will tell you what I think will happen and then you should test it. It appears that you have a grid with 0.1 m grid cells, and you want to treat an obstruction that is one grid cell thick as an obstruction that is actually, physically 0.001 m thick. If this is correct, FDS will treat this as a 0.001 thick obstruction, exposed on both sides to its surroundings. The reason is that when determining whether to apply exposed or void backings, FDS will round up the desired thickness (0.,001 m) to the nearest grid cell size (0.1 m). If, however, you made the obstruction 2 cells thick, FDS would apply the void condition.

[Ranhill]

Thanks, I will try that!