FDS User Guide: Explain HT3D limitations

Author: mcgratta

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -2534,7 +2534,7 @@ \subsection{Limitations}
 \item Avoid contact between 3-D and 1-D solids. If two sides of a 3-D solid touch 1-D solids, there will be no lateral heat conduction computed in that particular direction.
 \item If your 3-D obstruction extends beyond meshes that abut, FDS uses a special algorithm to identify all the meshes where this obstruction lives, and also those obstructions connected to it. However, if this algorithm fails to detect all the meshes and you recieve an error stating that there is a problem, add the parameter \ct{NEIGHBOR_SEPARATION_DISTANCE} to the \ct{MISC} line. Any mesh within this distance of another mesh will share geometry information for use in the 3-D heat conduction calculation. If you set this parameter to a value larger than the width of the computational domain, then all meshes will establish communication channels for exchanging boundary information.
 \item By default, the interior nodes are clustered near the surface and stretched out deeper within the solid. If you want to maintain uniform spacing, set \ct{CELL_SIZE} on the \ct{SURF} or \ct{OBST} line to indicate the desired interior node spacing. The \ct{CELL_SIZE} is typically chosen to be comparable to the gas phase cells. If the obstruction is thin; that is, less than one gas phase cell thick, the specified \ct{CELL_SIZE} will only apply to the heat conduction in the transverse, not normal, direction. The normal direction gridding will be controlled by the parameters \ct{STRETCH_FACTOR} and \ct{CELL_SIZE_FACTOR}. This may be useful in cases where the specified \ct{CELL_SIZE} is too coarse to resolve variations in surface definition along the normal dimension (see Section~\ref{checkerboard} for an example).
-\item \ct{HT3D} cannot be applied to an \ct{OBST} that is to \ct{BURN_AWAY}.
+\item \ct{HT3D} cannot be applied to an \ct{OBST} that is to \ct{BURN_AWAY}. In addition, if the solid undergoes significant shrinking or swelling, do not use \ct{HT3D}. ``Significant'' means that the number of internal cells changes, in which case the 3-D nodal structure breaks down. To determine if this happens, use the \ct{PROF}ile output feature (Sec.~\ref{info:PROF}) to visualize profiles of internal temperature or other solid phase quantities. These output files contain the internal node coordinates as a function of time.
 \item If a \ct{SURF} line specifies either \ct{HT3D=T} or \ct{VARIABLE_THICKNESS=T} and also specifies an \ct{HRRPUA}, \ct{MLPUA}, or \ct{MASS_FLUX}, you must specify a \ct{MATL_ID} on the \ct{SURF} line and the appropriate \ct{OBST} lines. The reason for doubly specifying the \ct{MATL_ID} is so that the \ct{SURF} line can be set up properly. Note that the specification of \ct{HRRPUA} or similar on the surface of a 3-D or variably thick solid means that no obstruction making up the solid can have specified internal reactions, i.e. pyrolysis.
 \end{enumerate}