Merge pull request #14349 from rmcdermo/master

FDS User Guide: update guidance on mesh interface errors and clarify default ignition delay

Author: rmcdermo

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -1056,7 +1056,7 @@ \subsection{Mesh Alignment}
 
 The following rules of thumb should also be followed when setting up a multiple mesh calculation:
 \begin{itemize}
-\item Avoid putting mesh boundaries where critical action is expected, especially fire. Sometimes fire spread from mesh to mesh cannot be avoided, but if at all possible try to keep mesh interfaces relatively free of complicated phenomena since the exchange of information across mesh boundaries is not yet as accurate as cell to cell exchanges within one mesh.
+\item When running multi-mesh calculations you need to closely monitor \ct{VELOCITY_ERROR} at mesh interfaces, which is reported in the \ct{CHID.out} file.  Check that you are not consistently hitting the\\\ct{MAX_PRESSURE_ITERATIONS}, which defaults to 10.  If you are, then this indicates a potential problem and you may consider increasing this value to 20 or even 100 in some cases.  Velocity errors are proportional to mass loss errors.  If your errors are too large for your application, you may need to tighten the \ct{VELOCITY_TOLERANCE} or else consider using a different pressure solver, see Sec.~\ref{pressure_solver}.
 \item In general, there is little advantage to overlapping meshes because information is only exchanged at exterior boundaries. This means that a mesh that is completely embedded within another receives information at its exterior boundary, but the larger mesh receives no information from the mesh embedded within. Essentially, the larger, usually coarser, mesh is doing its own simulation of the scenario and is not affected by the smaller, usually finer, mesh embedded within it. Details within the fine mesh, especially related to fire growth and spread, may not be picked up by the coarse mesh. In such cases, it is preferable to isolate the detailed fire behavior within one mesh, and position coarser meshes at the exterior boundary of the fine mesh. Then the fine and coarse meshes mutually exchange information.
 \item Be careful when using the shortcut convention of declaring an entire face of the domain to be an \ct{OPEN} vent. Every mesh takes on this attribute. See Sec.~\ref{info:VENT} for more details.
 \item If a planar obstruction is close to where two meshes abut, make sure that each mesh ``sees'' the obstruction. If the obstruction is even a millimeter outside of one of the meshes, that mesh does not account for it, in which case information is not transferred properly between meshes.
@@ -1072,7 +1072,7 @@ \subsection{Mesh Alignment}
 
 \subsubsection{Accuracy of the Multiple Mesh Calculation}
 
-Experiment with different mesh configurations using relatively coarse mesh cells to ensure that information is being transferred properly from mesh to mesh. There are two issues of concern. First, does it appear that the flow is being badly affected by the mesh boundary? If so, try to move the mesh boundaries away from areas of activity. Second, is there too much of a jump in cell size from one mesh to another? If so, consider whether the loss of information moving from a fine to a coarse mesh is tolerable.
+Experiment with different mesh configurations using relatively coarse mesh cells to ensure that information is being transferred properly from mesh to mesh. There are two issues of concern. First, does it appear that the flow is being badly affected by the mesh boundary? If so, check your velocity errors and consider tightening your velocity tolerance or chaning pressure solvers, see Sec.~\ref{pressure_solver}. Second, is there too much of a jump in cell size from one mesh to another? If so, consider whether the loss of information moving from a fine to a coarse mesh is tolerable.  Mesh refinement ratios of more than 4:1 to should be avoided if possible.
 
 
 
@@ -4746,7 +4746,7 @@ \chapter{Combustion}
 \label{info:REAC}
 \label{info:COMB}
 
-Combustion can be modeled in two ways. By default, the reaction of fuel and oxygen is infinitely fast and controlled only by mixing, hence the label {\em mixing-controlled}. The alternative is that the reaction is {\em finite-rate}. The latter approach usually requires very fine grid resolution that is not practical for large-scale fire applications. This chapter describes both methods, with an emphasis on the more commonly used mixing-controlled model.
+Combustion can be modeled in two ways. By default, the reaction of fuel and oxygen is infinitely fast and controlled only by mixing with zero ignition delay---you will often see this model described as ``mixed is burnt''. The alternative is that the reaction is {\em finite-rate}. The latter approach usually requires very fine grid resolution that is not practical for large-scale fire applications. This chapter describes both methods, with an emphasis on the more commonly used mixing-controlled model.
 
 There are two groups of parameters that govern combustion. The first, called the \ct{COMB} namelist group, contains parameters that pertain to any and all reactions. Specific parameters about a particular reaction are specified using the \ct{REAC} namelist group. There can only be one \ct{COMB} line, but multiple \ct{REAC} lines if there are multiple reactions. If you are modeling a fire, you {\em must} specify the fuel and basic stoichiometry using a \ct{REAC} line. You need not specify a \ct{COMB} line unless you want to modify mainly numerical parameters.