FDS User Guide: add comment about using ULMAT HYPRE for cases with large mesh blocks

Author: rmcdermo

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -9282,7 +9282,7 @@ \subsection{Optional Pressure Solvers}
         \item[$^3$] ``Velocity @ Mesh'' refers to errors in the normal component of velocity at mesh boundaries, so-called ``interpolated'' boundaries in FDS.  For inexact solvers, this error may be controlled using the parameter {\ct VELOCITY\_TOLERANCE}.  Mass conservation errors are proportional to the velocity tolerance.  Solvers listed as ``exact'' are \emph{global} solvers.  ``Global'' refers to a global matrix solution, one matrix for the whole domain, as opposed to one matrix per mesh.  Thus, there are no mesh boundary errors for this type of solver.
         \item[$^4$] ``Velocity @ Solid'' refers to errors in the normal component of velocity at solid boundaries, so-called ``penetration'' errors that are present in \emph{immersed boundary methods} and lead to mass conservation errors.  These errors are particularly important to control for tightly sealed pressure zones. For inexact solvers, this error may be controlled using the parameter {\ct VELOCITY\_TOLERANCE}. Mass conservation errors are proportional to the velocity tolerance.  Solvers listed as ``exact'' are \emph{unstructured} solvers.  ``Unstructured'' means the solid boundaries (e.g., {\ct OBST} boundaries) are treated as domain boundaries for the pressure Poisson equation.  Unstructured solvers are the opposite of \emph{immersed boundary methods}.
         \item[$^5$] ``Inseparability'' refers to handling the error compared to the solution of the \emph{inseparable} Poisson equation incurred by lagging the pressure in the baroclinic term.  This error may be controlled using the parameter {\ct PRESSURE\_TOLERANCE}.
-        \item[$^6$] ULMAT HYPRE uses the HYPRE library developed at Lawrence Livermore National Laboratories (LLNL).  Use this variant of ULMAT if the Intel MKL library is not available.
+        \item[$^6$] ULMAT HYPRE uses the HYPRE library developed at Lawrence Livermore National Laboratories (LLNL).  Use this variant of ULMAT if the Intel MKL library is not available \emph{or} if MKL runs out of memory or is too slow due to memory issues with large mesh blocks.  Our implementation of HYPRE uses an algebraic multigrid (AMG) preconditioned conjugate gradient (PCG) method, which does not require excessive amounts of memory.
         % \item[$^7$] PFFT stands for Parallel Fast Fourier Transform.  This solver is only possible for block structured domains.  The domain decomposition for the pressure solver differs from the transport solver.  It uses pencil-shaped domains that span each direction and thus perform FFT solves in each direction for the whole domain.  Data is transferred in each stage of the solution to the orthogonal directions.  This solver is \emph{global} but \emph{structured}.  So, mesh to mesh errors are eliminated but immersed boundary errors are not.
     \end{itemize}
 \end{tablenotes}