You signed in with another tab or window. Reload to refresh your session.You signed out in another tab or window. Reload to refresh your session.You switched accounts on another tab or window. Reload to refresh your session.Dismiss alert
Copy file name to clipboardExpand all lines: Manuals/FDS_User_Guide/FDS_User_Guide.tex
+3-1Lines changed: 3 additions & 1 deletion
Original file line number
Diff line number
Diff line change
@@ -9234,14 +9234,16 @@ \section{Accuracy of the Pressure Solver}
9234
9234
\subsection{Optional Pressure Solvers}
9235
9235
\label{optional_pressure_solver}
9236
9236
9237
-
The default Poisson solver in FDS ({\ct SOLVER='FFT'} on the {\ct PRES} line) is based on the package of linear algebra routines called CrayFishpak. However, in certain circumstances you may need to use one of several alternatives that is based on the Intel\textsuperscript{\textregistered} MKL Sparse Cluster Solver.
9237
+
The default Poisson solver in FDS ({\ct SOLVER='FFT'} on the {\ct PRES} line) is based on the package of linear algebra routines called CrayFishpak. However, in certain circumstances you may need to use one of several alternatives that is based on the Intel\textsuperscript{\textregistered} MKL Sparse Cluster Solver or the HYPRE solver library from Lawrence Livermore National Laboratories (LLNL) \cite{HYPRE}.
9238
9238
\begin{description}
9239
9239
\item[{\ct SOLVER='ULMAT'}] This solver is useful when the computational domain has relatively small, sealed cavities enclosed with thin (i.e. zero cell thick) walls. The small error in the normal component of velocity that you incur with the FFT solver can lead to unphysical changes in temperature, density and pressure that may eventually cause the calculation to become numerically unstable. For example, suppose you have hollow steel columns or hollow aluminum ducts with thin walls and you want to simulate the heat penetration within these spaces.
9240
9240
9241
9241
With the {\ct ULMAT} solver, the unknown values of the pressure live only in gas-phase cells, allowing for the normal components of velocity at a solid surface to be computed exactly with no penetration error. Strictly speaking, in this mode, FDS is no longer using an ``immersed boundary method'' for flow obstructions---the pressure solution is now ``unstructured,'' hence the {\ct U} in its name. This solver does not guarantee that the normal component of velocity matches perfectly at mesh boundaries, and the iterative procedure used by the default {\ct 'FFT'} solver is still used to drive the mesh interface velocity normals closer together. This solver option does allow for refinement at mesh boundaries and for internal sealed regions within the domain for which additional matrices must be computed.
9242
9242
9243
9243
The {\ct ULMAT} solver uses a substantial amount of memory per mesh. Its use should be limited to meshes of at most 150,000 cells. This number depends on the amount of random access memory (RAM) that you can devote to each MPI process, and you might want to experiment with larger meshes if you have, say, tens of gigabytes of RAM per MPI process.
9244
9244
9245
+
\item[{\ct SOLVER='ULMAT HYPRE'}] This solver uses the same strategy as ULMAT but the solver is implemented via LLNL's HYPRE library \cite{HYPRE}. ULMAT HYPRE uses the algebraic multigrid (AMG) preconditioned conjugate gradient (PCG) solver. This solver does not require LU decomposition and storage of a dense matrix, so the memory requirements are much less than the MKL solver. Hence, ULMAT HYPRE may be considered as an option for large mesh blocks (larger than 150,000 cells). Note, however, that for small mesh blocks HYPRE is somewhat slower than MKL.
9246
+
9245
9247
\item[{\ct SOLVER='GLMAT'}] This solver is for non-overlapping, non-stretched meshes at the same refinement level covering a single connected domain, i.e., a large rectangular box. With this solver, the pressure in both solid and gas cells is computed using an immersed boundary method for flow obstructions, the same as the default {\ct 'FFT'} solver. This mode produces the exact same pressure solution as the {\ct 'FFT'} solver would if the domain were one single mesh. That is, velocity errors at any mesh boundaries are eliminated. Note that currently a single discretization matrix is built for all gas cells defined on the model, therefore the solver is not meant to be used in cases where ther are internal sealed regions within the domain.
9246
9248
9247
9249
The {\ct GLMAT} solver is used mainly for testing and diagnostics. Because it solves the pressure over the entire computational domain, it is limited to about 1~million cells total unless your computer has an extraordinary amount of RAM.
0 commit comments