FDS Source: Always make a random seed, and RND_SEED.

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -8714,6 +8714,8 @@ \subsection{Default Initial Velocity Field}
 
 FDS initializes the flow field with a very small amount of ``noise'' to prevent the development of a perfectly symmetric flow when the boundary and initial conditions are perfectly symmetric. To turn this off, set {\ct NOISE=F} To control the amount of noise, set {\ct NOISE\_VELOCITY}. Its default value is 0.005~m/s.
 
+FDS uses a predefined random seed for the random number generator which ensures that each mesh is seeded with a different number. A predefined seed means that all simulations will use the same sequence of random numbers for a given mesh. You can change the random seed by specifying {\ct RND\_SEED} on the {\ct MISC} line. This defines a constant value that is added to the predefined random seed on each mesh.
+
 \subsection{Turning off the Flow Field}
 \label{info:freeze_velocity}
 
@@ -12077,7 +12079,8 @@ \section{\texorpdfstring{{\tt MISC}}{MISC} (Miscellaneous Parameters)}
 {\ct RAMP\_GZ}                                      & Character    & Section~\ref{info:GVEC}                 &            &                   \\ \hline
 {\ct RESTART}                                       & Logical      & Section~\ref{info:restart}              &            & {\ct F}           \\ \hline
 {\ct RESTART\_CHID}                                 & Character    & Section~\ref{info:restart}              &            & {\ct CHID}        \\ \hline
-{\ct SC}                                            & Real         & Section~\ref{info:LES}                  &            & 0.5               \\ \hline
+{\ct RND\_SEED}                                     & Integer      & Section~\ref{info:restart}              &            &                   \\ \hline
+{\ct SC}                                            & Real         & Section~\ref{info:NOISE}                &            & 0.5               \\ \hline
 {\ct SHARED\_FILE\_SYSTEM}                          & Logical      & Section~\ref{info:multimesh}            &            & {\ct T}           \\ \hline
 {\ct SIMULATION\_MODE}                              & Character    & Section~\ref{Sim_Mode}                  &            & {\ct 'VLES'}      \\ \hline
 {\ct SMOKE3D\_16}                                   & Logical      & Section~\ref{info:smoke16}              &            & {\ct F}           \\ \hline

Source/cons.f90

@@ -141,6 +141,7 @@ MODULE GLOBAL_CONSTANTS
 INTEGER, PARAMETER :: OBST_CONE_TYPE=3                 !< Flag for OB\%SHAPE_TYPE
 INTEGER, PARAMETER :: OBST_BOX_TYPE=4                  !< Flag for OB\%SHAPE_TYPE
 
+
 INTEGER :: FUEL_INDEX=0                    !< Index for FUEL in SIMPLE_CHEMISTRY model
 INTEGER :: O2_INDEX=0                      !< Index for O2 in SIMPLE_CHEMISTRY model
 INTEGER :: N2_INDEX=0                      !< Index for N2 in SIMPLE_CHEMISTRY model
@@ -159,6 +160,8 @@ MODULE GLOBAL_CONSTANTS
 INTEGER :: STOP_STATUS=NO_STOP             !< Indicator of whether and why to stop the job
 INTEGER :: INPUT_FILE_LINE_NUMBER=0        !< Indicator of what line in the input file is being read
 
+INTEGER :: RND_SEED=0                      !< User RANDOM_SEED
+
 ! Miscellaneous logical constants
 
 LOGICAL :: HVAC_DEBUG=.FALSE.               !< Output known hvac values to smokeview

Source/init.f90

@@ -4881,14 +4881,16 @@ SUBROUTINE INITIAL_NOISE(NM)
 CALL RANDOM_SEED(SIZE=SIZE_RND)
 ALLOCATE(SEED_RND(SIZE_RND),STAT=IZERO)
 CALL CHKMEMERR('INITIAL_NOISE','SEED_RND',IZERO)
-SEED_RND = 2819 * 13*NM
+SEED_RND = 2819 * 13*NM + RND_SEED
 CALL RANDOM_SEED(PUT=SEED_RND)
 DEALLOCATE(SEED_RND)
 
 DO I=1,NM
    CALL RANDOM_NUMBER(RN2)
 ENDDO
 
+IF (.NOT. NOISE) RETURN
+
 ! Point to local mesh variables
 
 CALL POINT_TO_MESH(NM)

Source/main.f90

@@ -319,7 +319,7 @@ PROGRAM FDS
 
 DO NM=LOWER_MESH_INDEX,UPPER_MESH_INDEX
    IF (TGA_SURF_INDEX>0) CYCLE
-   IF (NOISE) CALL INITIAL_NOISE(NM)
+   CALL INITIAL_NOISE(NM)
    IF (PERIODIC_TEST==1) CALL NS_ANALYTICAL_SOLUTION(NM,T_BEGIN,RK_STAGE=2)
    IF (PERIODIC_TEST==2) CALL UVW_INIT(NM,UVW_FILE)
    IF (PERIODIC_TEST==3) CALL COMPRESSION_WAVE(NM,0._EB,3)