Merge pull request #16271 from mcgratta/master

FDS User Guide: Clarify reference

Author: mcgratta

Manuals/FDS_User_Guide/FDS_User_Guide.tex

@@ -1727,7 +1727,7 @@ \subsubsection{Open Vents}
 
 The first special \ct{VENT} is invoked by the parameter \ct{SURF_ID='OPEN'}. This \ct{VENT} denotes a passive opening to the outside, and it can only be applied to the exterior boundary of the computational domain. By default, FDS assumes that the exterior boundary of the computational domain (the \ct{XB}s on the \ct{MESH} line where they are not adjacent to another \ct{MESH}) is a solid wall. To create a totally or partially open domain, use \ct{OPEN} vents on the exterior mesh boundaries. It is sometimes convenient to specify doors or windows that open out to the exterior of the computational domain by simply specifying it to be \ct{OPEN}. However, keep in mind that the pressure boundary condition on such an opening is imperfect, and it is recommended that if the flow through the doorway or window is important, you should extend the domain a few meters beyond the doorway or window. You would still have to use the \ct{OPEN} boundary on the extended region to open up one or more sides of the computational domain, but these openings would be far enough away from the modeled door or window that they would not affect the flow pattern.
 
-By default, it is assumed that ambient conditions exist beyond the \ct{'OPEN'} vent. However, in some cases, you may want to alter this assumption, for example, the temperature. If you assume a temperature other than ambient, specify \ct{TMP_EXTERIOR} along with \ct{SURF_ID='OPEN'}. You can modify the time history of this parameter using a ramp function, \ct{TMP_EXTERIOR_RAMP} (see Section~\ref{info:RAMP_Time} for details about time/temperature ramps). Use this option cautiously -- in many situations if you want to describe the exterior of a building, it is better to include the exterior explicitly in your calculation because the flow in and out of the doors and windows will be more naturally captured. See Sec.~\ref{info:stackeffect} for more details. If you want to specify a non-ambient pressure at the \ct{OPEN} boundary, see Sec.~\ref{info:pressure_boundary}.
+By default, it is assumed that ambient conditions exist beyond the \ct{'OPEN'} vent. However, in some cases, you may want to alter this assumption, for example, the temperature. If you assume a temperature other than ambient, specify \ct{TMP_EXTERIOR} along with \ct{SURF_ID='OPEN'}. You can modify the time history of this parameter using a ramp function, \ct{TMP_EXTERIOR_RAMP} (see Eq.~(\ref{temp_time_ramp}) as an example). Use this option cautiously -- in many situations if you want to describe the exterior of a building, it is better to include the exterior explicitly in your calculation because the flow in and out of the doors and windows will be more naturally captured. See Sec.~\ref{info:stackeffect} for more details. If you want to specify a non-ambient pressure at the \ct{OPEN} boundary, see Sec.~\ref{info:pressure_boundary}.
 
 The \ct{OPEN} pressure boundary condition is most stable for flows that are predominantly normal  to the vent, either mostly in or mostly out.  This is because the prescribed pressure at an \ct{OPEN} boundary is ill-conditioned (a small perturbation to the input may lead to large change in the output) if the flow is parallel to the vent.  Suppose, for example, that an outdoor flow is 10 m/s in the $x$ direction and $\pm 0.001$ m/s in the $z$ direction with an \ct{OPEN} top boundary.  The kinetic energy of this flow is roughly $k=50$ \unit{m^2/s^2}.  When the vertical velocity is positive (+0.001 m/s) then the prescribed boundary condition for the stagnation pressure is set to $\cH = k = 50$ \unit{m^2/s^2}.  But when the vertical velocity is negative (-0.001 m/s) then $\cH = 0$ (see \cite{FDS_Tech_Guide}).  For this reason, \ct{OPEN} vents should be used with care in outdoor applications.  See Section \ref{info:WIND} for an alternative approach.
 
@@ -4317,12 +4317,16 @@ \chapter{User-Specified Functions}
 \section{Time-Dependent Functions}
 \label{info:RAMP_Time}
 
-At the start of any calculation, the temperature is ambient everywhere, the flow velocity is zero everywhere, nothing is burning, and the mass fractions of all species are uniform. You can control the rate at which things turn on, or turn off, by specifying time histories either with pre-defined functions or with user-defined functions.  The parameters \ct{TAU_Q}, \ct{TAU_T}, and \ct{TAU_V} indicate that the heat release rate (\ct{HRRPUA}); surface temperature (\ct{TMP_FRONT}); and/or normal velocity (\ct{VEL}, \ct{VOLUME_FLOW}), or \ct{MASS_FLUX_TOTAL} are to ramp up to their prescribed values as follows, using the heat release rate, $\dot{Q}(t)$, as an example:
+At the start of any simulation, the temperature is ambient everywhere, the flow velocity is zero everywhere, nothing is burning, and the mass fractions of all species are uniform. You can control various time histories either with pre-defined functions or with user-defined functions, referred to as ``time ramps.'' For example, the heat release rate, $\dot{Q}(t)$, can be ramped up from zero as follows:
 \begin{equation}
     \dot Q(t) = \left\{ \begin{array}{ll} \dot Q_0 \, \left( t/\tau \right)^2      &  (\tau < 0) \\[.1in]
-        \dot Q_0 \, \tanh \left( t/\tau \right)  &  (\tau > 0) \end{array} \right.
+                                          \dot Q_0 \, \tanh \left( t/\tau \right)  &  (\tau > 0) \end{array} \right.
 \end{equation}
-$\dot Q_0$ is the user-specified heat release rate. If the fire ramps up following a $t$-squared curve, then it remains constant after \ct{TAU_Q} seconds.  These rules apply to \ct{TAU_T} and \ct{TAU_V} as well. The default value for all \ct{TAU}s is 1~s\footnote{You can change the default ramp-up time by setting \ct{TAU_DEFAULT} on the \ct{MISC} line.}.  If something other than a tanh or $t$-squared ramp up is desired, then a user-defined function can be input. To do this, set \ct{RAMP_Q}, \ct{RAMP_T} or \ct{RAMP_V} equal to a character string designating the ramp function to use for that particular surface type, then somewhere in the input file generate lines of the form:
+where $\dot{Q}_0$ is the user-specified heat release rate and $\tau$ is a time-scale with a default value of 1~s that can be specified via \ct{TAU_Q} on the \ct{SURF} line. The parameter \ct{TAU_T} controls the surface temperature (\ct{TMP_FRONT}) and \ct{TAU_V} controls the normal velocity (\ct{VEL}), volume flow (\ct{VOLUME_FLOW}), or total mass flux (\ct{MASS_FLUX_TOTAL}).
+
+If the fire ramps up following a $t$-squared curve, then it remains constant after |\ct{TAU_Q}| seconds.  These rules apply to \ct{TAU_T} and \ct{TAU_V} as well. The default value for all \ct{TAU}s is 1~s\footnote{You can change the default ramp-up time by setting \ct{TAU_DEFAULT} on the \ct{MISC} line.}.  
+
+If something other than a tanh or $t$-squared ramp up is desired, then a user-defined function can be input. To do this, set \ct{RAMP_Q}, \ct{RAMP_T} or \ct{RAMP_V} equal to a character string designating the ramp function to use for that particular surface type, then somewhere in the input file generate lines of the form:
 \begin{lstlisting}
     &RAMP ID='rampname1', T= 0.0, F=0.0 /
     &RAMP ID='rampname1', T= 5.0, F=0.5 /
@@ -4343,7 +4347,7 @@ \section{Time-Dependent Functions}
 \end{lstlisting}
 Use \ct{TAU_T} or \ct{RAMP_T} to control the ramp-ups for surface temperature. The surface temperature at time $t$, $T(t)$, is
 \be
-T(t) = T_0 + f(t) \left( T_{\rm f} - T_0 \right)
+T(t) = T_0 + f(t) \left( T_{\rm f} - T_0 \right)  \label{temp_time_ramp}
 \ee
 where $f(t)$ is the result of evaluating the \ct{RAMP_T} at time $t$, $T_0$ is the ambient temperature, and $T_{\rm f}$ is specified via \ct{TMP_FRONT} on the same \ct{SURF} line as \ct{RAMP_T}. Use \ct{TAU_MF(N)} or \ct{RAMP_MF(N)} to control the ramp-ups for either the mass fraction or mass flux of species \ct{N}. For example:
 \begin{lstlisting}