diff options
| -rw-r--r-- | doc/documention.tex | 72 | ||||
| -rw-r--r-- | interface.ccl | 2 | ||||
| -rw-r--r-- | param.ccl | 15 | ||||
| -rw-r--r-- | schedule.ccl | 4 | ||||
| -rw-r--r-- | src/Courant.c | 8 |
5 files changed, 55 insertions, 46 deletions
diff --git a/doc/documention.tex b/doc/documention.tex index 5629beb..44038b7 100644 --- a/doc/documention.tex +++ b/doc/documention.tex @@ -13,51 +13,61 @@ This thorn provides a routines for calculating the timestep for an evolution based on the spatial Cartesian grid spacing and a wave speed. -\section{Comments} - -There are currently two methods for calculating the timestep, either the -simple courant method, or a dynamic courant method. The method is chosen -using the keyword parameter {\tt time::method} -The timestep, is passed into the Cactus variable {\tt cctk\_delta\_time}. -Both timesteps are based on the Courant condition, which states that for -numerical stability, the chosen timestep should satisfy -$$ -\Delta t \le \mbox{min}(\Delta x^i)/\mbox{wave speed}/\sqrt(\mbox{dim}) -$$ -The two methods currently implemented are: +\section{Description} + +Thorn {\tt Time} uses one of four methods to decide on the timestep +to be used for the simulation. The method is chosen using the +keyword parameter {\tt time::timestep\_method}. (Note: In releases Beta 8 and +earlier the parameter used was {\tt time::courant\_method} \begin{itemize} -\item {\tt time::method = "standard"} (this is the default). The timestep is calculated once - at the start of the run in the {\tt BASEGRID} timebin, and is then held static - throughout the run. The algorithm, which uses the parameter {\tt time::dtfac} is +\item{} {\tt time::timestep\_method = ``given''} The timestep is fixed to the + value of the parameter {\tt time::timestep}. + +\item{} {\tt time::timestep\_method = ``courant\_static''} This is the default + method, which calculates the timestep once at the start of the + simulation, based on a simple courant type condition using + the spatial gridsizes and the parameter {\tt time::dtfac}. $$ \Delta t = \mbox{\tt dtfac} * \mbox{min}(\Delta x^i) $$ - Note that the parameter {\tt dtfac} should take into account the dimension - of the space being used, and the wave speed. - -\item {\tt time::method = "courant"}. The timestep is calculated dynamically at the - start of each iteration in the {\tt PRESTEP} timebin. The algorithm is + Note that it is up to the user to custom {\tt dtfac} to take + into account the dimension of the space being used, and the wave speed. + +\item{} {\tt time::timestep\_method = ``courant\_speed''} This choice implements a + dynamic courant type condition, the timestep being set before each + timestep using the spatial dimension of the grid, the spatial grid sizes, the + parameter {\tt courant\_fac} and the grid variable {\tt courant\_wave\_speed}. + The algorithm used is $$ -\Delta t = \mbox{\tt courant\_fac} * \mbox{min}(\Delta x^i)/\mbox{maximum wavespeed}/\sqrt(\mbox{dim}) +\Delta t = \mbox{\tt courant\_fac} * \mbox{min}(\Delta x^i)/\mbox{courant\_wave\_speed}/\sqrt(\mbox{dim}) $$ + For this algorithm to be successful, the variable {\tt courant\_wave\_speed} + must have been set by a thorn to the maximum wave speed on the grid. - -\item {\tt time::method = "courant\_time"}. The timestep is calculated dynamically at the - start of each iteration in the {\tt PRESTEP} timebin. The algorithm is +\item{} {\tt time::timestep\_method = ``courant\_time''} This choice is similar to the + method {\tt courant\_speed} above, in implementing a dynamic timestep. + However the timestep is chosen using $$ -\Delta t = \mbox{\tt courant\_fac} * \mbox{minimum time to cross a zone}/\sqrt(\mbox{dim}) +\Delta t = \mbox{\tt courant\_fac} * \mbox{\tt courant\_min\_time}/\sqrt(\mbox{dim}) $$ + where the grid variable {\tt courant\_min\_time} must be set by a thorn to + the minimum time for a wave to cross a gridzone. + +\end{itemize} -\section{Technical Details} +In all cases, Thorn {\tt Time} sets the Cactus variable {\tt cctk\_delta\_time} +which is passed as part of the macro {\tt CCTK\_ARGUMENTS} to thorns called +by the scheduler. -If a dynamic {\tt courant} condition is selected, a thorn must set the protected variable -{\tt courant\_wave\_speed} for the maximum wave speed before Time sets the timestep. +Note that for hyperbolic problems, the Courant condition gives a minimum +requirement for stability, namely that the numerical domain of dependency +must encompass the physical domain of dependency, or +$$ +\Delta t \le \mbox{min}(\Delta x^i)/\mbox{wave speed}/\sqrt(\mbox{dim}) +$$ -If a dynamic {\tt courant\_time} condition is selected, a thorn must set the protected variable -{\tt courant\_time} for the minimum time for a wave to cross a zone - before Time sets the timestep. \end{itemize} diff --git a/interface.ccl b/interface.ccl index db7f042..6ea9cdc 100644 --- a/interface.ccl +++ b/interface.ccl @@ -8,7 +8,7 @@ protected: REAL speedvars type=SCALAR { courant_wave_speed - courant_time + courant_min_time } "Speed to use for Courant condition" private: @@ -3,19 +3,18 @@ restricted: -KEYWORD courant_method "Method for calculating timestep" +KEYWORD timestep_method "Method for calculating timestep" { - "none" :: "Use given timestep" - "standard" :: "Courant condition at BASEGRID" - "courant" :: "Courant condition at PRESTEP (using wavespeed)" + "given" :: "Use given timestep" + "courant_static" :: "Courant condition at BASEGRID" + "courant_speed" :: "Courant condition at PRESTEP (using wavespeed)" "courant_time" :: "Courant condition at PRESTEP (using min time)" -} "standard" +} "courant_static" -BOOLEAN courant_outonly "Only output courant timestep?" +BOOLEAN timestep_outonly "Don't set a dynamic timestep, just output what it would be" { } "no" - private: REAL timestep "Absolute value for timestep" @@ -33,7 +32,7 @@ REAL courant_fac "The courant timestep condition dt = courant_fac*max(delta_spac 0:* :: "Probably only makes sense to be bigger than zero" } 0.9 -INT outcourant_every "How often to output courant timestep" +INT outtimestep_every "How often to output courant timestep" { 0:* :: "Zero means no output" } 0
\ No newline at end of file diff --git a/schedule.ccl b/schedule.ccl index 2399f7f..343bede 100644 --- a/schedule.ccl +++ b/schedule.ccl @@ -1,14 +1,14 @@ # Schedule definitions for thorn Time # $Header$ -if (CCTK_Equals(courant_method,"standard")) +if (CCTK_Equals(timestep_method,"courant_static")) { schedule Time_Simple at CCTK_BASEGRID after SpatialCoordinates { LANG: C } "Set timestep based on Courant condition" } -else if (CCTK_Equals(courant_method,"courant") || CCTK_Equals(courant_method,"courant_time")) +else if (CCTK_Equals(timestep_method,"courant_speed") || CCTK_Equals(timestep_method,"courant_time")) { schedule Time_Simple at CCTK_BASEGRID after SpatialCoordinates { diff --git a/src/Courant.c b/src/Courant.c index 81cfa60..eb778d3 100644 --- a/src/Courant.c +++ b/src/Courant.c @@ -47,18 +47,18 @@ void Time_Courant(CCTK_ARGUMENTS) } /* Calculate the courant timestep */ - if (CCTK_Equals(courant_method,"courant_time")) + if (CCTK_Equals(timestep_method,"courant_time")) { - *courant_dt = courant_fac*(*courant_time)/sqrt((CCTK_REAL )cctk_dim); + *courant_dt = courant_fac*(*courant_min_time)/sqrt((CCTK_REAL )cctk_dim); } - else if (CCTK_Equals(courant_method,"courant")) + else if (CCTK_Equals(timestep_method,"courant_speed")) { *courant_dt = courant_fac/(*courant_wave_speed)/sqrt((CCTK_REAL )cctk_dim); } /* Set the Cactus timestep */ - if (! courant_outonly) + if (! timestep_outonly) { cctkGH->cctk_delta_time = *courant_dt; |