1 The Infrastructure

The CoordGauge implementation schedules five groups:

CoordGauge
LapseSelect IN CoordGauge BEFORE PickCoordGauge
LapseApply  IN CoordGauge AFTER  PickCoordGauge
ShiftSelect IN CoordGauge BEFORE PickCoordGauge
ShiftApply  IN CoordGauge AFTER  PickCoordGauge

and one function

PickCoordGauge IN CoordGauge

and has two public grid scalars

selected_lapse
selected_shift

and two string parameters

lapse_list
shift_list

It also provides four aliased functions

int CoordGauge_RegisterLapse("lapse-name")
int CoordGauge_RegisterShift("shift-name")
CoordGauge_Lapse("lapse-name")
CoordGauge_Shift("shift-name")

(If someone can think of better names, please say so 8-)

Then each thorn which wants to apply a coordinate gauge condition registers itself, receiving a unique integer as an id, and schedules a selection routine and an application routine in the appropriate schedule groups.

The selection routine decides if this gauge condition should be applied at this time, and calls the CoordGauge_Lapse/Shift aliased function. (It should check that it is actually in the appropriate parameter as a minimum.)

The PickCoordGauge function traverses the list of lapses/shifts and selects the first one in the list which has called the CoordGauge_Lapse/Shift aliased function and sets the appropriate grid scalar to the id of this one.

The application routine checks to see if the grid scalar is set to its id, and if so, applies the gauge condition.

Evolution thorns could schedule CoordGauge at the appropriate point or points in their schedule.

An advantage of this scheme over the current one is that it provides the selection routines with a full set of variables from which to decide whether they should apply a guage or not. So it becomes very easy to choose to switch off maximal if the lapse has collapsed within a certain volume, etc.

This is simpler than the previous scheme as there is no arbitrary ’bid’ floating around. It also allows us to keep the logic of the final selection in one place, thus allowing people to override this logic if they need to.

2 Current Status

As of summer 2004, many thorns don’t use the above mechanism, instead they directly extend
ADMBase::lapse_evolution_method and/or ADMBase::shift_evolution_method. That is, the thorn implementing a coordinate condition thorn says in its param.ccl:

EXTENDS KEYWORD lapse_evolution_method "Which lapse condition to use"
{
"super-duper" :: "my super-duper new lapse condition"
} ""

(and/or the equivalent for shift_evolution_method).

The thorn then schedules a routine in some suitable schedule bin/group (probably CCTK_PRESTEP or MOL_PRESTEP) to check ADMBase::lapse_evolution_method and/or ADMBase::shift_evolution_method, and if they’re equal to the appropriate string, it does the coordinate condition.

3 Parameters

4 Interfaces

General

Implements:

coordgauge

Grid Variables

4.0.1 PUBLIC GROUPS

Adds header:

Slicing.h

5 Schedule

This section lists all the variables which are assigned storage by thorn EinsteinBase/CoordGauge. Storage can either last for the duration of the run (Always means that if this thorn is activated storage will be assigned, Conditional means that if this thorn is activated storage will be assigned for the duration of the run if some condition is met), or can be turned on for the duration of a schedule function.

Storage

Scheduled Functions

CCTK_WRAGH

  einstein_activateslicing

  initialize slicing, setup priorities for mixed slicings

CCTK_WRAGH

  einstein_setnextslicing

  identify the slicing for the next iteration

CCTK_PRESTEP

  einstein_setnextslicing

  identify the slicing for the next iteration

CCTK_PRESTEP (conditional)

  coordgauge

  coordinate gauge group

CoordGauge (conditional)

  lapseselect

  lapse selection group

CoordGauge (conditional)

  lapseapply

  lapse application group

CoordGauge (conditional)

  shiftselect

  shift selection group

CoordGauge (conditional)

  shiftapply

  shift application group