## CoordGauge

Date

### Abstract

This thorn provides an infrastructure for dynamic gauge selection. However, at present (summer 2004) many thorns don’t use this infrastructure.

### 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

 slicing_verbose Scope: private KEYWORD Description: Print information on current slicing Range Default: no yes print slicing info no do not print slicing info

 lapse_list Scope: restricted STRING Description: List of allowed lapses to use Range Default: (none) .* Any string

 shift_list Scope: restricted STRING Description: List of allowed shifts to use Range Default: (none) .* Any string

 lapse_evolution_method Scope: shared from ADMBASE KEYWORD Extends ranges: coordgauge Use dynamic stuff from CoordGauge

 shift_evolution_method Scope: shared from ADMBASE KEYWORD Extends ranges: coordgauge Use dynamic stuff from CoordGauge

### 4 Interfaces

Implements:

coordgauge

#### Grid Variables

##### 4.0.1 PUBLIC GROUPS
 Group Names Variable Names Details slicing_flags compact 0 active_slicing_handle description Slicing and shift stuff dimensions 0 distribution CONSTANT group type SCALAR timelevels 1 variable type INT

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

 Always: slicing_flags

#### Scheduled Functions

CCTK_WRAGH

einstein_activateslicing

initialize slicing, setup priorities for mixed slicings

 Language: c Type: function

CCTK_WRAGH

einstein_setnextslicing

identify the slicing for the next iteration

 After: einstein_activateslicing Language: c Type: function

CCTK_PRESTEP

einstein_setnextslicing

identify the slicing for the next iteration

 Language: c Type: function

CCTK_PRESTEP (conditional)

coordgauge

coordinate gauge group

 Type: group

CoordGauge (conditional)

lapseselect

lapse selection group

 Before: pickcoordgauge Type: group

CoordGauge (conditional)

lapseapply

lapse application group

 After: pickcoordgauge Type: group

CoordGauge (conditional)

shiftselect

shift selection group

 Before: pickcoordgauge Type: group

CoordGauge (conditional)

shiftapply

shift application group

 After: pickcoordgauge Type: group