[ET Trac] [Einstein Toolkit] #1656: CarpetInterp and MoL do not work properly together

[Einstein Toolkit]

#1656: CarpetInterp and MoL do not work properly together
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 Reporter:  hinder  |       Owner:  eschnett           
     Type:  defect  |      Status:  new                
 Priority:  major   |   Milestone:                     
Component:  Carpet  |     Version:  development version
 Keywords:          |  
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 I am trying to integrate interpolated quantities using MoL.  I call the
 interpolator in MoL_CalcRHS, and want it to perform only a spatial
 interpolation of the current content of timelevel 0.  This current content
 is what has been set by MoL; it is not the final value that will be at
 t_{n+1}  or was at t_N, but is the intermediate value that should be used
 when computing the RHS at a given MoL substep.  Since MoL does not set the
 Carpet time hierarchy values, CarpetInterp seems to get confused about how
 to do the interpolation.  I need to tell CarpetInterp not to interpolate
 in time at all, but to use timelevel 0 only.  Unfortunately, setting the
 interpolator option to use only one timelevel does not work.  There is
 commented-out code to "use cctk_time to decide whether to interpolate",
 which essentially guarantees that no time interpolation will happen (since
 the interpolation is requested for cctk_time, and the current time is
 cctk_time, so they are always equal).  Re-enabling this code allowed me to
 achieve 4th order convergence for integrated interpolated quantities,
 though I am not sure I understand everything that is going on in
 CarpetInterp.  I have added a parameter which controls this, and with this
 parameter set to the default, nothing changes (i.e. it is not going to
 change anyone's results unless they set the parameter).  I would like to
 commit this, so that collaborators can work off the same version.  Since
 the patch is very small, I hope this will not add too much unneeded
 complexity.  Is it OK to commit?  Patch is attached.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/1656>
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