trixi-framework/Trixi.jl

Make initial conditions with random numbers invariant wrt the order of AMR/analysis callbacks

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#342 opened on Nov 17, 2020

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Description

Thus the order of AMR and analysis seems to matter - why, however, is currently beyond me. If the analysis happens before AMR, there are a total of 111 time steps for the test (with tspan=(0.0, 0.2)). If it happens afterwards, it increases to 114 time steps. Thus, somehow the analysis step affects the solution in such a way that a subsequent AMR procedure is affected. Does it have to do with the analysis callback executing rhs!? As I said: I don't get it. Any ideas, @ranocha?

The analysis callback computes "error" norms by comparing to the initial condition - in particular when the analysis callback is initialized. Similarly, the AMR callback will call the initial condition when it's initialized to perform initial AMR. Since the initial condition uses random numbers, a different order of these callbacks will of course lead to different results.

Possible adaptations allowing us to move the callbacks around while still keeping the current order of initial AMR and analysis are

  • to disable initial calculation of error norms (which would be weird, I think)
  • to allow calling an AMR callback directly on an ODE to perform initial refinement

Other than that, I still think it's often not really appropriate to rely on a fixed stream of random numbers. It might be better here to just check some statistical errors etc. - or to disable initial AMR and load the corresponding data from a file.

Originally posted by @ranocha in https://github.com/trixi-framework/Trixi.jl/issues/338#issuecomment-728699593

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Issue type
bug
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4
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clear
Prerequisites
JuliaAdaptive mesh refinementCallbacks in Trixi.jl
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the initialization order of AMR and analysis callbacks in Trixi.jl. Review the linked issue #338 for context. Examine how the analysis callback computes error norms and how the AMR callback calls initial conditions. Consider implementing one of the suggested solutions: disable initial error norm calculation or allow calling AMR callback directly on ODE to perform initial refinement. Ensure that random number seeding is consistent or use a deterministic alternative.