From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Wed, 14 Aug 2024 16:08:11 +0000 (-0600)
Subject: Add a note to step-86.
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ae8739d0671acf34b4c7e317b02a21e4544bd972;p=dealii.git

Add a note to step-86.
---

diff --git a/examples/step-86/doc/results.dox b/examples/step-86/doc/results.dox
index d578a0c659..b7b57f6f00 100644
--- a/examples/step-86/doc/results.dox
+++ b/examples/step-86/doc/results.dox
@@ -234,7 +234,14 @@ ourselves to blame for that by setting
 @code
       set match final time        = false
 @endcode
-in the input file.
+in the input file. If hitting the end time exactly is important to us,
+setting the flag to `true` resolves this issue.
+
+We can even reason why PETSc eventually chooses a time step of around
+0.025: The boundary values undergo a complete cosine cycle within 0.5
+time units; we should expect that it takes around ten or twenty time
+steps to resolve each period of a cycle to reasonable accuracy, and
+this leads to the time step choice PETSc finds.
 
 Not all combinations of methods, time step adaptation algorithms, and
 other parameters are valid, but the main messages from the experiment