From fdb8336ada0abd169b2103eaf0d4be318e1d1700 Mon Sep 17 00:00:00 2001
From: "Aaron M. Lattanzi" <103702284+AMLattanzi@users.noreply.github.com>
Date: Mon, 9 Dec 2024 09:38:44 -0800
Subject: [PATCH] Base number of substeps on a theoretical max. (#2007)

* Base number of substeps on a theoretical max.

* Update based upon slack convo.

* Make failing tests use 6 substeps.
---
 Source/TimeIntegration/ERF_ComputeTimestep.cpp                | 4 +++-
 Tests/test_files/ABL_InflowFile/ABL_InflowFile.i              | 1 +
 .../MSF_Sub_IsentropicVortexAdv/MSF_Sub_IsentropicVortexAdv.i | 3 ++-
 Tests/test_files/TaylorGreenAdvecting/TaylorGreenAdvecting.i  | 2 +-
 .../TaylorGreenAdvectingDiffusing.i                           | 2 +-
 5 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/Source/TimeIntegration/ERF_ComputeTimestep.cpp b/Source/TimeIntegration/ERF_ComputeTimestep.cpp
index 28131b92a..96f37c1e4 100644
--- a/Source/TimeIntegration/ERF_ComputeTimestep.cpp
+++ b/Source/TimeIntegration/ERF_ComputeTimestep.cpp
@@ -201,7 +201,9 @@ ERF::estTimeStep (int level, long& dt_fast_ratio) const
          if (fixed_dt[level] > 0. && fixed_fast_dt[level] > 0.) {
              dt_fast_ratio = static_cast<long>( fixed_dt[level] / fixed_fast_dt[level] );
          } else if (fixed_dt[level] > 0.) {
-             dt_fast_ratio = static_cast<long>( 6 );
+             // Max CFL_c = 1.0 for substeps, but we enforce a min of 4 substeps
+             auto dt_sub_max = (estdt_comp/cfl);
+             dt_fast_ratio = static_cast<long>( std::max(fixed_dt[level]/dt_sub_max,4.) );
          }
 
          // Force time step ratio to be an even value
diff --git a/Tests/test_files/ABL_InflowFile/ABL_InflowFile.i b/Tests/test_files/ABL_InflowFile/ABL_InflowFile.i
index c75440cc7..4dbd8ea80 100644
--- a/Tests/test_files/ABL_InflowFile/ABL_InflowFile.i
+++ b/Tests/test_files/ABL_InflowFile/ABL_InflowFile.i
@@ -28,6 +28,7 @@ zhi.type = "SlipWall"
 
 # TIME STEP CONTROL
 erf.fixed_dt = 0.1  # fixed time step depending on grid resolution
+erf.fixed_mri_dt_ratio = 6
 
 # DIAGNOSTICS & VERBOSITY
 erf.sum_interval   = 1       # timesteps between computing mass
diff --git a/Tests/test_files/MSF_Sub_IsentropicVortexAdv/MSF_Sub_IsentropicVortexAdv.i b/Tests/test_files/MSF_Sub_IsentropicVortexAdv/MSF_Sub_IsentropicVortexAdv.i
index ce102ecca..46763bf72 100644
--- a/Tests/test_files/MSF_Sub_IsentropicVortexAdv/MSF_Sub_IsentropicVortexAdv.i
+++ b/Tests/test_files/MSF_Sub_IsentropicVortexAdv/MSF_Sub_IsentropicVortexAdv.i
@@ -20,7 +20,8 @@ zhi.type = "SlipWall"
 # TIME STEP CONTROL
 erf.substepping_type = Implicit
 erf.fixed_dt         = 0.0003
-
+erf.fixed_mri_dt_ratio = 6
+    
 # DIAGNOSTICS & VERBOSITY
 erf.sum_interval    = 1       # timesteps between computing mass
 erf.v               = 1       # verbosity in ERF.cpp
diff --git a/Tests/test_files/TaylorGreenAdvecting/TaylorGreenAdvecting.i b/Tests/test_files/TaylorGreenAdvecting/TaylorGreenAdvecting.i
index 49ba19cff..c9e553a21 100644
--- a/Tests/test_files/TaylorGreenAdvecting/TaylorGreenAdvecting.i
+++ b/Tests/test_files/TaylorGreenAdvecting/TaylorGreenAdvecting.i
@@ -15,7 +15,7 @@ zhi.type = "SlipWall"
 
 # TIME STEP CONTROL
 erf.fixed_dt           = 0.16    # fixed time step
-erf.mri_fixed_dt_ratio = 4
+erf.fixed_mri_dt_ratio = 6
 
 # DIAGNOSTICS & VERBOSITY
 erf.sum_interval   = 1       # timesteps between computing mass
diff --git a/Tests/test_files/TaylorGreenAdvectingDiffusing/TaylorGreenAdvectingDiffusing.i b/Tests/test_files/TaylorGreenAdvectingDiffusing/TaylorGreenAdvectingDiffusing.i
index fc44afc6b..81f64b4b7 100644
--- a/Tests/test_files/TaylorGreenAdvectingDiffusing/TaylorGreenAdvectingDiffusing.i
+++ b/Tests/test_files/TaylorGreenAdvectingDiffusing/TaylorGreenAdvectingDiffusing.i
@@ -15,7 +15,7 @@ zhi.type = "SlipWall"
 
 # TIME STEP CONTROL
 erf.fixed_dt           = .16    # fixed time step
-erf.mri_fixed_dt_ratio = 4
+erf.fixed_mri_dt_ratio = 6
 
 # DIAGNOSTICS & VERBOSITY
 erf.sum_interval   = 1       # timesteps between computing mass