From fbc58ec26d63333991e458ec66cf86c2cc8992ff Mon Sep 17 00:00:00 2001
From: "Marc T. Henry de Frahan" <marc.henrydefrahan@nrel.gov>
Date: Mon, 31 Jan 2022 15:51:16 -0700
Subject: [PATCH] Fix FPE when CFLs are zero (#558)

* Fix FPE when CFLs are zero

* Abort if using adaptive time stepping and the CFL is small
---
 amr-wind/core/SimTime.cpp | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/amr-wind/core/SimTime.cpp b/amr-wind/core/SimTime.cpp
index 3cf41ef838..a38827c19c 100644
--- a/amr-wind/core/SimTime.cpp
+++ b/amr-wind/core/SimTime.cpp
@@ -95,7 +95,15 @@ void SimTime::set_current_cfl(
     const amrex::Real cd_cfl = conv_cfl + diff_cfl;
     const amrex::Real cfl_unit_time =
         cd_cfl + std::sqrt(cd_cfl * cd_cfl + 4.0 * src_cfl);
-    amrex::Real dt_new = 2.0 * m_max_cfl / cfl_unit_time;
+    if ((m_adaptive) &&
+        (cfl_unit_time < std::numeric_limits<amrex::Real>::epsilon())) {
+        amrex::Abort(
+            "CFL is below machine epsilon and the time step is adaptive. "
+            "Please use a fixed time step or fix the case setup");
+    }
+    amrex::Real dt_new =
+        2.0 * m_max_cfl /
+        amrex::max(cfl_unit_time, std::numeric_limits<amrex::Real>::epsilon());
 
     // Restrict timestep during initialization phase
     if (m_is_init) {