From f6ef00c41a6a15cc19187430756e9321a2ecf0cd Mon Sep 17 00:00:00 2001
From: Hrvoje Jasak <h.jasak@wikki.co.uk>
Date: Thu, 17 Mar 2016 10:13:30 +0000
Subject: [PATCH] Bugfix: Round-off stabilisation in sqrt

---
 src/ODE/sixDOF/finiteRotation/finiteRotation.C | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/ODE/sixDOF/finiteRotation/finiteRotation.C b/src/ODE/sixDOF/finiteRotation/finiteRotation.C
index c03d0d037..c1f9a3900 100644
--- a/src/ODE/sixDOF/finiteRotation/finiteRotation.C
+++ b/src/ODE/sixDOF/finiteRotation/finiteRotation.C
@@ -82,7 +82,9 @@ Foam::vector Foam::finiteRotation::eulerAngles(const tensor& rotT)
     // Calculate roll angle
     rollAngle = atan2(rotT.yz(), rotT.zz());
 
-    const scalar c2 = sqrt(rotT.xx() + rotT.xy());
+    // Use mag to avoid negative value due to round-off
+    // HJ, 24/Feb/2016
+    const scalar c2 = sqrt(Foam::max(0, rotT.xx() + rotT.xy()));
 
     // Calculate pitch angle
     pitchAngle = atan2(-rotT.xz(), c2);