From 173973071b4b2c11cd2fc94a9b4dd3b5edd245a1 Mon Sep 17 00:00:00 2001
From: SVS87 <shleshsakpal@gmail.com>
Date: Mon, 15 Jun 2026 00:18:20 -0400
Subject: [PATCH] fix: correct axis normalization in
 s_compute_moment_of_inertia

Use sqrt(sum(axis**2)) instead of sqrt(sum(axis)) to compute
the unit vector along the rotation axis. The incorrect norm
produced a non-unit normal_axis, making the orthogonal projection
and computed moment of inertia wrong for 3D moving IBs with
non-analytic geometry.

Fixes #1481
---
 src/simulation/m_ibm.fpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/simulation/m_ibm.fpp b/src/simulation/m_ibm.fpp
index 99aeb1e382..d8bdaa2cc5 100644
--- a/src/simulation/m_ibm.fpp
+++ b/src/simulation/m_ibm.fpp
@@ -1143,7 +1143,7 @@ contains
             patch_ib(ib_idx)%moment = 1._wp
             return
         else
-            normal_axis = axis/sqrt(sum(axis))
+            normal_axis = axis/sqrt(sum(axis**2))
         end if
 
         ! if the IB is in 2D or a 3D sphere, we can compute this exactly