From 259672580fe849b894ddcc8b327bb6a589bf88c4 Mon Sep 17 00:00:00 2001
From: Teng Zhang <ztnuaa0211@gmail.com>
Date: Sun, 4 Jan 2026 17:02:09 +0800
Subject: [PATCH] Bug fix: improve the logic of sorting singular values in
 SIGMA sgs model.

    - This will makesure sigma_1 >= sigma_2 >= sigma_3.

    - Fix patch is provided by Min Zhang.
---
 .../turbulenceModels/Sigma/Sigma.C            | 23 +++++++------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/src/TurbulenceModels/turbulenceModels/Sigma/Sigma.C b/src/TurbulenceModels/turbulenceModels/Sigma/Sigma.C
index 4f1ee33b9..34497cf8c 100644
--- a/src/TurbulenceModels/turbulenceModels/Sigma/Sigma.C
+++ b/src/TurbulenceModels/turbulenceModels/Sigma/Sigma.C
@@ -57,21 +57,14 @@ void Sigma<BasicTurbulenceModel>::correctNut()
       }
 
       DiagonalMatrix<scalar> sigmas(SVD(gradUMatrix).S());
-      scalar sigma_1(max(sigmas));
-      scalar sigma_2(sigma_1);
-      scalar sigma_3(min(sigmas));
-
-      for (label ii = 0; ii<3; ii++)
-      {
-         if(sigmas[ii]>=sigma_3 && sigmas[ii]<=sigma_1) sigma_2 = sigmas[ii];
-
-         if(sigmas[ii]<0)
-         {
-            WarningInFunction
-            << "Negative sigular values!!! sigmas = \n"
-            << sigmas <<endl;
-         }
-      }
+   
+      scalar s_a = sigmas[0];
+      scalar s_b = sigmas[1];
+      scalar s_c = sigmas[2];
+
+      scalar sigma_1 = max(max(s_a, s_b), s_c);
+      scalar sigma_3 = min(min(s_a, s_b), s_c);
+      scalar sigma_2 = s_a + s_b + s_c - sigma_1 - sigma_3;
 
       if(abs(sigma_1)<ROOTVSMALL && !zeroSigma) zeroSigma = true;