Restore the slope sign in the SUPERBEE limiter

docs/documentation/equations.md

@@ -875,7 +875,7 @@ Primitive variable reconstruction is used to avoid spurious oscillations at inte
 2. **MC (Monotonized Central):** \f$\phi = \text{sign}(a)\,\min(2|a|,\;2|b|,\;\frac{1}{2}(|a|+|b|))\f$ if \f$ab > 0\f$
 3. **OSPRE (Van Albada):** \f$\phi = (a^2 b + a b^2)/(a^2 + b^2)\f$
 4. **Van Leer:** \f$\phi = 2ab/(a + b)\f$ if \f$ab > 0\f$
-5. **Superbee:** \f$\phi = \max(\min(2|a|,|b|),\;\min(|a|,2|b|))\f$ if \f$ab > 0\f$
+5. **Superbee:** \f$\phi = \text{sign}(a)\,\max(\min(2|a|,|b|),\;\min(|a|,2|b|))\f$ if \f$ab > 0\f$
 
 where \f$a\f$ and \f$b\f$ are the left and right slope differences.
 

src/simulation/m_muscl.fpp

@@ -203,6 +203,7 @@ contains
                                             slope = -1._wp*min(-min(2._wp*abs(slopeL), abs(slopeR)), -min(abs(slopeL), &
                                                                & 2._wp*abs(slopeR)))
                                         end if
+                                        if (slopeL < 0._wp) slope = -slope
                                     end if
 
                                     ! reconstruct from left side