Fix CSTR time derivative Jacobian (#310)

Co-authored-by: Jan Breuer <[email protected]>

src/libcadet/model/StirredTankModel.cpp

@@ -1824,8 +1824,8 @@ void CSTRModel::addTimeDerivativeJacobian(double t, double alpha, const ConstSim
 	double const* const c = simState.vecStateY + _nComp;
 	double const* const q = simState.vecStateY + 2 * _nComp;
 	const double v = simState.vecStateY[2 * _nComp + _totalBound];
-	const double vsolid = static_cast<double>(_constSolidVolume);
 	const double timeV = v * alpha;
+	const double timeVSolid = static_cast<double>(_constSolidVolume) * alpha;
 
 	// Assemble Jacobian: dRes / dyDot
 
@@ -1839,7 +1839,7 @@ void CSTRModel::addTimeDerivativeJacobian(double t, double alpha, const ConstSim
 		{
 			double const* const qi = q + _offsetParType[type] + _boundOffset[type * _nComp + i];
 			const unsigned int localOffset = _nComp + _offsetParType[type] + _boundOffset[type * _nComp + i];
-			const double vSolidParVolFrac = vsolid * static_cast<double>(_parTypeVolFrac[type]);
+			const double vSolidParVolFrac = timeVSolid * static_cast<double>(_parTypeVolFrac[type]);
 			for (unsigned int j = 0; j < _nBound[type * _nComp + i]; ++j)
 			{
 				mat.native(i, localOffset + j) += vSolidParVolFrac; // dRes / dqDot
@@ -1851,7 +1851,7 @@ void CSTRModel::addTimeDerivativeJacobian(double t, double alpha, const ConstSim
 			}
 		}
 
-		mat.native(i, _nComp + _totalBound) += alpha * c[i]; // dRes / dV
+		mat.native(i, _nComp + _totalBound) += alpha * c[i]; // dRes / dVlDot
 	}
 
 	// Bound states