[Mat/Fluid] Add LinearConcentrationDependentDensitity model.

MaterialLib/Fluid/Density/LinearConcentrationDependentDensity.h

+/**
+ * @copyright
+ * Copyright (c) 2012-2017, OpenGeoSys Community (http://www.opengeosys.org)
+ *            Distributed under a Modified BSD License.
+ *              See accompanying file LICENSE.txt or
+ *              http://www.opengeosys.org/project/license
+ *
+ */
+
+#pragma once
+
+#include <string>
+
+#include "BaseLib/ConfigTree.h"
+
+#include "MaterialLib/Fluid/FluidProperty.h"
+
+namespace MaterialLib
+{
+namespace Fluid
+{
+/// Linear concentration dependent density model.
+class LinearConcentrationDependentDensity final : public FluidProperty
+{
+public:
+    /**
+     * @param reference_density  \f$\rho_0\f$
+     * @param reference_concentration \f$C_0\f$
+     * @param fluid_density_difference_ratio  \f$ \bar \alpha \f$ in reference
+     * Coupled groundwater flow and transport: 2. Thermohaline and 3D convection
+     * systems
+     */
+    explicit LinearConcentrationDependentDensity(
+        const double reference_density, double reference_concentration,
+        const double fluid_density_difference_ratio)
+        : _reference_density(reference_density),
+          _reference_concentration(reference_concentration),
+          _fluid_density_difference_ratio(fluid_density_difference_ratio)
+    {
+    }
+
+    /// Get model name.
+    std::string getName() const override
+    {
+        return "Linear concentration dependent density";
+    }
+
+    /// Get density value.
+    /// \param var_vals Variable values in an array. The order of its elements
+    ///                 is given in enum class PropertyVariableType.
+    double getValue(const ArrayType& var_vals) const override
+    {
+        const double C = var_vals[static_cast<int>(PropertyVariableType::C)];
+        return _reference_density * (1 +
+                                     _fluid_density_difference_ratio *
+                                         (C - _reference_concentration));
+    }
+
+    /// Get the partial differential of the density with respect to
+    /// concentration.
+    /// \param var_vals  Variable values  in an array. The order of its elements
+    ///                   is given in enum class PropertyVariableType.
+    /// \param var       Variable type.
+    double getdValue(const ArrayType& var_vals,
+                     const PropertyVariableType var) const override
+    {
+        (void)var_vals;
+        if (var != PropertyVariableType::C)
+            return 0.0;
+        return _reference_density * _fluid_density_difference_ratio;
+    }
+
+private:
+    const double _reference_density;
+    const double _reference_concentration;
+    const double _fluid_density_difference_ratio;
+};
+
+}  // end namespace
+}  // end namespace