ExponentialAtmosphere.hpp

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// Exponential Atmosphere Model
// Simple exponential atmosphere for quick estimates and validation
#pragma once
 
#include <janus/janus.hpp>
 
namespace vulcan::exponential_atmosphere {
 
// ============================================================================
// Physical Constants
// ============================================================================

inline constexpr double RHO_0 = 1.225;

inline constexpr double P_0 = 101325.0;

inline constexpr double T_0 = 288.15;

inline constexpr double DEFAULT_SCALE_HEIGHT = 8500.0;

inline constexpr double G_0 = 9.80665;

inline constexpr double R_AIR = 287.058;

inline constexpr double GAMMA = 1.4;
 
// ============================================================================
// Atmospheric State Struct
// ============================================================================

template <typename Scalar> struct AtmosphericState {
    Scalar temperature;    
    Scalar pressure;       
    Scalar density;        
    Scalar speed_of_sound; 
};
 
// ============================================================================
// Public API - Individual Property Accessors
// ============================================================================

template <typename Scalar>
Scalar density(const Scalar &altitude,
               double scale_height = DEFAULT_SCALE_HEIGHT) {
    return RHO_0 * janus::exp(-altitude / scale_height);
}

template <typename Scalar>
Scalar pressure(const Scalar &altitude,
                double scale_height = DEFAULT_SCALE_HEIGHT) {
    return P_0 * janus::exp(-altitude / scale_height);
}

template <typename Scalar>
Scalar temperature(const Scalar &altitude,
                   double scale_height = DEFAULT_SCALE_HEIGHT) {
    // Isothermal assumption - return constant temperature
    // Use janus multiplication to ensure proper type for symbolic inputs
    (void)scale_height; // Unused
    return altitude * 0.0 + T_0;
}

template <typename Scalar>
Scalar speed_of_sound(const Scalar &altitude,
                      double scale_height = DEFAULT_SCALE_HEIGHT) {
    // a = sqrt(gamma * R * T)
    // For isothermal atmosphere, this is constant
    Scalar T = temperature(altitude, scale_height);
    return janus::sqrt(GAMMA * R_AIR * T);
}
 
// ============================================================================
// Public API - Combined State Accessor
// ============================================================================

template <typename Scalar>
AtmosphericState<Scalar> state(const Scalar &altitude,
                               double scale_height = DEFAULT_SCALE_HEIGHT) {
    Scalar T = temperature(altitude, scale_height);
    Scalar exp_factor = janus::exp(-altitude / scale_height);
 
    return AtmosphericState<Scalar>{.temperature = T,
                                    .pressure = P_0 * exp_factor,
                                    .density = RHO_0 * exp_factor,
                                    .speed_of_sound =
                                        janus::sqrt(GAMMA * R_AIR * T)};
}
 
// ============================================================================
// Utility Functions
// ============================================================================

inline double compute_scale_height(double temperature = T_0,
                                   double gravity = G_0) {
    return R_AIR * temperature / gravity;
}

template <typename Scalar>
Scalar altitude_from_density(const Scalar &rho,
                             double scale_height = DEFAULT_SCALE_HEIGHT) {
    return -scale_height * janus::log(rho / RHO_0);
}

template <typename Scalar>
Scalar altitude_from_pressure(const Scalar &P,
                              double scale_height = DEFAULT_SCALE_HEIGHT) {
    return -scale_height * janus::log(P / P_0);
}
 
} // namespace vulcan::exponential_atmosphere