[Thermo] Add speed of sound property

Add ThermoPhase::soundSpeed virtual function with documentation
Add IdealGasPhase implementation
Add to cython interface
Add units declaration

include/cantera/thermo/IdealGasPhase.h

@@ -401,6 +401,8 @@ public:
         return 1.0 / temperature();
     }
 
+    virtual double soundSpeed() const;
+
     //! @}
     //! @name Chemical Potentials and Activities
     //!

include/cantera/thermo/ThermoPhase.h

@@ -292,6 +292,17 @@ public:
         throw NotImplementedError("ThermoPhase::thermalExpansionCoeff");
     }
 
+    //! Return the speed of sound. Units: m/s.
+    /*!
+     * The speed of sound is defined as 
+     * \f[
+     * c = \sqrt{\left(\frac{\partial P}{\partial\rho}\right)_s}
+     * \f]
+     */
+    virtual double soundSpeed() const {
+        throw NotImplementedError("ThermoPhase::soundSpeed");
+    }
+
     //! @}
     //! @name Electric Potential
     //!

interfaces/cython/cantera/composite.py

@@ -492,7 +492,7 @@ class SolutionArray(SolutionArrayBase):
         'gibbs_mass', 'cv', 'cv_mole', 'cv_mass', 'cp', 'cp_mole', 'cp_mass',
         'critical_temperature', 'critical_pressure', 'critical_density',
         'P_sat', 'T_sat', 'isothermal_compressibility',
-        'thermal_expansion_coeff', 'electric_potential',
+        'thermal_expansion_coeff', 'sound_speed', 'electric_potential',
         # From Kinetics
         'heat_release_rate',
         # From Transport

interfaces/cython/cantera/onedim.py

@@ -773,8 +773,8 @@ for _attr in ['density', 'density_mass', 'density_mole', 'volume_mass',
               'entropy_mass', 'g', 'gibbs_mole', 'gibbs_mass', 'cv',
               'cv_mole', 'cv_mass', 'cp', 'cp_mole', 'cp_mass',
               'isothermal_compressibility', 'thermal_expansion_coeff',
-              'viscosity', 'thermal_conductivity', 'heat_release_rate',
-              'mean_molecular_weight']:
+              'sound_speed', 'viscosity', 'thermal_conductivity', 
+              'heat_release_rate', 'mean_molecular_weight']:
     setattr(FlameBase, _attr, _array_property(_attr))
 FlameBase.volume = _array_property('v') # avoid confusion with velocity gradient 'V'
 FlameBase.int_energy = _array_property('u') # avoid collision with velocity 'u'

interfaces/cython/cantera/thermo.pxd

@@ -76,6 +76,7 @@ cdef extern from "cantera/thermo/ThermoPhase.h" namespace "Cantera":
         double molarVolume() except +translate_exception
         double isothermalCompressibility() except +translate_exception
         double thermalExpansionCoeff() except +translate_exception
+        double soundSpeed() except +translate_exception
         double electricPotential() except +translate_exception
         void setElectricPotential(double) except +translate_exception
 

interfaces/cython/cantera/thermo.pyx

@@ -1713,6 +1713,11 @@ cdef class ThermoPhase(_SolutionBase):
         def __get__(self):
             return self.thermo.thermalExpansionCoeff()
 
+    property sound_speed:
+        """Speed of sound [m/s]."""
+        def __get__(self):
+            return self.thermo.soundSpeed()
+
     property min_temp:
         """
         Minimum temperature for which the thermodynamic data for the phase are

site_scons/site_tools/UnitsInterfaceBuilder.py

@@ -32,7 +32,7 @@ def UnitsInterfaceBuilder(env, target, source):
         "concentrations": '"kmol/m**3"', "critical_pressure": '"Pa"',
         "critical_temperature": '"K"', "critical_density": 'self.basis_units + "/m**3"',
         "electric_potential": '"V"', "electrochemical_potentials": '"J/kmol"',
-        "isothermal_compressibility": '"1/Pa"', "max_temp": '"K"',
+        "isothermal_compressibility": '"1/Pa"', "sound_speed": '"m/s"', "max_temp": '"K"',
         "mean_molecular_weight": '"kg/kmol"', "min_temp": '"K"',
         "molecular_weights": '"kg/kmol"', "partial_molar_cp": '"J/kmol/K"',
         "partial_molar_enthalpies": '"J/kmol"', "partial_molar_entropies": '"J/kmol/K"',
@@ -52,7 +52,7 @@ def UnitsInterfaceBuilder(env, target, source):
         "P_sat", "T", "T_sat", "atomic_weight", "chemical_potentials", "concentrations",
         "critical_pressure", "critical_temperature", "critical_density",
         "electric_potential", "electrochemical_potentials", "isothermal_compressibility",
-        "max_temp", "mean_molecular_weight", "min_temp", "molecular_weights",
+        "sound_speed", "max_temp", "mean_molecular_weight", "min_temp", "molecular_weights",
         "partial_molar_cp", "partial_molar_enthalpies", "partial_molar_entropies",
         "partial_molar_int_energies", "partial_molar_volumes", "reference_pressure",
         "thermal_expansion_coeff", "cp", "cv", "density", "h", "s", "g", "u", "v",

src/thermo/IdealGasPhase.cpp

@@ -37,6 +37,12 @@ doublereal IdealGasPhase::cv_mole() const
     return cp_mole() - GasConstant;
 }
 
+double IdealGasPhase::soundSpeed() const {
+    return sqrt(
+        cp_mole() / cv_mole() * GasConstant / meanMolecularWeight() * temperature()
+    );
+}
+
 doublereal IdealGasPhase::standardConcentration(size_t k) const
 {
     return pressure() / RT();