Add function to compute mole fractions from mass and vice-versa (#1041)

mirgecom/eos.py

@@ -572,6 +572,8 @@ class PyrometheusMixture(MixtureEOS):
     .. automethod:: species_enthalpies
     .. automethod:: get_species_source_terms
     .. automethod:: get_temperature_seed
+    .. automethod:: mole_fractions_from_mass_fractions
+    .. automethod:: mass_fractions_from_mole_fractions
     """
 
     def __init__(self, pyrometheus_mech, temperature_guess=300.0):
@@ -870,3 +872,16 @@ def get_species_source_terms(self, cv: ConservedVars, temperature: DOFArray):
 
         return make_conserved(dim, rho_source, energy_source, mom_source,
                               species_sources)
+
+    def mole_fractions_from_mass_fractions(self, species_mass_fractions):
+        """Get mole fractions from mass fractions."""
+        mix_mol_weight = \
+            self._pyrometheus_mech.get_mix_molecular_weight(species_mass_fractions)
+        return self._pyrometheus_mech.get_mole_fractions(mix_mol_weight,
+                                                         species_mass_fractions)
+
+    def mass_fractions_from_mole_fractions(self, species_mole_fractions):
+        """Get mass fractions from mole fractions."""
+        mol_weights = self.get_species_molecular_weights()
+        mmw = sum(species_mole_fractions*mol_weights)
+        return species_mole_fractions*mol_weights/mmw

test/test_eos.py

@@ -100,13 +100,16 @@ def inf_norm(x):
 
         cantera_soln.TPX = tempin, pressure, x
         can_t, can_rho, can_y = cantera_soln.TDY
+        can_x = cantera_soln.X
         can_p = cantera_soln.P
 
         pin = can_p * ones
         tin = can_t * ones
-        yin = can_y * ones
+        xin = can_x * ones
         rhoin = can_rho * ones
 
+        yin = eos.mass_fractions_from_mole_fractions(xin)
+
         # First, check density
         mass = eos.get_density(pin, tin, yin)
         abs_err_m = inf_norm(mass - can_rho)
@@ -173,13 +176,24 @@ def inf_norm(x):
 
         cantera_soln.TPX = tempin, pressure, x
         can_t, can_rho, can_y = cantera_soln.TDY
+        can_x = cantera_soln.X
         can_p = cantera_soln.P
 
         pin = can_p * ones
         tin = can_t * ones
-        yin = can_y * ones
+        xin = can_x * ones
         rhoin = can_rho * ones
 
+        yin = eos.mass_fractions_from_mole_fractions(xin)
+        for i in range(nspecies):
+            abs_err_y = inf_norm(yin[i] - can_y[i])
+            assert abs_err_y < 1.0e-14
+
+        xin_p = eos.mole_fractions_from_mass_fractions(yin)
+        for i in range(nspecies):
+            abs_err_x = inf_norm(xin_p[i] - can_x[i])
+            assert abs_err_x < 1.0e-14
+
         mass = eos.get_density(pin, tin, yin)
         abs_err_m = inf_norm(mass - can_rho)
         assert abs_err_m/can_rho < 1.0e-14