freezing tests volume-to-mass refactor. closes #1262 (#1358)

Author: slayoo

tests/unit_tests/backends/test_freezing_methods.py

@@ -6,7 +6,6 @@
 from PySDM import Builder, Formulae
 from PySDM.dynamics import Freezing
 from PySDM.environments import Box
-from PySDM.physics import constants_defaults as const
 from PySDM.physics import si
 from PySDM.products import IceWaterContent
 
@@ -68,7 +67,7 @@ def test_freeze_singular(backend_class):
         dt = 1 * si.s
         dv = 1 * si.m**3
         T_fz = 250 * si.K
-        vol = 1 * si.um**3
+        water_mass = 1 * si.mg
         multiplicity = 1e10
         steps = 1
 
@@ -81,7 +80,7 @@ def test_freeze_singular(backend_class):
         attributes = {
             "multiplicity": np.full(n_sd, multiplicity),
             "freezing temperature": np.full(n_sd, T_fz),
-            "volume": np.full(n_sd, vol),
+            "water mass": np.full(n_sd, water_mass),
         }
         products = (IceWaterContent(name="qi"),)
         particulator = builder.build(attributes=attributes, products=products)
@@ -92,9 +91,10 @@ def test_freeze_singular(backend_class):
         particulator.run(steps=steps)
 
         # assert
-        np.testing.assert_almost_equal(
-            np.asarray(particulator.products["qi"].get()),
-            [n_sd * multiplicity * vol * const.rho_w / dv],
+        np.testing.assert_approx_equal(
+            actual=np.asarray(particulator.products["qi"].get()),
+            desired=n_sd * multiplicity * water_mass / dv,
+            significant=7,
         )
 
     @staticmethod
@@ -123,7 +123,7 @@ def test_freeze_time_dependent(backend_class, double_precision, plot=False):
         )
 
         # dummy (but must-be-set) values
-        vol = (
+        initial_water_mass = (
             44  # for sign flip (ice water has negative volumes), value does not matter
         )
         d_v = 666  # products use conc., dividing there, multiplying here, value does not matter
@@ -165,7 +165,7 @@ def low(t):
             attributes = {
                 "multiplicity": np.full(n_sd, int(case["N"])),
                 "immersed surface area": np.full(n_sd, immersed_surface_area),
-                "volume": np.full(n_sd, vol),
+                "water mass": np.full(n_sd, initial_water_mass),
             }
             particulator = builder.build(attributes=attributes, products=products)
             env["RH"] = 1.0001
@@ -178,7 +178,7 @@ def low(t):
 
                 ice_mass_per_volume = particulator.products["qi"].get()[cell_id]
                 ice_mass = ice_mass_per_volume * d_v
-                ice_number = ice_mass / (const.rho_w * vol)
+                ice_number = ice_mass / initial_water_mass
                 unfrozen_fraction = 1 - ice_number / number_of_real_droplets
                 output[key]["unfrozen_fraction"].append(unfrozen_fraction)