TST: introduce testing to variable density tanks.

Author: phmbressan

data/motors/liquid_motor_example/n2o_fluid_parameters.csv

@@ -0,0 +1,101 @@
+Temperature (K),Pressure (Pa),Density (kg/m³)
+193.150000,101325.000000,2.839688
+205.372222,101325.000000,2.659196
+217.594444,101325.000000,2.501930
+229.816667,101325.000000,2.363172
+242.038889,101325.000000,2.239591
+254.261111,101325.000000,2.128693
+266.483333,101325.000000,2.028539
+278.705556,101325.000000,1.937588
+290.927778,101325.000000,1.854594
+303.150000,101325.000000,1.778532
+193.150000,990733.333333,1206.532256
+205.372222,990733.333333,1168.931406
+217.594444,990733.333333,1129.335464
+229.816667,990733.333333,1087.042281
+242.038889,990733.333333,24.480299
+254.261111,990733.333333,22.807515
+266.483333,990733.333333,21.414919
+278.705556,990733.333333,20.221402
+290.927778,990733.333333,19.178742
+303.150000,990733.333333,18.255258
+193.150000,1880141.666667,1207.901606
+205.372222,1880141.666667,1170.560205
+217.594444,1880141.666667,1131.305292
+229.816667,1880141.666667,1089.479276
+242.038889,1880141.666667,1044.107836
+254.261111,1880141.666667,993.574261
+266.483333,1880141.666667,44.818526
+278.705556,1880141.666667,41.521255
+290.927778,1880141.666667,38.847302
+303.150000,1880141.666667,36.598433
+193.150000,2769550.000000,1209.256890
+205.372222,2769550.000000,1172.168610
+217.594444,2769550.000000,1133.244437
+229.816667,2769550.000000,1091.867709
+242.038889,2769550.000000,1047.140735
+254.261111,2769550.000000,997.611513
+266.483333,2769550.000000,940.542069
+278.705556,2769550.000000,67.699444
+290.927778,2769550.000000,61.910544
+303.150000,2769550.000000,57.447399
+193.150000,3658958.333333,1210.598454
+205.372222,3658958.333333,1173.757228
+217.594444,3658958.333333,1135.154031
+229.816667,3658958.333333,1094.209859
+242.038889,3658958.333333,1050.096018
+254.261111,3658958.333333,1001.503955
+266.483333,3658958.333333,946.059899
+278.705556,3658958.333333,878.329006
+290.927778,3658958.333333,90.504952
+303.150000,3658958.333333,81.893190
+193.150000,4548366.666667,1211.926631
+205.372222,4548366.666667,1175.326641
+217.594444,4548366.666667,1137.035143
+229.816667,4548366.666667,1096.507838
+242.038889,4548366.666667,1052.978332
+254.261111,4548366.666667,1005.263627
+266.483333,4548366.666667,951.292446
+278.705556,4548366.666667,886.629576
+290.927778,4548366.666667,130.554270
+303.150000,4548366.666667,112.079628
+193.150000,5437775.000000,1213.241741
+205.372222,5437775.000000,1176.877402
+217.594444,5437775.000000,1138.888779
+229.816667,5437775.000000,1098.763608
+242.038889,5437775.000000,1055.791888
+254.261111,5437775.000000,1008.901026
+266.483333,5437775.000000,956.273388
+278.705556,5437775.000000,894.237174
+290.927778,5437775.000000,812.409895
+303.150000,5437775.000000,153.463224
+193.150000,6327183.333333,1214.544092
+205.372222,6327183.333333,1178.410038
+217.594444,6327183.333333,1140.715888
+229.816667,6327183.333333,1100.978992
+242.038889,6327183.333333,1058.540519
+254.261111,6327183.333333,1012.425369
+266.483333,6327183.333333,961.030387
+278.705556,6327183.333333,901.279439
+290.927778,6327183.333333,825.421408
+303.150000,6327183.333333,689.418589
+193.150000,7216591.666667,1215.833981
+205.372222,7216591.666667,1179.925053
+217.594444,7216591.666667,1142.517369
+229.816667,7216591.666667,1103.155693
+242.038889,7216591.666667,1061.227727
+254.261111,7216591.666667,1015.844801
+266.483333,7216591.666667,965.586483
+278.705556,7216591.666667,907.849718
+290.927778,7216591.666667,836.658448
+303.150000,7216591.666667,729.094125
+193.150000,8106000.000000,1217.111692
+205.372222,8106000.000000,1181.422931
+217.594444,8106000.000000,1144.294074
+229.816667,8106000.000000,1105.295297
+242.038889,8106000.000000,1063.856717
+254.261111,8106000.000000,1019.166555
+266.483333,8106000.000000,969.961109
+278.705556,8106000.000000,914.018666
+290.927778,8106000.000000,846.619843
+303.150000,8106000.000000,753.272731

docs/user/motors/tanks.rst

@@ -80,7 +80,9 @@ Fluid are then passed to tanks when they are defined.
   a dataset or external libraries. 
   In this case, the fluid would be defined as such:
 
-  >>> Fluid(name="Liquid Nitrous Oxide", density=lambda t, p: 44 * p / (8.314 * t))
+  >>> Fluid(name="N2O", density=lambda t, p: 44 * p / (8.314 * t))
+  >>> from CoolProp.CoolProp import PropsSI # external library
+  >>> Fluid(name="N2O", density=lambda t, p: PropsSI('D', 'T', t, 'P', p, 'N2O'))
 
   In fact, the density parameter can be any ``Function`` source, such as a
   ``callable``, csv file or an array of points. See more on 

rocketpy/mathutils/function.py

@@ -3695,8 +3695,9 @@ def savetxt(
                     + " must be provided."
                 )
             # Generate the data points using the callable
-            x = np.linspace(lower, upper, samples)
-            data_points = np.column_stack((x, self(x)))
+            data_points = self.set_discrete(
+                lower, upper, samples, mutate_self=False
+            ).source
         else:
             # If the source is already an array, use it as is
             data_points = self.source

rocketpy/motors/fluid.py

@@ -1,4 +1,5 @@
 import numpy as np
+from scipy.constants import atm, zero_Celsius
 
 from ..mathutils.function import Function
 from ..plots.fluid_plots import _FluidPlots
@@ -69,7 +70,7 @@ def density_function(temperature, pressure):
 
         self._density_function = Function(
             density_function,
-            interpolation="shepard",
+            interpolation="linear",
             extrapolation="natural",
             inputs=["Temperature (K)", "Pressure (Pa)"],
             outputs="Density (kg/m³)",
@@ -90,7 +91,7 @@ def get_time_variable_density(self, temperature, pressure):
         Function
             Density of the fluid in kg/m³ as function of time.
         """
-        if callable(temperature.source):
+        if callable(temperature.source) and callable(pressure.source):
             return Function(
                 lambda time: self.density_function.get_value(
                     temperature.source(time), pressure.source(time)
@@ -99,13 +100,16 @@ def get_time_variable_density(self, temperature, pressure):
                 outputs="Density (kg/m³)",
             )
         else:
+            time_scale = np.unique(
+                np.concatenate((temperature.x_array, pressure.x_array))
+            )
             density_time = self.density_function.get_value(
-                temperature.y_array, pressure.y_array
+                temperature.get_value(time_scale), pressure.get_value(time_scale)
             )
             return Function(
                 np.column_stack(
                     (
-                        temperature.x_array,
+                        time_scale,
                         density_time,
                     )
                 ),
@@ -138,7 +142,20 @@ def __str__(self):
         return f"Fluid: {self.name}"
 
     def to_dict(self, **kwargs):  # pylint: disable=unused-argument
-        return {"name": self.name, "density": self.density}
+        discretize = kwargs.get("discretize", False)
+        if not isinstance(self.density, (int, float, np.number)):
+            density = self.density_function
+        else:
+            density = self.density
+
+        if discretize and isinstance(density, Function):
+            density = density.set_discrete(
+                ((100, atm * 0.9), (zero_Celsius + 30, atm * 100)),
+                (25, 25),
+                mutate_self=False,
+            )
+
+        return {"name": self.name, "density": density}
 
     @classmethod
     def from_dict(cls, data):