How to use the Model object¶
This page describes the usage of the Model object. This object provides a
pure Python API, so some basic knowledge about Python is assumed.
Model paradigm¶
A Model consists one or more features. For instance, an aircraft model
might have the feature propulsion and the feature wing. Such a feature
consists of one or more properties. Concrete values can be assigned to these
properties. So, in our example propulsion might have the properties thrust
and type, and the wing feature might have properties span and
mount_point (e.g. for an engine, landing gear, or some other technical
system). We may assign some values to each of these properties. For instance,
we may set thrust to \(50\,\textrm{kN}\) or the span to
\(20\,\textrm{m}\). How do we set these values in a Python script?
# Get an instance of the Model object
model = Model()
# From the model instance, we can create a 'propulsion' instance. We can
# now set actual value to the properties 'trust' and propulsion 'type'.
prop = model.set_feature('propulsion')
prop.set('thrust', 50e3)
prop.set('type', 'turbofan engine')
# Now we create a new 'wing'. We can set the 'span' and add mount points
wing = model.add_feature('wing')
wing.set('span', 20)
wing.add('mount_point', (4, 2, 4))
wing.add('mount_point', (4, 6, 4))
wing.add('mount_point', (4, 20, 4))
Fig. 4 Simple aircraft model with a propulsion features, and a wing feature¶
Notice the method add_feature(). When calling this method, we create a
instance of the feature we want to add (here 'propulsion' and
'wing'). The feature instance has set() and add() methods to assign
a value to some property. The model object will check the value. That means
that you must provide a number for the property trust. If you try to assign,
say a string ('50e3'), an error will be thrown, and the model will not
continue to be built.
Fig. 5 A model object can have multiple features, and each feature can have multiple properties¶
Model and feature methods¶
The Model object and its features provide only few methods. The main thing
to remember is that there are set* and add* methods. The set*
method will always apply if there can only be one instance, and the add*
method will apply if there can be multiple instances. For example, an aircraft
can have more than one wing, hence the add_feature() method applies. To
create another wing, we simply call the method again. However, some feature may
only exists once. In our dummy aircraft model, we impose the restriction of
only adding one propulsion feature (arguably, an aircraft model could have
multiple propulsion instances, but here we assume otherwise). To highlight that
propulsion is a singleton feature, we use the set_feature() method.
Trying to use add_feature('propulsion') will result in an error and the
model will not continue to build. Property values in a feature can be assigned
with the add() and set() methods, depending on whether the properties
are singleton or not. In the example above, the wing may have multiple mount
points, but there can only be a single wing span.
Model |
Feature |
|
Singleton |
|
|
Non-singleton |
|
|
You can retrieve feature instances from your model at any time. Note that you must have created these instances first with the methods discussed above. The table below shows the available methods you may use.
Model |
Feature |
|
Singleton |
|
|
Non-singleton |
|
|
Running the model¶
The last thing you need to know is how to run the model. Once you set up the
entire, you can call the run() method. This method will start the actual
evaluation of the model.
model = Model()
... # Add features and assign values here
results = model.run()
Results¶
The run() method returns an object with which you can interact pretty much
in the same way as the Model object. Results are group into “features”
which have properties. You can retrieve data using the get() and iter()
methods mentioned above.