# 6. pyDAE User Guide¶

## 6.1. Importing DAE Tools modules¶

pyDAE modules can be imported in the following way:

from daetools.pyDAE import *

This will set the python sys.path for importing the platform dependent c++ extension modules (i.e. .../daetools/pyDAE/Windows_win32_py34 and .../daetools/solvers/Windows_win32_py34 in Windows, .../daetools/pyDAE/Linux_x86_64_py34 and .../daetools/solvers/Linux_x86_64_py34 in GNU/Linux), import all symbols from all pyDAE modules: pyCore, pyActivity, pyDataReporting, pyIDAS, pyUnits and import some platfom independent modules: logs, variable_types and dae_simulator.

Alternatively, only the daetools module can be imported and classes from the pyDAE extension modules accessed using fully qualified names. For instance:

import daetools

model = daetools.pyDAE.pyCore.daeModel("name")

Once the pyDAE module is imported, the other modules (such as third party linear solvers, optimisation solvers etc.) can be imported in the following way:

# Import Trilinos LA solvers (Amesos, AztecOO):
from daetools.solvers.trilinos import pyTrilinos

# Import SuperLU linear solver:
from daetools.solvers.superlu import pySuperLU

# Import SuperLU_MT linear solver:
from daetools.solvers.superlu_mt import pySuperLU_MT

# Import IPOPT NLP solver:
from daetools.solvers.ipopt import pyIPOPT

# Import BONMIN MINLP solver:
from daetools.solvers.bonmin import pyBONMIN

# Import NLOPT set of optimisation solvers:
from daetools.solvers.nlopt import pyNLOPT

Since domains, parameters and variables in DAE Tools have a numerical value in terms of a unit of measurement (quantity) the modules containing definitions of units and variable types must be imported. They can be imported in the following way:

from daetools.pyDAE.variable_types import length_t, area_t, volume_t
from daetools.pyDAE.pyUnits import m, kg, s, K, Pa, J, W

The complete list of units and variable types can be found in Variable types and Module pyUnits modules.

## 6.2. Developing models¶

In DAE Tools models are developed by deriving a new class from the base pyCore.daeModel class. An empty model definition is presented below:

class myModel(daeModel):
def __init__(self, name, parent = None, description = ""):
daeModel.__init__(self, name, parent, description)

# Declaration/instantiation of domains, parameters, variables, ports, etc:
...

def DeclareEquations(self):
# Declaration of equations, state transition networks etc.:
...

The process consists of the following steps:

1. Calling the base class constructor:

daeModel.__init__(self, name, parent, description)

2. Declaring the model structure (domains, parameters, variables, ports, components etc.) in the pyCore.daeModel.__init__() function:

One of the fundamental ideas in DAE Tools is separation of the model specification from the activities that can be carried out on that model: this way several simulation scenarios can be developed based on a single model definition. Thus, all objects are defined in two stages:

Therefore, parameters, domains and variables are only declared here, while their initialisation (setting the parameter value, setting up the domain, assigning or setting an initial condition etc.) is postponed and will be done in the simulation class

All objects must be declared as data members of the model since the base pyCore.daeModel class keeps only week references and does not own them:

def __init__(self, name, parent = None, description = ""):
self.domain    = daeDomain(...)
self.parameter = daeParameter(...)
self.variable  = daeVariable(...)
... etc.

and not:

def __init__(self, name, parent = None, description = ""):
domain    = daeDomain(...)
parameter = daeParameter(...)
variable  = daeVariable(...)
... etc.

because at the exit from the pyCore.daeModel.__init__() function the objects will go out of scope and get destroyed. However, the underlying c++ model object still holds references to them which will eventually result in the segmentation fault.

3. Specification of the model functionality (equations, state transition networks, and OnEvent and OnCondition actions) in the pyCore.daeModel.DeclareEquations() function.

Initialisation of the simulation object is done in several phases. At the point when this function is called by the framework the model parameters, domains, variables etc. are fully initialised. Therefore, it is safe to obtain the values of parameters or domain points and use them to create equations at the run-time.

A simplest DAE Tools model with a description of all steps/tasks necessary to develop a model can be found in the What’s the time? (AKA: Hello world!) tutorial (whats_the_time.py).

### 6.2.1. Parameters¶

Parameters are time invariant quantities that do not change during a simulation. Usually a good choice what should be a parameter is a physical constant, number of discretisation points in a domain etc.

There are two types of parameters in DAE Tools:

• Ordinary
• Distributed

The process of defining parameters is again carried out in two phases:

#### 6.2.1.1. Declaring parameters¶

Parameters are declared in the pyCore.daeModel.__init__() function. An ordinary parameter can be declared in the following way:

self.myParam = daeParameter("myParam", units, parentModel, "description")

Parameters can be distributed on domains. A distributed parameter can be declared in the following way:

self.myParam = daeParameter("myParam", units, parentModel, "description")
self.myParam.DistributeOnDomain(myDomain)

# Or simply:
self.myParam = daeParameter("myParam", units, parentModel, "description", [myDomain])

#### 6.2.1.2. Initialising parameters¶

Parameters are initialised in the pyActivity.daeSimulation.SetUpParametersAndDomains() function. To set a value of an ordinary parameter the following can be used:

myParam.SetValue(value)

where value can be a floating point value or the quantity object, while to set a value of distributed parameters (one-dimensional for example):

for i in range(myDomain.NumberOfPoints):
myParam.SetValue(i, value)

where the value can be either a float (i.e. 1.34) or the pyUnits.quantity object (i.e. 1.34 * W/(m*K)). If the simple floats are used it is assumed that they represent values with the same units as in the parameter definition.

Nota bene

DAE Tools (as it is the case in C/C++ and Python) use zero-based arrays in which the initial element of a sequence is assigned the index 0, rather than 1.

In addition, all values can be set at once using:

myParam.SetValues(values)

where values is a numpy array of floats/quantity objects.

#### 6.2.1.3. Using parameters¶

The most commonly used functions are:

Notate bene

The functions pyCore.daeParameter.__call__() and pyCore.daeParameter.array() return pyCore.adouble and pyCore.adouble_array objects, respectively and does not contain values. They are only used to specify equations’ residual expressions which are stored in the pyCore.adouble.Node() / pyCore.adouble_array.Node() attributes.

Other functions (such as pyCore.daeParameter.npyValues and pyCore.daeParameter.GetValue) can be used to access the values data during the simulation.

All above stands for similar functions in pyCore.daeDomain and pyCore.daeVariable classes.

1. To get a value of the ordinary parameter the pyCore.daeParameter.__call__() function (operator ()) can be used. For instance, if the variable myVar has to be equal to the sum of the parameter myParam and 15:

$myVar = myParam + 15$

in DAE Tools it is specified in the following acausal way:

# Notation:
#  - eq is a daeEquation object created using the model.CreateEquation(...) function
#  - myParam is an ordinary daeParameter object (not distributed)
#  - myVar is an ordinary daeVariable (not distributed)

eq.Residual = myVar() - (myParam() + 15)

2. To get a value of a distributed parameter the pyCore.daeParameter.__call__() function (operator ()) can be used again. For instance, if the distributed variable myVar has to be equal to the sum of the parameter myParam and 15 at each point of the domain myDomain:

$myVar(i) = myParam(i) + 15; \forall i \in [0, n_d - 1]$

in DAE Tools it is specified in the following acausal way:

# Notation:
#  - myDomain is daeDomain object
#  - eq is a daeEquation object distributed on the myDomain
#  - i is daeDistributedEquationDomainInfo object (used to iterate through the domain points)
#  - myParam is daeParameter object distributed on the myDomain
#  - myVar is daeVariable object distributed on the myDomain
i = eq.DistributeOnDomain(myDomain, eClosedClosed)
eq.Residual = myVar(i) - (myParam(i) + 15)

This code translates into a set of n algebraic equations.

Obviously, a parameter can be distributed on more than one domain. In the case of two domains:

$myVar(d_1,d_2) = myParam(d_1,d_2) + 15; \forall d_1 \in [0, n_{d1} - 1], \forall d_2 \in [0, n_{d2} - 1]$

the following can be used:

# Notation:
#  - myDomain1, myDomain2 are daeDomain objects
#  - eq is a daeEquation object distributed on the domains myDomain1 and myDomain2
#  - i1, i2 are daeDistributedEquationDomainInfo objects (used to iterate through the domain points)
#  - myParam is daeParameter object distributed on the myDomain1 and myDomain2
#  - myVar is daeVariable object distributed on the myDomaina and myDomain2
i1 = eq.DistributeOnDomain(myDomain1, eClosedClosed)
i2 = eq.DistributeOnDomain(myDomain2, eClosedClosed)
eq.Residual = myVar(i1,i2) - (myParam(i1,i2) + 15)

3. To get an array of parameter values the function pyCore.daeParameter.array() can be used, which returns the pyCore.adouble_array object. The ordinary mathematical functions can be used with the pyCore.adouble_array objects: pyCore.Sqrt(), pyCore.Sin(), pyCore.Cos(), pyCore.Min(), pyCore.Max(), pyCore.Log(), pyCore.Log10(), etc. In addition, some additional functions are available such as pyCore.Sum() and pyCore.Product().

For instance, if the variable myVar has to be equal to the sum of values of the parameter myParam for all points in the domain myDomain, the function pyCore.Sum() can be used.

The pyCore.daeParameter.array() function accepts the following arguments:

• plain integer (to select a single index from a domain)
• python list (to select a list of indexes from a domain)
• python slice (to select a portion of indexes from a domain: startIndex, endIindex, step)
• character * (to select all points from a domain)
• empty python list [] (to select all points from a domain)

Basically all arguments listed above are internally used to create the pyCore.daeIndexRange object. pyCore.daeIndexRange constructor has three variants:

1. The first one accepts a single argument: pyCore.daeDomain object. In this case the returned pyCore.adouble_array object will contain the parameter values at all points in the specified domain.
2. The second one accepts two arguments: pyCore.daeDomain object and a list of integer that represent indexes within the specified domain. In this case the returned pyCore.adouble_array object will contain the parameter values at the selected points in the specified domain.
3. The third one accepts four arguments: pyCore.daeDomain object, and three integers: startIndex, endIndex and step (which is basically a slice, that is a portion of a list of indexes: start through end-1, by the increment step). More info about slices can be found in the Python documentation. In this case the returned pyCore.adouble_array object will contain the parameter values at the points in the specified domain defined by the slice object.

Suppose that the variable myVar has to be equal to the sum of values in the array values that holds values from the parameter myParam at the specified indexes in the domains myDomain1 and myDomain2:

$myVar = \sum values$

There are several different scenarios for creating the array values from the parameter myParam distributed on two domains:

# Notation:
#  - myDomain1, myDomain2 are daeDomain objects
#  - n1, n2 are the number of points in the myDomain1 and myDomain2 domains
#  - eq1, eq2 are daeEquation objects
#  - mySum is daeVariable object
#  - myParam is daeParameter object distributed on myDomain1 and myDomain2 domains
#  - values is the adouble_array object

# Case 1. An array contains the following values from myParam:
#  - the first point in the domain myDomain1
#  - all points from the domain myDomain2
# All expressions below are equivalent:
values = myParam.array(0, '*')
values = myParam.array(0, [])

eq1.Residual = mySum() - Sum(values)

# Case 2. An array contains the following values from myParam:
#  - the first three points in the domain myDomain1
#  - all even points from the domain myDomain2
values = myParam.array([0,1,2], slice(0, myDomain2.NumberOfPoints, 2))

eq2.Residual = mySum() - Sum(values)

The case 1. translates into:

$mySum = myParam(0,0) + myParam(0,1) + ... + myParam(0,n_2 - 1)$

where n2 is the number of points in the domain myDomain2.

The case 2. translates into:

$\begin{split}mySum = & myParam(0,0) + myParam(0,2) + myParam(0,4) + ... + myParam(0, n_2 - 1) + \\ & myParam(1,0) + myParam(1,2) + myParam(1,4) + ... + myParam(1, n_2 - 1) + \\ & myParam(2,0) + myParam(2,2) + myParam(2,4) + ... + myParam(2, n_2 - 1)\end{split}$

### 6.2.2. Variable types¶

Variable types are used in DAE Tools to describe variables and they contain the following information:

• Name: string
• Units: pyUnits.unit object
• LowerBound: float
• UpperBound: float
• InitialGuess: float
• AbsoluteTolerance: float

Declaration of variable types is commonly done outside of the model definition (in the module scope).

#### 6.2.2.1. Declaring variable types¶

A variable type can be declared in the following way:

# Temperature type with units Kelvin, limits 100-1000K, the default value 273K and the absolute tolerance 1E-5
typeTemperature = daeVariableType("Temperature", K, 100, 1000, 273, 1E-5)

### 6.2.3. Distribution domains¶

There are two types of domains in DAE Tools:

• Simple arrays
• Distributed domains (used to distribute variables, parameters, and equations in space)

Distributed domains can form uniform grids (the default) or non-uniform grids (user-specified). In DAE Tools many objects can be distributed on domains: parameters, variables, equations, even models and ports. Distributing a model on a domain (that is in space) can be useful for modelling of complex multi-scale systems where each point in the domain have a corresponding model instance. In addition, domain points values can be obtained as a numpy one-dimensional array; this way DAE Tools can be easily used in conjunction with other scientific python libraries: NumPy, SciPy and many other.

Again, the domains are defined in two phases:

• Declaring a domain in the model
• Initialising it in the simulation

#### 6.2.3.1. Declaring domains¶

Domains are declared in the pyCore.daeModel.__init__() function:

self.myDomain = daeDomain("myDomain", parentModel, units, "description")

#### 6.2.3.2. Initialising domains¶

Domains are initialised in the pyActivity.daeSimulation.SetUpParametersAndDomains() function. To set up a domain as a simple array the function pyCore.daeDomain.CreateArray() can be used:

# Array of N elements
myDomain.CreateArray(N)

while to set up a domain distributed on a structured grid the function pyCore.daeDomain.CreateStructuredGrid():

# Uniform structured grid with N elements and bounds [lowerBound, upperBound]
myDomain.CreateStructuredGrid(N, lowerBound, upperBound)

where the lower and upper bounds can be simple floats or quantity objects. If the simple floats are used it is assumed that they represent values with the same units as in the domain definition. Typically, it is better to use quantities to avoid mistakes with wrong units:

# Uniform structured grid with 10 elements and bounds [0,1] in centimeters:
myDomain.CreateStructuredGrid(10, 0.0 * cm, 1.0 * cm)

Nota bene

Domains with N elements consists of N+1 points.

It is also possible to create an unstructured grid (for use in Finite Element models). However, creation and setup of such domains is an implementation detail of corresponding modules (i.e. pyDealII).

In certain situations it is not desired to have a uniform distribution of the points within the given interval, defined by the lower and upper bounds. In these cases, a non-uniform structured grid can be specified using the attribute pyCore.daeDomain.Points which contains the list of the points and that can be manipulated by the user:

# First create a structured grid domain
myDomain.CreateStructuredGrid(10, 0.0, 1.0)

# The original 11 points are: [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0]
# If the system is stiff at the beginning of the domain more points can be placed there
myDomain.Points = [0.0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.60, 1.00]

The effect of uniform and non-uniform grids is given in Fig. 6.1 (a simple heat conduction problem from the tutorial_3 has been served as a basis for comparison). Here, there are three cases:

• Black line: the analytic solution
• Blue line (10 intervals): uniform grid - a very rough prediction
• Red line (10 intervals): non-uniform grid - more points at the beginning of the domain

Fig. 6.1 Effect of uniform and non-uniform grids on numerical solution (zoomed to the first 5 points)

It can be clearly observed that in this problem the more precise results are obtained by using denser grid at the beginning of the interval.

#### 6.2.3.3. Using domains¶

The most commonly used functions are:

Nota bene

The functions pyCore.daeDomain.__call__(), pyCore.daeDomain.__getitem__() and pyCore.daeDomain.array() can only be used to build equations’ residual expressions. On the other hand, the attributes pyCore.daeDomain.Points and pyCore.daeDomain.npyPoints can be used to access the domain points at any point.

The arguments of the pyCore.daeDomain.array() function are the same as explained in Using parameters.

1. To get a point at the specified index within the domain the pyCore.daeDomain.__getitem__() function (operator []) can be used. For instance, if the variable myVar has to be equal to the sixth point in the domain myDomain:

$myVar = myDomain[5]$

the following can be used:

# Notation:
#  - eq is a daeEquation object
#  - myDomain is daeDomain object
#  - myVar is daeVariable object
eq.Residual = myVar() - myDomain[5]

### 6.2.4. Variables¶

There are various types of variables in DAE Tools. They can be:

• Ordinary
• Distributed

and:

• Algebraic
• Differential
• Constant (that is their value is assigned by fixing the number of degrees of freedom - DOF)

Again, variables are defined in two phases:

• Declaring a variable in the model
• Initialising it, if required (by assigning its value or setting an initial condition) in the simulation

#### 6.2.4.1. Declaring variables¶

Variables are declared in the pyCore.daeModel.__init__() function. An ordinary variable can be declared in the following way:

self.myVar = daeVariable("myVar", variableType, parentModel, "description")

Variables can also be distributed on domains. A distributed variable can be declared in the following way:

self.myVar = daeVariable("myVar", variableType, parentModel, "description")
self.myVar.DistributeOnDomain(myDomain)

# Or simply:
self.myVar = daeVariable("myVar", variableType, parentModel, "description", [myDomain])

#### 6.2.4.2. Initialising variables¶

Variables are initialised in the pyActivity.daeSimulation.SetUpVariables() function:

• To assign the variable value/fix the degrees of freedom the following can be used:

myVar.AssignValue(value)

or, if the variable is distributed:

for i in range(myDomain.NumberOfPoints):
myVar.AssignValue(i, value)

# or using a numpy array of values
myVar.AssignValues(values)

where value can be either a float (i.e. 1.34) or the pyUnits.quantity object (i.e. 1.34 * W/(m*K)), and values is a numpy array of floats or pyUnits.quantity objects. If the simple floats are used it is assumed that they represent values with the same units as in the variable type definition.

• To set an initial condition use the following:

myVar.SetInitialCondition(value)

or, if the variable is distributed:

for i in range(myDomain.NumberOfPoints):
myVar.SetInitialCondition(i, value)

# or using a numpy array of values
myVar.SetInitialConditions(values)

where the value can again be either a float or the pyUnits.quantity object, and values is a numpy array of floats or pyUnits.quantity objects. If the simple floats are used it is assumed that they represent values with the same units as in the variable type definition.

• To set an absolute tolerance the following can be used:

myVar.SetAbsoluteTolerances(1E-5)

• To set an initial guess use the following:

myVar.SetInitialGuess(value)

or, if the variable is distributed:

for i in range(0, myDomain.NumberOfPoints):
myVar.SetInitialGuess(i, value)

# or using a numpy array of values
myVar.SetInitialGuesses(values)

where the value can again be either a float or the pyUnits.quantity object and values is a numpy array of floats or pyUnits.quantity objects.

#### 6.2.4.3. Using variables¶

The most commonly used functions are:

Nota bene

The functions pyCore.daeVariable.__call__(), pyCore.dt(), pyCore.d(), pyCore.d2(), pyCore.daeVariable.array(), pyCore.dt_array(), pyCore.d_array() and pyCore.d2_array() can only be used to build equations’ residual expressions. On the other hand, the functions pyCore.daeVariable.GetValue, pyCore.daeVariable.SetValue and pyCore.daeVariable.npyValues can be used to access the variable data at any point.

All above mentioned functions are called in the same way as explained in Using parameters. More information will be given here on getting time and partial derivatives.

1. To get a time derivative of the ordinary variable the function pyCore.dt() can be used. For instance, if a time derivative of the variable myVar has to be equal to some constant, let’s say 1.0:

${ d(myVar) \over {d}{t} } = 1$

the following can be used:

# Notation:
#  - eq is a daeEquation object
#  - myVar is an ordinary daeVariable
eq.Residual = dt(myVar()) - 1.0

2. To get a time derivative of a distributed variable the pyCore.dt() function can be used again. For instance, if a time derivative of the distributed variable myVar has to be equal to some constant at each point of the domain myDomain:

${\partial myVar(i) \over \partial t} = 1; \forall i \in [0, n]$

the following can be used:

# Notation:
#  - myDomain is daeDomain object
#  - n is the number of points in the myDomain
#  - eq is a daeEquation object distributed on the myDomain
#  - d is daeDEDI object (used to iterate through the domain points)
#  - myVar is daeVariable object distributed on the myDomain
d = eq.DistributeOnDomain(myDomain, eClosedClosed)
eq.Residual = dt(myVar(d)) - 1.0

This code translates into a set of n equations.

Obviously, a variable can be distributed on more than one domain. To write a similar equation for a two-dimensional variable:

${d(myVar(d_1, d_2)) \over dt} = 1; \forall d_1 \in [0, n_1], \forall d_2 \in [0, n_2]$

the following can be used:

# Notation:
#  - myDomain1, myDomain2 are daeDomain objects
#  - n1 is the number of points in the myDomain1
#  - n2 is the number of points in the myDomain2
#  - eq is a daeEquation object distributed on the domains myDomain1 and myDomain2
#  - d is daeDEDI object (used to iterate through the domain points)
#  - myVar is daeVariable object distributed on the myDomaina and myDomain2
d1 = eq.DistributeOnDomain(myDomain1, eClosedClosed)
d2 = eq.DistributeOnDomain(myDomain2, eClosedClosed)
eq.Residual = dt(myVar(d1,d2)) - 1.0

This code translates into a set of n1 * n2 equations.

3. To get a partial derivative of a distributed variable the functions pyCore.d() and pyCore.d2() can be used. For instance, if a partial derivative of the distributed variable myVar has to be equal to 1.0 at each point of the domain myDomain:

${\partial myVar(d) \over \partial myDomain} = 1.0; \forall d \in [0, n]$

we can write:

# Notation:
#  - myDomain is daeDomain object
#  - n is the number of points in the myDomain
#  - eq is a daeEquation object distributed on the myDomain
#  - d is daeDEDI object (used to iterate through the domain points)
#  - myVar is daeVariable object distributed on the myDomain
d = eq.DistributeOnDomain(myDomain, eClosedClosed)
eq.Residual = d(myVar(d), myDomain, discretizationMethod=eCFDM, options={}) - 1.0

# since the defaults are eCFDM and an empty options dictionary the above is equivalent to:
eq.Residual = d(myVar(d), myDomain) - 1.0

Again, this code translates into a set of n equations.

The default discretisation method is center finite difference method (eCFDM) and the default discretisation order is 2 and can be specified in the options dictionary: options["DiscretizationOrder"] = integer. At the moment, only the finite difference discretisation methods are supported by default (but the finite volume and finite elements implementations exist through the third party libraries):

• Center finite difference method (eCFDM)
• Backward finite difference method (eBFDM)
• Forward finite difference method (eFFDM)

### 6.2.5. Ports¶

Ports can be used to provide an interface of a model, that is the model inputs and outputs. The models with ports can be used to create flowsheets where models are inter-connected by (the same type of) ports. Ports can be defined as inlet or outlet depending on whether they represent model inputs or model outputs. Like models, ports can contain domains, parameters and variables.

In DAE Tools ports are defined by deriving a new class from the base pyCore.daePort class. An empty port definition is presented below:

class myPort(daePort):
def __init__(self, name, parent = None, description = ""):
daePort.__init__(self, name, type, parent, description)

# Declaration/instantiation of domains, parameters and variables
...

The process consists of the following steps:

1. Calling the base class constructor:

daePort.__init__(self, name, type, parent, description)

2. Declaring domains, parameters and variables in the pyCore.daePort.__init__() function

The same rules apply as described in the Developing models section.

Two ports can be connected by using the pyCore.daeModel.ConnectPorts() function.

#### 6.2.5.1. Instantiating ports¶

Ports are instantiated in the pyCore.daeModel.__init__() function:

self.myPort = daePort("myPort", eInletPort, parentModel, "description")

### 6.2.6. Event ports¶

Event ports are also used to connect two models; however, they allow sending of discrete messages (events) between models. Events can be triggered manually or when a specified condition is satisfied. The main difference between event and ordinary ports is that the former allow a discrete communication between models while latter allow a continuous exchange of information. Messages contain a floating point value that can be used by a recipient. Upon a reception of an event certain actions can be executed. The actions are specified in the pyCore.daeModel.ON_EVENT() function.

Two event ports can be connected by using the pyCore.daeModel.ConnectEventPorts() function. A single outlet event port can be connected to unlimited number of inlet event ports.

#### 6.2.6.1. Instantiating event ports¶

Event ports are instantiated in the pyCore.daeModel.__init__() function:

self.myEventPort = daeEventPort("myEventPort", eOutletPort, parentModel, "description")

### 6.2.7. Equations¶

There are four types of equations in DAE Tools:

• Ordinary or distributed
• Continuous or discontinuous

Distributed equations are equations which are distributed on one or more domains and valid on the selected points within those domains. Equations can be distributed on a whole domain, on a portion of it or even on a single point (useful for specifying boundary conditions).

#### 6.2.7.1. Declaring equations¶

Equations are declared in the pyCore.daeModel.DeclareEquations() function. To declare an ordinary equation the pyCore.daeModel.CreateEquation() function can be used:

eq = model.CreateEquation("MyEquation", "description")

while to declare a distributed equation:

eq = model.CreateEquation("MyEquation")
d = eq.DistributeOnDomain(myDomain, eClosedClosed)

Equations can be distributed on a whole domain or on a portion of it. Currently there are 7 options:

• Distribute on a closed (whole) domain - analogous to: $$x \in [x_0, x_n]$$
• Distribute on a left open domain - analogous to: $$x \in (x_0, x_n]$$
• Distribute on a right open domain - analogous to: $$x \in [x_0, x_n)$$
• Distribute on a domain open on both sides - analogous to: $$x \in (x_0, x_n)$$
• Distribute on the lower bound - only one point: $$x \in \{ x_0 \}$$
• Distribute on the upper bound - only one point: $$x \in \{ x_n \}$$
• Custom array of points within a domain: $$x \in \{ x_0, x_3, x_7, x_8 \}$$

where $$x_0$$ stands for the LowerBound and $$x_n$$ stands for the UpperBound of the domain.

An overview of various bounds is given in the table below. Assume that we have an equation which is distributed on two domains: x and y. The table shows various options while distributing an equation. Green squares represent portions of a domain included in the distributed equation, while white squares represent excluded portions.

 x = eClosedClosed; y = eClosedClosed $$x \in [x_0, x_n], y \in [y_0, y_n]$$ x = eOpenOpen; y = eOpenOpen $$x \in ( x_0, x_n ), y \in ( y_0, y_n )$$ x = eClosedClosed; y = eOpenOpen $$x \in [x_0, x_n], y \in ( y_0, y_n )$$ x = eClosedClosed; y = eOpenClosed $$x \in [x_0, x_n], y \in ( y_0, y_n ]$$ x = eLowerBound; y = eClosedOpen $$x = x_0, y \in [ y_0, y_n )$$ x = eLowerBound; y = eClosedClosed $$x = x_0, y \in [y_0, y_n]$$ x = eUpperBound; y = eClosedClosed $$x = x_n, y \in [y_0, y_n]$$ x = eLowerBound; y = eUpperBound $$x = x_0, y = y_n$$

#### 6.2.7.2. Defining equations (equation residual expressions)¶

Equations in DAE Tools are given in implicit (acausal) form and specified as residual expressions. For instance, to define a residual expression of an ordinary equation:

${\partial V_{14} \over \partial t} + {V_1 \over V_{14} + 2.5} + sin(3.14 \cdot V_3) = 0$

the following can be used:

# Notation:
#  - V1, V3, V14 are ordinary variables
eq.Residal = dt(V14()) + V1() / (V14() + 2.5) + sin(3.14 * V3())

To define a residual expression of a distributed equation:

${\partial V_{14}(x,y)) \over \partial t} + {V_1 \over V_{14}(x,y) + 2.5} + sin(3.14 \cdot V_3(x,y)) = 0; \forall x \in [0, nx], \forall y \in (0, ny)$

the following can be used:

# Notation:
#  - V1 is an ordinary variable
#  - V3 and V14 are variables distributed on domains x and y
eq = model.CreateEquation("MyEquation")
dx = eq.DistributeOnDomain(x, eClosedClosed)
dy = eq.DistributeOnDomain(y, eOpenOpen)
eq.Residal = dt(V14(dx,dy)) + V1() / ( V14(dx,dy) + 2.5) + sin(3.14 * V3(dx,dy) )

where dx and dy are pyCore.daeDEDI (which is short for daeDistributedEquationDomainInfo) objects. These objects are used internally by the framework to iterate over the domain points when generating a set of equations from a distributed equation. If a pyCore.daeDEDI object is used as an argument of the operator (), dt, d, d2, array, dt_array, d_array, or d2_array functions, it represents a current index in the domain which is being iterated. Hence, the equation above is equivalent to writing:

# Notation:
#  - V1 is an ordinary variable
#  - V3 and V14 are variables distributed on domains x and y
for dx in range(0, x.NumberOfPoints): # x: [x0, xn]
for dy in range(1, y.NumberOfPoints-1): # y: (y0, yn)
eq = model.CreateEquation("MyEquation_%d_%d" % (dx, dy) )
eq.Residal = dt(V14(dx,dy)) + V1() / ( V14(dx,dy) + 2.5) + sin(3.14 * V3(dx,dy) )

The second way can be used for writing equations that are different for different points within domains.

pyCore.daeDEDI class has the pyCore.daeDEDI.__call__ (operator ()) function defined which returns the current index as the adouble object. In addition, the class provides operators + and - which can be used to return the current index offset by the specified integer. For instance, to define the equation below:

$V_1(x) = V_2(x) + V_2(x+1); \forall x \in [0, nx)$

the following can be used:

# Notation:
#  - V1 and V2 are variables distributed on the x domain
eq = model.CreateEquation("MyEquation")
dx = eq.DistributeOnDomain(x, eClosedOpen)
eq.Residal = V1(dx) - ( V2(dx) + V2(dx+1) )

#### 6.2.7.3. Supported mathematical operations and functions¶

DAE Tools support five basic mathematical operations (+, -, *, /, **) and the following standard mathematical functions: sqrt, pow, log, log10, exp, min, max, floor, ceil, abs, sin, cos, tan, asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh, atan2, erf).

To define conditions the following comparison operators: < (less than), <= (less than or equal), == (equal), != (not equal), > (greater), >= (greater than or equal) and the following logical operators: & (logical AND), | (logical OR), ~ (logical NOT) can be used.

Nota bene

Since it is not allowed to overload Python’s operators and, or and not they cannot be used to define logical conditions; therefore, the custom operators &, | and ~ are defined and should be used instead.

#### 6.2.7.4. Interoperability with NumPy¶

The adouble class is designed with numpy library in mind. It redefines all standard mathematical functions available in numpy (i.e. sqrt, exp, log, log10, sin, cos etc.) so that the numpy functions also operate on the adouble objects. Therefore, the adouble class can be used as an ordinary data type in numpy. In addition, numpy and DAE Tools mathematical functions are interchangeable. In the example given below, the Exp() and numpy.exp() function calls produce identical results:

# Notation:
#  - Var is an ordinary variable
#  - result is an ordinary variable
eq = self.CreateEquation("...")
eq.Residual = result() - numpy.exp( Var() )

# The above is identical to:
eq.Residual = result() - Exp( Var() )

Often, it is desired to apply numpy/scipy numerical functions on arrays of adouble objects. In those cases the functions such as array(), d_array(), dt_array(), Array() etc. are NOT applicable since they return adouble_array objects. However, numpy arrays can be created and populated with adouble objects and numpy functions applied on them. In addition, an adouble_array object can be created from resulting numpy arrays of adouble objects, if necessary.

For instance, to define the equation below:

$sum = \sum\limits_{i=0}^{N_x-1} \left( V_1(i) + 2 \cdot V_2(i)^2 \right)$

the following code can be used:

# Notation:
#  - x is a continuous domain
#  - V1 is a variable distributed on the x domain
#  - V2 is a variable distributed on the x domain
#  - sum is an ordinary variable
#  - ndarr_V1 is one dimensional numpy array with dtype=object
#  - ndarr_V2 is one dimensional numpy array with dtype=object
#  - Nx is the number of points in the domain x

# 1.Create empty numpy arrays as a container for daetools adouble objects
ndarr_V1 = numpy.empty(Nx, dtype=object)
ndarr_V2 = numpy.empty(Nx, dtype=object)

# 2. Fill the created numpy arrays with adouble objects
ndarr_V1[:] = [V1(x) for x in range(Nx)]
ndarr_V2[:] = [V2(x) for x in range(Nx)]

# Now, ndarr_V1 and ndarr_V2 represent arrays of Nx adouble objects each:
#  ndarr_V1 := [V1(0), V1(1), V1(2), ..., V1(Nx-1)]
#  ndarr_V2 := [V2(0), V2(1), V2(2), ..., V2(Nx-1)]

# 3. Create an equation using the common numpy/scipy functions/operators
eq = self.CreateEquation("sum")
eq.Residual = sum() - numpy.sum(ndarr_V1 + 2*ndarr_V2**2)

# If adouble_aray is needed after operations on a numpy array, the following two functions can be used:
# Both return an adouble_array object.

#### 6.2.7.5. Details on autodifferentiation support¶

Fig. 6.2 Equation evaluation tree in DAE Tools

The equation F in Fig. 6.2 is a result of the following DAE Tools equation:

eq = model.CreateEquation("F", "F description")
eq.Residal = dt(V14()) + V1() / (V14() + 2.5) + Sin(3.14 * V3())

As it has been described in the previous sections, domains, parameters, and variables contain functions that return adouble/adouble_array objects used to construct the evaluation trees. These functions include functions to get a value of a domain/parameter/variable (operator ()), to get a time or a partial derivative of a variable (functions dt(), d(), or d2()) or functions to obtain an array of values, time or partial derivatives (array(), dt_array(), d_array(), and d2_array()).

Another useful feature of DAE Tools equations is that they can be exported into MathML or Latex format and easily visualised.

#### 6.2.7.6. Defining boundary conditions¶

Assume that a simple heat transfer needs to be modelled: heat conduction through a very thin rectangular plate. At one side (at y = 0) we have a constant temperature (500 K) while at the opposite end we have a constant flux (1E6 W/m2). The problem can be described by a single distributed equation:

# Notation:
#  - T is a variable distributed on x and y domains
#  - rho, k, and cp are parameters
eq = model.CreateEquation("MyEquation")
dx = eq.DistributeOnDomain(x, eClosedClosed)
dy = eq.DistributeOnDomain(y, eOpenOpen)
eq.Residual = rho() * cp() * dt(T(dx,dy)) - k() * ( d2(T(dx,dy), x) + d2(T(dx,dy), y) )

The equation is defined on the y domain open on both ends; thus, the additional equations (boundary conditions at y = 0 and y = ny points) need to be specified to make the system well posed:

$\begin{split}T(x,y) &= 500; \forall x \in [0, nx], y = 0 \\ -k \cdot {\partial T(x,y) \over \partial y} &= 1E6; \forall x \in [0, nx], y = ny\end{split}$

To do so, the following equations can be used:

# "Bottom edge" boundary conditions:
bceq = model.CreateEquation("Bottom_BC")
dx = bceq.DistributeOnDomain(x, eClosedClosed)
dy = bceq.DistributeOnDomain(y, eLowerBound)
bceq.Residal = T(dx,dy) - Constant(500 * K)  # Constant temperature (500 K)

# "Top edge" boundary conditions:
bceq = model.CreateEquation("Top_BC")
dx = bceq.DistributeOnDomain(x, eClosedClosed)
dy = bceq.DistributeOnDomain(y, eUpperBound)
bceq.Residal = - k() * d(T(dx,dy), y) - Constant(1E6 * W/m**2)  # Constant flux (1E6 W/m2)

### 6.2.8. PDE on unstructured grids using the Finite Elements Method¶

DAE Tools support numerical simulation of partial differential equations on unstructured grids using the Finite Elements method. Currently, DAE Tools use the deal.II library for low-level tasks such as mesh loading, assembly of the system stiffness and mass matrices and the system load vector, management of boundary conditions, quadrature formulas, management of degrees of freedom, and the generation of the results. After an initial assembly phase the matrices are used to generate DAE Tools equations which are solved together with the rest of the system. All details about the mesh, basis functions, quadrature rules, refinement etc. are handled by the deal.II library and can be to some extent configured by the modeller.

The unique feature of this approach is a capability to use DAE Tools variables to specify boundary conditions, time varying coefficients and non-linear terms, and evaluate quantities such as surface/volume integrals. This way, the finite element model is fully integrated with the rest of the model and multiple FE systems can be created and coupled together. In addition, non-linear finite element systems are automatically supported and the equation discontinuities can be handled as usual in DAE Tools.

DAE Tools provide four main classes to support the deal.II library:

1. dealiiFiniteElementDOF_nD

In deal.II it represents a degree of freedom distributed on a finite element domain. In DAE Tools it represents a variable distributed on a finite element domain.

2. dealiiFiniteElementSystem_nD (implements daeFiniteElementObject)

It is a wrapper around deal.II FESystem<dim> class and handles all finite element related details. It uses information about the mesh, quadrature and face quadrature formulas, degrees of freedom and the FE weak formulation to assemble the system’s mass matrix (Mij), stiffness matrix (Aij) and the load vector (Fi).

3. dealiiFiniteElementWeakForm_nD

Contains weak form expressions for the contribution of FE cells to the system/stiffness matrices, the load vector, boundary conditions and (optionally) surface/volume integrals (as an output).

4. daeFiniteElementModel

daeModel-derived class that use system matrices/vectors from the dealiiFiniteElementSystem_nD object to generate a system of equations in the following form:

$\left[ M_{ij} \right] \left\{ {dx_j}\over{dt} \right\} + \left[ A_{ij} \right] \left\{ x_j \right\} = \left\{ F_i \right\}$

This system is in a general case a DAE system, although it can also be a linear or a non-linear system (if the mass matrix is zero).

A typical use-case scenario consists of the following steps:

1. The starting point is a definition of the dealiiFiniteElementSystem_nD class (where nD can be 1D, 2D or 3D). That includes specification of:
• Mesh file in one of the formats supported by deal.II (GridIn)
• Degrees of freedom (as a python list of dealiiFiniteElementDOF_nD objects). Every dof has a name which will be also used to declare DAE Tools variable with the same name, description, finite element space FE (deal.II FiniteElement<dim> instance) and the multiplicity
• Quadrature formulas for elements and their faces
2. Creation of daeFiniteElementModel object (similarly to the ordinary DAE Tools model) with the finite element system object as the last argument.
3. Definition of the weak form of the problem using the functions provided by DAE Tools (for 1D, 2D and 3D):
• phi(): corresponds to shape_value in deal.II
• dphi(): corresponds to shape_grad in deal.II
• d2phi(): corresponds to shape_hessian in deal.II
• phi_vector(): corresponds to shape_value of vector dofs in deal.II
• dphi_vector(): corresponds to shape_grad of vector dofs in deal.II
• d2phi_vector(): corresponds to shape_hessian of vector dofs in deal.II
• div_phi(): corresponds to divergence in deal.II
• JxW(): corresponds to the mapped quadrature weight in deal.II
• xyz(): returns the point for the specified quadrature point in deal.II
• normal(): corresponds to the normal_vector in deal.II
• function_value(): wraps Function<dim> object that returns a value
• dof(): returns daetools variable at the given index (adouble object)
• dof_approximation(): returns FE approximation of a quantity as a daetools variable (adouble object)
• dof_hessian_approximation(): returns FE hessian approximation of a quantity as a daetools variable (adouble object)
• vector_dof_approximation(): returns FE approximation of a vector quantity as a daetools variable (adouble object)
• vector_dof_gradient_approximation(): returns FE approximation of a vector quantity as a daetools variable (adouble object)
• adouble(): wraps any daetools expression to be used in matrix assembly
• tensor1(): wraps deal.II Tensor<rank=1>
• tensor2(): wraps deal.II Tensor<rank=2>
• tensor2(): wraps deal.II Tensor<rank=3>

More information about the finite element method in DAE Tools can be found in the API reference and in Finite Element Tutorials (in particular the Tutorial deal.II 1 source code).

### 6.2.9. State Transition Networks¶

Discontinuous equations are equations that take different forms subject to certain conditions. For example, to model a flow through a pipe one can observe three different flow regimes:

• Laminar: if Reynolds number is less than 2,100
• Transient: if Reynolds number is greater than 2,100 and less than 10,000
• Turbulent: if Reynolds number is greater than 10,000

From any of these three states the system can go to any other state. This type of discontinuities is called a reversible discontinuity and can be described using IF(), ELSE_IF(), ELSE() and END_IF() functions:

IF(Re() <= 2100)                    # (Laminar flow)
#... (equations go here)

ELSE_IF(Re() > 2100 & Re() < 10000) # (Transient flow)
#... (equations go here)

ELSE()                              # (Turbulent flow)
#... (equations go here)

END_IF()

The comparison operators operate on pyCore.adouble objects and Float values. Units consistency is strictly checked and expressions including Float values are allowed only if a variable or parameter is dimensionless. The following expressions are valid:

# Notation:
#  - T is a variable with units: K
#  - m is a variable with units: kg
#  - p is a dimensionless parameter

# T < 0.5 K
T() < Constant(0.5 * K)

# (T >= 300 K) or (m < 1 kg)
(T() >= Constant(300 * K)) | (m < Constant(0.5 * kg))

# p <= 25.3 (use of the Constant function not necessary)
p() <= 25.3

Reversible discontinuities can be symmetrical and non-symmetrical. The above example is symmetrical. However, to model a CPU and its power dissipation one can observe three operating modes with the following state transitions:

• Normal mode
• switch to Power saving mode if CPU load is below 5%
• switch to Fried mode if the temperature is above 110 degrees
• Power saving mode
• switch to Normal mode if CPU load is above 5%
• switch to Fried mode if the temperature is above 110 degrees
• Fried mode
• Damn, no escape from here... go to the nearest shop and buy a new one! Or, donate some money to DAE Tools project :-)

What can be seen is that from the Normal mode the system can either go to the Power saving mode or to the Fried mode. The same stands for the Power saving mode: the system can either go to the Normal mode or to the Fried mode. However, once the temperature exceeds 110 degrees the CPU dies (let’s say it is heavily overclocked) and there is no return. This type of discontinuities is called an irreversible discontinuity and can be described using STN(), STATE(), END_STN() functions:

STN("CPU")

STATE("Normal")
#... (equations go here)
ON_CONDITION( CPULoad() < 0.05,       switchToStates = [ ("CPU", "PowerSaving") ] )
ON_CONDITION( T() > Constant(110*K),  switchToStates = [ ("CPU", "Fried") ] )

STATE("PowerSaving")
#... (equations go here)
ON_CONDITION( CPULoad() >= 0.05,      switchToStates = [ ("CPU", "Normal") ] )
ON_CONDITION( T() > Constant(110*K),  switchToStates = [ ("CPU", "Fried") ] )

STATE("Fried")
#... (equations go here)

END_STN()

The function pyCore.daeModel.ON_CONDITION() is used to define actions to be performed when the specified condition is satisfied. In addition, the function pyCore.daeModel.ON_EVENT() can be used to define actions to be performed when an event is trigerred on a specified event port. Details on how to use pyCore.daeModel.ON_CONDITION() and pyCore.daeModel.ON_EVENT() functions can be found in the OnCondition actions and OnEvent actions sections, respectively.

### 6.2.10. OnCondition actions¶

The function ON_CONDITION() can be used to define actions to be performed when a specified condition is satisfied. The available actions include:

• Changing the active state in specified State Transition Networks (argument switchToStates)
• Re-assigning or re-ininitialising specified variables (argument setVariableValues)
• Triggering an event on the specified event ports (argument triggerEvents)
• Executing user-defined actions (argument userDefinedActions)

Nota bene

OnCondition actions can be added to models or to states in State Transition Networks (pyCore.daeSTN or pyCore.daeIF):

• When added to a model they will be active throughout the simulation
• When added to a state they will be active only when that state is active

Nota bene

switchToStates, setVariableValues, triggerEvents and userDefinedActions are empty by default. The user has to specify at least one action.

For instance, to execute some actions when the temperature becomes greater than 340 K the following can be used:

def DeclareEquations(self):
...

self.ON_CONDITION( T() > Constant(340*K), switchToStates     = [ ('STN', 'State'), ... ],
setVariableValues  = [ (variable, newValue), ... ],
triggerEvents      = [ (eventPort, eventMessage), ... ],
userDefinedActions = [ userDefinedAction, ... ] )

where the first argument of the ON_CONDITION() function is a condition specifying when the actions will be executed and:

• switchToStates is a list of tuples (string ‘STN Name’, string ‘State name to become active’)
• setVariableValues is a list of tuples (daeVariable object, adouble object)
• triggerEvents is a list of tuples (daeEventPort object, adouble object)
• userDefinedActions is a list of user defined objects derived from the base daeAction class

For more details on how to use ON_CONDITION() function have a look on tutorial_13.

### 6.2.11. OnEvent actions¶

The function ON_EVENT() can be used to define actions to be performed when an event is triggered on the specified event port. The available actions are the same as in the ON_CONDITION() function.

Nota bene

OnEvent actions can be added to models or to states in State Transition Networks (pyCore.daeSTN or pyCore.daeIF):

• When added to a model they will be active throughout the simulation
• When added to a state they will be active only when that state is active

Nota bene

switchToStates, setVariableValues, triggerEvents and userDefinedActions are empty by default. The user has to specify at least one action.

For instance, to execute some actions when an event is triggered on an event port the following can be used:

def DeclareEquations(self):
...

self.ON_EVENT( eventPort, switchToStates     = [ ('STN', 'State'), ... ],
setVariableValues  = [ (variable, newValue), ... ],
triggerEvents      = [ (eventPort, eventMessage), ... ],
userDefinedActions = [ userDefinedAction, ... ] )

where the first argument of the ON_EVENT() function is the daeEventPort object to be monitored for events, while the rest of the arguments is the same as in the ON_CONDITION() function.

For more details on how to use ON_EVENT() function have a look on tutorial_13.

## 6.3. Configuration files¶

Various options used by DAE Tools objects are located in the daetools/daetools.cfg config file (in JSON format). The configuration file can be obtained using the global function daeGetConfig():

cfg = daeGetConfig()

which returns the daeConfig object. The configuration file is first searched in the HOME directory, the application folder and finally in the default location. It also can be specified manually using the function daeSetConfigFile(). However, this has to be done before the DAE Tools objects are created. The current configuration file name can be retrieved using the ConfigFileName attribute. The options can also be programmatically changed using the Get/Set functions i.e. GetBoolean()/SetBoolean():

cfg = daeGetConfig()
checkUnitsConsistency = cfg.GetBoolean("daetools.core.checkUnitsConsistency")
cfg.SetBoolean("daetools.core.checkUnitsConsistency", True)

Supported data types are: Boolean, Integer, Float and String. The whole configuration file with all options can be printed using:

cfg = daeGetConfig()
print(cfg)

The sample configuration file is given below:

{
"daetools":
{
"core":
{
"checkForInfiniteNumbers": false,
"eventTolerance": 1E-7,
"pythonIndent": "    ",
"checkUnitsConsistency": true,
"resetLAMatrixAfterDiscontinuity": true,
"printInfo": false,
"deepCopyClonedNodes": true
},
"activity":
{
"timeHorizon": 100.0,
"reportingInterval": 1.0,
"objFunctionAbsoluteTolerance": 1E-8,
"constraintsAbsoluteTolerance": 1E-8,
"measuredVariableAbsoluteTolerance": 1E-8
},
"datareporting":
{
},
"logging":
{
"tcpipLogPort": 51000
},
"minlpsolver":
{
"printInfo": false
},
"IDAS":
{
"relativeTolerance": 1E-5,
"nextTimeAfterReinitialization": 1E-7,
"printInfo": false,
"numberOfSTNRebuildsDuringInitialization": 1000,
"SensitivitySolutionMethod": "Staggered",
"SensErrCon": false,
"maxNonlinIters": 3,
"sensRelativeTolerance": 1E-5,
"sensAbsoluteTolerance": 1E-5,
"MaxOrd": 5,
"MaxNumSteps": 1000,
"InitStep": 0.0,
"MaxStep": 0.0,
"MaxErrTestFails": 10,
"MaxNonlinIters": 4,
"MaxConvFails": 10,
"NonlinConvCoef": 0.33,
"SuppressAlg": false,
"NoInactiveRootWarn": false,
"NonlinConvCoefIC": 0.0033,
"MaxNumStepsIC": 5,
"MaxNumJacsIC": 4,
"MaxNumItersIC": 10,
"LineSearchOffIC": false
},
"superlu":
{
"factorizationMethod": "SamePattern_SameRowPerm",
"useUserSuppliedWorkSpace": false,
"workspaceSizeMultiplier": 3.0,
"workspaceMemoryIncrement": 1.5
},
"BONMIN":
{
"IPOPT":
{
"print_level": 0,
"tol":1E-5,
"linear_solver": "mumps",
"hessianApproximation": "limited-memory",
}
},
"NLOPT":
{
"printInfo": true,
"xtol_rel": 1E-6,
"xtol_abs": 1E-6,
"ftol_rel": 1E-6,
"ftol_abs": 1E-6,
"constr_tol": 1E-6
}
}
}

## 6.4. Units and quantities¶

There are three classes in the framework: base_unit, unit and quantity. The base_unit class handles seven SI base dimensions: length, mass, time, electric current, temperature, amount of substance, and luminous intensity (m, kg, s, A, K, mol, cd). The unit class operates on base units defined using the base seven dimensions. The quantity class defines a numerical value in terms of a unit of measurement (it contains the value and its units).

There is a large pool of base units and units defined (all base and derived SI units) in the pyUnits module:

• m
• s
• cd
• A
• mol
• kg
• g
• t
• K
• sr
• min
• hour
• day
• l
• dl
• ml
• N
• J
• W
• V
• C
• F
• Ohm
• T
• H
• S
• Wb
• Pa
• P
• St
• Bq
• Gy
• Sv
• lx
• lm
• kat
• knot
• bar
• b
• Ci
• R
• rd
• rem

and all above with 20 SI prefixes:

• yotta = 1E+24 (symbol Y)
• zetta = 1E+21 (symbol Z)
• exa = 1E+18 (symbol E)
• peta = 1E+15 (symbol P)
• tera = 1E+12 (symbol T)
• giga = 1E+9 (symbol G)
• mega = 1E+6 (symbol M)
• kilo = 1E+3 (symbol k)
• hecto = 1E+2 (symbol h)
• deka = 1E+1 (symbol da)
• deci = 1E-1 (symbol d)
• centi = 1E-2 (symbol c)
• milli = 1E-3 (symbol m)
• micro = 1E-6 (symbol u)
• nano = 1E-9 (symbol n)
• pico = 1E-12 (symbol p)
• femto = 1E-15 (symbol f)
• atto = 1E-18 (symbol a)
• zepto = 1E-21 (symbol z)
• yocto = 1E-24 (symbol y)

for instance: kmol (kilo mol), MW (mega Watt), ug (micro gram) etc.

New units can be defined in the following way:

rho = unit({"kg":1, "dm":-3})

The constructor accepts a dictionary of base_unit : exponent items as its argument. The above defines a new density unit $$\frac{kg}{dm^3}$$.

The unit class defines mathematical operators *, / and ** to allow creation of derived units. Thus, the density unit can be also defined in the following way:

mass   = unit({"kg" : 1})
volume = unit({"dm" : 3})
rho = mass / volume

Quantities are created by multiplying a value with desired units:

heat = 1.5 * J

The quantity class defines all mathematical operators (+, -, *, / and **) and mathematical functions.

heat = 1.5 * J
time = 12 * s
power = heat / time

Units-consistency of equations and logical conditions is strictly enforced (although it can be switched off, if required). For instance, the operation below is not allowed:

power = heat + time

since their units are not consistent (J + s).

## 6.5. Data Reporters¶

There is a large number of available data reporters in DAE Tools:

The best starting point in creating custom data reporters is daeDataReporterLocal class. It internally does all the processing and offers to users the Process property (daeDataReceiverProcess instance) which contains all domains, parameters and variables in the simulation.

The following functions have to be implemented (overloaded):

• Connect(): Connects the data reporter. In the case when the local data reporter is used it may contain a file name, for instance.
• Disconnect(): Disconnects the data reporter.
• IsConnected(): Checks if the data reporter is connected or not.

All functions must return True if successful or False otherwise.

An empty custom data reporter is presented below:

class MyDataReporter(daeDataReporterLocal):
def __init__(self):
daeDataReporterLocal.__init__(self)

def Connect(self, ConnectString, ProcessName):
...
return True

def Disconnect(self):
...
return True

def IsConnected(self):
...
return True

To write the results into a file the daeDataReporterFile base class can be used. It writes the data into a file in the WriteDataToFile() function called in the Disconnect() function. The only function that needs to be overloaded is WriteDataToFile() while the base class handles all other operations.

daeDataReceiverProcess class contains the following properties that can be used to obtain the results data from a data reporter:

The example below shows how to save the results to the Matlab .mat file:

class MyDataReporter(daeDataReporterFile):
def __init__(self):
daeDataReporterFile.__init__(self)

def WriteDataToFile(self):
mdict = {}
for var in self.Process.Variables:
mdict[var.Name] = var.Values

import scipy.io
scipy.io.savemat(self.ConnectString,
mdict,
appendmat=False,
format='5',
long_field_names=False,
do_compression=False,
oned_as='row')

The filename is provided as the first argument (connectString) of the Connect() function.