Difference between revisions of "ASCEND/C2/Writing-basic-model/English"

Title slide

Learning Objectives

We will learn how to

• Build a basic model
• Add methods to the basic model and
• Write dimensionally consistent equations

System Requirement

• Ubuntu Linux OS v. 12.04
• ASCEND v. 0.9.8

Pre-requisites

Please visit ascend4.org

For relevant tutorials please visit our website

http://spoken-tutorial.org

Example

In this example, let us solve 3 simultaneous equations with 3 unknowns

We will use a text editor like gedit to write our model

I already have the code for the example in a text file

Let me explain the code

Here we require the basic system.a4l library

.a4l extension denotes the ascend4 library

the name of the library is written in double quotes

ASCEND considers all the statements as one sentence if semi colon is not put after each statement

You can give any name to the model

Comments are not executed

They make the model easier to understand

We will now define variables for our model

The definition of the type solver_var is predefined in the system.a4l library

IS_A is an operator in ASCEND which is used to define variable types

x+y+z = a*d

x+y+2*z = 5 and

5*x+2*y-3*z = 0

Note that we have not mentioned how to solve this given set of equations.

We have also not mentioned which variables are to be specified and which are to be calculated

This can be seen from the message in the panel below

ASCEND displays the results

A converged message is displayed after the successful completion of the calculation

Keping the values of two variables y and d as default value and calculated the value for rest of the variables

Fix d by selecting the fixed option

Repeat the same for x

Then, right click on d and select properties from the drop down menu

Under values, assign a value to d

Here I am giving the value 1

Now in solving section, select fixed

Click on Apply and OK

Now similarly repeat the same procedure for fixing the value of x

Note that the fixed variables are shown in green color and free variables, calculated by ASCEND are shown in blue color

Also note that the final result has been displayed with a tick mark and the fixed variables have been denoted by a closed lock icon

We can also specify the fixed and free variables in the code and assigning value to the variables

In ASCEND the solution procedure is separate from the problem statement

Methods contain procedures to solve the model

Open the file my_model.a4c in a text editor

We write the methods after equations

Delete the parenthesis and asterik mark in front of methods and at the end of the code

We also assign values to the variables as a = 1 and d=3,note that to assign the values to a particular variable, we use a : and = sign together

I will refer := to sign as is equal to from here on

Under method on_load we run the method specify

This is the method that describes ASCEND the action to be taken when a model is loaded

Click on run

Observe that ASCEND has fixed the variable a and d and calculated the values for x,y and z

Now we will add a new method called values

Type method values and then type a = 1, d= 2 and end values

Now under on_load type run values after run specify

You will see ASCEND has fixed variable a and d and assigned the values as mentioned under values section in the code

Here we will compute the mass of a cylinder

I already have the code for this model

Let me explain the code

Open cylinder.a4c with a text editor

atoms is a refined version of systems library

and contains the definition of various variable types that represent different physical units

For example we write v is a volume

m is a mass etc.

Volume, mass etc. are different variable types defined in atoms.a4l

This helps in ensuring that the equations written in ASCEND model are dimensionally consistent

After this we write equations and methods as explained earlier

Note that while specifying values for these variable types, we need to mention the corresponding physical units in curly brackets

Here we write V = 100 cm^3 and p = 100kg/m^3

Note that V is in CGS units while p is in SI units

The user doesn't have to worry about the units as ASCEND itself performs the unit conversion

Now once again run the on_load method

and solve the problem

Note that ASCEND by default uses SI units until specified otherwise

Summary

We have learned how to

• Build a basic model
• Add methods to the basic model and
• Write dimensionally consistent equations

Assignment

Edit the model cylinder in a text editor and make it dimensionally inconsistent

For example, write m is a distance instead of mass

Now run this dimensionally inconsistent model in ASCEND and observe the output

Add more variables and equations to the model cylinder.a4c and solve it in ASCEND

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