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

Let me explain the code

Here we require the basic system.a4l 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 the variables for our model.

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

IS underscore 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

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 result.

A converged message is displayed after the successful completion of the calculations.

• squared the system automatically
• by keping the values of two variables y and d as default values
• and calculated the values for rest of the variables.

Fix d by selecting the fixed option.

Repeat the same for x.

• make x and d as our fixed variables'Bold text by assigning values to them and solve for a, y and z'.

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 results 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 assign values 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.

I will refer := (colon and equal to) sign as is equal to from here on.

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 variables a and d and calculated the values for x, y and z.

Type METHOD values and then type a = 1, d= 2 and END values

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

Here we will compute the mass of a cylinder.

Let me explain the code.

Open cylinder.a4c with a text editor.

• as compared to systems library, used earlier in the last model

• atoms is a refined version of systems library.
• and contains the definition of various variable types
• that represent different physical units

• 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 models are dimensionally consistent.

Here we write V = 100 cm cube and p = 100 kg per metre cube.

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

In this tutorial,

We have learnt how to

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

Assignment

1. Edit the model cylinder in a text editor and make it dimensionally inconsistent
2. For example, write m is a distance instead of mass
3. Now run this dimensionally inconsistent model in ASCEND and observe the output
4. Add more variables and equations to the model cylinder.a4c and solve it in ASCEND

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