Code a simple flow solver

Computational analysis of flow problems has become an important piece of equipment in the toolbox for science and engineering. The problem of finding approximate solutions to the equations for fluid flow may be reduced to evaluating a numerical recipe, following from a suitable discretization for the problem. These pages aim to provide a tutorial-styled course on numerical solvers for flow problems. It is not only a step-by-step guide on howto write a flow solver in the C-programming language, also, a discussion on the various solver-design choices is included. C is chosen here as it is the perfect didactical language for learning how to program due to its strict syntax.

Pre-requisites

Apart from time, interest and a good spirit, a few easier-to-come-by assets are needed to follow along with this course. First, you will need to have a computer that provides you with a C compiler. On a Linux system you may open a terminal-emulator (terminal) and type

$ gcc -v

and see something like,

Using built-in specs.
...
gcc version 8.3.0 (Debian 8.3.0-6) 

Otherwise you can install it on a Debian-based distro,

$ sudo apt install gcc 

If you do not know what the above means, please consult the internet and search for “Install C compiler on your OS”, where you replace the italicized text (your OS) with the name of your operating system. From hereon, I will assume the C compiler is called gcc, although (like always) worthwhile alternatives exists. Furthermore, a so-called text editor is needed. This is a program that lets you write a plain text file. Examples include atom, vim and emacs. The course will mainly go between writing in the editor and typing commands in a termninal. If you prefer a different workflow (e.g. using an IDE), you may need to translate some instructions.

Chapter 0: systems check

This zeroth chapter introduces (and tests) the workflow. You may skip it if you already familiar with C programming. Open your text editor and make a new file that we will call hello.c. Type,

#include <stdio.h>

int main() {
    puts ("This is not a flow solver");
}

The program code consists of an #include directive which tells the C compiler to load the standard input-output header file (<stdio.h>), a function declaration (main()), and a corresponding code block that is marked by curly braces ({ .. }) and a function call to the “put string” function (puts), that takes a text string as an argument inbetween the round braces ((...)). Altough the C compiler knows that puts only takes a string as an argument, we need to mark our input string with the double quotation marks ("..."). Now we can save and exit the editor and come back to the terminal, where we compile our code. Make sure you are in the same folder as where you keep hello.c,

$ gcc hello.c

The absence of error messages marks a successfully run command. If you do get an error, read it carefully and try to fix it. The generated executable can be run from the command line as well,

$ ./a.out

Any C executable starts with executing the main() function, so pretty soon you should see appear,

This is not a flow solver

on your terminal screen. Great! Lets move on …

Continue to chapter 1

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