Gerris Flow Solver Programming Course for Dummies

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Contents

Preface


This course is about Gerris, a general-purpose fluid mechanics code developped by Stephane Popinet at NIWA, Wellington, New Zealand .

The intended audience is typical first year graduate students with very little experience of C or some Fortran knowledge, but willing to work hard and learn.

Gerris is a free, open source code available at [[1]] .

Course 1



Introduction


This course is about how Gerris ([[2]] ) is programmed. It is intended to help in understanding the Gerris source code and learning how to modify it usefully.

This course is not about the numerical methods in Gerris (however it would be good for students of this course to learn about them, for instance in the J. Comput. Phys. article) .

Gerris is closely linked to Gts, the GNU triangulated surface library, also written by Stephane Popinet

Gerris and Gts are programmed in a style analogous to that of Glib, Gnome and GTK . It is a style of C programming that offers several advantages:

 * Most aspects of Object-Oriented Programming (OOP) , such as the existence of classes with their own methods and inheritance.
 * The ability to interface to other programming languages. (As far as I know, this feature is not used in Gerris/Gts)

To implement Object-Oriented Programming, Gerris/Gts uses its own “Object system”. This system is analogous to the Glib object system (Gobject), but not identical to it. Learning more about Gobject can be very useful.

References

References to the Gobject system (in order of ease of reading):


[[3]]

[[4]]

[[5]]

Useful tips


Navigate in emacs using the TAGS file

 - Create the TAGS file : 

% cd src % make tags

 - open any *.c or *.h file with emacs.
 - Position the cursor on a function or variable. 
 - do ESC . to find its definition. (or M-. using the emacs Meta key convention ) 
 - do ESC *   to return to the previous location (s) . 


No order in which to read the code

There is no good order in which to read the code (I have not found it yet)


Keep a C precedence and associativity table nearby

A lot of macros and functions such g_assert come from the Glib. Keep a bookmark to the Glib documentation: [[6]]


C basics

Structures


struct Point {

 char  name;
 double  x, y;

};

An example of usage

main() {

struct Point my_point; /* declaration */

my_point.x = 0. ; my_point.y = 1.; my_point.name = ‘A’; }

name, x and y are members of the structure of type “Point” called my_point. We also give it a name that can be exported (printed, passed to other functions) as a character. This example shows why it is useful to use structures to store several relevant informations or data together.

An example: the structure GfsNorm in Gerris

struct _GfsNorm {

 gdouble bias, first, second, infty, w;

}; typedef struct _GfsNorm GfsNorm

From domain.h.

GfsNorm gfs_domain_norm_residual (GfsDomain * domain, FttTraverseFlags flags, gint max_depth, gdouble dt, GfsVariable * res) {

 GfsNorm n;
 gpointer data[2];
 g_return_val_if_fail (domain != NULL, n);
 g_return_val_if_fail (res != NULL, n);
 
 gfs_norm_init (&n);
 data[0] = res;
 data[1] = &n;
 gfs_domain_cell_traverse (domain, FTT_PRE_ORDER, flags, max_depth, 

(FttCellTraverseFunc) add_norm_residual, data);

  1. ifdef HAVE_MPI
 domain_norm_reduce (domain, &n);
  1. endif /* HAVE_MPI */
 gfs_norm_update (&n);
 dt *= dt;
 n.bias *= dt;
 n.first *= dt;
 n.second *= dt;
 n.infty *= dt;
 return n;

}

From domain.c

Notice the use of

 * glib basic types gdouble, gpointer
 * glib functions g_return_val_if_fail

Structures are related to each other

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