Gnome Flow Users Guide

3. Using Gnome Flow

IMPORTANT NOTICE

Don't panic if you get a warning when you start Gnome FLow for the first time, saying that GFP_NOT_FOUND_IN_DATADIR - Please load a ... gradient manually!. It only means that there might be some sort of misconfiguration in your Gnome installation. During installation, the program installs some gradient (.gfp) files in `gnome-config --datadir`/gflow/palettes and some objects in `gnome-config --datadir`/gflow/objects. This is also the place, where Gnome Flow looks for the data as default (ie. when you run the program for the first time, and don't have any sessions saved) using the gnome_datadir_file("/palettes/somefile") library call. These two datadirs should be the same, but I have noticed some (as of writing only one) Madrake 7.2 installation, where the files were installed correctly, but the library call did not work properly. If this happens, just locate the files and load one of them - it will be remembered for the next time! And don't forget to load a gradient for both the Stream, Vorticity and Pressure!

3.1 Quickstart

This section is a short tutorial on how you can begin doing some simulations assuming, that you passed the situation described in the IMPORTANT NOTICE.
The default parameter settings should be all right to begin with, but if you have a slow computer, you might consider changing the Iter parameter to something like 1000 milliseconds.

Start Gnome Flow by typing gflow &
Load a simple object like cylinder.gfd
Begin iterating by pressing Calculate
The first results should be plotted in 10 seconds (you can chage this by changing the value of Plot)
You can change any parameters except Iter while the program is calculating. It is however discouraged to change the physical parameters during the same calculation.
If you want to change Iter, you must Stop and, after changing the value, Resume the calculation. (The difference between Calculate and Resume is that Calculate resets the values of the Stream and Vorticity while Resume does not.)
When the iteration converges ie. the maximal changes in Stream and Vorticity are less the Stream Lim. and Vort. Lim., the program stops iterating automaticly, and you will be asked, wether to calculate the pressure or not. The calculation of the pressure runs after the same timer values as for the stream and vorticity.
You can save your results as ASCII files containing XYZ data by pressing Save. After entering a filename, you will have four files: filename.psi filename.xi filename.p filename.par containing the data from the Stream, Vorticity, Pressure and the Parameters.
Now have done your first simulation and can go on playing with other objects, parameters and colors!

3.2 Not so Quick-Start

When started for the first time, Gnome Flow loads the default values for the physical and iteration parameters. The following table gives an overview of the parameters:

Inc. Vel.	Incident velocity
Visc	Viscosity
Reynolds	Reynolds number V₀/Nu
Omega	Relaxation parameter
Stream Lim.	Limit for stopping (Stream)
Vort. Lim.	Limit for stopping (Vorticity)
Pres. Lim.	Limit for stopping (Pressure)
Iter	How often to make an iteration
Plot	How often to plot the graphs

The last two parameters needs some comments. I wanted to make the program interactive, so that the parameters could be changed while calculating. That would not have been possible with a simple for-loop (in that case the program would have been frozen during calculations), so instead a timer calls the iteration functions at regular intervals. These intervals can be optimized (Iter) for the particular computer, the program is running on. The Plot parameter is another timer value for how often to update the graphics. It should be much larger than the Iter parameter.

To make some calculations, load an object by pressing Open. You can begin an iteration by pressing Calculate. You can change the parameters during calculation (except for the timer), and you can also stop a calculation by pressing Stop. Sometimes, if your Omega is not small enough, the iterations will stop by themselves because of divergence! If you want to improve a calculation, lower the limits and press Resume. The difference between Calculate and Resume is, that Calculate resets the values of the Stream, Vorticity and Pressure, while Resume continues with the present values. When you load a new object, you should always begin calculations with Calculate.

You can save your results in plain ASCII files containing XYZ data by pressing Save. The program will create four files containing the stream (filename.psi), vorticity (filename.xi), pressure (filename.p) and the parameters (filename.par). You can then plot your result in your favourite plotting program.

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