# Tutorial

## Dispersion relation ﬁnder tutorial

While the graphic interface for the dispersion relation ﬁnder is very simple, the use of the program is not entirely intuitive. This tutorial will walk the user through an example in an effort to familiarize him or her with the application’s operation. It will be necessary to download the ﬁles at the top of the page in order to follow along. Also, please note that the time required to plot depends on the speed of the computer running it. For a 1 GHz laptop, ‘argument plots’ require less than 10 seconds, and the actual dispersion relation plotting requires 25 minutes. The vertical bar to the right of the display window will be yellow when the program is processing a command.

- Download the ﬁles Download the following ﬁles into an easily accessible area.
- Deﬁning a dispersion function
- Inputing a function in the dispersion relation ﬁnder program
- Zooming in on roots
- Tracing Once the roots of interest have been isolated, click on the ‘Trace’ button. This will initiate the DART algorithm, which will proceed to ﬁnd the roots in the selected area. Depending on the size of the area and the degeneracy of the roots, this can take anywhere from 5 seconds to 1 minute. Once the roots have been located, the program will start to increment from \(\omega_{\textrm{min}}\) to \(\omega_{\textrm{max}}\) and will trace the migration of the roots. If at any point a root is lost, a new ‘argument plot’ will appear in the left hand column centered at the location of the last known root. It is up to the user to ﬁnd the vortex corresponding to the lost root, to zoom in on it, and to click ‘Trace’ again. The program will resume where it left off. The location of the root in the complex plane with increasing \(\omega\) is plotted in the left hand window during this process. The real and imaginary parts of \(k\) are depicted vs. \(\omega\) in the right hand window.
- Stopping and saving
- Reset

Sample Dis.nb

DART.nb

Open the ﬁle ‘Sample Dis.nb’. The deﬁned function, ‘dis’, is a sample dispersion relation drawn from Choueiri’s paper on the DART technique. Before proceeding it is necessary to run this notebook (press shift + enter) so that the function ‘dis’ will be deﬁned for future use.

Open the ﬁle ‘DART.nb’ and type the line shown in the ﬁgure below into the box titled ‘function’. The deﬁnition of the function must follow the standard Mathematica format,‘f[k ,w ] :=’. Also, the function’s name must always be f. When the function is input, choose minimum and maximum values for \(\omega_{\textrm{min}}\) and \(\omega_{\textrm{max}}\) respectively and click ‘enter’. In the left hand side, an ‘argument plot’ will appear with the real values of \(k\) on the x-axis and the imaginary values on the y-axis. At each point in the grid, a line is drawn that makes an angle with the real axis equal to the argument of the function at the corresponding value of k and ωmin. Vortices indicate where suspected roots are.

The next step is to isolate the roots of interest. Click once on the ‘argument plot’. This point will form the corner of a ‘zoom’ box. Move the cursor around and a pink rectangle will follow it. When the desired roots have been highlighted, click again. Within 1020 seconds, the ‘argument plot’ will be redrawn with boundaries equal to the boundaries of the zoom box. General zooming can also be performed by clicking the ‘+’ and ‘’ buttons to the left of the argument plot.

At any point, the user can stop the tracer by pressing ‘Stop’ and save the information to a text ﬁle by clicking ‘Save to File’. This will generate a dialog box that allows the user to select the location and name of the ﬁle. The output has three columns. The ﬁrst contains the values of ω. The second and the third have the real and imaginary values of \(k\) respectively for the root corresponding to \(\omega\). Note: It is possible to trace multiple roots at once with this program. However, the output of the text ﬁle will always have three columns, i.e. tracing multiple roots will not generate new sets of columns.

Once the tracer has completed its task or been stopped, the user can reset the program to display the argument plot at \(\omega_{\textrm{min}}\) again (or a diﬀerent value if desired). This can be accomplished in two ways. The ﬁrst is to click ‘reset’. This erases all of the data such that it no longer appears graphically and can no longer be saved. The second method is to click ‘Save trace and reset’. This will reset the program to \(\omega_{\textrm{min}}\) but retain the information on the calculated roots in an array. It will also continue to display the trace in the right hand window. This is beneﬁcial as it allows real time comparison of dispersion relations.

This concludes the brief tutorial. The user can use this as a guide for exploring other dispersion relations. It is recommended that the user deﬁne a function outside of DART and then input it into the ‘function’ ﬁeld–just as done in step 2. This will save room in the ﬁeld and minimize possible errors in deﬁning the function. If there are any any questions or comments, please feel free to contact the author at bjorns@princeton.edu. This is an open source code and free for download and updates.