ATMS 502 / CSE 566: Using the VisIt visualization program

VisIt is "a free interactive parallel visualization and graphical analysis tool for viewing scientific data on Unix and PC platforms." We will be using VisIt to help plot fields from our class program assignments.

Information

The VisIt home page is here; a Wikipedia page on it may be found here.
The VisIt user community wiki page is visitusers.org, and contains a short web tutorial.
Official documentation is available off the home site pages here.

Downloads

The downloads page contains many choices include source code and also executables for Linux, Mac and Windows systems.

Getting test data

You may find a few test data sets at: http://rfd.atmos.uiuc.edu/Visit-data/

Test data set: PeriodicBC

Below I describe visualizing the PeriodicBC.nc data set (.nc is a suffix for NetCDF data files). Click on any image to see it full sized.

First download the desired data set to your system. On Windows systems in our classroom, you will need to first create a scratch sub-directory for yourself (likely under D:) and put the data file there. After starting VisIt (on the mac, from Terminal), open the data file.
The windows should look something like shown at right. The PeriodicBC.nc file will be open, showing 5 data times (labeled cycle 0000 ... cycle 0004 in the control window on the left side of your screen). You next click on the "Add" button under Plots (again in the left control window).
Here I have selected the Contour plot type, and the field T (the only field of interest in this particular data set). In response, VisIt puts a line "Contour - T" in the Plots control window on the left. Nothing else happens yet, as you have the option to load up other graphics and/or to alter the plotting parameters for the plots, or to begin plotting. In this case, simply click on the Draw button under the Plots control window on the left side of your screen.
The result of clicking Draw. The default plot view shows you a sphere, looking down on the 3-D domain from above (so Z-axis is into the screen, X is to the right, and Y is towards the top). The data field being plotted ranges from 0 to -20, but all the "colder" values are within the sphere, so you see only the one surface (a cutaway would show more detail inside). Click on the DVD-like controls on the left window to step through the 5 data times. In so doing you will see the sphere disappear out the top (+Y) boundary and reappear in the bottom (-Y) boundary.
The viewpoint has been altered by clicking the mouse inside the plot window (on the right side of your screen) and moving the mouse while it is held down. This 'drags' the object you are viewing as if you were palming a ball. I have rotated the surface down and left so the view is from +X, -Y and above. The time has been stepped forward to cycle 0002 (3rd data time), when the sphere appears sliced open as it enters the -Y boundary.
Same viewpoint but when altering the surface (in Visit, "Contour") plot parameters. I have double-clicked on Contour - T in the Plots window and altered the parameters: Selected by was changed from N levels to Value(s), the value (constant surface) was chosen to be -2, the color table was set fixed by clicking Single, and I then clicked Apply and Dismiss to see my plot. The surface designating all points where the T field is -2.0 is shown; lower (colder) values are hidden within this sphere.

Test data set: Atms502fall09.nc

I have opened the data file Atms502fall09.nc . First I have made a simple 3-D surface plot. Under the Plots control window on left side of screen, I clicked on the Add button, chose Contour, and selected the W field (vertical velocity in meters/s). I then clicked on Draw, taking the default plot settings.

The viewpoint is from -Y, looking towards +Y; X is to the right, and +Z is towards the top. When done with this plot, click on Delete.

In this example, one field is being painted ('mapped onto') another. The 3-D surface is of the T field; the colors on it represent the V field, looking towards +Y and down. To produce this plot, click on Add; choose Pseudocolor, then V. Now click on "+/- Operators" and select Slicing, and then Isosurface. Then click on the white triangle to the left of the line "Pseudocolor - Isosurface(V)" to open up the plot settings below it, and double-click on Isosurface. Change Select by to Value(s), enter a value in the text box next to it as -3, and then click on Variable, select Scalars, and choose T. Click Apply, and Dismiss, and then Plot. The shape represents the temperature (T) field, with blues showing where negative V values lie, and tan colors representing V>0 on this same T surface.

Test data set: Atms502outflowA.nc

This data set shows colliding outflows in a quiet (calm) environment. I have set a pseudocolor choice of W (with Scale limits in pseudocolor of -15 to +15), mapped onto an Isosurface of T at -1.5˚. View is towards +X, +Y and down.

Test data set: Atms502outflowB.nc

This data set shows two combined outflows reaching the surface in the center of the domain. Plot choices are identical to Atms502outflowA.nc above.

Test data set: Atms502testc3.nc

Two colliding outflows with different characteristic V (y-direction) velocities. Plot choices are identical to the Atms502outflow cases above.