Other Research Interests

Dynamics of Granular Media

The behaviour of granular systems is a hot topic in physics these days. Check out the collaborative "Powder Page" on the subject. Also check out Sand Land and Granular Matter, featuring experiments, movies, and technical stuff.

Recent site listing from March 2000 Physics Today article:

o utexas o uchicago o rutgers

o hu-berlin o haverford o uni-stuttgart

o duke

Dan Starr, Andrew Markiel, and I are working with Gerald Seidler's group (the Microstructural Kinetics Laboratory in UW Physics) to study the topology of close packing. Check out the movies on Dan's home page! (unformatted).

On a related issue...

Inelastic Collapse

Ironically, a "perfect" collision code will inevitably run up against the phenomenon of inelastic collapse when the particle density is sufficiently high. This numerical artifact occurs when the code attempts to perform essentially an infinite number of collisions in a finite interval owing to continued dissipation from rapid successive collisions between a subset of the particles. In unpublished work, I have shown this can occur in 3D, following on from original 2D work by S. McNammara and W. Young (Phys Rev E 50:R28). One way to get around inelastic collapse is to "look ahead" for possible trouble spots (collisions between the same two particles over a very small interval) and then force the next collision to be perfectly elastic. Another more sophisticated approach is to store collisional energy as vibration modes in the particles and release this energy stochastically in subsequent collisions.

As a test of my code's ability to perform granular dynamics, I have been simulating the formation of sandpiles, like this one. Click on the snapshot to the left for a movie of an even more sophisticated test I generated recently. Check back here for more updates in the future!

Recently UW undergraduate Dan Starr has joined the crusade to understand granular dynamics and rubble piles. Check out hi s home page!

Fractal Aggregates

By allowing particles to stick at the point of contact in an N-body simulation, it is possible to build up fluffy aggregates that are reminiscent of fluffy grains studied in the laboratory. Such grains may be important in the early stages of planetesimal growth, since their fractal nature allow a limited kind of runaway agglomeration even while being stirred up by the turbulent gas in the primordial nebula. Eventually these grains would grow large enough to decouple from the gas, settle to the midplane, and begin to coagulate into larger planetesimals. We think!

Extra-solar Planets

I had the privelege of working with some of the pioneers in the search for extra-solar planets when I was an undergraduate at the University of British Columbia. At that time I worked on time-series analysis of their Precision Radial Velocity data and I also modeled the instrumental profile of their hydrogen-fluoride absorption cell and spectrograph. Since then my interests in this area have turned to explaining why so many of the extra-solar planets have such unexpected orbits. It seems the exciting new observations have simply resulted in more questions!

Yet More Interests

I am interested in N-body problems in general, and although much of my work to date has been in the field of planetesimal dynamics I am certainly interested in other areas. I worked with Bob Thompson at Cambridge to simulate the tidal stripping of small spiral galaxies in order to explain the morphological features of Centaurus A and its environs. My early code has been used by other groups to study similar problems, most notably by Rodrigo Ibata and Geraint Lewis to model the disruption of dwarf galaxies by our own galaxy. In earlier work with Geraint Lewis I adapted my tree code to a ln(r) potential for gravitational microlensing studies. In the future I would like to experiment with SPH or other hydrodynamical techniques in order to study giant planet formation.

Data Visualization & Public Outreach

I have considerable interest in methods of data visualization. To help with the interpretation of results from my numerical work I have developed visualization techniques that have led to the production of several videos illustrating models of tidal breakup and planet formation. Some of the animations feature full 3D rendering with ray tracing. The videos have been popular at talks, especially in more public forums such as astronomy society meetings. I have also generated VRML worlds to aid visualization. Finally, I have developed public-domain software for ``live'' animation of computer images. I enjoy working on these projects and sharing the results with the public. I hope to further refine these techniques and software to make them more efficient and easier to use, both for my own work and for use by others.

Last modified: May 11, 2001