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NBodyLab is a web-based N-body simulation testbed for undergraduate astronomy students. It was developed for the Astronomical Computing Initiative at Pomona College and used initially to support the curriculum for a course in cosmology. Students can use the testbed to simulate stellar dynamics: dynamics of single galaxies (relaxation, scattering and collapse); interacting galaxies (collisions and mergers); and to explore interesting structures like tidal tails.
At Pomona, a liberal arts college, students work primarily with Microsoft products and some students, even physics seniors, are unfamiliar with Unix and/or programming. NBodyLab runs server-side, under Linux, and supports a CGI-based web interface for ease of use by students with little background in computing. Its open-source framework was designed to enable students with programming ability to learn scientific computing and refine the software components. NEMO programs provide commonly used data models and analysis functions. NEMO is a widely used, advanced stellar dynamics toolbox that runs under Unix. Students with astrophysics knowledge can add additional data models and analysis programs from NEMO, or write their own, and they are encouraged to master NEMO natively under Unix. A menu of data sets is also offered, including, for example, disk-halo-bulge models of Andromeda and the Milky Way, produced by the package GalactICS (Kuijken & Dubinski 1995). To accelerate the particle-particle force calculations, a desktop supercomputer card, the MD-GRAPE2, may optionally be used.
The model and/or datasets are transformed, if requested, and stacked into one data file in preparation for the numerical integration.
The force exerted on each of the N particles by all of the other N-1 particles is computed for each time step, from Newton's law of gravitation. By Newton's second law of motion, the force calculation has yielded the acceleration for each particle. Two first-order differential equations involving acceleration and velocity must be solved, using numerical integration, to estimate new positions and velocities of each particle for the next time step. Integration options include NEMO's hackcode1, 2nd order leapfrog, 2nd, 4th and 6th order symplectic, 4th order Runge-Kutta, and others. Except for hackcode1, all may run optionally with the MD-GRAPE2.
The MD-GRAPE2 is designed specifically to accelerate force calculations between a list of particles. The MD-GRAPE2 (molecular dynamics-gravity pipeline) card runs at 64 GFlops, peak, has a PCI interface, and can provide the computational power of, roughly, a 24 node Beowulf cluster for direct summation of forces. The MD-GRAPE board has memory for five hundred thousand particle positions. The card is manufactured in Japan, costs less then $20,000, and can also be used for forces used in molecular dynamics simulations.
The user specifies the plot observation plane, plot axes range, and whether the following should be produced: animated GIF, image and velocity maps, radial profiles, slit spectrogram simulation, and debugging information. These analyses products and displays are presented on the resulting web pages and are produced by corresponding NEMO programs.
An ASCII dataset is produced by the initial and final states. A binary-only Windows OpenGL viewer supplied by the MD-GRAPE2 vendor allows the user to visualize, rotate and scale the results in three-space.
Technical assistance from Dr. Bruce Elmegreen, IBM, RIKEN (the Japanese Institute of Physical and Chemical Research), and the Peta Computing Institute is gratefully acknowledged. NBodyLab was developed by Interconnect Technologies Corporation through a grant to Dr. Bryan Penprase of Pomona College from the Fletcher Jones Foundation.
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