A Java implementation of the N-body integrator of Hut, P., Makino, J. & McMillan, S.
- nbody_sh1.C: an N-body integrator with a shared but variable time step
- (the same for all particles but its size changing in time),
- using the Hermite integration scheme (4th order integrator).
- ref.: Hut, P., Makino, J. & McMillan, S., 1995,
- Astrophysical Journal Letters 443, L93-L96.
- note: in this first version, all functions are included in one file,
- without any use of a special library or header files. *_____________________________________________________________________________
- USAGE: This is a JAVA program run from the command line.
- The minimum required command line is:
- java nbody_sh1 input.dat
- Additional options can be specified in the following order:
- dt_param dt_tot dt_out dt_dia
- usage: nbody_sh1 [dt_param: step_size_control_parameter]
- [dt_out: output_interval] [dt_tot: total_duration]
- [dt_dia: time interval between diagnostics]
- "step_size_control_parameter" is a coefficient determining the
- the size of the shared but variable time step for all particles
- "diagnostics_interval" is the time between output of diagnostics,
- in the form of kinetic, potential, and total energy; with the
- -x option, a dump of the internal particle data is made as well
- "output_interval" is the time between successive snapshot outputs
- "total_duration" is the integration time, until the program stops
- Input/output are read/written from/to the standard i/o streams.
- Since all options have sensible defaults, the simplest way to run
- the code is by only specifying the i/o files for the N-body
- snapshots:
- java nbody_sh1 data_in.dat > data_out.dat
- The diagnostic information is written to file error.txt
- Note: if any of the times specified in the -e, -o, or -t options are not an
- an integer multiple of "step", output will occur slightly later than
- predicted, after a full time step has been taken. And even if they
- are integer multiples, round-off error may induce one extra step. *_____________________________________________________________________________
- External data format:
- The program expects input of a single snapshot of an N-body system,
- in the following format: the number of particles in the snapshot n;
- the time t; mass mi, position ri and velocity vi for each particle i,
- with position and velocity given through their three Cartesian
- coordinates, divided over separate lines as follows:
- n
- t
- m1 r1_x r1_y r1_z v1_x v1_y v1_z
- m2 r2_x r2_y r2_z v2_x v2_y v2_z
- ...
- mn rn_x rn_y rn_z vn_x vn_y vn_z
Code to generate specific types of snapshot has been included in the repository:
writeSphere: generates a spherical distribution of particles mkPlummer: generates a distribution of particles according to the Plummer model.
The outputs of these java programs can then be input into nbody_sh1. *
- Output of each snapshot is written according to the same format.
- Internal data format:
- The data for an N-body system is stored internally as a 1-dimensional
- array for the masses, and 2-dimensional arrays for the positions,
- velocities, accelerations and jerks of all particles.
- In order for the internal data to be passed to and from a method whilst
- being changed (without resorting to global variables),
- it is necessary to pass variables by reference which meant that t, epot and
- coll_time all had to be stored as single value arrays.
June 2016 David Reid (from v1 of the Art of Computational Physics by Piet Hut and Jun Makino.
animatePlot2.m is an Octave file used to generate plots of each snapshot output by nbody_sh1. These plots can then be combined into an animation using appropriate software.