nbsim: A N-Body Simulation

A command line tool simulating the gravitational interactions of planetary bodies.

Setup

To get started, clone this repo and compile the code. With g++: g++ -O3 -I ./include/nbsim src/* -o nbsim Note that the -O3 flag is important here to maximize performance. If using other compilers, make sure the optimization setting is as high as possible. Then to run the program, run: ./nbsim <timestep> <theta> <iterations> [options] where:

• timestep - the amount of time elapsed between each simulation evalution in seconds.
• theta - parameter dictating granularity of simulation. Limited to be in interval [0,1]. Larger value leads to better accuracy, but significantly lower performance. Value of 1 is equivalent to brute-force algorithm. 0.5 is common value.
• iterations - the number of iterations to run the simulation for.

Run ./nbsim -h to learn more about options, or see the Full Options List.

Quick Start

Here are some commands to get started quickly without worrying about different runtime options:

• To run with random input: ./nbsim 10 0.5 100 -r 10 -o temp.json This will randomly generate 10 objects, simulate their gravitational iteraction for 10 iterations with 10 seconds in between each evaluation, and then outputs the simulation results to the "temp.json" file.
• To run with custom input: ./nbsim 10 0.5 100 -i input.json -o temp.json This will simulate the gravitational interactions between the objects specified in "input.json" for 10 iterations with 10 seconds in between each evaluation, and then outputs the simulation results to the "temp.json" file. A sample "input.json" is given here: (Earth and Moon*)

{
    "bodies": [
        {
            "mass": 5.972e24,
            "position": {
                "x": 0,
                "y": 0,
                "z": 0
            },
            "velocity": {
                "x": 0,
                "y": 0,
                "z": 0
            },
            "acceleration": {
                "x": 0,
                "y": 0,
                "z": 0
            }
        },
        {
            "mass": 7.347e22,
            "position": {
                "x": 0,
                "y": 1737100,
                "z": 0
            },
            "velocity": {
                "x": 1022,
                "y": 0,
                "z": 0
            },
            "acceleration": {
                "x": 0,
                "y": 0,
                "z": 0
            }
        }
    ]
}

* - The moon's radius of orbit is on average 1,737 kilometers and its velocity is on average 1,022 meters per second. While not wholly circular, as assumed for there to be a fixed orbit, choosing an approximate starting radius should allow for approximate behavior to be simulated.

Full Options List

• -o,--output <filename> Specifies filename, the output file for simulation results. If not specified,prints output to console.
• -i,--input <filename> Specifies filename, the input file to read objects from
• -r,--random <n> Randomly generates n objects to simulate. Default simulation width is set to 1e10 meters, but can be expanded by specifying the -w option
• -v,--verbose Prints verbose output messages on simulation progress.
• -h,--help Prints a help message listing options and arguments.