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Velocity dependent drag force (C)

This is a very simple example on how to implement a velocity dependent drag force. The example uses the IAS15 integrator, which is ideally suited to handle non-conservative forces. No gravitational forces or collisions are present.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>
#include "rebound.h"

void additional_forces(struct reb_simulation* const r);
void heartbeat(struct reb_simulation* const r);

double tmax = 40.;

int main(int argc, char* argv[]){
    struct reb_simulation* r = reb_create_simulation();
    // Setup constants
    r->dt             = 1e-4;        // initial timestep.
    r->integrator        = REB_INTEGRATOR_IAS15;
    r->gravity        = REB_GRAVITY_NONE;

    // Setup callback function for velocity dependent forces.
    r->additional_forces     = additional_forces;
    r->force_is_velocity_dependent = 1;
    // Setup callback function for outputs.
    r->heartbeat        = heartbeat;
    r->usleep        = 10000;        // Slow down integration (for visualization only)

    struct reb_particle p = {0};
    p.m      = 0;    // massless
    p.x     = 1;
    p.vx     = -1;
    reb_add(r, p);

    // Delete previous output
    system("rm -v r.txt");

    // Do the integration
    reb_integrate(r, tmax);

void additional_forces(struct reb_simulation* const r){
    // Simplest velocity dependent drag force.
    double dragcoefficient = 1;
    struct reb_particle* const particles = r->particles;
    const int N = r->N;
    for (int i=0;i<N;i++){
        particles[i].ax = -dragcoefficient*particles[i].vx;
        particles[i].ay = -dragcoefficient*particles[i].vy;
        particles[i].az = -dragcoefficient*particles[i].vz;

void heartbeat(struct reb_simulation* const r){
    // Output some information to the screen every 100th timestep
    if(reb_output_check(r, 100.*r->dt)){
        reb_output_timing(r, tmax);
    // Output the particle position to a file every timestep.
    const struct reb_particle* const particles = r->particles;
    FILE* f = fopen("r.txt","a");
    fprintf(f,"%e\t%e\t%e\n",r->t,particles[0].x, particles[1].vx);

This example is located in the directory examples/dragforce