Test Runs

April 2001

In our simulations, the orbits of bodies are perturbed by halo fluctuations and by the numerical approximations made in force calculation and orbital integration. The fluctuations are the effect we wish to study, while the numerical errors are artifacts which tend to mask the signal from the halo. Here, short calculations are used to determine the time-step and force-calculation parameters.


These calculations adopted the initial conditions with κ = 1.35 used in the initial data tests. Below, results are shown for four runs with different time-steps and force-calculation parameters; the `quick scan' runs use a fast version of the tree code which constructs minimal interaction lists, while the run with θ = 0.8 imposes a further opening-angle criterion which improves the force accuracy. All runs included quadrupole moments.

Δt = 1/16
quick scan
Δt = 1/32
quick scan
Δt = 1/32
κ = 0.8
Δt = 1/64
quick scan

These images are linked to animations showing how the binding energy of each body changes over 8 time units. On the whole, all four runs yield similar results. But in the run with the longest time-step a small number of bodies suffer relatively large changes in binding energy; this does not happen in the other runs. Thus a time-step Δt = 1/32 seems necessary for accurate integration, but further refinements may not be needed.


Joshua E. Barnes (barnes@ifa.hawaii.edu)

Last modified: May 8, 2002
http://www.ifa.hawaii.edu/~barnes/research/halo_noise/test_runs.html