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Code Configuration

This section describes the meaning of Sailfish configuration parameters.

The tunits parameter

Time Coordinate System

The State struct contains a time member representing "raw" simulation time. With G=1 and M=1, gas parcels at r=1 have orbital periods of ==2π== in raw time.

For user convenience, there's a "user time" t = state.time / tunits where tunits has default value 1.0. Setting tunits=6.283185 means status messages show t=1 after one orbit.

Key Features

  • Negative values: tunits values are multipliers of 2π (e.g., tunits=-1 equals tunits=6.283185)
  • Product files: Store user time in HDF5 key __time__
  • Time series: Output user time in "time" column
  • Checkpoints: Store raw simulation time in state/time

The central_object parameter

Point Mass Configurations

Three configurations control the gravitational setup:

"single" - Single point mass at grid center

Perfect for studying accretion disks around isolated objects.

"binary" - Two point masses with separation a (default 1.0, configurable)

  • Mass ratio set by mass_ratio parameter (secondary/primary, range 0.0-1.0)
  • Ideal for circumbinary disk simulations

"merger" - Like binary, but separation decreases following GW inspiral

  • For t < 0: a(t) = a0 * pow(-t, 0.25) where a0 = orbit_size_at_unit_time(q)
  • At t=0: Merger occurs forming single point mass

The beta parameter

Viscosity Control

Controls the bulk vs. dynamic viscosity ratio in the stress tensor formulation.

The stress tensor is calculated as:

Viscous Stress Tensor
τij = μ (∂xj/∂vi + ∂xi/∂vj + (β - 2/3) δij ⋅ ∇⋅v)

where β = ζ/μ (bulk/dynamic viscosity ratio).

Typical Values

Setup Type beta Value Purpose
Disk setups (steady, KITP) 0.0 Minimal bulk viscosity
Spreading ring 5/3 Significant bulk viscosity

Quick Reference

  • Use tunits=-1 for orbital time units
  • Set central_object=binary for circumbinary disks
  • Choose beta=0 for most disk simulations