You can release a distribution of particle sizes by selecting one of the options Specify particle diameter or Specify particle mass from the Particle Size Distribution list in the physics interface Additional Variables section. While solving for particle diameter or mass, a text field for the Accretion rate R (SI unit: kg/s) is shown in the settings window for the Particle Properties feature.

The accretion rate is the time derivative of particle mass. If the particle mass mp (SI unit: kg) is solved for, its governing equation is

If the particle diameter dp (SI unit: m) is solved for, its governing equation is

where ρp (SI unit: kg/m3) is the particle density.

When the particle diameter or mass is solved for by selecting Specify particle diameter or Specify particle mass, the interpretation of Newton’s second law of motion for each particle is

The physical interpretation of Equation 5-17 is that the mass gained or lost by the model particle is moving with the particle’s velocity, whereas the physical interpretation of Equation 5-18 is that the mass gained or lost by the model particle is stationary with respect to the coordinate system in which the geometry is defined.

When solving for particle diameter or mass, Equation 5-17 is solved. To instead solve Equation 5-18, so that mass gained or lost by the particle affects its velocity such that the momentum of the model particle is conserved, select Uniform size from the Particle size distribution list and instead either enter a time-dependent expression for the particle mass directly in the settings for the Particle Properties node, or express the particle mass in terms of a user-defined auxiliary dependent variable.