Statistics – Computation
Scientific paper
Jun 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991pggp.rept..424w&link_type=abstract
In NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1990 p 424-425 (SEE N92-10728 01-91)
Statistics
Computation
Astronomical Models, Coagulation, Computer Programs, Computerized Simulation, Cosmology, Kolmogoroff Theory, Nebulae, Particle Collisions, Turbulence Effects, Aerodynamics, Algorithms, Collision Parameters, Computational Fluid Dynamics, Gas Dynamics, Particle Size Distribution, Turbulent Diffusion, Turbulent Mixing, Vortices
Scientific paper
An improved numerical code was constructed to model coagulation and settling of particles in disk nebula containing generic turbulence with arbitrary velocities in the gas. The turbulence is assumed to have a Kolmogorov eddy spectrum. Relative velocities of particles, which lead to collisions and possible coagulation, are computed as due all significant causes in their appropriate regimes: thermal motion, shear and inertial effects in turbulent eddies, and systematic motions due to settling and non-keplerian rotation of the gas. Significant improvements to this program were produced. One significant problem was the disparity of timescales for turbulent mixing and coagulation. To accurately compute the former, the timestep must be shorter than the smallest spatial scale (layer thickness) divided by the turbulent velocity. However, the size distribution often varies due to coagulation on much longer timescales. To minimize the computational overhead associated with collisions between particles of all sizes, a dual timestep was introduced. Collisional changes in the size distribution are computed once in every N substeps, where the substep is controlled by the turbulent diffusion velocity, and N is determined by the rate of collisions. This algorithm allowed simulations to be extended to longer times and later stages.
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