A Numerical Study on the Motion of Snow Aggregates Freely Falling in Air

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Abstract
This paper reports the results of the first quantitative study on the flow fields of freely falling snow aggregates. The motions are investigated by performing numerical simulations of four snow aggregate models consisting of 2, 3, 4, and 5 conformally shaped dendrites of different sizes. The dendrites are attached primarily in a horizontal configuration, with some at small angles, to resemble commonly observed snowflakes. These models are assumed to represent an early stage of aggregate formation, with the largest dimension of the aggregates approximately 2.15–2.3 mm, corresponding to Reynolds numbers in the range Re = 44–54. The flow fields around the falling aggregates are determined by numerically solving the unsteady Navier–Stokes equations using a solver capable of six‐degrees‐of‐freedom (6‐DOF) motion. Distributions of pressure, velocity, and vorticity around the falling aggregates are shown, and empirical formulas for the drag coefficient, terminal velocity, and ventilation coefficient are provided. No simple relationship between the kinematic properties and aggregate dimension exists because of the highly asymmetric shapes and the many possible arrangements of the component dendrites. All aggregates appear to move in a preferred horizontal direction, possibly determined by the specific geometry of each particle. Such fall attitudes have important implications for the collision growth of snowflakes.

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