Geometric descriptors for the prediction of snowflake drag

The dynamics of solid particles of complex shapes such as airborne snowflakes are governed by aerodynamic drag forces that are a function of the relative velocity of the particle in the flow and the particle drag coefficient, which depends on the particle geometry and its orientation. In this study, artificial snowflakes are produced by additive manufacturing and their drag coefficients are obtained by measuring the terminal velocity in a liquid container, matching the Reynolds number typically encountered in natural occurrences. The experimental results show that the convex hull of the particle is suitable to accurately predict the drag force with existing correlations. Since it is unfeasible to accurately measure the three-dimensional geometries of natural snowflakes, the approximation with the convex hull provides a useful simplification. Furthermore, the known shapes of the artificial snowflakes are used to develop correlations to estimate the most relevant three-dimensional descriptors to predict the drag of snowflakes from a two-dimensional projection onto an arbitrary plane. Graphic abstract