2-D and 3-D modelling of low density ice accretion on rotating wires with variable surface irregularities

New results on two stochastic models, a 2-D and 3-D one, developed to simulate ice accrection processes at low values of impaction parameters are reported. The 2-D model of ice accretion on rotating cylindrical objects introduces fluid dynamics analytically by calculating the particle impaction direction as per the aerodynamic conditions; a polydisperse spectrum of particles is also used. Stokes numbers K in the range 1 ≤ K ≤ 7.8 were used in the numerical experiments. The numerical results on rotating collectors suggested an original theory of lobe formation, being interpreted as the outcome of an amplified growth of surface instabilities. An equation relating the angular frequency of lobes occurrence to the irregularities of the collector surface is introduced and tested for an experimental deposit with good result. A parameterization of the lobes growth is given by means of the droplets aerodynamics. The 3-D model employs a monodisperse droplet spectrum: the dependence of the density on the angular position with respect to the stagnation line shows that angular dependence of some properties of the 2-D pure ballistic aggregate is kept even in the 3-D. The value at maximum density in the numerical deposits matches values found in experimental deposits.