Morphology and density of ice accreted on cylindrical collectors at low values of impaction parameter. II: Rotating depositsxs

The morphology and density of deposits grown on rotating cylinders of 0.5 cm radius, collecting droplets of median volume radius Δ = 10 μm, are studied. The wind speed and temperature are regulated so that Macklin's parameter and Stokes number are 0.5 < X < 7 μm m s −1 K −1 and 0.8 < K < 3.2 respectively. The knobby structure typical of rotating deposits is shown to form after the formation of the first 0.5–1 mm rime layer. Pronounced lobes, separated by wide entrances are observed at K < 3. Entrances change into air gaps for larger K values, as is shown also by comparison with previous results obtained for K > 4. Rime density measurements are made by the X‐ray micrography technique. The first rime layer and the lobe interior density is represented by a ρ( X ) curve located between those of Prodi and Levi and Macklin. The former is approached by the experimental point at X = 6.5, K = 3.2. The mean deposit density is evaluated by averaging the results obtained by scanning the X‐ray micrographs, at different distances d from the point of deposit initiation. The mean density is shown to decrease markedly with d for K ± 3, as a consequence of the knobby structure, so that the average density of deposits 3–4 mm thick approaches Macklin's curve. This effect is very much reduced for K > 4, where lobes are separated by thin air gaps. These results are discussed by taking into account the shadow effect, and its dependence on K . Formulae for ρ( X ) for different values of K and deposit thickness are proposed.