Morphology and density of ice accreted on cylindrical collectors at low values of impaction parameter. I: Fixed deposits

Abstract

The morphology and density of ice depositions grown on fixed cylinders are studied in the ranges of Macklin's parameter, X, and of the Stokes number, K, of 0.5 <X<7μ ms−1 K−1 and 0.8° K £3.2 respectively. For K<3, the deposit morphology was characterized by concave growth fronts and inward direction of lateral feathers. For larger K-values, the growth fronts changed to convex profiles, and ice feathers to an outward direction. The density was measured in the first millimetre of rime, using the X-ray micrography technique. The stagnation-region density (θ = 0) was found to depend on X only, the ρ(X) curve being independent of the K-value. For X<2, ρdecreased slowly down to ρ ±; 0.3gcm−3, probably approaching a limiting value. The results ‘off the stagnation region’ depend on K. For K<3, air gaps appear between lateral feathers and ρdecreases with the angular distance, from the stagnation line. For K > 4, the air-gap effect was reduced to the very flanks of the deposit, and ρwas nearly independent of θ. The possible dependence of the observed behaviour on the shadow effect, generating air gaps, and on the supposed variation with θ of the droplet impact speed is discussed. It is shown that experimental aspects of deposit morphology agree well enough with stochastic model results obtained by previous authors, some differences probably being related to the oversimplification of the model, where the droplet spectrum is represented by the median volume droplet radius. Relations between the morphology and density of the deposit and between these properties and the parameters X and K are discussed.