Sponginess and Drop Shedding of Gyrating Hailstones in a Pressure-Controlled Icing Wind Tunnel

Abstract

Artificial hailstones were grown in an icing wind tunnel under simulated natural conditions, starting from oblate ice spheroids with major and minor diameters of 2.0 and 1.3 cm respectively, while undergoing symmetric gyration. The experiments were performed at air temperatures from ?25&deg to ?2°C, air pressures from 35.5 to 102.0 kPa, liquid water contents from 0.5 to 20 g m ?3 and for hailstone spin and nutation/precession rates up to 35 Hz. The net collection efficiency, ice fraction and final aspect ratio were determined for each icing trial. The experimentally determined boundary between growth of ice and spongy deposits agrees well with the theoretical Schumann-Ludlam limit (SLL), adapted for spheroids. This is not necessarily expected since the SLL is based on bulk considerations. Six distinct deposit growth regimes [dry (ice, with some air), moist (wet surface but solid interior), spongy, spongy-shedding, soaked-shedding (soaked: high water content deposit) and dry-shedding] were found by varying air temperature, liquid water content and nutation/procession rate of the hailstones. The rotational effects were divided into three distinct ranges: low (0.5 to 9 Hz), intermediate (9 to 20 Hz) and high (>20 Hz). The fraction of unfrozen accreted water in wet growth that is shed from a hailstone as opposed to being incorporated in spongy ice was determined. Within the experimental conditions the mass amount of liquid water in spongy ice was found to be limited to 50%. Aerodynamic molding, which is the shaping of a nonrigid body by pressure forces, is significant for a variety of wet growth conditions. Other main findings are 1) the shedding of water drops and spongy ice formation can occur concurrently, 2) shedding canine observed during growth of solid deposits if the rotation rates are high enough, 3) millimeter- sized water drops are produced by shedding and 4) the hailstone's angular motion has a significant influence on its growth characteristics.