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power of its diameter (Rayleigh scattering) [61]. Currently, radars can only adequately
measure icing clouds purely consisting of supercooled drops [79], and has
a possibility of overestimation [129].
Summary
The MVD of SLD has a bimodal distribution, with two peaks of 10 µm to 30 µm
and 100 µm to 400 µm. The 99% LWC of SLD icing cloud is 0.2 g m −3 , much lower
than that of small droplet, 0.7 g m −3 . The temperature at which SLD clouds are
created is most frequently higher than −10 ◦ C. The possibility of occurrence of SLD
clouds is generally lower than 2% among all icing cases, although a peak of 7%
was reported. The occurrence and distribution of icing clouds is not regular, and
the best way to be aware of its existence is more precise forecast. Normal relative
velocity between cloud and aircraft is from 50m/s to 100m/s corresponding to
ascent and descent phases, since the normal icing clouds are located at altitudes
below 4000 m while the cruise height of an airliner is above 10 000 m.
2.1.2 Experimental Investigation on SLD Icing
The drop impact process in the in-flight icing is conventionally understood that a
cloud droplet carried by the airflow approaches the aircraft and deposits on the
solid surface upon impact. Depending on the temperature and drop size, the
droplet can freeze directly upon impact or flows toward the aft surface, forming
runback water, which freezes gradually along its path. Freezing upon impact induces
rime ice, which has a more uniform shape and fragile structure, and is thus
less dangerous. Freezing of the run back water leads to glaze ice with a complex
shape, especially ice horns on the leading edge. Glaze ice severely alters the
aerodynamic outline of the airfoil and is more dangerous than rime ice. The occurrence
of glaze ice is significantly higher than rime ice because supercooled cloud
droplets exist mostly at higher temperatures. For SLD icing condition, the glaze ice
is dominant.
Significant efforts have been dedicated to the simulation of the ice shape in the
past six decades. Accurate prediction of ice shape is crucial for the assessment of
aerodynamic degradation of iced airfoils and consequently determines the design
of anti-icing or de-icing system. In order to calculate the amount of ice that accretes
on the airfoil, a thin layer of control volumes are placed adjacent to the wing (or
iced) surface, as depicted in Figure 2.1. For each control volume there is impinging
water, incoming water from the upstream control volume, outgoing water to the
2.1. Supercooled Large Droplets in Aircraft Icing 11