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Project Overview Aircraft Icing Meteorological Aspects Prof. Dr. Serkan ÖZGEN Dept. Aerospace Engineering, METU October 2020 Outline • Composition and structure of the atmosphere • Water cycles in the atmosphere • Cloud formation and classification • Icing clouds • Frontal icing conditions • Icing cloud conditions defined in FAR 25, Appendix C – Continuous icing envelopes – Intermittent icing envelopes Serkan ÖZGEN 2 Introduction • Four basic factors to be considered for aircraft icing: – Temperature: influences the type and intensity of ice, – Liquid water content: indicates the severity of icing, type and shape, – Droplet size: determines the type and rate of icing through the droplet collection efficiency, – Type of aircraft. Serkan ÖZGEN 3 Composition of the atmosphere • Atmosphere is (by volume, dry air) 78% Nitrogen, 20.946% Oxygen, 0.934% Argon and 0.03% Carbon Dioxide. • Atmosphere may also contain up to 4% of moist water (typically 1%). • Unlike other constituents, water can be found in all three phases (gas, liquid, solid) in the atmosphere. • The atmosphere may also contain other types of materials such as salt crystal, dust, and smoke particles that act as nuclei around which water droplet or ice crystals form (Aitken particles). Serkan ÖZGEN 4 Characteristics sizes and concentrations of atmospheric constituents Serkan ÖZGEN 5 Structure of the atmosphere Serkan ÖZGEN 6 Water cycles in the atmosphere • In the homosphere, defined as the layer where relative concentration of the most abundant gases are uniform for an average distance of 80 km, water vapor concentration is variable (from 0% to 4%) and decreases with height. • Under different circumstances, water vapor in the atmosphere may be disturbed and its state changes by evaporation- condensation, melting-freezing or sublimation-deposition. Serkan ÖZGEN 7 Water cycles in the atmosphere Serkan ÖZGEN 8 Water cycles in the atmosphere • Heat consumed in one place in the atmosphere during evaporation or sublimation may be released in different places during condensation or deposition. • This is considered as an effective way of transporting heat over great distances. When winds transport moist air to other region it forms clouds by condensation producing rain and snow. • This cycle of moving and transforming water is called hydrologic cycle. Serkan ÖZGEN 9 Cloud formation • When the amount of water vapor is increased, the air reaches the state of saturation, which is the maximum amount of water vapor that can exist in the atmosphere. When the amount of water vapor is further increased or the air is cooled, excess water vapor condensates into droplets or transforms into ice crystals by deposition. • Clouds are formed by condensation of the invisible water vapor into visible water droplets, snow, or ice crystals. This formation requires sufficient water vapor, a cooling process and the presence of nuclei in the atmosphere. Serkan ÖZGEN 10 Cloud types The clouds may be categorized as: • High clouds: above 6 km (20,000 ft), • Middle clouds: 2-6 km (6,500-20,000 ft), • Low clouds: below 2 km (6,500 ft), • Clouds of vertical extent. Serkan ÖZGEN 11 Cloud types Nomenclature: • Cirrus: feathery or fibrous, • Stratus: stratified or in layers, • Cumulus: heaped up, • Nimbus: rain, • Alto: high. Serkan ÖZGEN 12 Cloud types Serkan ÖZGEN 13 Characteristics of high clouds (>6 km) Serkan ÖZGEN 14 Cirrus cloud Serkan ÖZGEN 15 Cirrocumulus cloud Serkan ÖZGEN 16 Cirrostratus cloud Serkan ÖZGEN 17 Characteristics of mid clouds (2<h<6 km) Serkan ÖZGEN 18 Altocumulus cloud Serkan ÖZGEN 19 Altostratus cloud Serkan ÖZGEN 20 Characteristics of low clouds (<2 km) Serkan ÖZGEN 21 Stratocumulus cloud Serkan ÖZGEN 22 Nimbostratus cloud Serkan ÖZGEN 23 Stratus cloud Serkan ÖZGEN 24 Characteristics of clouds of vertical development Serkan ÖZGEN 25 Cumulus cloud Serkan ÖZGEN 26 Cumulonimbus cloud Serkan ÖZGEN 27 Frontal icing conditions - warm front, horizontal extent Serkan ÖZGEN 28 Frontal icing conditions - cold front, vertical extent Serkan ÖZGEN 29 Icing cloud conditions defined in FAR 25 • Consist of 6 figures, • Has been in use since 1964 for selecting values of icing related clouds for design of ice protection systems, • They indicate the probable maximum (99 %) value of liquid water content that is to be expected as an average over a specified reference distance for a given temperature and droplet size in the cloud. Serkan ÖZGEN 30 Icing cloud conditions defined in FAR 25 • For continous maximum conditions the reference distance is 17.4 nm (20 miles), • For intermittent maximum conditions it is 2.6 nm (3 miles), • In icing applications the actual droplet size distribution in clouds is represented by a single variable called the droplet median volume diameter. Serkan ÖZGEN 31 Icing cloud conditions defined in FAR 25 • Overall average for stratiform clouds is 15m, cumuliform clouds is 19m. • The cloud characteristics were measured by NACA and Weather Bureau in 1950s. Serkan ÖZGEN 32 Continuous maximum (stratiform) atmospheric icing conditions Serkan ÖZGEN 33 Continuous maximum (stratiform) atmospheric icing conditions Serkan ÖZGEN 34 Continuous maximum (stratiform) atmospheric icing conditions Serkan ÖZGEN 35 Intermittent maximum (cumuliform) atmospheric icing conditions Serkan ÖZGEN 36 Intermittent maximum (cumuliform) atmospheric icing conditions Serkan ÖZGEN 37 Intermittent maximum (cumuliform) atmospheric icing conditions Serkan ÖZGEN 38 Appendix C curves converted to distance based format (MVD=15m) Serkan ÖZGEN 39 Natural 99% limits vs altitude (MVD=15-20m) Serkan ÖZGEN 40 Natural 99% LWC and HE limits for selected MVD (T=0o to -10oC) Serkan ÖZGEN 41 The entire supercooled cloud database Serkan ÖZGEN 42.
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