Browsing by Author "Luxford, Geoffrey"

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    Experimental and modelling investigation of the deformation, drag and break-up of drizzle droplets subjected to strong aerodynamics forces in relation to SLD aircraft icing
    (Cranfield University, 2005-03) Luxford, Geoffrey; Hammond, David W.
    The distortion, drag and break-up of drizzle droplets subjected to strong aerodynamic forces was investigated to understand the pre-impact behaviour of droplets in aircraft icing from supercooled freezing drizzle. The objective was to obtain a formulation and data for the drag properties of droplets distorted by the aerodynamic forces, which were beyond the scope of available experimental and modelling methods. A practical and efficient semi-empirical computer model was developed for small water droplets in air, 100µm < D < 500µm, at moderate Reynolds numbers, 350 < Re < 1500, and high Weber numbers 3 < We < 20. This used available experimental terminal velocity data for free-falling droplets, extrapolated to higher Weber numbers, and the numerical solution for sessile droplets on a horizontal unwettable surface, with corrections for the Reynolds number. A theory for bag break-up was developed based on the Rayleigh-Taylor instability of the windward droplet surface. The critical Bond number was 13.7, with a critical diameter of 10.1mm for free-falling water droplets, compared to the experimental value of 10mm diameter. The equivalent Weber number was 14.2 for free falling water droplets. Aerodynamic interaction between the closely-spaced droplets from a vibrating nozzle droplet generator resulted in irregular spacing and coalescence of droplets. In an alternative design a laminar jet impinged on a rotating slotted disk to achieve the necessary droplet spacing, but the significant size variability of the droplets degraded the experimental measurements. High-speed videos, to 50,000pps, and photographs were obtained of droplet distortion, break-up, coalescence and splashes using a high-intensity LED strobe flash. A specially-designed convergent wind tunnel was developed for experimental measurements, to validated the drag model and provide data for droplets distorted by aerodynamic forces. The convergent profile produced a rapidly-increasing Weber number at a sufficiently slow rate to avoid transients or droplet vibrations. A special instrument was developed, with three equispaced parallel laser beams and photo detectors, to determine the droplet velocity and acceleration. Droplet drag characteristics were measured up to Weber numbers of 16. Good agreement was obtained between droplet drag model and experimental results. The greatest discrepancy was about 20% at a Weber number of about 8.
  • Thumbnail Image
    ItemOpen Access
    Experimental and modelling investigation of the deformation, drag and break-up of drizzle droplets subjected to strong aerodynamics forces in relation to SLD aircraft icing
    (Cranfield University, 2005-03) Luxford, Geoffrey; Hammond, David W.
    The distortion, drag and break-up of drizzle droplets subjected to strong aerodynamic forces was investigated to understand the pre-impact behaviour of droplets in aircraft icing from supercooled freezing drizzle. The objective was to obtain a formulation and data for the drag properties of droplets distorted by the aerodynamic forces, which were beyond the scope of available experimental and modelling methods. A practical and efficient semi-empirical computer model was developed for small water droplets in air, 100μm < D < 500μm, at moderate Reynolds numbers, 350 < Re < 1500, and high Weber numbers 3 < We < 20. This used available experimental terminal velocity data for free-falling droplets, extrapolated to higher Weber numbers, and the numerical solution for sessile droplets on a horizontal unwettable surface, with corrections for the Reynolds number. A theory for bag break-up was developed based on the Rayleigh-Taylor instability of the windward droplet surface. The critical Bond number was 13.7, with a critical diameter of 10.1mm for free-falling water droplets, compared to the experimental value of 10mm diameter. The equivalent Weber number was 14.2 for free falling water droplets. Aerodynamic interaction between the closely-spaced droplets from a vibrating nozzle droplet generator resulted in irregular spacing and coalescence of droplets. In an alternative design a laminar jet impinged on a rotating slotted disk to achieve the necessary droplet spacing, but the significant size variability of the droplets degraded the experimental measurements. High-speed videos, to 50,000pps, and photographs were obtained of droplet distortion, break-up, coalescence and splashes using a high-intensity LED strobe flash. A specially-designed convergent wind tunnel was developed for experimental measurements, to validated the drag model and provide data for droplets distorted by aerodynamic forces. The convergent profile produced a rapidly-increasing Weber number at a sufficiently slow rate to avoid transients or droplet vibrations. A special instrument was developed, with three equispaced parallel laser beams and photo detectors, to determine the droplet velocity and acceleration. Droplet drag characteristics were measured up to Weber numbers of 16. Good agreement was obtained between droplet drag model and experimental results. The greatest discrepancy was about 20% at a Weber number of about 8.
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    ItemOpen Access
    Modelling, imaging and measurement of distortion, drag and break-up of aircraft- icing droplets
    (2005-01-01T00:00:00Z) Luxford, Geoffrey; Hammond, David W.; Ivey, Paul C.
    The distortion, drag and break-up of supercooled drizzle droplets in aircraft icing was investigated in ambient conditions. An effective efficient computer procedure was developed for the distortion and drag of small droplets, < 1mm, at low Reynolds numbers, < 1000, and high Weber numbers, > 10. High-speed videos and photographs were obtained with an improved high-intensity LED strobe. Experimental measurements validated the drag model for droplets distorted by a rapidly accelerating airflow in a convergent wind tunnel. To prevent droplet coalescence, due to wake interactions, droplets were generated with a steady jet impinging of a rotating slotted disk.
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    ItemOpen Access
    Role of droplet distortion and break-up in large droplet aircraft icing
    (2004-01-01T00:00:00Z) Luxford, Geoffrey; Hammond, David W.; Ivey, Paul C.
    Experimental and analytical evidence is presented to illustrate droplet distortion and potential break-up in the flow conditions similar those of a drizzle droplet in the vicinity of an aerofoil. The droplet size range considered is 100u m to 500u m. The current research is at ambient temperature but with equivalent parametric conditions to SLD icing. While the results support the contention that smaller droplets, <100u m, remain essentially spherical, the larger droplets become sufficiently distorted to affect their drag characteristics and can break up due to the transient aerodynamic forces.