Investigation of the Motion and Decay of the Vortex Wake of a Light Twin-Engine Aircraft
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Investigation of the Motion and Decay of the Vortex Wake of a Light Twin-Engine Aircraft

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  • English

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    • Edition:
      Final report; Apr 1973- Oct 1974
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    • NTL Classification:
      NTL-AVIATION-AVIATION;NTL-AVIATION-Aviation Safety/Airworthiness;
    • Abstract:
      The properties of the smoke-marked trailing vortex wake generated by a light, twin-engine aircraft (Aero Commander 560F) were investigated experimentally. Velocity and temperature fields in the wake were measured by an other instrumented probe aircraft. Ground-based and airborne cameras recorded the motion and decay of the smoke-marked vortices. Meteorological measurements were made by a number of ground-based and airborne instruments. Analysis of these measurements shows that, for the particular conditions encountered, prediction of atmospheric turbulence aloft (up to 70 m, say) from surface measurements of wind is feasible within bounds of error acceptable for some operational purposes. As has been noted previously, the local meteorological conditions significantly affect the development and ultimate disposition of the wake. In these experiments, a clear correlation between wake tilting and wind shear transverse to the wake was observed. Single persistent vortices were regularly observed in shear conditions. The measured descent speed of wakes decreased with time in a stable atmosphere, and the spacing between the vortex pair and the size of the buoyant wake oval both increased slightly as the wake descended. Termination of the organized motion in the wakes was always brought about by vortex instabilities, with vortex breakdown (bursting) predominating for the aircraft scale and flight conditions investigated. Increasing levels of ambient atmospheric turbulence had the expected result of decreasing wake lifetimes, and the behavior correlated quantitatively with similar measurements for other aircraft. The experiments also showed that wakes descending in a stably stratified atmosphere acquire buoyancy and then subsequently begin to lose it through mixing before they break up. These results provide further confirmation of a previously presented categorization of vortex wake behavior into three evolutionary stages --an initial period of motion similar to that of a classical inviscid wake, which subsequently evolves into a stage in which turbulent mixing between the wake and its surroundings begins to significantly affect its characteristics, and then terminates as the organized motion decays through turbulence or instabilities. Also, further validation of the Crow-Bate theory for wake decay by the sinuous instability of the vortex filaments was provided by the experiments. Needed areas of theoretical exploration, building on the observations reported here, include the behavior of wakes in wind shear and the impact of ambient turbulence on vortex core breakdown.
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