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An Assessment of Dominant Mechanisms in Vortex-Wake Decay

  • P. B. MacCreadyJr.

Abstract

Predicting the transport and decay of the organized vortex system for various aircraft types, flight modes, and meteorological conditions requires clear identification of the various significant factors and mechanisms involved in the transport/decay process. Outside of the ground effect, the dominant factors include the cir­culation and core characteristics of the vortices, the turbulence in the wake and in the environment, and the thermal stability of the environment. The environmental factors are sometimes dominant; the vortex wakes from an airplane operating in two different meteorological regimes can differ by an order of magnitude in decay and by an order of magnitude in descent distance. Thus any theory of vortex-wake decay in the real atmosphere must consider the meteorological factors, and field observations on decay must be interpreted with consideration of these factors.

There are two distinct mechanisms advanced for vortex wake decay-- the slowing down of the vortices by mixing action of eddy viscosity, and the interaction of the vortices with each other, which disorganizes the organized vortex flow field. The slowing down by eddy viscosity is conceptually simple, but is difficult to treat quantitatively because of uncertainty as to how the eddy vis­cosity coefficient depends on the aircraft characteristics or on the atmospheric turbulence. The vortex interaction mechanism commonly operates by the development of perturbations in the vortex lines on a scale an order of magnitude greater than the vortex separation distance. Existing theory seems useful in predicting the wavelength and shape of the breakdown, but not the time of breakdown. Factors involved in initiating breakdown may include atmospheric turbulence, wake turbulence, periodic lift variations, Benard cell-type structure arising from buoyancy, and core charac­teristics (especially size and axial flow).

Atmospheric stability causes relative buoyancy of the wake, with subsequent strong effects on the vertical transport of the vortices and on their horizontal separation. The effects also depend intimately upon the mixing between the wake and the environ­ment. Thus the environmental turbulence and the wake turbulence both play a role in the mechanism.

Operational rules for terminal operations of aircraft in strong turbulence should be rather easy to establìsh, even with the present limited stage of understanding. However, greatly improved understanding is required for predicting the worst (slow­est decay) cases. Present reasoning suggests these will occur during the climbout of heavy aircraft, flying slowly in “clean” configuration, in marine air conditions where zero turbulence and near-neutral stability may often coexist.

Keywords

Eddy Viscosity Vortex Pair Atmospheric Turbulence Atmospheric Stability Vortex Sheet 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Plenum Press, New York 1971

Authors and Affiliations

  • P. B. MacCreadyJr.
    • 1
  1. 1.Meteorology Research, Inc.USA

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