Izvorni znanstveni članak
REPRESENTING THE DISPERSION OF EMISSIONS FROM AIRCRAFT ON RUNWAYS
A. Graham
; Centre for Air Transport and the Environment (CATE), Manchester Metropolitan University, Manchester, United Kingdom
M. Bennett
; Centre for Air Transport and the Environment (CATE), Manchester Metropolitan University, Manchester, United Kingdom
S. Christie
; Centre for Air Transport and the Environment (CATE), Manchester Metropolitan University, Manchester, United Kingdom
Sažetak
Aircraft in their takeoff ground run constitute an unavoidable strong source of emissions subject to a highly variable
motion. The intermittent nature of release makes it difficult to measure the emissions and establish their impact on mean
concentrations in the vicinity of airports. Practically, it is also difficult to perform experimental studies near taxiways and runways.
Remote observations have nevertheless recently been obtained by a rapidly-swept UV Lidar, and analysis of these has necessitated
and informed a parallel modelling effort. Aircraft exhausts disperse in a complex manner, as they are subject not only to transport
processes of the ambient atmosphere, but also to those associated with the aircraft itself (with diffusion in ambient turbulence to be
expected once turbulence resulting from the aircraft falls to ambient levels). They have a downstream Lagrangian momentum
associated with the engine thrust, and steadily acquire a vertical momentum as a result of their buoyancy. Exhaust streams merge
and interact strongly with the ground to form a common emission plume within about a wingspan downstream of engines’ exits.
Before the aircraft reaches a threshold speed, is rotated upward and lifts off, the downstream (thrust) forcing and upstream source
acceleration are approximately constant, and a first-order nonlinear partial differential equation may be expressed capturing the
turbulent diffusion of the plume in the reference frame of the source. The downstream forcing exceeds the buoyant forcing, so the
plume remains in contact with the ground, but is heightened and narrowed by buoyant rise. During rotation and liftoff, the net
downstream forcing declines as a significant airframe drag arises, and the source acceleration plummets. More importantly, lift on
the airframe and the associated shed circulation cause exhausts to move downward and, in proximity to ground, outward, so their
dynamics decouple from those of exhausts released earlier, with re-coupling unlikely before the aircraft has turned in its flight path.
Once rotation is initiated, this argues for a simplifying (and partly analytic) treatment of the exhaust plume generated earlier, such
that the turbulent diffusion of a given elemental plume segment is taken to match that of an infinitely long flow tube (with the latter
ascribed the same buoyancy density and downstream Lagrangian momentum density – or mass flux – throughout).
Ključne riječi
Hrčak ID:
64320
URI
Datum izdavanja:
12.12.2008.
Posjeta: 731 *