Plume Rise from Aircraft Sources

One of the key needs in applying existing dispersion models to
aircraft sources is the plume rise from the engine exhaust. In some
models (e.g., AERMOD), the plume rise is neglected, which can lead to
significant overestimation of surface concentrations on or near
airport property. For example, Arunachalam et al. (2013) found this
to be the case at the Los Angeles Airport, where the highest modeled
concentrations were roughly an order of magnitude greater than the
observations. Thus, a realistic plume rise prediction albeit simple is
required to improve the modeled surface concentration field.

In this paper, we present a fluid-mechanical entrainment (FEM) model
of plume rise based on the governing equations for plume mass,
momentum, and energy or buoyancy fluxes through an aircraft plume
cross section. The equations are similar to those presented
previously by Briggs (1984), Weil (1988), and others, but the earlier
models applied to stationary and vertically-oriented sources or
"stacks." The FEM differs from these models in that it accounts for:
1) the horizontal high-speed heated jet and momentum from the aircraft
engines, 2) a different air entrainment mechanism for the high-speed
jet (entrainment proportional to the jet velocity), and 3) a mobile
source. All of these differences are necessary but the third --
mobility and high speed of the source -- is the most unique.

Analytical and numerical trajectories of the plume rise are obtained
for a constant aircraft acceleration down the runway, where the
aircraft attains a velocity of about 74 m/s just before take off. The
trajectory is controlled by the exhaust momentum, buoyancy, and an "effective
wind speed" which equals the ambient wind plus a fraction of the
aircraft speed. The effective speed is dominated by the aircraft speed
over most of the trajectory since it is much greater than the local
wind. Comparisons of modeled plume centerline height and vertical
spread with lidar observations of plume height for a B777 aircraft at
Heathrow Airport (Bennett et al., 2010) show approximate
agreement. Further results and trajectories for other aircraft will be
shown to demonstrate the model applicability to a wide range of
aircraft types.