Transport and effective diffusion of aircraft emissions

The transport and effective diffusion of exhaust are analyzed in the wake flow of a large-bodied aircraft which flies through a stably stratified, sheared, and turbulent atmosphere. The analysis is based on data sets from large-eddy simulations of the wake in the atmosphere. Diffusion and dilution measures are obtained from a chemically inert species concentration, Most of the exhaust is concentrated and isolated in the two wing tip vortices (the "primary wake"). However, as the vortices sink through a stably stratified atmosphere, a baroclinic torque develops between the vortices and the surrounding flow and detrains about 10 to 30% of the exhaust mass from the vortices into the ambient air (the well mixed "secondary wake"). Consequently, the entrainment rates computed for the primary and;secondary wakes differ by orders of magnitude. In the period between 1.5 and 3 min the vortices collapse into aircraft turbulence. The trapped emissions of the primary wake are now released and diffused by ambient turbulence and shear. After about 5 min the exhaust concentration has been diluted by 2 x 10(-5) and 4 x 10(-6) compared to the value at the nozzle exit for the primary and secondary wakes, respectively, and covers areas of about 5 x 10(4) m(2) and 2 x 10(4) m(2) Under flow conditions typically found at cruising heights the emissions are diluted to background concentrations within 2 and 12 hours for wind shear between 0.002 and 0.01 s(-1). The spatial plume extension does not-exceed the lower mesoscale range (20 km horizontally and 0.3 km vertically). Good to excellent agreement is achieved between the numerical results and in situ measured data.