Studies of aircraft wake chemistry and dispersion

Abstract

Use of aerospace technology to study aircraft wakes is reviewed. Although aerospace vehicles are suspected of potentially altering the environment, they have an important role in its study. The discussions focus on methods, because few published results are available. It is shown how aerospace vehicles can be used to provide data for increased understanding of the atmosphere and of aircraft exhaust trails where knowledge is inadequate to evaluate fully the potential impact of the engine emissions. Models of aircraft near field exhaust wakes are characterized by jet, vortex, and dispersion regimes. Wake growth in the jet regime is self-determined and rapid, whereas further spreading is inhibited in the vortex regime because of circulating vortex motion. Wake diffusion in the dispersion regime is initially influenced by aircraft induced turbulence but is dominated later by small scale atmospheric turbulence. Computed fluid mechanical results show the importance of effects such as wake buoyancy, wind shear, turbulence, and traffic corridor exhaust buildup on dispersion of the wake. In the jet regime the exhaust characteristics and thermochemistry serve to illustrate initial chemical changes involving potential pollutant species. From computations including flowfield parameters and more than 100 chemical reactions, histories of representative exhaust species are shown for a jet engine in stratospheric, supersonic flight. The influence of photochemistry on certain products in an undiluted hypothetical wake results in significant chemical changes, thereby illustrating the large local effects which might occur if dispersion were not rapid. Aircraft wake species sampling flights show concentrations of nitrogen oxides and carbon oxide in tens to thousands of parts-per-billion in the near field. These data show that for many species, available instruments are adequate for making near field wake measurements. Problems limiting the applications of aerospace technology to atmospheric studies include chemical reaction rate uncertainties, measurement platform limits and availability, certain gas and aerosol measurement difficulties, and uncertainties in our present understanding of atmospheric processes.