This paper is a brief review of recent wake vortex research as it affects the operational problem of spacing aircraft to increase airport capacity and throughput. The paper addresses the questions of what do we know about wake vortices and what don't we know about wake vortices. The introduction of Heavy jets in the late 1960s stimulated the study of wake vortices for safety reasons and the use of pulsed lidars and the maturity of computational fluid dynamics in the last three decades have led to extensive data collection and analyses which are now resulting in the development and implementation of systems to safely decrease separations in the terminal environment. Although much has been learned about wake vortices and their behavior, there is still more to be learned about the phenomena of aircraft wake vortices.

Although wake vortices are known to decay more rapidly near the ground than away from the ground, the details of the ground interaction are not well understood. Propeller anemometer arrays located under the approach path have been used to study vortex transport and provide some information about the vortex interaction with the ground, such as the generation of secondary vortices via boundary-layer detachment. A propeller anemometer array at John F. Kennedy International Airport using 8.5-m poles was augmented with 1) a sonic anemometer measuring three-dimensional wind and temperature at 10 Hz and 2) a vertical array of vertical wind and crosswind anemometers, mounted at four additional levels (4.2, 3.2, 1.05, and 0.5 m). The sonic anemometer gave 1) measurements of turbulence inside the vortex flowfield and 2) indications of vertical variations in the ambient headwind and temperature, which were brought down to the measurement level by the descent of the vortex recirculation oval. In general, under conditions of low to moderate turbulence, the turbulence level inside the wake vortex flowfield is greater than that in the ambient wind. The vertical anemometer array showed that the crosswind profile under a wake vortex in ground effect has a very thin boundary layer, much thinner than that of the ambient wind. It also provided some details concerning the wind profile of the secondary vortex.

No abstract

Aircraft wake vortex behavior in ground effect between two parallel runways at Frankfurt/Main International Airport was studied. The distance and time of vortex demise were examined as a function of crosswind, aircraft type, and a measure of atmospheric turbulence. Vortex decay in ground effect is little influenced by ambient turbulence and is seen to be a stochastic process.

For more than two decades cw doppler lidars have been used to study the decay of wake vortices generated by jet transport aircraft. With appropriate scan and data processing strategies, the vortex tangential velocity profile can be measured every few seconds. Under low turbulence conditions conducive to long vortex persistence the observed decay process is contrary to that predicted by classical theory and observed at low Reynolds numbers in wind tunnels and for small aircraft: namely, that the core grows in size and the maximum tangential velocity decreases while the total circulation remains roughly constant. For full-sizedjet transport aircraft the vortex core often remains stable while the outer portion of the vortex decays, thereby reducing the total circulation. Data are presented from B-747 alleviation flight tests conducted in the 1970s and for other aircraft collected in the early 1990s. The stability of the core is consistent with the essentially laminar core flow observed via flow visualization.