Decay Characteristics of Wake Vortices from Jet Transport Aircraft

In the 1970s the B-747-100 alleviation tests, sponsored by NASA with FAA participation, provided unique continuous-wave lidar data, in which the vortex cores could be measured because they were marked with smoke. Measurements for both normal and alleviated wakes showed that the vortex core is persistent whereas the outer circulation of the vortex decays. This form of decay is exactly the opposite of what is observed at low Reynolds number for small aircraft or in wind tunnels. The stability of the core is consistent with the essentially laminar core observed via flow visualization. The circulation profile for the normal wake is well fitted by the Burnham-Hallock model both after initial rollup and after significant decay of the total circulation. The core size for the normal B-747-100 wake is larger (5% of wingspan) than typically observed for aircraft with zero or two wing-mounted engines. The observed wake decay means that the total vortex circulation is approximately equal to the popular vortex strength parameter: average circulation over vortex radii from 5 to 15 m.