Aircraft Vortex Wake Decay Near the Ground

PDFTech ReportDOT-TSC-FAA-77-08FAA-RD-77-46DOT-TSC-1008WakesVorticesTurbulenceUnited States. Federal Aviation AdministrationTombach, I.Lissaman, P. B. S.Mullen, J. B.Barker, S. J.John A. Volpe National Transportation Systems Center (U.S.)NTL-AVIATION-Air Traffic ControlNTL-AVIATION-Aviation Energy and EnvironmentVolpe National Transportation Systems Center, Technical Reference CenterA multi-faceted experimental and analytical research program was carried out to explore the details of aircraft wake vortex breakdown under conditions representative of those which prevail at low altitudes in the vicinity of airports. Three separate approached were taken simultaneously. Flight tests with Lockheed L-18 Lodestar and Boeing 747 aircraft flying over ground-based instrumentation provided data on overall vortex behavior, on the vortex ages at the time of onset of instabilities, and on the changes in the vortex velocity theories identified conditions under which vortices could undergo unstable decay. Experimental tests in a water tank looked at the instability of vortices, and also shed light on vortex motion near the ground. Finally, a heuristic modeling approach resulted in a simple representation of the relationship between the times of vortex breakdowns and the ambient turbulence levels. Although a detailed mechanism for vortex breakdowns was not found, a universal function, usable for all aircraft, was developed for predicting vortex breakdown times within a factor of two error. It was also shown that vortex breakdowns do not generally result in total dissipation of the vortex energy, but rather a residual organized motion of significant intensity often persists after bursting of a smokemarked vortex has been noted to occur