Wake vortex physics

By means of large-eddy simulations the decay of wingtip vortices and the distribution of the turbine exhaust under the influence of turbulence in a stratified atmosphere is discussed. It is distinguished between atmospheric turbulence and a random turbulent flow originating from the boundary-layer around the aircraft. The ambient turbulence is weak, anisotropic, and decaying with sizes of the most energetic eddies at the order of the separation between the two wingtip vortices. The boundary-layer turbulence in wrapped around the vortices with the maximum intensity at the core diameter. The atmospheric eddies disturb the parallel vortex tubes by advection. Once distorted at that scale, the vortex induced velocities amplify the disturbance according to Crow's instability. The vortex tubes break up after about 1.5 minutes and form rings; the continuous trail of exhaust is reorganized in a row of single puffs. Without any atmospheric turbulence, the wake does not experience a sinusoidal instability but the parallel vortex tubes approach and start to dissolve after 2 minutes when they touch. The dissolution is triggered by the small-scale turbulent friction owing to the boundary-layer turbulence of the aircraft. The exhaust trail remains aligned along the flight track. The numerical data are compared to in-situ measurements. (orig.)Available from TIB Hannover: RR 6341(96) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman