Aircraft Wake Vortices in the Atmospheric Boundary Layer

Wake vortices generated by flying aircraft pose a potential risk for following aircraft due to the strong and coherent rotating flow structure behind the wake-generating aircraft. The probability to encounter wake vortices increases during the final approach close to the ground. There, the vortices tend to move in opposite directions and to rebound but a weak crosswind may compensate the lateral advection of the up-wind vortex, leaving it right in the flight corridor of the airport. For an optimum and safe spacing of aircraft during approach and landing, full understanding of wake vortex behaviour in the atmospheric boundary layer close to the ground is mandatory. In particular the wind conditions along the glide path and close to the ground control the wake transport and decay.

This paper introduces wall-resolved large eddy simulations to investigate the behavior of wake vortices in ground proximity at a variety of wind conditions. The six considered strengths of wind, ranging between 0.5 and 4 times the initial wake vortex descent speed, w₀, include practically and theoretically significant wind speeds. A crosswind of 0.5 w₀ may lead to windward stall posing a potential hazard to subsequently landing aircraft, whereas theoretical considerations predict that at 4 w₀ the rebound of the luff vortex is completely suppressed. The same range of wind speeds is also used to investigate the effects of headwind and diagonal wind in order to discriminate between effects of environmental turbulence increasing with wind speed and the direction of the wind shear. It is shown that vortex ascent, descent, rebound and decay characteristics are controlled by (i) the interaction of the vortices with secondary vorticity detaching from the ground, (ii) the redistribution of vorticity of the boundary layer which is altering the path of the primary vortices by mutual velocity induction, and (iii) the interaction of the vortices with the environmental turbulence.