LES investigation of the transport and decay of various-strengths wake vortices in ground effect and subjected to a turbulent crosswind

Bricteux, Laurent [UMONS] Duponcheel, Matthieu [UCL] De Visscher, Ivan [WaPT] Winckelmans, Grégoire [UCL]

This study is concerned with the investigation of two-vortex systems (2VS) of various strengths that are released near the ground and evolve in the presence of a turbulent crosswind. We analyze the physics of the vortices interactions with the turbulent wind and with the ground during the rebound phase, and that lead to the fully developed turbulent flow and interactions. The transport and decay of the vortices are also analyzed. The turbulent wind itself is obtained by direct numerical simulation using a half channel flow. The flow is then supplemented with the 2VS, using vortices with a circulation distribution that is representative of vortices after roll-up of a near wake. The vortex strengths, Γ0, are such that Re Γ = Γ0/ν = 2.0 × 104 for the baseline; there is then a case with twice weaker vortices, and a case with twice stronger vortices. The simulations are run in wall-resolved Large Eddy Simulation (LES) mode. The baseline is in line with the wall-resolved LES study of a similar case [A. Stephan et al., “Aircraft wake-vortex decay in ground proximity—Physical mechanisms and artificial enhancement,” J. Aircr. 50(4), 1250–1260 (2013)]. They highlighted the significant effect that the near-wall streaks of the wind have on the development of instabilities in the secondary vortices, and the ensuing turbulence. Our analysis complements theirs by also showing the significant effect that the wind turbulent structures, away from the ground and that are stretched by the primary vortices, also have on the destabilization of the secondary vortices. Comparisons are also made with the most recent study [F. N. Holzäpfel et al., “Wind impact on single vortices and counter-rotating vortex pairs in ground proximity,” in 7th AIAA Atmospheric and Space Environments Conference, AIAA Aviation (American Institute of Aeronautics and Astronautics, 2015)], where Re Γ = 2.0 × 104 for all cases and where it is the wind intensity that is varied. Diagnostics on the vortex trajectories and circulation decay are provided, for the mean and for the envelopes of behaviour. The results are discussed and compared with the recent literature. In particular, for the case with relatively twice stronger wind relative to the vortices, the upwind vortex quickly looses its coherence when it comes closest to the ground and does not rebound; the physics of that are explained by a long wave instability excited by the turbulent wind. Finally, a case where the baseline wake is released at a lower altitude is also studied, to support an analysis on what is the proper length scale to use, and initial time, when comparing results of wakes released at different altitudes: indeed, when normalized using those quantities, the trajectory and decay curves of this case are seen to collapse very well with those of the baseline.