Investigation of "vortex self turbulence" and Reynolds number effects for wake decay and transport IGE

Thiry, Olivier [UCL] Duponcheel, Matthieu [UCL] Winckelmans, Grégoire [UCL]

One of the key elements in any proposal aiming at reducing separation between aircraft in landing and/or take-off is the wake vortex decay and transport when in ground effects (IGE). Indeed, the severity metrics that are used to quantify the severity of potential wake vortex encounters (WVE) require, as one of the main WVE “impact parameters”, the decayed circulation of the vortex at the time of encounter. So far, the detailed 3-D simulations, through advanced LES, of wake vortices interacting with a ground has been carried out at “moderately high” Reynolds numbers (e.g., of the order of 2-3 10^4) and certainly with good success. Cases with cross-wind and/or with head wind have also been simulated, and for weak to medium winds. The results of such LES have allowed to develop, and also partially calibrate, physics-based simplified models for predicting the enhanced decay rate of the wake vortices that occurs due to their strong interaction with the ground; and that typically starts just after their “rebound”. The calibration was then also further improved using the decay rates derived from LiDAR data of measurement campaigns involving real aircraft wakes IGE, thus at far much higher Reynolds number (e.g., of the order of 2-3 10^7). The Reynolds number limitation of such LES is due to the fact that they are “wall-resolved”, meaning that the near wall region is well resolved down to within the viscous sub-layer of the boundary layer that is generated by the wake vortices. We also proposed that the fast decay rates obtained through such LES, and in various wind conditions, are representative of what happens for real aircraft wakes. Such “property” is indeed expected in turbulence, as the decay rate due to turbulent interactions will become insensitive to Reynolds number when it is high enough. The question then being: is 2-3 10^4 high enough? We will present a methodology that has been developed to perform LES of wake vortices at higher Reynolds numbers than what was done so far (at least 10 times higher). The methodology also allows to investigate wakes that interact with a rough ground (e.g., grass). Various cases will be considered. The decay history of the vortices will be presented, as well as their transport, and comparisons will be made with what was obtained before using wall-resolved LES.