An experimental study of trailing vortex dynamics on cruise and high-lift wing configurations

Towing tank experiments are conducted to investigate the effect of the wing span-wise load profile on the structure and strength of trailing vortices. In the framework of the lifting line theory, the local circulation Γ_y (with y the span-wise coordinate) depends on the local chord, effective incidence and lift coefficient. For a commercial aircraft, the wing geometry varies depending on the phase of flight (eg. deployment of high-lift devices for take-off/landing) which modifies Γ_y. Theoretical models found in the literature allow to relate the configurations of trailing vortices about this span-load. However, few experimental validation exists for complex cases. The question of how a given span load influences the vortex wake structure and its dynamics is of high interest. Furthermore this question also relates to that of the wake control strategies. In this work, we investigate the wake dynamics resulting from two different span-loads to answer these questions. The baseline case is a NACA 4412 rectangular wing with no twist. A second model creates a load variation about this reference by twisting a specific inboard section. On top of the main external tip vortices, this introduces additional vortices downstream. SPIV measurements in sections of the wake generated by the towed wings are made to asses the vortex characteristics such as radius, circulation, swirl number and trajectory in each configuration. The development of the vortex wake is investigated from the roll-up stage up to 170 spans downstream at a chord based Reynolds number of R_ec = 10⁵ . In both wing configurations, the towing velocity U₀ and angle of attack α are modified to produce the same total lift as the baseline case and compare the distribution of downstream vorticity. In the case of the high-lift wing, the ratio of circulation between the flap and wingtip vortices is studied. Results show that at high α axial flow deficit grows. The swirl number decreases as a consequence but stagnates towards the value of 1.5 which is a threshold for instability onset. Under similar total loading conditions, vortices trailing behind the high-lift wing have a higher circulation, core size, and axial velocity than those of the untwisted wing.