Flow dynamics past a simplified wing body junction

Abstract

Junction flows may suffer from secondary flows such as horseshoe vortices and corner separations that can dramatically impair the performances of aircrafts. The present article brings into focus the unsteady aspects of the flow at the intersection of a wing and a flat plate. The simplified junction flow test case is designed according to a literature review to favor the onset of a corner separation. The salient statistical and fluctuating properties of the flow are scrutinized using large eddy simulation and wind tunnel tests, which are carried out at a Reynolds number based on the wing chord c and the free stream velocity U∞ of Rec=2.8×105. As the incoming boundary layer at Reθ=2100 (θ being the boundary layer momentum thickness one-half chord upstream the junction) experiences the adverse pressure gradient created by the wing, a three dimensional separation occurs at the nose of the junction leading to the formation of a horseshoe vortex. The low frequency, large scale bimodal behavior of the horseshoe vortex at the nose of the junction is characterized by multiple frequencies within f.δ/U∞=[0.05−0.1] (where δ is the boundary layer thickness one-half chord upstream the wing). Downstream of the bimodal region, the meandering of the core of the horseshoe vortex legs in the crossflow planes is scrutinized. It is found that the horseshoe vortex oscillates around a mean location over an area covering almost 10% of the wing chord in the tranverse plane at the trailing edge at normalized frequencies around f.δ/U∞=0.2–0.3. This so-called meandering is found to be part of a global dynamics of the horseshoe vortex initiated by the bimodal behavior. Within the corner, no separation is observed and it is shown that a high level of anisotropy (according to Lumley’s formalism) is reached at the intersection of the wing and the flat plate, which makes the investigated test case challenging for numerical methods. The conditions of apparition of a corner separation are eventually discussed and we assume that the vicinity of the horseshoe vortex suction side leg might prevent the corner separation. It is also anticipated that higher Reynolds number junction flows are more likely to suffer from such separations.