3D Lagrangian Particle Tracking within transitional flows

Abstract

The development and production of laminar flow wings is one important goal in Europe’s challenges to significantly reduce the fuel burn and the emission of future transport aircraft. For the aerodynamic design of laminar flow wings (NLF or HLFC), the understanding of the boundary layer transition process is essential and the further improvement of numerical prediction tools is necessary. While the transition over smooth surfaces is nowadays well understood, the understanding of the effect of surface imperfections on the laminar boundary layer is still vague. Both the effect of roughness (e.g. due to small rivet heads or insects) and the effect of surface steps (e.g. between two adjacent wing panels) are important. Two different types of imperfections were studied by 3D Lagrangian Particle Tracking (LPT) experiments, using similar setups. On the one hand, the transitional flow developing downstream of a backward facing step (BFS) of height ℎ 5 mm at several Reynolds numbers (Re_h~ 850 - 4.150) was investigated. On the other hand, the BFS was replaced by of a small cube (1.5 mm sides). In both cases, the incoming boundary layer was kept laminar, such that a transition occurs in the wake of the step and the cube. It is known that the flow around these imperfections depends on various parameters such as the dimension ratios L/ℎ, delta/ℎ and theta/ℎ, the Reynolds number, the free stream turbulence and its specific spectral distribution, etc. The transition of the boundary layer can be caused downstream due to the growth of Tollmien-Schlichting-waves in two-dimensional boundary layers or, in addition, by crossflow instabilities in three-dimensional boundary layers. The investigation of the development of disturbances due to instabilities requires the measurement of highly resolved velocity fields both in space and in time and, preferably, the skin friction distribution on the surface. For a related publication, see (Schröder et al. 2020).