The Cross-Flow (CF) instability is the main cause of transition from the laminar to the turbulent regime in flows around swept wings. It has been investigated following three different approaches: experimental campaigns, numerical simulations and theoretical predictions. 12 In the past decades researchers have done several efforts in all these three directions in order to better understand the development of these modes, their growth in the boundary layer and the way they eventually cause breakdown and transition to turbulence. Experimental campaigns have been pursued by a restricted number of groups on different models given the need of having very low turbulence tunnels for this kind of studies: the group of D. Arnal1 at ONERA and the one at Novosibirsk5 used the coupling of a flat plate and a second body at the opposite side of the wind tunnel designed in such a way to give to the flow a pressure gradient with respect to the free stream direction; the group of W. Saric at Arizona State University and Texas A&M University9, 10 chose a Natural Laminar Flow airfoil set to a negative angle of attack and in a study perfomed at KTH Stockholm a similar shape was chosen; 11 at DLR the group of Bippes2, 4 used different geometries to study the relative effect of CF instability with respect to other sources of destabilization (Tollmien-Schlichting (TS) waves, attachment line flow or Görtler vortices).

In this paper it is presented a description of the design of a swept wing model to install in the Delft University of Technology LTT facility, a subsonic low-turbulence closed-loop wind tunnel. First the airfoil is designed (section IIA) following the specifications of Bippes. 2The model has to be representative of an infinite wing and as such it needs the installation at its tips of wall liners shaped as the inviscid streamlines around the wing as reported by Mateer8 and Romano et al. 11 The procedure to achieve these contoured wall liners is presented in section IIB. In section III the boundary layer developing on the model for the experimental conditions is computed by means of an implicit finite differences solver. The section IV is instead dedicated to the assessment of the boundary layer stability to standing CF waves by means of linear stability theory (LST). In chapter V the validation of the model design is presented. Surface pressure measurements, oil flow visualization and boundary layer measurements have been performed: two different test flows were investigated to establish the effectiveness and the conditions for stationary cross-flow turbulent breakdown.