Effects of Extended Laminar Flow on Wing Buffet-Onset Characteristics

Global instability analysis is used to investigate the effects of extended regions of laminar flow on both unswept and swept infinite-span wings. The formulation is based on the Reynolds-averaged Navier-Stokes equations and differs from earlier studies on fully turbulent flows in the activation of the trip term in the Spalart-Allmaras eddy-viscosity transport equation. The trip term ensures a rapid transition to turbulence at an arbitrary-specified location on the wing surface, analogous a trip strip in an experiment. A parametric study is conducted for the ONERA OAT15A extruded airfoil, as an example. Results show that an extended region of laminar flow leads to a reduction in the critical angle of attack for the buffet onset, as compared with the fully turbulent conditions. All modes of instability show enhanced growth as a result of the laminar flow. However, increased lift (at fixed angle of attack) associated with an extended region of laminar flow results in a higher lift coefficient at buffet onset as compared with the fully turbulent case. Results show that the laminar flow effects on the buffet onset are linked to the suction-side laminar flow and are largely independent of the boundary-layer state on the pressure side of the wing.