A computational investigation of receptivity in high-speed flow near a swept leading-edge.

摘要

Laminar-flow wings have the promise of reducing viscous drag forces in cruise for commercial aircraft. However, the success of a laminar-flow wing depends critically on the external disturbance environment and how these disturbances influence the transition from laminar to turbulent flow. The process by which external disturbances are converted into instability waves, which are the precursors to turbulence, is called receptivity.;The receptivity and early evolution of stationary crossflow vortices is investigated through numerical solutions of the linearized Navier-Stokes equations for a swept leading-edge. Consistent with findings for other geometries, convex surface curvature stabilizes crossflow vortex growth while nonparallel effects are destabilizing. In contrast, the initial amplitude of crossflow vortices downstream of a localized surface roughness site is found to be greater in the presence of convex surface curvature, while the nonparallel meanflow near a leading-edge is found to strongly reduce the initial amplitude of crossflow vortices. These competing effects--curvature and nonparallelism--tend to counteract one another, but, for the conditions studied here, the nonparallel effect is dominant.;Comparisons between linearized Navier-Stokes solutions and recent theoretical receptivity analysis, based on the parallel-flow equations, show that the theoretical method over-predicts the initial amplitude of stationary crossflow vortices by as much as 77% for long wavelength disturbances. The error in the theoretical prediction is reduced for shorter wavelengths, since nonparallel effects are relatively less important, but remains as high as 30% near the leading-edge. Although the parallel theory provides a conservative estimate of the initial amplitude of crossflow vortices, it is concluded that accurate theoretical prediction of crossflow receptivity near the leading-edge of a swept wing requires the inclusion of nonparallel effects.;The research described in this thesis focuses on the receptivity of the three-dimensional boundary layer near the leading edge of a high-speed swept wing. In particular, the influence of surface roughness near the leading edge is examined as it relates to the formation of stationary crossflow vortices in the boundary layer. Recent experiments indicate that surface roughness is the primary cause of stationary crossflow vortices which are observed to dominate the laminar to turbulent transition process on swept wings. Therefore, the mechanisms responsible for the formation of crossflow vortices and the accurate prediction of their initial amplitude are essential for the development of laminar-flow wings.