Large-eddy simulation results for laminar-to-turbulent transition in a spatially developing boundary layer are presented. The disturbances are ingested into a laminar ow through an unsteady suction-and-blowing strip. The ltered, three-dimensional timedependent N a vier-Stokes equations are integrated numerically using spectral, high-order nite-di erence, and three-stage low-storage Runge-Kutta methods. The bu er-domain technique is used for the out ow boundary condition. The localized dynamic model used to parameterize the subgrid-scale stresses begins to have a signi cant impact at the beginning of the nonlinear transition or intermittency region. The ow structures commonly found in experiments are also observed in the present simulation; the computed linear instability modes and secondary instability lambda-vortex structures are in agreement with the experiments, and the streak-like-structures and turbulent statistics compare with both the experiments and the theory. The physics captured in the present LES are consistent with the experiments and the full Navier-Stokes simulation DNS , at a signi cant fraction of the DNS cost. A comparison of the results obtained with several SGS models shows that the localized model gives accurate results both in a statistical sense and in terms of predicting the dynamics of the energy-carrying eddies, without ad hoc adjustments.

The spatial development of laminar-turbulent transition in a 45 swept-wing boundary layer is investigated using the large-eddy simulation approach. Both stationary and traveling disturbances are initiated by steady and random forcings. The numerical simulations reproduce the surface shear streaks typical of the crossow instability observed in experiments. Downstream of the transition location the wall shear stress is found to turn towards the streak direction. The strength of the stationary vortices grows exponentially, initially independent of the unsteady forcing amplitude and saturates at dierent levels during the later development, depending on the magnitude of the traveling vortices. Streamwise velocity contours show the evolution from a wave-like structure to the \half-mushroom" structure for the lower-amplitude, traveling-wave case, leading to double inection points in the wallnormal velocity proles prior to transition; the nonlinear interactions involving the stationary vortices are also much stronger, as indicated by the amplication of higher harmonics of the primary mode. The frequency spectrum of the traveling waves show a high frequency secondary instability prior to transition that, however, is not prominent in the higheramplitude traveling-wave case.

The spatial evolution of the disturbances that lead to boundary-layer transition on a swept wedge is computed by large-eddy simulations (LES). Stationary and travelling crossflow-vortex disturbances are generated using steady and random-amplitude suction and blowing on the wedge. For a fixed initial amplitude of the stationary vortex and low-amplitude unsteady disturbances, the LES show the evolution of stationary-dominated crossflow disturbances similar to previous simulations and experiments: linear amplification is followed by vortex roll-over and doubly inflectional velocity profiles just prior to transition. A high-frequency secondary instability is associated with the double inflection points in the velocity profiles. The harmonic modes of the primary disturbance were found to be amplified, while no energy was found in any subharmonic mode. The physical phenomena were significantly different when the stationary and travelling vortices have comparable initial amplitudes: in this case, the vortex roll-over does not occur and transition is dominated by the travelling-wave component.

The large-eddy simulation of the spatial evolution of a stationary cross ow v ortex packet in a three-dimensional boundary layer was performed. Although a coarse grid was used compared to that required by a direct numerical simulation the essential features of the disturbance evolution, such as the spanwise disturbance spreading and the vortex rollover, were captured accurately. The eddy viscosity became signi cant only in the late nonlinear stages of the simulation.