Simulation of boundary layer transition induced by periodically passing wakes

The interaction between an initially laminar boundary layer developing spat ially on a flat plate and wakes traversing the inlet periodically has been simulated numerically. The three-dimensional, time-dependent Navier-Stokes equations were solved with 5.24 x 10(7) grid points using a message passing interface on a scalable parallel computer. The flow bears a close resembla nce to the transitional boundary layer on turbomachinery blades and was des igned following, in outline, the experiments by Liu & Rodi (1991). The mome ntum thickness Reynolds number evolves from Re-theta = 80 to 1120. Mean and second-order statistics downstream of Re-theta = 800 are of canonical flat -plate turbulent boundary layers and are in good agreement with Spalart (19 88). In many important aspects the mechanism leading to the inception of turbule nce is in agreement with previous fundamental studies on boundary layer byp ass transition, as summarized in Alfredsson & Matsubara (1996). Inlet wake disturbances inside the boundary layer evolve rapidly into longitudinal puf fs during an initial receptivity phase. In the absence of strong forcing fr om free-stream vortices, these structures exhibit streamwise elongation wit h gradual decay in amplitude. Selective intensification of the puffs occurs when certain types of turbulent eddies from the free-stream wake interact with the boundary layer flow through a localized instability. Breakdown of the puffs into young turbulent spots is preceded by a wavy motion in the ve locity field in the outer part of the boundary layer. Properties and streamwise evolution of the turbulent spots following breakd own, as well as the process of completion of transition to turbulence, are in agreement with previous engineering turbomachinery flow studies. The ove rall geometrical characteristics of the matured turbulent spot are in good agreement with those observed in the experiments of Zhong et al. (1998). Wh en breakdown occurs in the outer layer, where local convection speed is lar ge, as in the present case, the spots broaden downstream, having t The vague appearance of an arrowhead pointing upstream. The flow has also b een studied statistically. Phase-averaged velocity fields and skin-friction coefficients in the transitional region show similar features to previous cascade experiments. Selected results from additional thought experiments a nd simulations are also presented to illustrate the effects of streamwise p ressure gradient and free-stream turbulence.