The interaction between an initially laminar boundary layer developing spatially 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 5.24×107 grid points using a message passing interface on a scalable parallel
computer. The flow bears a close resemblance to the transitional boundary layer on
turbomachinery blades and was designed following, in outline, the experiments by Liu
& Rodi (1991). The momentum thickness Reynolds number
evolves from Reθ = 80
to 1120. Mean and second-order statistics downstream of Reθ = 800 are of canonical
flat-plate turbulent boundary layers and are in good agreement with Spalart (1988).
In many important aspects the mechanism leading to the inception of turbulence is
in agreement with previous fundamental studies on boundary layer bypass transition,
as summarized in Alfredsson & Matsubara (1996). Inlet wake disturbances inside
the boundary layer evolve rapidly into longitudinal puffs during an initial receptivity
phase. In the absence of strong forcing from free-stream vortices, these structures exhibit
streamwise elongation with 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 velocity
field in the outer part of the boundary layer.
Properties and streamwise evolution of the turbulent spots following breakdown,
as well as the process of completion of transition to turbulence, are in agreement with
previous engineering turbomachinery flow studies. The overall geometrical characteristics
of the matured turbulent spot are in good agreement with those observed in the
experiments of Zhong et al. (1998). When breakdown occurs in the outer layer, where
local convection speed is large, as in the present case, the spots broaden downstream,
having the vague appearance of an arrowhead pointing upstream.
The flow has also been 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 and simulations are
also presented to illustrate the effects of streamwise pressure gradient and free-stream