In the present work, the evolution of the boundary layer over a low-pressure turbine blade is
studied using direct numerical simulations, with the aim of investigating the unsteady flow
field induced by the rotor-stator interaction. The freestream flow is characterized by the
high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream
of the turbine blades. To include the presence of the wake without employing an ad-hoc
model, we simulate both the moving bars and the stationary blades in their respective
frames of reference and the coupling of the two domains is done through appropriate
boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part
of the blade is suppressed. Moreover, the presence of the wake introduces alternating
regions in the streamwise direction of high- and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress.
The analysis of the velocity fields allows the characterization of the streaky structures
forced in the boundary layer by turbulence carried by upstream wakes. The breakdown
events are observed once positive streamwise velocity fluctuations reach the end of the
blade. Both the fluctuations induced by the migration of the wake in the blade passage
and the presence of the streaks contribute to high values of the disturbance velocity
inside the boundary layer with respect to a steady inflow case. The amplification of the
boundary layer disturbances associated with different spanwise wavenumbers has been
computed. It was found that the migration of the wake in the blade passage stands for
the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the lowand high-speed regions associated with the wakes. [DOI: 10.1115/1.4056108
