The influence of disturbances carried by periodically incoming wakes on the separating flow around a turbine blade

The effect of fluctuations caused by periodically incoming wakes on the separating flow around a turbine blade are studied using direct numerical simulations of incompressible flow—at a Reynolds number of Re = 51,831—in the T106A low-pressure turbine cascade. To be able to differentiate between the effect of small-scale fluctuations and large-scale fluctuations, in one of the simulations small-scale disturbances have been removed from the wake before it enters the computational domain. It is shown that the large-scale disturbance, associated with the presence of the mean wake, triggers the Kelvin–Helmholtz instability of the separated shear layer along the downstream half of the suction side, which is also observed in experiment. On the suction side, the small-scale fluctuations carried by the wakes are found to seed the transition to turbulence of the rolled-up shear layer flow, while on the pressure side the fluctuations are responsible for the formation of longitudinal vortical structures [Wu, X., Durbin, P.A., 2001. Evidence of longitudinal vortices evolved from distorted wakes in a turbine passage. J. Fluid Mech. 446, 199–228]. Finally, the production of kinetic energy inside the passage between blades is also found to depend on the presence of fluctuations inside the wakes.