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. (C) 2006 Elsevier Inc. All rights reserved.