Numerical investigation of active flow control for low-pressure turbine blade separation

In the present paper, active flow control (AFC) for low-pressure turbine (LPT) applications is investigated using two separate computational approaches. In the first approach, a boundary layer on a flat plate is subjected to the same streamwise pressure gradient as measured on the suction side of a Pak-B LPT blade at a Reynolds number of 25,000 (based on axial chord). The relevant mechanisms for AFC using steady and pulsed vortex generator jets (VGJs) are investigated by direct numerical simulations. Results indicate two main mechanisms associated with VGJ control: free stream momentum entrainment due to streamwise structures for steady, angled VGJs and early (by-pass) boundary layer transition for pulsed VGJs. Two-dimensional simulations show that the natural instability of the separated shear layer can effectively be exploited for the purpose of flow control. In the second approach, the entire flow through a linear LPT cascade is investigated. Both two- and three-dimensional simulations are presented and compared to experimental data. At a Reynolds number of 25,000 and a blade spacing of 88% axial chord, good agreement is observed between numerical and experimental results, except in the separated region near the trailing edge. The separated flow through a LPT cascade with a blade spacing of 110% axial chord is controlled using pulsed blowing through a slot upstream of the separation location. Results indicate that with AFC, an increase of 30% in the time-averaged ratio of lift over drag can be achieved.