Numerical investigation of low-pressure turbine separation control

Low-pressure turbines are a common element of many modern jet engines. Flow separation from the suction side of the constituent blades at low Reynolds number conditions can noticeably deteriorate overall engine performance. Under such conditions active control of laminar separation may eliminate or reduce associated losses resulting in increased engine performance. We investigated separation control for the PackB LPT blade geometry at a Reynolds number based on axial chord of 25,000 where laminar separation was observed in the experiments. For our investigations we performed numerical simulations using a higher-order-accurate finite-volume compressible Navier-Stokes code developed in our laboratory. A grid resolution study for the uncontrolled flow indicated grid convergence for our simulations. Pulsed vortex generator jets are shown to result in an earlier transitioning of the flow and successful separation control. The jet amplitude was found to influence the intensity of spanwise coherent structures downstream of the actuator location. An even more efficient separation control can be accomplished by harmonic blowing through a slot (or alternatively plasma actuators). The astounding effectiveness of the latter control scheme is attributed to the suppression of three-dimensional structures which weaken the spanwise structures. Transition was also delayed by streamwise vortices, which were introduced by volume forces. As the flow does not amplify such structures the energy input required for obtaining streamwise vortices of sufficient strength was found to be more than two orders of magnitude larger than for the harmonic blowing through a slot. This, however, is not a real concern as such structures can likely be generated with passive devices such as vortex generators.