Numerical Simulation of Low-Pressure Turbine Blade Separation Control

Abstract

Low-pressure turbines are a common and important element of many modern jet engines. Any performance improvement will lead to net savings in aircraft operating costs. At low operating Reynolds numbers or for reduced stage solidities laminar separation from the constituent blades 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. A high-order-accurate finite volume Navier-Stokes code for investigating separation control for the PackB blade geometry at a Reynolds number based on axial chord of 25,000 was employed. For this Reynolds number laminar separation was observed in the experiments. First, a grid resolution study for the uncontrolled flow was carried out, which shows grid convergence for our simulations. Then separation control by pulsed vortex generator jets was investigated. It is shown how this control results in an earlier transitioning of the flow and the introduction of spanwise coherent structures. The increased wall-normal mixing associated with these two mechanisms results in an effective separation control. An even more efficient separation control can be accomplished by harmonic blowing through a slot. The astounding effectiveness of the latter control scheme is attributed to an amplification of the disturbance input through a hydrodynamic instability mechanism and to the suppression of three-dimensional structures, which weaken the spanwise structures. Finally, separation control by streamwise vortices that were generated by volume forces was investigated. As the flow does not amplify such structures the energy input required for accomplishing an effective control was found to be large compared with the other flow control techniques. Because the structures are steady they can, however, likely be generated with passive devices such as vortex generators.