Active separation control for lifting surfaces at Low-Reynolds number operating conditions

The low-pressure turbine (LPT) stage is a common element of many modern jet engines. Its performance at cruise conditions is of great economical importance. Low-Reynolds number conditions and high blade loading can result in laminar separation from the suction side and performance degradation. For external aerodynamics problems, such as airfoils, low-Reynolds number conditions and large angles of attack can similarly lead to laminar flow separation, and in the worst case complete stall. Successful control of separation from lifting surfaces at such detrimental conditions promises significant savings in operating expenses and improved safety. We are employing computational fluid dynamics (CFD) for investigating passive and active flow control for lifting surfaces. Because of the many design parameters available for flow control strategies, such as the spacing and their dimensions, a trial and error optimization is ill-fated. It has been convincingly demonstrated in the past that through a deepened understanding of the underlying fundamental physical mechanisms the effectiveness of flow control can be greatly improved. In addition, an improved understanding will likely result in entirely new and innovative flow control devices and strategies. Using CFD we investigated separation control using vortex generator jets (VGJs) and harmonic blowing through a slot for a typical LPT blade. With pulsed and harmonic VGJs and for moderate blowing ratios as well as for harmonic blowing through a slot the generation of spanwise coherent structures that were amplified by the flow appeared to be the dominant mechanism for separation control. When the forcing amplitude was increased, the two-dimensional coherence was reduced and turbulent mixing appeared to become the more dominant mechanism. The hole spacing was found to have little effect as long as it was smaller than the length of the separated flow region.