Boundary layers on the suction surface of low pressure turbine (LPT) blades are known to be susceptible to laminar separation. This is mainly due to the fact that, during high-altitude cruise, Reynolds numbers in LPTs can drop to very low values, on the order of 25,000. The resulting laminar boundary layer separation is associated with dramatic losses in turbine performance. Numerous experimental studies of separation on LPT blades clearly suggest that flow control might be beneficial for preventing or delaying separation. Lake et al. (1999) reported investigations involving modified blade surfaces (dimples) and concluded that boundary layer separation was significantly reduced. Such passive techniques, however, are ultimately limited by the fact that increased viscous losses may incur penalties at higher Reynolds numbers where unmodified (uncontrolled) blades yield satisfactory turbine performance. More recently, the influence of active control devices (pulsed vortex generator jets (VGJs)) on the separation behavior has been studied extensively by Bons et al. (1999, 2001a, 2001b). These experiments have shown that so-called pulsed VGJs have a dramatic effect on low Reynolds number separation in LPTs. A reduction in wake losses of up to 60% was measured. However, many of the underlying physical mechanisms responsible for these striking-experimental results are not understood.
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