This work involved the documentation and control of flow separation that occurs over low pressure turbine (LPT) blades at low Reynolds numbers. A specially constructed linear cascade was utilized to study the flow field over a generic LPT cascade consisting of Pratt & Whitney “PakB” shaped blades. Flow visualization, pressure measurements, LDV measurements, and hot-wire measurements were conducted to examine the flow fields with and without separation control. Experimental conditions were chosen to give a range of chord Reynolds numbers from 10,000 to 100,000, and a range of freestream turbulence intensities from 0.08% to 2.85%.
The blade pressure distributions were measured and used to define a region of separation that depends on the freestream conditions. The location of separation was found to be relatively insensitive to the experimental conditions. However, the reattachment location was very sensitive to the Reynolds number and the turbulence intensity.
Separation control was performed using plasma actuators. Both steady and unsteady actuation were implemented and found to work well. For the steady actuators, it was found that the separation control is the most effective when applied slightly upstream the separation location. There exists a threshold plasma amplitude for the actuator to take effect for separation control. However, the effectiveness of the actuator is saturated when the plasma amplitude is greater than certain value. The effectiveness of the steady actuator is not sensitive to the orientation of the plasma electrodes.
For the unsteady actuators, there exists an optimum excitation frequency at which the unsteady actuator was the most effective. The optimum ex- citation frequency was corresponded to the unity Strouhal number. It was also found that lowest plasma duty cycle (10% in this work) was as effective as the highest plasma duty cycle (50% in this work). This has an advantage for reducing the power to the actuators.
The comparison between the steady and unsteady actuators showed that the unsteady actuators worked better than the steady ones. The mechanisms of the steady and unsteady plasma actuators were studied. It was suggested by the experimental results that the mechanism for the steady actuators is turbulence tripping, while the mechanism for the unsteady actuators is to generate a train of spanwise structures that promote mixing.