The use of hot-wire anemometry to investigate unsteady wake-induced boundary-layer development on a high-lift LP turbine cascade

Recent research has revealed positive effects of unsteady flow on the devel opment of boundary layers in turbine cascades especially at conditions with a laminar suction side separation bubble at low Reynolds numbers. Compared to steady flow, a reduction of total pressure loss coefficient over a broa d range of Reynolds numbers has been shown. Taking into account the positiv e effects of wake-induced transition already during the design process, new high lift bladings with nearly the same low losses at unsteady inlet flow conditions could be achieved. This lends to a reduction of weight and cost of the whole turbine module for a constant stage loading. Unsteady flow in turbomachines is caused by the relative motion of rotor and stator rows. Fo r simulating a moving blade row upstream of a linear cascade in the High-Sp eed Cascade Wind Tunnel of the Universitat der Bundeswehr Munchen, a wake g enerator has been designed and built. The wakes are generated with bars, mo ving with a velocity of up to 40 m/s in the test section upstream of the ca scade inlet plane. Unsteady flow causes the transition on the surface of th e suction side of a low-pressure turbine blade to move upstream whenever an incoming wake is present on the surface; moreover a laminar separation bub ble can be diminished or even, suppressed In order to detect the effects of wakes on the boundary layer development a new hot wire data acquisition sy stem is required. Due to the fact that hot wires give a good insight into b oundary layer development, a new hot-wire data acquisition system has been set up. The anemometry system can acquire four channels simultaneously, the refore being capable of logging a triple hot-wire sensor and a bar trigger simultaneously. One further channel is utilized for a once-per-revolution t rigger. The once-per-revolution trigger is used to start the measurement of one data block. Using the well-established ensemble-averaging technique, 3 00 ensembles each consisting of five wake passing periods have been acquire d. Ensemble averaging can be directly performed without any data reduction. The adaptation of this new hot-wire anemometry data acquisition system to the High-Speed Cascade Wind Tunnel of the Universitat der Bundeswehr Munche n is pointed out. First, results on unsteady periodic boundary layer develo pment of a highly loaded low-pressure turbine cascade under unsteady inlet flow conditions are presented. During the present investigation four bounda ry layer traverses, ranging from x/l(ax) = 0.82 to x/l(ax) = 0.99 (suction side), at steady and unsteady inlet flow conditions (U-bar = 10 m/s) at ant outlet Reynolds number of Re-2th = 100,000 have been conducted [S0889-504X (00)00204-X].