UNSTEADY WAKE-INDUCED BOUNDARY-LAYER-TRANSITION IN HIGH LIFT LP TURBINES

The development of the unsteady section side boundary layer of a highl y loaded LP turbine blade has been investigated in a rectilinear casca de experiment. Upstream rotor wakes were simulated with a moving-bar w ake generator A variety of cases with different wake-passing frequenci es, different wake strength, and different Reynolds numbers were teste d. Boundary layer surveys have been obtained with a single hot-wire pr obe. Wall shear stress has been investigated with surface-mounted hot- film gages. Losses have been measured. The suction surface boundary la yer development of a modern highly loaded LP turbine blade is shown to be dominated by effects associated with unsteady wake-passing. Wherea s without wakes the boundary layer features a large separation bubble at a typical cruise Reynolds number, the bubble was largely suppressed if subjected to unsteady wake-passing at a typical frequency and wake strength. Transitional patches and becalmed regions, induced by the w ake, dominated the boundary layer development. The becalmed regions in hibited transition and separation and are shown. to reduce the loss of the wake-affected boundary layer. An optimum wake-passing frequency e xists at cruise Reynolds numbers. For a selected wake-passing frequenc y and wake strength, the profile loss is almost independent of Reynold s number, This demonstrates a potential to design highly loaded LP tur bine profiles without suffering large losses at low Reynolds numbers.