Laminar separation bubbles form on the back surfaces of aero-engine LP turbine blades. In recent years significant weight and cost reductions and performance improvements have been achieved through a better understanding of the behavior of such separation bubbles. A project is underway at the Universita¨t Stuttgart to study a possible technique to suppress laminar separation bubbles using actuated transition. This paper reports on DNS results with and without actuation for different frequencies, amplitudes and Reynolds numbers, revealing the nature of the transitional process. Early results from an experimental simulation are included. In addition numerical simulations of fluidic oscillators which are capable to provide the required frequencies at a size which would fit into an LP turbine are presented.

The reduction of blade counts in the LP turbine is one possibility to cut down weight and therewith costs. At low Reynolds numbers the suction side laminar boundary layer of high lift LP turbine blades tends to separate and hence cause losses in turbine performance. To limit these losses, the control of laminar separation bubbles has been the subject of many studies in recent years. A project is underway at the University of Stuttgart that aims to suppress laminar separation at low Reynolds numbers (60,000) by means of actuated transition. In an experiment a separating flow is influenced by disturbances, small in amplitude and of a certain frequency, which are introduced upstream of the separation point. Small existing disturbances are therewith amplified, leading to earlier transition and a more stable boundary layer. The separation bubble thus gets smaller without need of a high air mass flow as for steady blowing or pulsed vortex generating jets. Frequency and amplitude are the parameters of actuation. The non-dimensional actuation frequency is varied from 0.2 to 0.5, whereas the normalized amplitude is altered between 5, 10 and 25% of the free stream velocity. Experimental investigations are made by means of PIV and hot wire measurements. Disturbed flow fields will be compared to an undisturbed one. The effectiveness of the presented boundary layer control will be compared to those of conventional ones. Phase-logged data will give an impression of the physical processes in the actuated flow.