Aircraft manufacturers and airlines demand for highly efficient engines  to address the more and more stringent requirements in terms of economy and environmental compatibility. This  target requires light and compact propulsion systems, leading to the necessity to reduce the weight of the single components. Within the compressor module, the number of the required stages can be minimized by a higher stage pressure ratio at constant total pressure ratio. This can be done by a stronger turning of the flow within the passage of a supercritical compressor cascade. Nevertheless, separation needs to be avoided, one approach being active flow control.

At the Technical University of Berlin projects investigating active flow control concepts have been executed within the Collaborative Research Center 557, Control of turbulent shear flows.

At the department of Aeronautics and Astronautics of  the Technical University of Berlin, a linear stator cascade test facility for active flow control investigations was developed and tested at high speed flow conditions. The compressor blades were designed as critically loaded controled diffusion airfoils  (CDA)  for an upstream Mach number of M = 0.75 and a Reynolds number of Re = 1.1 • 10^6 based on axial chord.

As a basis for the development of a closed-loop active flow control method, a detailed investigation of the flow field without actuation is necessary. Five-hole probes have been used to determine the pressure field and the flow direction of the wake behind the compressor blade. Furthermore an analysis of the baseline cascade flow by oil flow pattern reveals complex three-dimensional secondary flow phenomena. On the suction side of the blades close to the leading edge, a laminar separation bubble is observed. Where blade and side walls meet, corner stall is developing. Caused by the blade aspect ratio of 0.8, the passage flow is strongly influenced by these vortices. Uniform main passage flow is thus constricted to the midspan region, leading to a reduction of the passage efficiency. The aim is  to investigate how the secondary flow phenomena  can be supressed by active flow control methods to receive a higher stage pressure ratio or lower total pressure losses respectively and where limits of these methods can be found.

For this purpose, actuators for steady and pulsed blowing through a side wall blowing nozzle  were developed. Active flow control investigations have been carried out in the cascade test rig for different incidences and inflow Mach numbers and variations of the jets mass flow rate, the velocity ratio and the actuation frequency to investigate the influence of the injected air on the cascade flow.

Person of contact: Dr.-Ing. Christine Tiedemann