Motivation
Aircraft engines as well as stationary flow machinery show undesired states of flow with a large influence on noise appearance and aerodynamic loss production. Separations of flow and boundary layers are prone to have negative effects on vibrations and aerodynamic performance.
Turbulent structures in technical types of flow interact with the structure. In axial flow machines wakes of stator and rotor blades with increased levels of turbulence impinge on subsequent stages. High alternating pressures attaining the leading edges create significant acoustic emissions.
In addition to the internal aerodynamic considerations all types of noise emission of especially flying gas turbines have a large impact on their operating efficiency and the environmental acceptability.
Methods
Flow control can be a suitable method for reducing noise emission, increase of efficiency and therefore reduction of fuel consumption. Passive as well as active flow control methods can be applied to various types of flow.
Active methods use an amount of external energy in order to alter the flow and the sound field. Flow injection or flow suction as well as acoustic converters are associated with active flow control.
Changing the flow channel geometry or introducing control devices such as turbulators, vortex generators and boundary layer fences are counted among passive flow control measures.
Projects and applications
In the future more attention will be paid to active flow control in compressor stages. In a publicly funded project boundary layer flow suction is applied in a linear high speed compressor cascade for efficiency increase.
"Riblet" surfaces are used as a passive method for flow control. These microscopically grooved structures are based on the skin of fast sharks. They decrease the turbulent drag of overflown surfaces. The Berlin oil channel is a special test bed for the research of those flow control structures.
At the small scale fan test rig flow injection at the blade tips is used for flow improvement. The operating performance is enhanced with a positive influence on the acoustic emissions. Similar effects can be detected at trailing edge blowing rotor configurations. The reduction of flow inhomogenities suppresses a substantial amount of noise development at the subsequent stator stages.
Previous investigations
In earlier projects "Gurney Flaps" have been subject to investigation. They are a high lift concept based on small trailing edge devices perpendicular to the camberline of an airfoil. Wind tunnel experiments have been accompanied by numerical and analytical approaches. It has been shown that there is a possible increase in the lift-to-drag ratio of an airfoil with applied Gurney Flaps.
In a different set of tests "backflow flaps" based on birds plumage have been examined in the wind tunnel. This passive method can help successfully to prevent or delay flow separation on airfoils at high angles of attack. In-flight experiments have shown the feasability of the concept on large aspect ratio wings.
Further reading
Previous investigations (Only German Versions available)