Right from the outset of the era of jet airplanes, a reduction in fuel consumption was an essential goal of aircraft construction. Modern medium- and long-haul aircraft spend approximately 98% of their operation time in cruise flight at Mach numbers of up to 0.85. Hence, apart from the use of efficient engines and the application of composite materials that reduce the structural weight of the airplane, the fuel consumption of a commercial aircraft can be reduced by an improved aerodynamic performance, i.e., reduced drag, in transonic cruise flight. This aerodynamic performance is mainly determined by the state of the boundary layer on the aircraft’s wings. Thus, the skin friction drag of future aircraft could be reduced significantly if it was possible to alter the turbulent boundary layer at high Reynolds numbers without great energy expenditure.
To a certain degree, the skin friction produced by turbulent boundary layers can be influenced by passive flow control mechanisms like surfaces with a defined riblet structure. Nevertheless, the drag reduction potential of these passive mechanisms is rather limited. Further drag reduction can only be achieved by active flow control methods or by a combination of active and passive methods. For high Reynolds numbers, oscillating surfaces seem to be a promising approach. Recent numerical and experimental investigations at low Reynolds numbers revealed that oscillations of the surface perpendicular to the free stream direction possess an influence on the development of vortical structures in the boundary layer in the vicinity of the surface.
Further studies showed that these oscillations also reduce the skin friction of a turbulent boundary layer on a flat plate Transverse surface waves, i.e., waves travelling in spanwise direction with amplitudes normal to the surface and thus the near-wall flow, also posses the potential to reduce the drag produced by turbulent boundary layers. However, the considerable drag-reducing potential of this active mechanism that impinges on the coherent structures in the boundary layer, cannot be used, yet.
Although the understanding of turbulent boundary layers has made extensive progress in the recent years, there still exist great challenges as far as the production of active micro-mechanical systems and the control of chaotic systems are concerned. Moreover, the complexity and pronounced non-linear behavior of transonic flow fields which contain shock waves on the suction side of the wing require the detailed analysis of further flow phenomena.
The scope of this project is to carry out a reference experiment that demonstrates the dragreducing effect of surface oscillations which are generated using miniature piezo actuators and which act on the flow field over a flat plate at Reynolds numbers in the order of O(106). The control of the actuators and the determination of the oscillating surface material will be performed by the Central Institute for Electronics (Dr. Schiek) and by the Institute of Energy Research (Prof. Beck), both located at the Forschungszentrum Jülich GmbH. All experiments will be carried out at the Institute of Aerodynamics, RWTH Aachen University (Dr. Klaas).
The experimental investigations of this study served as a preliminary for the proposal of a new DFG research unit in the field of drag reduction by active flow control. This research unit will be carried out in close cooperation between the Institute of aerodynamics, which will perform the experimental analysis of flows with transverse surface waves on flat plates and airfoils, and the Central Institute of Electronics as well as the Institute of Energy Research of the Forschungszentrum Jülich, which will work on the development of the actuator system and the complex architecture of the setup as well as the determination of the fatigue of the material.