Basic research into performance of fluidic oscillators

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At high aerodynamic loading and at low Reynolds numbers turbine blades are prone to flow separation on the diffusing part of the suction surface, which is detrimental for efficiency and performance. Boundary layer control (BLC) is a suitable option to counteract this phenomenon by triggering transition from laminar to turbulent flow and to provoke an earlier flow reattachment in the deceleration region of the blade. This can reduce the separated flow region and the associated high losses significantly. Passive devices, such as turbulators, are very beneficial at low Reynolds numbers, but increase profile losses at higher Reynolds numbers by triggering transition regardless of the boundary layer state. Active BLC devices can be deactivated when they are not needed, avoiding these additional losses. Continuous blowing is known as an effective measure, but it requires unfavorable high mass flow rates. Pulsed blowing can reduce the mass flow requirements significantly, but it is challenging in terms of suitable actuation devices. This is due to the fact that the transition process from laminar to turbulent state is usually characterized by so-called Tollmien-Schlichting instabilities, which exhibit frequencies of up to 10 kHz and even higher in typical low pressure turbine bladings. In order to excite transition, actuation devices have to provide frequencies of this order of magnitude. Therefore, mechanical actuators are not capable of providing such high excitation frequencies on turbine blades. For the first time the applicant succeeded in achieving those frequencies with specifically designed and miniaturized fluidic oscillators. The main objective of this research project is to perform basic research into the performance of fluidic oscillators used as device for active BLC on low pressure turbine bladings using experimental methods. The applicant aims to gain a better understanding of the fundamental phenomena occurring inside this kind of fluidic oscillator in order to be able to predict frequency and amplitude of the oscillations preferably quantitatively, at least qualitatively. This will allow utilizing such devices as effective and efficient BLC method for loss reduction in turbine bladings. At the end of the project, the main influence factors are supposed to be identified and the potential of fluidic oscillators to reduce profile losses will be determined. Once technically realized the increase of turbine efficiency by powerful flow control with fluidic oscillators contributes directly to a reduction of pollutants and CO2 emissions. Thus, the proposed project can make substantial contributions to the Förderlinie Ökoeffizientes Fliegen. Assuming a successful completion of the project, it is possible to apply the gained know-how and to utilize this technology for the design of real engines in cooperation with an industrial partner in upcoming phases of the national civil aeronautical research program LuFo of the BMWi.