Basic research into performance of fluidic oscillators

在高气动载荷和低雷诺数条件下，涡轮叶片在吸力面扩散部分容易发生流动分离，不利于效率和性能的提高。边界层控制（BLC）是抵消这一现象的一个合适的选择，通过触发从层流到湍流的转变，并在叶片的减速区域引发更早的流动重新附着。这可以显著减少分离流区和相关的高损失。无源器件，如湍流，在低雷诺数时非常有利，但在高雷诺数时，无论边界层状态如何，都会触发跃迁，从而增加剖面损失。有源BLC设备可以在不需要时停用，避免这些额外的损失。连续吹气被认为是一种有效的措施，但它需要不利的高质量流量。脉冲吹气可以显著降低质量流量要求，但在合适的驱动装置方面具有挑战性。这是因为从层流到湍流状态的过渡过程通常以所谓的Tollmien-Schlichting不稳定性为特征，这种不稳定性在典型的低压涡轮叶片中表现出高达10 kHz的频率，甚至更高。为了激发跃迁，驱动装置必须提供这个数量级的频率。因此，机械执行器无法在涡轮叶片上提供如此高的激励频率。申请人首次成功地通过专门设计和小型化的流体振荡器实现了这些频率。本研究项目的主要目的是通过实验方法对低压涡轮叶片主动BLC装置——流体振荡器的性能进行基础研究。申请人的目的是更好地了解这种流体振荡器内部发生的基本现象，以便能够定量地，至少是定性地预测振荡的频率和幅度。这将允许利用这样的装置作为有效和高效的BLC方法，以减少涡轮机叶片的损失。在项目结束时，应确定主要影响因素，并确定流体振荡器减少剖面损失的潜力。一旦在技术上实现了通过强大的流体振荡器流动控制来提高涡轮效率，直接有助于减少污染物和二氧化碳的排放。因此，拟议的项目可以对Förderlinie Ökoeffizientes Fliegen作出重大贡献。假设该项目成功完成，有可能应用所获得的专业知识，并在BMWi的国家民用航空研究计划LuFo的下一阶段与工业伙伴合作，将这项技术用于设计真正的发动机。

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.