Experimental Investigations of a High Frequency Master-Slave Fluidic Oscillator to Achieve Independent Frequency and Mass Flow Characteristics

Abstract

High frequency fluidic oscillators have been of scientific interest for many decades. Especially over the last couple of years fluidic oscillators became more important for active flow control applications. At the Institute of Jet Propulsion of the University of the German Federal Armed Forces Munich studies on different kinds of flow control methods were carried out on aerodynamically highly loaded low pressure turbine blades. On the basis of these studies, the most efficient way to trigger transition at low Reynolds numbers was found to be with fluidic oscillators at frequencies up to 10 kHz. Still, it is an open issue whether it is most efficient to trigger Tollmien-Schlichting waves, stimulate Kelvin-Helmholtz instabilities or simply induce a frequency independent disturbance in form of a periodic impulse for boundary layer control on aero-dynamically highly loaded low pressure turbine blades. To find an answer to these questions, a high frequency master-slave fluidic oscillator is introduced with an independent frequency and mass flow characteristic. Any frequency from the master oscillator’s characteristic can be chosen and the mass flow rate can be controlled with the slave oscillator. Contrary to concepts with fast switching valves or piezo actuators, this actuator is based on a working principle without the necessity of any moving and life limited parts. Based on experimental results, the characteristics of the master as well as the coupled oscillator are shown. The predictable operation of the coupled device is demonstrated in detail for a constant overall mass flow rate at discrete frequencies of 5 and 6 kHz. In addition, it is also shown that the mass flow can be varied with one master-slave arrangement by a factor of six while keeping the frequency constant at 5 or 6 kHz, respectively. Besides proof of concept these investigations focus on relevant parameters for active boundary layer and transition control. The frequency and velocity spectra of the coupled device are presented for constant frequency and constant mass flow operating points. Based on these results the improvement potential of the coupled oscillator for fundamental research on this topic is discussed.