A Study of Fluidic Oscillators as an Alternative Pulsed Vortex Generating Jet Actuator for Flow Separation Control

Abstract

The current study experimentally examines a control loaded bistable fluidic oscillator and an array of bistable fluidic amplifiers driven by the control loaded fluidic oscillator to validate their potential as an alternative flow control actuator. These fluidic devices are considered to be able to generate a periodically oscillating jet without a necessity of any mechanical moving parts and their prototype designs are provided by Professor Vaclav Tesar from Academy of Sciences of the Czech Republic.The output jet characteristics of the control loaded fluidic oscillator has been experimentally evaluated by means of hotwire measurements with single wire probes oriented normal to the plane of output orifices for supply flow rate range of 0.7g/s ~ 6g/s with a feedback loop length as a parameter which varied from 0.3m ~ 1.78m. The experiment have successfully demonstrated that the prototype fluidic oscillator can generate periodically oscillating jets from its conjugating pair of orifices in an alternating manner with a 50% pulse duty cycle, which its amplitude varies proportionally to the supply flow rate. The period of the pulsation found to vary linearly with a supply flow rate up to a power nozzle Reynolds number of 3.0X104 where the frequency saturation initiates giving a constant Strouhal number of ~0.7 with further increase in supply flow rate. The experimental results demonstrate an existence of a strong correlation between the propagation time, , and the switching time, , in determining the saturation frequency, fsat, for a chosen feedback loop length. Furthermore the current study demonstrates a simple way of possibly approximating the natural switching time of a control loaded fluidic oscillator without making an internal flow measurements, which may require further verification though, and found it to be approximately 1ms for the current test model. In general, the frequency response of the oscillating output jet reduced linearly with increasing feedback loop length when running at a relatively low values of , while changing inverse proportionally with L at high values.The experimental study of an array of bistable fluidic amplifiers verified that there exists a load mismatching due to a concentric reduction in the output orifice diameter for its intended function of delaying flow separation over an aerofoil, which caused attenuation in the output pulse characteristic. Although this change in the output characteristic may not be crucial when it comes down to an actual flow control application or can even be advantageous in certain cases the author attempted to remove the disparity in the output jet characteristic with a normal operation condition and suggests a design modification as described in chapter 5. As a by-product of this study it was found that the ratio of the output orifice diameter to the cross sectional area of the exit aperture conduit of a device should be greater than ~0.5 to avoid attenuated output jet characteristic and as a supporting evidence it was found that amplifier with a smaller height, i.e., lower power nozzle aspect ratio, can accommodate a smaller orifice diameter without disturbed output jet characteristic.

Date of Award	1 Aug 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Shan Zhong (Supervisor)