High-Bandwidth Unsteady Microactuators for Active Control of High-Speed Flows

Unsteady actuators with a high amplitude output and tunable frequency are needed for the effective and efficient control of many sub and supersonic flows. The development, design, characterization, modeling and implementation of a novel High-bandwidth micro fluidic actuator are described in this dissertation. A systematic study is followed to understand the resonance phenomena in microscales with the intent to utilize them for designing unsteady microjet actuators for the active control of low and high-speed flows. A supersonic microjet, issuing from a convergent nozzle of 1mm exit diameter, is studied extensively in a number of canonical configurations. The microjet, impinging normally on a flat surface with a flush mounted probe, under expanded into a short cylindrical cavity, grazing the sharp edges of a circular hole –are the configurations examined in this study. Some of these configurations of impinging microjets produced resonance that lead to the generation of high-amplitude, disturbances at very high frequencies for a range of geometric and flow parameters. The jet structure, strength and location of the standoff shock and the Mach disc etc. are observed to be closely related to the generation of high-amplitude flow unsteadiness. The impinging flowfield, characterized by discrete tonal features was found to be coupled with the highly unstable plate shock oscillations, visualized using a high magnification microschlieren system. It was found that if the plate shock is located inside a specific region within the first shock cell, the flowfield is more likely to become highly unsteady and often lead to resonance. This region is referred to as the region of instability (RoI). Expanding upon the various canonical configurations examined and adding more geometric complexity (to enhance and better control the resonance phenomena), a first generation actuator is designed. This micro fluidic actuator system essentially consists of an underexpanded primary source jet impinging upon and exiting a short cylindrical cavity where the array of unsteady supersonic secondary microjets emanates through multiple micro orifices, in the lower cavity surface. The frequency and the amplitude response of secondary microjets are examined in detail over a range of geometric and flow parameters such as the source jet nozzle to cavity distance h; the nozzle pressure ratio NPR; cavity length L; actuator volume V and the inflow to outflow area ratio Sc/Sm. A preliminary actuator design criteria is first derived from these results to design actuators for the active control of various supersonic and subsonic flows. The results clearly show that microjets produced by this actuator possess very high mean momentum (they are supersonic for most cases, hence >300 m/s) as well as a very significant unsteady component (more than 30% of the mean velocity). By the proper selection of geometric and fluid parameters, the frequency can be 'fine tuned' to any value in between 1-60 kHz in the present studies. A lumped element modeling (LEM) approach is used to describe the aero-acoustic characteristics of the microactuator that can produce high amplitude pulsed microjets at a design frequency. In the LEM model the microactuator is considered to be an aero-acoustic resonator with a well defined natural frequency. The natural resonance frequency of the microactuator is derived from the basic principles of physical acoustics using an impedance model based on an electrical-acoustic analogy which considers it as a lumped system consisting of a number of elemental building blocks. The total acoustic impedance experienced by the acoustic mass of these elements is derived using the circuit analogy in terms of the geometric and flow parameters involved in the problem. This model uses an impedance minimum principle for calculating the resonance frequency with maximum amplitude. The LEM model for actuators with different geometry is compared with the corresponding experimental data, showing very good agreement for the entire range of the parametric space examined in this study. Two supersonic test beds, an impinging supersonic jet and a cavity subjected to supersonic flow, were used to evaluate the flexibility and control efficiency of these actuators. For an impinging jet with base line unsteadiness around 3-6 kHz, actuator modules were designed, using the scaling principles developed earlier in the study, to produce microjet arrays in a frequency range similar to the base flow. These actuators were then integrated in the model near the nozzle exit. Activation of pulsed microjets into the flowfield resulted in interesting results such as: annihilation of primary tones and appearance of new tones in the impinging flowfield spectra. The frequency of these new tones neither matches the baseline nor the actuator frequency. It is considered that the high frequency actuation has changed the shear layer instability properties that essentially affect the evolution and dynamics of vortical structures in the jet shear layer. The energy content of the new tone is less than the original peaks resulting in an over all sound pressure level (OASPL) reduction of 4-5 dB, for most testing conditions. In the second application of the actuator i.e. for the supersonic cavity, two actuator modules that can generate 12 pulsed microjets at a design frequency range of 4- 6 kHz were integrated to the leading edge of a Mach 1.5 cavity. Unsteady pressure measurement inside the cavity shows that by pulsed actuation the dominant cavity tones were suppressed nearly 7 dB along with an overall broad band reduction of 4-6 dB. Also, it is found that the actuators operated in a pulsed mode performed better than in a steady mode. It is expected that an optimum design of an actuator with a better venting capability can do an improved control of this resonating flowfield. In summary, the potential capability of the microactuator to produce High-bandwidth pulsed microjets at a design frequency and amplitude as well as its use in supersonic flow control is clearly demonstrated in this dissertation. The ability to produce unsteady flow with significant mean and unsteady components, where the dynamic range can be easily varied makes these actuators promising for a number flow control applications. This study should be continued further to realize the final goal of designing a high momentum/High-bandwidth 'ultimate actuator' with frequency, amplitude and phase control.