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A systematic study on the design, development, and characterization of high-momentum, high-bandwidth microactuators for high-speed flow control is described in this paper. Beginning with building-block experiments, multiple resonant flow phenomena are used in the actuator design to arrive at an actuator configuration that provides the desired flow properties. The first-generation actuator design consists of an underexpanded source jet incident upon a cavity. The lower surface of this cavity contains micronozzles through which the unsteady microjets (400 m) issue. Results show that microjets produced by this actuator have a high mean momentum (300-400 m=s) and a significant unsteady component (20-30% of the mean). Experiments were conducted over a large range of parameters in terms of cavity length, source jet nozzle pressure ratio, and impingement distance. The results unequivocally demonstrate the ability to vary the frequency and the amplitude of the mean and unsteady momentum of microjets issuing from this actuator. By varying the dimensions of the actuator by few hundred microns and/or source jet pressure by roughly 1 atm, one is able to vary the frequency rather precisely over a range of 5-20 kHz. A correlation based on Strouhal number and jet column length is suggested for the design of actuators. Actuators in the frequency range of a few to well over 50 kHz have been designed and characterized. It is believed that the frequency range may be extended down to O100 Hz and up to O100 kHz using this actuator approach.

The flowfield associated with supersonic impinging jets has been of interest to both engineers and researchers for some time due to its wide range of practical applications and its complex nature from a fundamental fluid dynamic point of view. An example of supersonic impinging jets occurs in short takeoff and vertical landing aircraft, for which the highly oscillatory flowfield and the associated acoustic loads are also accompanied by a dramatic loss in lift during hover, severe ground erosion of the landing surface, and hot gas ingestion into the engine inlets. Another characteristic feature of this flowfield is an intensive heat transfer between the jet and the impingement surface. In the past we have examined impinging jets and their control using microjets at cold conditions; the present study is a step toward examining this flowfield and the effectiveness of microjet control at increasingly realistic thermal conditions. An ideally expanded, Mach 1.5 primary jet issuing from an axisymmetric nozzle was heated up to a stagnation temperature of 500 K. Mean and unsteady temperature and pressure measurements were obtained on a lift plate representative of the undersurface of an aircraft and on the ground plane over a range of nozzle-to-plate distances (representing aircraft hover conditions). In addition, near-field noise was also measured using a microphone. The velocity field of the impinging jet for both cold and hot conditions was mapped using particle image velocimetry. Our results show that the temperature recovery factor at the stagnation point on the ground plane is strongly dependent on the temperature ratio and nozzle-to-plate distance, similar to observations in subsonic impinging jets. The hover lift loss for hot jets is much higher than for cold jets, nearly 75% of the primary jet thrust at small nozzle-to-plate distances. The pressure fluctuations generated by hot impinging jets are also substantially higher than their cold counterparts. As in cold jets, pressure and noise spectra for hot jets show discrete, high-amplitude acoustic tones (generally known as impinging tones) at frequencies varying with jet temperature. The activation of microjet control shows a substantial reduction in pressure fluctuations both in terms of overall sound pressure levels (up to 20 dB on the ground plane and 15 dB on the lift plate) and the attenuation of discrete, high-amplitude impinging tones (up to 32 dB). High-temperature peaks were observed in the temperature spectra at frequencies corresponding to impingement tones in the pressure and noise spectra; these were also substantially attenuated with microjet control. As much as 50% of the lift loss was recovered by using control for hot jets at smaller nozzle-to-plate distances. In general, the results provide evidence of the feasibility of using this active control approach under increasingly realistic conditions to achieve desired reductions in noise, unsteady pressures, and thermal loads.

Supersonic impinging jet(s) inherently produce a highly unsteady flow field. The occurrence of such flows leads to many adverse effects for short take-off and vertical landing (STOVL) aircraft such as: a significant increase in the noise level, very high unsteady loads on nearby structures and an appreciable loss in lift during hover. In prior studies, we have demonstrated that arrays of microjets, appropriately placed near the nozzle exit, effectively disrupt the feedback loop inherent in impinging jet flows. In these studies, the effectiveness of the control was found to be strongly dependent on a number of geometric and flow parameters, such as the impingement plane distance, microjet orientation and jet operating conditions. In this paper, the effects of some of these parameters that appear to determine control efficiency are examined and some of the fundamental mechanisms behind this control approach are explored. Through comprehensive two- and three-component velocity (and vorticity) field measurements it has been clearly demonstrated that the activation of microjets leads to a local thickening of the jet shear layer, near the nozzle exit, making it more stable and less receptive to disturbances. Furthermore, microjets generate strong streamwise vorticity in the form of well-organized, counter-rotating vortex pairs. This increase in streamwise vorticity is concomitant with a reduction in the azimuthal vorticity of the primary jet. Based on these results and a simplified analysis of vorticity transport, it is suggested that the generation of these streamwise vortices is mainly a result of the redirection of the azimuthal vorticity by vorticity tilting and stretching mechanisms. The emergence of these longitudinal structures weakens the large-scale axisymmetric structures in the jet shear layer while introducing substantial three-dimensionality into the flow. Together, these factors lead to the attenuation of the feedback loop and a significant reduction of flow unsteadiness.

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