Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Synthetic jet is a quasi-steady jet of fluid generated by oscillating pressure drop across an orifice, produced by a piston-like actuator. A unique advantage of the synthetic jet is that it is able to transfer linear momentum without requiring an external fluid source, and has therefore attracted much research within the past decade. Principal applications include aerodynamic flow boundary-layer separation control, heat transfer enhancement, mixing enhancement, and flow-generated sound minimization. In this thesis, the method of deriving the volume flux equation for a duct is first reviewed, combined with this method, a simplified synthetic jet model is presented, based on the principles of aerodynamic sound, the pressure fluctuation in the acoustic far field is predicted. This model is then been used to predict the minimum synthetic jet cavity resonance frequency, acoustic power, acoustic efficiency, root-mean-square jet speed, acoustic spectrum and their dependence on the following independent parameters: the duct length and radius, the aperture radius, the piston vibration frequency, and the maximum piston velocity. / 2031-01-01

An experimental investigation was undertaken to determine the ability of Synthetic Jet Actuators to control the aerodynamic properties of a wing. The Synthetic Jet Actuator (SJA) was placed at two separate positions on a wing comprised of a NACA0015 airfoil. The first of the jet positions is located at 12% of the chord, hereby referred to as the leading edge Synthetic Jet Actuator. The second exit position is located at 99% chord of an airfoil and hereby is referred to as the trailing edge Synthetic Jet Actuator. The two locations produced different benefits as the angle of attack of the wing was increased. The leading edge Synthetic Jet Actuator delayed the onset of stall of an airfoil, suppressing stall up to 25 degrees angle of attack. The control of the aerodynamic characteristics was achieved by influencing the amount of the separated flowfield region. The effects of the dynamic stall vortex were investigated with wind tunnel testing during the pitching motion of an airfoil to determine how the flow reacts dynamically. The trailing edge synthetic jet actuator was investigated as a form of low angle “hingeless” control. The study investigated the effect of the jet momentum coefficient on the ability of the synthetic jet to modify the lifting and pitching moment produced from the wind tunnel model. The data indicates that, with the present implementation, the SJA-jet flap generates moderate lift and moment coefficient increments that should be suitable for hinge- less control. It was also shown that, for the current experimental setup and a given jet momentum coefficient, continuous blowing is more effective than oscillatory blowing/sucking. The data shows that combining the SJA with a Gurney flap does not result in performance enhancement.

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A long aspect ratio synthetic jet is produced through an axial slit along part of the length of a cylinder. The jet is excited acoustically by a pair of loudspeakers mounted at the cylinder terminations. The study compares between the performance of two different slits with aspect ratios of 273 and 773. The comparison is based on the spanwise distribution of the mean jet velocity and phase between the jet velocity fluctuations and the excitation signal. Three different frequencies and amplitudes are used to excite the speakers covering the range of frequencies used in the control application. For both cases studied the mean centerline velocity of the jet increases with increasing the amplitude of the exciting signal, but decreases with increasing its frequency. Moreover, velocity deficits of up to 30% are evident as the midspan of the cylinder is approached from either end. Similar trends are also observed for the centerline phase distributions of the velocity fluctuations, with deficits of up to 130°. However, it is observed that for the long slit case the deficits in both the velocity and phase distributions are much larger than those for the short one. The synthetic jet is then mounted in the upstream cylinder of a tandem cylinder arrangement to be used as a control actuator for controlling the vibrations of the downstream cylinder. A simple feedback control mechanism is used at a Reynolds number of about 6.3x104. This Reynolds number corresponds to the case' where the downstream cylinder’s response is dominated with two frequency components, one at the resonance frequency of the cylinder, which is excited by broadband turbulence in the flow, and the other at the vortex shedding frequency. Both slits studied for the characterization experiments are used to compare their performance as control actuators. Both jets produce comparable reductions in the vibration of the downstream cylinder. A reduction of about 20% in the total RMS amplitude of the vibrations signal is achieved. This amounts to a reduction of about 50% in the resonant peak and an average value of about 40% in the vortex shedding peak. The optimal values of gain and time lag of the controller are then used to investigate the effect of the jet on the flow. It is found that the short slit jet produced an effect that was traced up to 1.875 diameters downstream, while the effect of the long slit jet dropped dramatically very close to the upstream cylinder. / Thesis / Master of Applied Science (MASc)

<p>The production of a novel micropump based on the synthetic jet principle is investigated both numerically and experimentally. The proposed micropump consists of a synthetic jet actuator driven by a vibrating diaphragm issuing into an inverted T- shaped channel structure forming the inlet/outlet channels of the pump.</p> <p>The software package Ansys is used to perform numerical investigations of the operation of the proposed micropump. Simulations were performed to study the effect of changing the inlet/outlet channel dimensions as well as the operating frequency, amplitude and duty cycle of the excitation signal. Inlet/outlet channel widths ranging from 200 to 800 μm and operating amplitude and frequency of excitation of the 5 mm square membrane driving the synthetic jet actuator ranging from 20 to 60 μm and from 20 to 60 Hz respectively were investigated.</p> <p>Based on the findings of the numerical simulations, a prototype design was chosen and produced. Prototype production using microfabrication techniques as well as micromachining was investigated. The final prototype was micromachined using plexiglass as the working material. An experimental setup was constructed to test the performance of the produced prototype, which allowed for measuring the produced flow rate, pressure head, actuation amplitude and frequency.</p> <p>The findings of the numerical simulations verified the possibility to produce a working micropump with flow rates of up to 1.3 ml/min. Simulation results also showed the dependence of the produced flow rate on both the inlet and outlet channel widths. An increase in the inlet channel width resulted in a gain in the average flow rate through the pump while an increase in the outlet channel width results in a reduction in the flow rate. Increases in either the actuation amplitude or frequency of excitation both resulted in an improvement in the produced flow rate. Changes in the ejection duty cycle, or the ejection time relative to the suction time during an actuation cycle, were found to influence the flow rate produced by the pump. A shorter ejection time produced a higher flow rate from the pump as compared to a longer ejection time. It was also found that changes in dimensions or operating parameters affected the fluctuations in the flow rate through the pump associated with the pulsating nature of the synthetic jet. Experimental investigations confirmed the findings of the numerical simulations in terms of the flow rate and the trends in the dependence of the flow rate on operating parameters. Values of maximum back pressure of up to 500 Pa were also reported experimentally and membrane driving powers of up to 122 μW were calculated numerically.</p> / Doctor of Philosophy (PhD)

In order to extend fluid-based flow control techniques that have been demonstrated at low subsonic speeds to high speed flows, it is necessary to develop actuators having sufficient momentum to control and manipulate high speed flows. Two fluidic actuation approaches are developed where the control jet may reach supersonic velocities and their performance is characterized. The first actuator is a compressible synthetic (zero net mass flux) jet. This is an extension of previous work on synthetic jets with an increase in driver power yielding substantial pressurization of the cavity such that the flow is compressible. The jet is generated using a piston/cylinder actuator, and the effects of variation of the orifice diameter, actuation frequency, and compression ratio are investigated. Operation in the compressible regime uniquely affects the time-dependent cylinder pressure in that the duty cycle of the system shifts such that the suction phase is longer than the blowing phase. The structure of the jet in the near-field is documented using particle image velocimetry and Schlieren flow visualization. In the range investigated, the stroke length is sufficiently long that the jet flow is dominated by a starting jet rather than a starting vortex (which is typical of low-speed synthetic jets). A simple, quasi-static numerical model of the cylinder pressure is developed and is in generally good agreement with the experimental results. This model is used to assess system parameters which could not be measured directly (e.g., the dynamic gas temperature and mass within the cylinder) and for predictions of the actuator performance beyond the current experimental range. Finally, an experiment is described with self-actuated valves mounted into the cylinder head which effectively icrease the orifice area in suction and overcome some of the limitations inherent to compressible operation. The second actuation concept is the combustion-driven jet actuator. This device consists of a small-scale (nominally 1 cc) combustion chamber which is filled with premixed fuel and oxidizer. The mixture is ignited using an integrated spark gap, creating a momentary high pressure burst within the combustor that drives a high-speed jet from an exhaust orifice. At these scales, the entire combustion process is complete within several milliseconds and the cycle resumes when fresh fuel/oxidizer is fed into the chamber and displaces the remaining combustion products. The actuator performance is characterized by using dynamaic measurements of the combustor pressure along with Schlieren flow visualization, limited dynamic thrust measurements, and flame photography. The effects of variation in the following system parameters are investigated: fuel type and mixture ratio, exhaust orifice diameter, chamber aspect ratio, chamber volume, fuel/air flow rate, ignition/combustion frequency, and spark ignition energy. The resulting performance trends are documented and the basis for each discussed. Finally, a proof-of-concept experiment demonstrates the utility of teh combustion-driven jet actuators at low-speed for transitory reattachment of a separated flow over an airfoil at high angles of attack.

An experimental investigation was undertaken to test the effectiveness of a novel design for controlling the aerodynamics of an airfoil. A synthetic jet actuator (SJA) was placed inside a NACA 0015 airfoil with its jet at 12.5% of the chord length, hereby referred to as the leading edge actuator. Four centrifugal fans across the span were mounted at 70% of the chord and the jet formed by them was located at 99% of the chord, hereby referred to as the trailing edge actuator. The effects of these actuators on the aerodynamic properties were studied, separately and then in conjunction, with varying angles of attack. The leading edge actuator delays the onset of stall up to 24 degrees, the maximum angle of attack that could be attained. The control of the aerodynamics was achieved by controlling the amount of separated region. There was no effect of the actuation at lower angles of attack. The trailing edge actuator provides aerodynamic control at both low and high angles of attack. The study investigated the effect of jet momentum coefficient on the aerodynamic properties for various angles of attack. The data obtained shows that lift control (in both positive and negative direction) was achieved even at low angles. The actuator enhances the aerodynamic properties by changing the pressure distribution as well as by delaying flow separation. Study of the combined actuation shows that the synthetic jet actuator was very effective in delaying stall when the trailing edge jet was ejected from the upper surface. For the case when the jet is ejected from the lower surface, there is less control. This can be accounted for by the difference in aerodynamic loading for both cases.

Like virtually every other human activity, air transport has an impact on the environment and similar to all other industries environmental impacts and economic issues are exerting more pressure on aircraft sector to meet the demands and implicated conditions. Secondly in today’s competitive industrial performance index new modern techniques are being introduced to improve the aerodynamics so that the efficiency of the newly designed aircrafts could be enhanced. The active flow control techniques have been proved vital towards achieving more effective air flow on the aircraft wing and that eventually helps to increase the lift coefficient at full scale flight. Synthetic jet actuators have been experimentally proved a promising technique towards achieving flow separation delay on the surface they have been deployed on. For the operation the synthetic jet actuators offer a unique characteristic in that they make use of the ambient work fluid and that deny the need of any extra fluid from outside the system and that helps in two fold. Firstly the need to make additional arrangements for air supply through the complex piping system has been ruled out completely. Secondly in the system the addition of any new weight that usually associates with the introduction of any new technique has been avoided. In this work firstly the dye visualization technique is used to study the interaction of the synthetic jet with both types of boundary layers that is laminar and turbulent. Secondly PIV measurements are performed to quantitatively analyze the evolution of vortical structures in the boundary layer. The aim is to understand the fluid dynamics involved in the interaction of the vortical structures with the neal wall fluid that ultimately re-attach the flow with the surface. Lastly an artificial flow separation is generated on the deflected flat plate surface and the synthetic jet is deployed to observe the separation delay on the surface. Various vortical structures have been generated by operating the actuator at varying parameters and issued into the boundary layer upstream of the separation line. The effectiveness of each type of vortices has been evaluated quantitatively to work out the optimum parameters at which the actuator must be operated to achieve the best control effect at the given free-stream condition.

Both the need for mixing highly viscous liquids more effectively and the advance of micro-scale applications urge the development of technologies for liquid mixing at low Reynolds numbers. However, currently engineering designs which offer effective jet mixing without structural and operational complexity are still lacking. In this project, the method of enhancing liquid mixing using active pulsation in the laminar flow regime is explored experimentally. This work started by improving the inline pulsation mechanism in an existing confined jet configuration whereby the fluid from a primary planar jet and two surrounding secondary planar jets are pulsated by active fluid injection control via solenoid valves in the out-of-phase mode. The influence of Reynolds number, pulsation modes, frequency, duty cycle on mixing is then investigated using PLIF and PIV experimental techniques. A combination of different mixing mechanisms is found to be at play, including sequential segmentation, shearing and stretching, vortex entrainment and breakup. At a given net flow Reynolds number, an optimal frequency exists which scales approximately with a Strouhal number (Str=fh/Uj) about 1. This optimal frequency reflects the compromise of the vorticity strength and segmentation length. Furthermore, a lower duty cycle is found to produce a better mixing due to a resultant higher instantaneous Reynolds number in the jet flow. Overall, the improvement of the rig has resulted in an excellent mixing being achieved at a net flow Reynolds number of 166 which is at least order of magnitude lower than in the original rig. In order to achieve fast laminar mixing at even lower Reynolds numbers, the active pulsation mechanism using lateral synthetic jet pairs is designed and tested at a net flow Reynolds number ranging from 2 to 166 at which a good mixing is achieved. The influence of actuation frequency and amplitude, and different jet configuration is evaluated using PLIF and PIV experimental techniques. At the mediate to high Reynolds numbers tested in this study, the interaction and subsequent breakup of vortices play a dominant role in provoking mixing. In contrast, at the lower end of Reynolds numbers the strength of vortex rollup is weakened significantly and as a result folding and shearing of sequential segments provide the main mechanism for mixing. Therefore it is essential to use multiple lateral synthetic jet pairs to achieve good mixing in both mixing channel and synthetic jet cavity at this Reynolds number. It is found that an increase in both the actuation magnitude and frequency improves mixing, thereby the velocity ratio represents the relative strength of the pulsation velocity to the mean flow velocity is crucial for mixing enhancement. In order to identify actuation conditions for good mixing, a regression fit is conducted for the correlation between the dimensionless parameters, net flow Reynolds number Ren, stroke length L and Strouhal number Str. Over the tested range of the net flow Reynolds number from 2 to 83, the relationship of parameters is found and the velocity ratio at least above 2.0. Suggested by the comparatively small exponent, net flow Reynolds number is less influential than stroke length and Strouhal number. The success in obtaining excellent mixing using lateral synthetic jet pairs at low Reynolds numbers in the present work has opened up a promising prospect of their applications in various scenarios, including mixing of highly viscous liquids at macro-scale and micro-mixing.

Despite many advances in aviation noise control over the past 50 years, the industry is continually striving to reduce noise emissions. Turbofan engine acoustic liners are efficient attenuators of engine noise. Plasma actuators have been used as flow control devices in other settings and will now be studied as an enhancement for acoustic liners. A plasma actuator can excite oscillatory flow or a single direction (bias flow). Both flow types are studied as possible means to excite turbofan liners in order to improve the acoustic performance. Experiments revealed the oscillatory flow as the dominant factor in controlling resonator performance. The phase control of the actuator signal is an important parameter when dealing with the oscillatory flow. The actuator is first applied to a single resonator and then a set of six resonators. The experiments show that with the correct phase, the actuators improved the performance of a single resonator by 3 dB to 5 dB. The results for the array of actuators/resonators mirror the results of a single device. Beyond the improvements in performance, a number of other factors affect the usefulness of the plasma actuator technology in a turbofan environment. The ability of the actuator to produce plasma is susceptible to small imperfections in the device, and this property will likely be amplified in a perforated sheet with embedded actuators. Additional weight and energy consumed by the actuators is another factor to consider. Finally, plasma actuator operation produces ozone, so environmental effects deserve consideration as well. / Master of Science