John T. Solomon

High-Bandwidth Pulsed Microactuators for High-Speed Flow Control

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 μm) 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 O(100 Hz) and up to O(100 kHz) using this actuator approach.

Flow Physics of a Pulsed Microjet Actuator for High-Speed Flow Control

Flow control actuators based on a small-diameter source jet and a cylindrical cavity structure take advantage of the flow resonance within the cylindrical cavity to generate a variable-frequency, pulsed high-momentum microjet issuing through the cavity orifice. The flow-acoustic coupling, which leads to resonance within the cavity of the actuator, is the main driving mechanism behind the pulsed microjet. In the present study, a computational methodology based on high-order numerical techniques is used to simulate a highly unsteady and compressible pulsed actuator flowfield. Simulation generated flowfield results are analyzed to further understand the complex flow physics governing the pulsed actuator operation. The simulation provides significant details about the highly unsteady and complex microscale actuator flowfield, which are not observable from the experiments. Qualitative comparisons made between the simulated flowfield visualizations and the experimental microschlieren images show a reasonable le...

Control of Supersonic Resonant Flows Using High Bandwidth Micro-actuators

Practical application of active flow control of high speed flows is dependent upon the development of simple and robust actuators that can produce high momentum and are reliable, low cost, and responsive and can be easily integrated. This paper presents an experimental investigation of the characterization and implementation of high bandwidth micro-actuators for the control of supersonic resonant flows. The striking feature of this micro-actuator is its high momentum mean flow along with high amplitude and a tunable frequency unsteady component. First generation micro-actuators are designed and their performance is tested in controlling the highly unsteady impinging jet flow field. The results show that the impinging tones are completely eliminated with the actuation of these micro-actuators, whereas, new peaks at a frequency different from the actuation frequency and its harmonics are observed in the spectra, the occurrence of which need to be further explored. A reduction of 3-4 dB in overall sound pressure levels (OASPL) is achieved over the range of test conditions.

Visual Study of Resonance Dominated Microjet Flows Using Laser-Based Micro-Schlieren

A laser-based micro-schlieren system has been developed to visually analyze the unsteady micro-flowfields of a high frequency pulsed microjet actuator, a sparkjet actuator, and a 1 mm impinging jet. Due to the small scale and high velocity of these flows it was necessary to develop an imaging technique that gives high spatial and temporal resolution. In this study, a short duration light source was coupled with a micro-schlieren system. Laser-induced breakdown in argon is used as the light source, which generates broadband light with a pulse width of approximately 10 nanoseconds and sets the exposure time for the image. With this unique light source, it is possible to freeze the structures present in each of these smallscale, high-speed, resonant flows. Through this technique various complex flow features such as shock oscillation displacements, jet front velocities, phase correlations between the aeroacoustic structures etc. were measured for various microscale resonant flows.

High bandwidth micro-actuators for active flow control

This paper describes an experimental study conducted at the Advanced Aero propulsion Lab (AAPL) for the design and development of actuator systems capable of producing high bandwidth, high momentum microjet arrays for active flow control applications. A systematic approach for designing micro-actuators with high unsteady and mean momentum efflux is followed. Beginning with a simple configuration, i.e., supersonic impinging microjets, we added more geometric complexity to the actuator design to finally arrive at an actuator configuration that provides the desired flow properties. Our first generation actuator design consists of a primary source jet, incident upon a cylindrical cavity. The lower surface of this cavity contains micronozzles through which the unsteady microjets (400 m) issue. Results clearly show that microjets produced by this actuator contain very high mean momentum (300-400 m/s) as well as a very significant unsteady component (70-100 m/s). Experiments were conducted over a large range of parameters, in terms of cavity length, source jet NPR and source jet impingement distance. The results unequivocally demonstrate the ability to vary the frequency as well as the amplitude of the mean and unsteady momentum of the microjets issuing from this actuator. By varying the dimensions of the actuator by only few hundred microns, we were able to tune the frequency of the unsteady component over intervals of 10-15 kHz. 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 of flow control applications.

Implementing Resonant Enhanced Pulsed Micro-Actuators for the Control of Supersonic Impinging Jets

Recent work at the Advanced Aero-Propulsion Laboratory (AAPL) at Florida State University has produced a micro-actuator design utilizing micro scale cavity resonance phenomena for active control of various high-speed flowfields such as supersonic impinging jets and cavity flows. These micro-scale actuators are capable of producing pulsed supersonic microjets over a wide range of design frequencies which can be chosen depending on the frequencies that are relevant to the application. Pulsed microjet control has the potential to produce improved flow and/or noise control as has been shown with steady microjet control, while introducing less mean momentum into the system by actuating at the natural frequency of the system. This study focuses on the design, characterization, and implementation of these actuators into the STOVL impinging jet flowfield. The results of this implementation are compared to the no control case, steady control, and the first generation implementation of pulsed microjets into our STOVL facility. By operating these pulsed microjet actuators at 6.1 kHz, impinging tones were reduced by up to 23 dB, and overall sound pressure levels were reduced by up to 7 dB as compared to the baseline flow. Compared to steady microjet control, pulsed control showed improved reduction of discrete tones of about 5 dB and overall sound pressure levels within 2 dB of those found in the steady case. Additionally, this second generation implementation does not exhibit the new peaks generated by first generation control, and further reduces OASPL by about 5 dB.

Resonance-Enhanced High-Frequency Micro-Actuators with Active Structures

Research in actuator development over the past few years has been driven towards increasing their amplitude and bandwidth thus enabling users to refine and adapt actuators for a wide array of applications. Recent developments at the Advanced Aero Propulsion Laboratory (AAPL) at Florida State University (FSU) have produced a micro-actuator that is capable of producing pulsed, supersonic microjets by utilizing a number of micro-scale, flow-acoustic resonance phenomena – this is referred to as the Resonance-Enhanced Microjet (REM) actuator. Studies at AAPL have shown that the micro-actuator volume is among the principal parameters in determining the actuator’s maximum-amplitude frequency component. Smart materials (specifically piezoelectric ceramic stack actuators) have been implemented into the micro-actuator to actively change its geometry, thus permitting a rapid change in the output frequency of the micro-actuator. The distinct feature of this design is that the smart materials are not used to produce the primary perturbation or flow from the actuator (which has in the past limited the control authority of other designs) but to change its dynamic properties. In this initial implementation of smart structures in the REM actuators, various static and dynamic control inputs to the piezostacks illustrate that the actuator frequency can be varied by almost 100 Hz. The very fast response times of the piezoelectric materials are shown to enable rapid tuning of the microactuator. Detailed correlations examining the relationship between the piezoelectric actuators’ control signal and the micro-actuator flowfield are presented. It is anticipated that future improvements in the design and strategic implementation of smart structures in REM actuators will significantly improve their performance allowing for rapid frequency modulation over a larger dynamic range.

Development and Characterization of High Bandwidth Micro-Actuator

This paper describes an experimental study conducted at the Advanced Aero Propulsion Laboratory (AAPL) on the design and development of actuator systems capable of producing high bandwidth, high momentum microjet arrays for active flow control applications. Using a simple geometry of a cavity with arrays of micro nozzles at the bottom end along with a primary source jet, highly unsteady microjets were produced in a frequency range of 6–60 kHz. The unsteady microjets, which are supersonic, have a mean velocity in the range of 300–400 m/sec with an unsteady component between 50–100 m/sec. Such actuators show considerable promise for flow control applications, especially in the supersonic domain. Notable characteristics of this design are its simplicity and the flexibility in controlling the frequency and amplitude suitable for the application of interest. The influence of feedback loop driven shear layer instability and other possible resonant mechanisms on the micro-actuator frequency response are outlined in the present paper. The location of cavity orifice within the Region of Instability (ROI), which is found to be the pressure recovery region of first shock cell of the source jet, plays a very important role in the output frequency and amplitude of the actuator. The length of the actuator cavity is another parameter that strongly influences the frequency of the actuator output. By using high resolution Micro-Schlieren images it was found that the frequency variation with Nozzle Pressure Ratio (NPR) is related to the shock cell properties of primary jet. As a result of this study, we have a better understanding of the geometric and flow parameters governing the unsteady properties of the actuator flow; an understanding that will be used to specifically tailor actuator design for various applications.Copyright

Experiments on Resonance Enhanced Pulsed Microjet Actuators in Supersonic Cross flow

An array of high-bandwidth, high-momentum pulsed supersonic microjets, called Resonance Enhanced Microjets (REM), were implemented to study the effect of pulsed actuation in a M=1.5 supersonic cross flow over a flat plate. Cross correlated, phase locked flow imaging and unsteady pressure measurements were used to characterize the shock wave-boundary layer interactions resulting from the pulsed actuation. Pulsed microjet actuators in supersonic cross flow have generated oblique shocks whose strength and shock angle are observed to have an invariable correlation with the pulsing phase of the microjets. Unsteady pressure measured downstream also shows a strong correlation to the pulsing phase of the actuator and the resulting oblique shock properties. These experiments clearly point to the potential capabilities of the resonance enhanced microjet actuator to manipulate the unsteady properties of the boundary layer of a supersonic flow.

Simulations of Pulsed Actuators for High-Speed Flow Control

High-speed flow control actuators based on a small-diameter source jet (on the order of one millimeter) impinging over a cylindrical cavity with an orifice at the cavity bottom take advantage of the resonance characteristics of supersonic impinging jets to generate a pulsed high-momentum microjet issuing through the cavity orifice. Such flow control actuators that generate unsteady microjets have undergone extensive experimental investigation and are intended for use in high-speed flow and noise control problems such as supersonic impinging jets and supersonic cavity flows. In the present study, a computational methodology based on highorder numerical techniques is utilized to simulate pulsed actuator flow fields and generate detailed flow field data regarding the operation of these control devices. We report our findings from the simulation of a baseline single-orifice pulsed actuator in this paper. The goal of this study is to utilize the simulation findings to gain a better understanding of the fluid dynamics related to pulsed actuator operation. The knowledge gained from this computational study will be utilized towards the design of further optimized actuators and development of a new generation of actuators that will implement active materials to generate broadband pulsed microjets.