Microjet Actuators Research Articles

Enhancing Flow Separation Control Using Hybrid Passive and Active Actuators in a Matrix Configuration

Efficient control of flow separation holds significant economic promise. This study investigates flow separation mitigation using an experimental platform featuring a combination of passive and active actuators arranged in a matrix configuration. The platform consists of 5 × 6 hybrid actuator units, each integrating a height-adjustable vortex generator and a micro-jet actuator. Inspired by the distributed pattern of V-shaped scales on shark skin, these actuator units are strategically deployed in a matrix configuration to reduce flow separation on a backward-facing ramp. Distributed pressure taps encircling the hybrid actuators monitor the flow state. Parametric analyses examine the effect of different control strategies. By adopting appropriate passive and active actuation patterns, effective pressure recovery on the ramp surface can be achieved. The most significant flow control outcome occurs when the actuators operate under combined active and passive excitation, harnessing the benefits of both control strategies. Particle image velocimetry (PIV) results confirm a notable reduction in flow separation under the best-controlled case. These findings suggest a promising future for flow control devices employing combined passive and active actuation in matrix-like configurations.

Volumetric Velocity Measurements of a Three-Dimensional Shock-Wave/Boundary-Layer Interaction with Flow Actuation Using Two-Camera Plenoptic Particle Image Velocimetry

Two-camera plenoptic particle image velocimetry is used to study a three-dimensional (3-D) shock-wave/boundary-layer interaction (SBLI) generated by a single unswept fin. The SBLI considered was the swept interaction induced by a sharp fin at an angle of attack of 15 deg in Mach 2 flow. The fin generates an oblique shock with a wave angle of approximately 45 deg that interacts with the boundary layer on the incident flat plate. As an extension of previous work, these experiments were conducted to examine the effects of momentum-based flow actuation on the 3-D flowfield, as well as reduce the data uncertainty and improve the overall data quality via addition of a second camera. The following three cases are presented for this work: 1) the baseline interaction with the presence of the inactive microjet actuators (physical insert present), 2) the mean SBLI flowfield under the influence of microjet actuation upstream of the oblique shock, and 3) the mean flowfields corresponding to phase-locked images captured at four different phases of the actuator pressure cavity signal. The results show good correlation with the literature of the SBLI with actuation, including a possible shift or rotation of the shock feature during actuation, and an increased velocity deficit and boundary-layer height. The resonance-enhanced microjet actuation also had a notable impact on the low-speed flow region near the fin. This work presents one of the first 3-D measurements of a fin-generated SBLI influenced by flow actuation.

Three-Dimensional Flowfield in a Fin-Generated Shock Wave/Boundary-Layer Interaction Using Tomographic PIV

Three-dimensional flowfield measurements within a separated swept shock wave/boundary-layer interaction are acquired using a four-camera tomographic particle image velocimetry system in the present study. The turbulent, Mach 2 incoming flow has a unit Reynolds number of , and the shock interaction is produced by a sharp, unswept fin with a deflection angle of 15 deg from the freestream flow. Three volumes are examined to focus on different regions of the flow: 1) conically developed flow away from the wall spanning almost the entire interaction, 2) near the fin surface, a region that remains relatively unexplored, and 3) parallel to the floor in the near-wall region of the interaction. The flowfield is investigated in depth by visualizing velocity isosurfaces, with special emphasis on the growth of the interaction as one moves away from the interaction origin, which is a unique feature of swept interactions with strong crossflow. Initial results of the flowfield response to microjet actuation at four separate locations within the interaction are also discussed. The effect of introducing disturbances using upstream actuators is found to be the most prominent in the region near flow separation, where the streamlines are modified and overall separation size is reduced.

Thrust Augmentation of Micro-Resistojets by Steady Micro-Jet Blowing into Planar Micro-Nozzle

The present work investigates the impact of steady micro-jet blowing on the performance of a planar micro-nozzle designed for both liquid micro-thrusters and nitrogen cold-gas micro-resistojets. Two micro-injectors have been placed into the divergent region along the sidewalls, injecting a secondary flow of propellant perpendicularly to the wall where they have been located. The micro-jet actuator configuration is characterized by the dimensionless momentum coefficient cμ. The best performance improvement is retrieved at the maximum cμ for both water vapor (Δ%T,jet = +22.6% and Δ%Isp,Tjet = +2.9% at cμ = 0.168) and nitrogen gaseous flows (Δ%T,jet = +36.1% and Δ%Isp,Tjet = +9.1% at cμ = 0.297). The fields of the Mach number and the Schlieren computations, in combination with the streamline visualization, reveal the formation of two vortical structures in the proximity of secondary jets, which energize the core flow and enhance the expansion process downstream secondary jets. The compressible momentum thickness along the width-wise direction θxy in presence of secondary injection reduces as a function of cμ. In particular, it becomes smaller than the one computed for the baseline configuration at cμ > 0.1, decreasing up to about and -57% for the water vapor flow at cμ = 0.168, and -64% for the nitrogen gaseous flow at cμ = 0.297.

Active Flow Control of a High-Lift Supercritical Airfoil with Microjet Actuators

Active flow control has the potential for substantial performance gains and meeting the challenges of next-generation air vehicles. High-lift airfoils employ trailing edge flaps during takeoff and landing, which are stowed during cruise. The present experimental investigation was carried out to examine the active flow control effectiveness on the NASA Energy Efficient Transport airfoil fitted with a simple hinged flap. Experiments were carried out at two flap deflection angles of 20 and 30° at a Reynolds number based on aerodynamic chord of . The microjet parameters varied during this study were the location, orientation, and blowing ratio of the jets. Measurements include velocity and vorticity fields obtained using planar and stereoscopic particle image velocimetry. The baseline flow is separated over a third of the flap at a deflection angle of 20°, and over the entire flap at a deflection angle of 30°. Microjet control is able to completely re-attach the flow at both flap deflection angles and significantly reduce the airfoil drag. The mechanism for control effectiveness is the re-energizing of the boundary layer through the development of counter-rotating vortex pairs.

Numerical Study on the Effectiveness of Vectoring Primary Flow with Dilator by Using Microjet Actuator

In the present paper, orient control of dilator primary flow in microjet were simulated numerically by using microjet actuator with the X-L whole flow field calculation model. The effects of different expansion half angles of the primary flow channel, microjet actuator amplitude, micro jet actuator frequency, and the position of micro jet actuator and primary flow velocity on deflection of primary flow were analyzed. It is found that expansion half angle has great effect on the effectiveness of vectoring primary flow with dilator using microjet actuator; if the expansion half angle is small, the wall-attached flow of micro jet plays a main role in driving the primary flow flows along dilator wall; if the expansion half angle is big enough, the coupling effect between microjet and shear layer plays the main role. With the increase of the velocity of the actuator, the controlling efficiency of microjet is expanded. Besides, dilator decreases the distance sensibility between actuator and primary flow. With the increase of primary flow velocity, the controlling efficiency of microjet is decreased.

Numerical study on Vectoring the primary flows with dilator using microjet actuators

The synthetic flow field of Vectoring control of a primary flows with dilator using microjet actuators was Numerical studied by the overall flow field calculation model-X-L model. The result shows that the primary flows with dilator deflected larger than no one. And the angle of dilator has an optimal value. The reason of primary flows deflected was analyzed. There are three stage sections that the “low pressure closed reticular flow region” results in non-equilibrium of pressure along the primary jet’s orifice; the microjet roll sucked and the primary flow coupled in each other in the near catchment area; the microjet and the primary flow coupled in each other in the far catchment area.

Control of incident shock-induced boundary-layer separation using steady micro-jet actuators at M∞ = 3.5

Experiments were carried out to control an incident shock-induced separation associated with 22° shock generator in a Mach 3.5 flow using an array of steady micro-jet actuators. Four micro-jet actuator configurations based on the variation in their pitch angle [Formula: see text], skew angle [Formula: see text] and span-wise spacing were used. Each of these configurations were placed 14 δ upstream of the interaction and operated with injection pressures ( Poj) varying from 140 to 643 kPa. While no major variations in separation characteristics were observed for Poj < 140 kPa, significant modifications were observed beyond [Formula: see text] of 140 kPa and until 208.5 kPa. Amongst all the four control configurations, micro-jet vortex generator 2 ([Formula: see text] showed the best control with a 2 δ downstream shift in separation point location relative to no-control. The shift is also accompanied with a change in maximum zero-crossing frequency towards higher frequency (almost twice), a reduction in the intermittency length and an increase in the correlation value between the boundary layer just upstream of the interaction and the intermittent region. These results indicate that the effectiveness of micro-jet vortex generator 2 is probably due to the improved entrainment levels in the shear layer induced by the micro-vortices which are generated downstream of these devices. The increase of the skew angle [Formula: see text] from 180° to 270° for the same pitch angle of β = 45° (micro-jet vortex generator 3) seems to have no major impact on the separation characteristics. The reduction in the span-wise spacing (micro-jet vortex generator 4) resulted in deterioration of the flow field due to the jet-to-jet interaction with increasing injection pressures.

Nonlinear Adaptive Approach to Microjet-Based Flow Separation Control

Boundary-layer separation, a critical phenomenon in the operation of aerodynamic surfaces, limits the performance of compressor and turbine blades, fixed and rotary wings, as well as bluff bodies moving through a fluid. Flow separation leads to increased drag, decreased lift, and unpredictable vibrations due to unsteadiness. On these systems, effective control of separation could provide greater maneuverability and performance, and reduced vibration. Separated flow is a macroscale phenomenon governed by complex flow interactions, but it can be controlled by microscale actuation. Recently, the emergence of closed-loop methods has enhanced robustness. Modern processors enable the use of sophisticated adaptive control methods that achieve separation control with adaptive models. This paper considers control of flow separation over a NACA-0025 airfoil using microjet actuators. Experimental results are presented for a novel approach to nonlinear model predictive control, referred to as adaptive sampling-based model predictive control, which applies the minimal resource allocation network algorithm for nonlinear system identification and the sampling-based model predictive optimization algorithm to achieve effective nonlinear control. Through pressure data and flow characterization from wind-tunnel experiments, effective and robust separation control is demonstrated. The method’s computational efficiency is sufficient for successful real-time experimental implementation.

Development and characterization of high-frequency resonance-enhanced microjet actuators for control of high-speed jets

For flow control applications requiring high-frequency excitation, very few actuators have sufficient dynamic response and/or control authority to be useful in high-speed flows. Due to this reason, experiments involving high-frequency excitation, attempted in the past, have been limited to either low-frequency actuation with reasonable control authority or moderate-frequency actuation with limited control authority. The current work expands on the previous development of the resonance-enhanced microactuators to design actuators that are capable of producing high-amplitude pulses at much higher frequencies [\(\mathcal {O}\) (10 kHz)]. Using lumped element modeling, two actuators have been designed with nominal frequencies of 20 and 50 kHz. Extensive benchtop characterization using acoustic measurements as well as optical diagnostics using a high-resolution micro-schlieren setup is employed to characterize the dynamic response of these actuators. The actuators performed at a range of frequencies, 20.3–27.8 and 54.8–78.2 kHz, respectively. In addition to providing information on the actuator flow physics and performance at various operating conditions, this study serves to develop easy-to-integrate high-frequency actuators for active control of high-speed jets. Preliminary testing of these actuators is performed by implementing the 20-kHz actuator on a Mach 0.9 free jet flow field for noise reduction. Acoustic measurements in the jet near field demonstrate attenuation of radiated noise at all observation angles.

Velocity field measurements on high-frequency, supersonic microactuators

The resonance-enhanced microjet actuator which was developed at the Advanced Aero-Propulsion Laboratory at Florida State University is a fluidic-based device that produces pulsed, supersonic microjets by utilizing a number of microscale, flow-acoustic resonance phenomena. The microactuator used in this study consists of an underexpanded source jet that flows into a cylindrical cavity with a single, 1-mm-diameter exhaust orifice through which an unsteady, supersonic jet issues at a resonant frequency of 7 kHz. The flowfields of a 1-mm underexpanded free jet and the microactuator are studied in detail using high-magnification, phase-locked flow visualizations (microschlieren) and two-component particle image velocimetry. These are the first direct measurements of the velocity fields produced by such actuators. Comparisons are made between the flow visualizations and the velocity field measurements. The results clearly show that the microactuator produces pulsed, supersonic jets with velocities exceeding 400 m/s for roughly 60 % of their cycles. With high unsteady momentum output, this type of microactuator has potential in a range of ow control applications.

A penalty model of synthetic micro-jet actuator with application to the control of wake flows

A simple model of synthetic (zero-net mass-flux) micro-jet is proposed and implemented in a large-eddy simulation spectral solver of the incompressible Navier–Stokes equations. Essentially, it is simply required to introduce a penalty like term in the momentum equation, with a variable penalty coefficient that culminates during the expulsion phase. Examples of applications are considered for the control of several turbulent wake flows, the backward facing step, the D-shaped body and the square back Ahmed body.

Mixing Characteristics of Active Microjet-Based Actuators in a Supersonic Backward-Facing-Step Flow

Mixing in self-ignited supersonic combustors continues to be an important process that influences the performance and limitations for propulsion applications. A common approach for current generation combustors involves the employment of fuel-jet-injection schemes that enhance shear-layer mixing. These fuel jets issuing in a supersonic crossflow induce a bow shock coupled with an oblique shock causing total pressure losses and consequently reducing scramjet-combustor efficiency. This paper explores a novel approach for enhancing mixing using transverse active microjet-based actuators injecting in a supersonic backward-facing-step flow, while reducing pressure losses through weak-shock formation. Shear-layer mixing and flow features were characterized using particle image velocimetry and shadowgraph imaging. Relative comparisons were made with and without microjet actuation under nonreacting conditions. The main flow mechanisms attributed to enhancing mixing are explained. A triple-microjet-injection configuration, in the form of three microjet arrays supplied with different pressures, was investigated. The multiple-microjet-injection configuration formed a virtual ramp at the trailing edge of the backward-facing step that further enhanced mixing and increased the shear-layer thickness.

Microjet-Based Active Flow Control on a Fixed Wing UAV

A fixed wing, remote controlled, unmanned aerial vehicle (UAV), has been retrofitted with a system of microjet-based actuators to test microjet efficacy for flow control during flight. This allows one to evaluate the flow control system in a more complex and uncontrolled environment relative to the “clean” laboratory flowfield. The system is composed of off-the-shelf components and provides some appreciation of the challenges associated with implementing such an active control scheme in more “practical” configurations. A wing section with actuators was first tested in a low-speed wind tunnel to characterize microjet control efficacy in the laboratory using surface flow visualizations and particle image velocimetry (PIV). The laboratory results show that the microjet actuators are an effective means of controlling separation with fairly low supply pressures and flow rates. Only relatively robust and straightforward diagnostics can be used to determine the flow conditions on the UAV during flight. As such, tufts are installed on the aircraft’s wing to serve as a qualitative way of measuring control efficacy. Results from the flight tests confirm that this flow control system is capable of delaying flow separation in the complex flows occurring during flight.

Separation Control on a Low-Pressure Turbine Blade using Microjets

Flow separation that occurs over low-pressure turbine blades at low Reynolds numbers has been a cause of concern due to its detrimental effect on engine performance. In the present study, the effect of microjet actuation, a low-mass, high-momentum device, is demonstrated for the elimination of separation on a low-pressure turbine blade over a wide range of Reynolds numbers using a range of complementary diagnostics, which include the surface pressure, velocity field, and wake-loss measurements. The U.S. Air Force Research Laboratory L1A low-pressure turbine blade used in this study is a highly aftloaded profile that experiences a nonreattaching separation at approximately 60% axial chord at low Reynolds numbers. Baseline blade pressure distributions as well as wake-loss coefficient measurements show that the blade experiences nonreattaching separation for Reynolds numbers based on axial chord less than 50,000 for a freestream turbulence intensity of 1% with steady inlet conditions. Microjet-based control was activated at various blowing ratios for each Reynolds number of interest. The integrated wake-loss coefficient was reduced by 85% with microjet control at low Reynolds numbers. Ensemble-averaged particle image velocimetry velocity fields show an elimination of reverse flow on the blade’s suction surface when steady microjets were activated at the optimum blowing ratio during low Reynolds number conditions. Turbulence statistics found through particle image velocimetry showed a concomitant dramatic reduction in unsteady velocities with control. Boundary-layer reattachment is attributed to the increase of local streamwise vorticity caused by the microjets (jets in crossflow), which lead to enhanced freestream entrainment to the near wall. The boundary layer is further energized due to enhanced turbulent mixing far downstream of the microjets. It was also observed that excessive control at high blowing ratios actually leads to a much larger separation. Prior studies applying active control on low-pressure turbines have shown the effectiveness of pitched and skewed vortex-generating jets for eliminating flow separation. The current study demonstrates that simple, normally issuing microjets with zero pitch angles and very low mass flow can be as effective as vortex-generating jet actuators for low-pressure turbine flow control.

Time-step coupling for hybrid simulations of multiscale flows

A new method is presented for the exploitation of time-scale separation in hybrid continuum-molecular models of multiscale flows. Our method is a generalisation of existing approaches, and is evaluated in terms of computational efficiency and physical/numerical error. Comparison with existing schemes demonstrates comparable, or much improved, physical accuracy, at comparable, or far greater, efficiency (in terms of the number of time-step operations required to cover the same physical time). A leapfrog coupling is proposed between the ‘macro’ and ‘micro’ components of the hybrid model and demonstrates potential for improved numerical accuracy over a standard simultaneous approach. A general algorithm for a coupled time step is presented. Three test cases are considered where the degree of time-scale separation naturally varies during the course of the simulation. First, the step response of a second-order system composed of two linearly-coupled ODEs. Second, a micro-jet actuator combining a kinetic treatment in a small flow region where rarefaction is important with a simple ODE enforcing mass conservation in a much larger spatial region. Finally, the transient start-up flow of a journal bearing with a cylindrical rarefied gas layer. Our new time-stepping method consistently demonstrates as good as or better performance than existing schemes. This superior overall performance is due to an adaptability inherent in the method, which allows the most-desirable aspects of existing schemes to be applied only in the appropriate conditions.

Design and Characterization of High-Bandwidth, Resonance Enhanced Pulsed Microactuators: A Parametric Study

An extensive study on a microactuator that can generate high-momentum, high-frequency perturbations over a large bandwidth is presented in this paper. Such an actuator can potentially be used for the active control of various shear and boundary-layer flows that involve separation, mixing, and noise generation. The resonance enhanced microjet actuator described in this paper is a simple microfluidic system consisting of an underexpanded source jet flowing into a specially configured cavity integrated with multiple micronozzles, through which unsteady pulsed supersonic jets issue. The resonance frequency of these microjets could be varied over a large range (approximately 1–60 kHz) by changing the geometric and flow parameters of the microactuator system. Mean and unsteady properties of the microactuator are examined, including time-resolved flow visualizations and synchronous pressure and noise measurements; collectively, they provide a better understanding of the actuator dynamics. The present study also explores the design space and performance, as well as some of the design limitations of this actuator. Based on this parametric, a correlation is suggested that may be used for designing such actuators for various applications.

A supersonic broadband microjet actuator using piezohydraulic actuation

A supersonic piezohydraulic microjet flow control actuator has been developed using a 400-µm-diameter micronozzle capable of producing pulsed microjet flow over a broad frequency range from quasi-static to 1.6 kHz. Such flow control actuators are useful in providing ideal control inputs for controlling flow properties to improve aerodynamic performance, for example, reduce or eliminate flow separation and mitigate noise from propulsion systems and other aircraft components. The design of the piezohydraulic microjet is presented in combination with experimental results to quantify the internal system dynamic characteristics in comparison with the microjet flow. The microjet actuator couples a piezoelectric stack actuator and a hydraulic circuit to amplify the 20-µm stack actuator displacement to an amplitude that is necessary to throttle flow through the 400-µm-diameter micronozzle. Unsteady pressure measurements at the microjet exit are correlated with the piezoelectric stack actuator displacement and voltage input to provide comparisons between the internal electromechanical actuation and external pulsed flow behavior. Phase-conditioned micro-Schlieren imagery is also utilized to quantify the flow field. The results illustrate broadband supersonic pulsed microjet actuation frequencies that approach 1 kHz: frequencies that are much higher than currently achieved with broadband actuators with this control authority.

Investigation of micro-jet active control of a precessing jet using PIV

A circular jet entering an open-ended concentric circular chamber can rotate or precess about the jet axis for certain flow conditions and chamber configurations. Active flow control of a precessing jet provides the ability to influence the flow field inside the chamber and the resulting flow after the chamber exit. Twelve micro-jets surrounding the jet at the chamber inlet are used as actuation. At the chamber exit, four pressure probes and three-component velocity measurement using stereo particle image velocimetry (stereo-PIV) is used to monitor the flow. A phase plane method using signals from the pressure sensors is developed to monitor the location of the jet high-velocity region in real-time. Phase-locked stereo-PIV, triggered by the micro-jet actuation signal, is used to investigate the flow field and validate the pressure phase plane results. The effectiveness of the micro-jet actuation and the validation of the pressure phase plane measurements demonstrate actuation and the sensing needed for future closed-loop control of the precessing jet.

초음속 마이크로제트 유동의 수치해석적 가시화

Supersonic microjets acquire considerable research interest from a fundamental fluid dynamics perspective, in part because the combination of highly compressible flow at low-to-moderate Reynolds number is not very common, and in part due to the complex nature of the flow itself. In addition, microjets have a great variety engineering applications such as micro-propulsion, MEMS(Micro-Electro Mechanical Systems) components, microjet actuators and fine particle deposition and removal. Numerical simulations have been carried out at moderate nozzle pressure ratios and for different nozzle exit diameters to investigate and to understand in-depth of aerodynamic characteristics of supersonic microjets.