Technique for Enhanced Flow Control Efficiency Through Thermal Actuation

In this study, passive and active flow control methods were used together to manipulate the flow around a circular cylinder. The experiments were conducted in a wind tunnel for the Reynolds number range of 4000 and 10000 based on the diameter of the circular cylinder (D). A splitter plate was used as passive flow control device and its length was chosen to be about 3.75D. Plasma actuators were placed on the circular cylinder at a position of ±90° as an active flow control device. Combining the active and passive flow control methods, a greater reduction of the drag coefficient was achieved compared to that of the cases when using these methods separately. For Reynolds numbers of 5000 and 10000, the hybrid method gives a reduction in drag of 48% and 45%, respectively. The velocity measurements were carried out by using the hot-wire anemometry and velocity profiles were obtained in the wake region. The flow was visualized by using a smoke wire method. The results revealed that the wake region of the circular cylinder with plasma actuator and splitter plate has a narrower width than the plain cylinder and with splitter plate. Also, it can be seen from spectral analysis that the vortex shedding frequency was suppressed significantly by usage of the hybrid flow control method was used.

The paper gives a brief overview about the challenges and the concepts for future aeroengine applications based on microjets. Technical requirement and difficulties of the potential applications of Active Flow Control (AFC) in turbomachines are discussed. The importance of an active stabilized turbocompressor for an efficiency increase is described. Future concepts of advanced turbocompressors with the application of active and passive flow control, partially based on MEMS devices are briefly sketched. Finally, the numerical methods necessary to unveil the physical background of AFC are presented.

Advanced technologies such as Active Flow Control (AFC) offer the potential to revolutionize next generation aircraft design through enhancing aircraft maneuver, control, and operation. DARPA’s CRANE (Control of Revolutionary Aircraft with Novel Effectors) program seeks to incorporate AFC into an aircraft design, then build and fly an X-Plane to demonstrate the potential capabilities and benefits of AFC technology. As a performer in the Phase and Phase 1 programs, Lockheed Martin has made significant progress on the development of a full-scale, tailless aircraft with an integrated AFC system to enable unique maneuvers without the use of conventional control surfaces. The Lockheed Martin-led team developed an approach that consisted of four key activities. 1. Design Library. Developing a multi-variate aerodynamic database (design library) for Active Flow Control. This was an offline process using CFD tools and significant HPC computing resources to model the complex aerodynamics and boundary layer affects associated with AFC. Response Surface Models were developed to integrate the aerodynamic effects of AFC into the Conceptual Design Process. 2. Conceptual Design Methods. Developing new methods and tools that would properly account for the effects of AFC. This included updating Stability and Control models to determine control power required and control power available, updating flight control model simulations for implementation of AFC, updated MDAO models to incorporate the changes to aero, propulsion, mass properties, and updating Operations Analysis models to model new maneuvers and behaviors achievable with AFC. These new methods were used in conducting trade studies for the design of the AFC system, the propulsion system, and the air vehicle. 3. RPV Verification. Performing test activities that help verify and calibrate our analytical methods, as well as reducing key risks. Multiple wind tunnel tests were conducted to validate the numerical simulations and gather data to capture the forces and moments generated by the various fluidic effectors on the vehicle. A subscale, dynamically scaled RPV (Remotely Piloted Vehicle) with integrated AFC was built and flown to gather additional data on aspects of interest such as flight controls and control authority, while serving as risk reduction for the larger scale X-Plane. 4. X-Plane Design. Complete the design of an AFC-enabled X-Plane. The X-Plane will serve as validation of our methods as well as demonstrate the potential for AFC to enable new and compelling mission capabilities. The work in Phase 1 matures the X-Plane design in preparation for a Preliminary Design Review. This presentation will provide an overview of the Lockheed Martin team’s progress, while highlighting challenges, solutions developed, and lessons learned under the DARPA CRANE program.

A parameter dependent Riccati equation approach is taken to analyze the stability properties of an output feedback adaptive control law design. The adaptive controller is intended to augment a nominal, fixed gain, observer based output feedback control law. Although the formulation is in the setting of model following adaptive control, the realization of the adaptive controll er does not require implementing the reference model. In this regard, the cost of implementing the adaptive controller above that of a fixed gain control law is far less than that of other methods. The error signals are shown to be uniformly ultimately bounded and an expression for the ultimate bound is provided. The control design process and theoretical results are illustrated using a model for wing-rock dynamics. Research in adaptive output feedback control of uncertain nonlinear systems is motivated by the many emerging applications that employ novel actuation devices for active control of flexible structures and fluid flows. These applications include actuators such as piezo-electric films and s ynthetic jets, which are typically nonlinearly coupled to t he plant dynamics they are intended to control. Models for these applications vary from accurate low frequency models to models that crudely approximate the true dynamics even at low frequencies. Examples of applications include active damping of flexible structures, control of aeroservoelasti c aircraft, and active control of flows. Adaptive control can be used to satisfy performance requirements in the presence of large scale parameter uncertainty, and improved safety in the event of actuator failure.

This paper deals with a theoretical analysis and experiments of a non-contact active feedback control, which is based on flow and aero-dynamical control, to suppress a leakage-flow-induced vibration of translational 1DOF system. In the proposed active feedback control method, the leakage-flow-induced vibration is suppressed by using a movable plate, as an actuator, set at inlet and outlet of the passage to control fluid force acting on the controlled target and fluid flow in the passage actively. The strategy of this active control method is based on active flow and aero-dynamical control which cancels the exciting fluid force acting on the rigid body. The root locus and critical flow velocity with and without control are calculated with changing the controller gain and phase-shift to evaluate the control performance, and experiments are conducted to verify the theoretical calculations. As a result, it is indicated that the leakage-flow-induced vibration is effectively suppressed by the proposed non-contact active control method and the suppression effect is very sensitive to the phase-shift and actuator position. For the inlet flow control, the vibration is effectively suppressed when the phase-shift is positive, and for the outlet flow control, the vibration is suppressed when the phase-shift is negative. The suppression effect of the inlet flow control is greater than that of outlet flow control.

A computational study was performed of a flow-excited Helmholtz resonator. This setup was used to model the phenomenon that occurs when a sunroof is open, or rear passenger compartment window lowered, in an automobile. The results of the computational study were validated through comparison to experimental and numerical results found in literature. After validating the setup of the computational study, the research then focused on using methods of active flow control to reduce the sound pressure levels and fluctuations within the resonator. Specifically, this was accomplished through the use of a small elongated jet placed upstream of the resonator opening. After seeing changes in the pressure levels using this active flow control technique, the velocity of the jet was altered to study its effect on the level of pressure reduction. The final results from this computational study show the desired outcome of a reduction in pressure and it was concluded that air from the jet that is added to the boundary layer produces an effect similar to that of running the simulation at a higher free stream velocity, but without active flow control. Thus, using the jet for active flow control will provide the ability to reduce pressure levels across a spectrum of free stream velocities.

A computational method for predicting the aerodynamic performance of a vertical tail due to active flow control has been developed. Enhanced directional control using flow-control technology can potentially result in reduced fuel consumption and improved airplane performance. The numerical method is based on the unsteady Reynolds-averaged Navier–Stokes formulation for overset grid systems. The flow control is modeled for an array of synthetic jet actuators mounted at the trailing edge of the stabilizer. Results of the validation for various flow conditions, actuation parameters, and port layouts are used to establish the accuracy of the numerical tool. The side-force increments on the tail due to flow control at various sideslip conditions are presented. Sensitivities to flow-control layouts, especially with respect to spanwise flow development, are also investigated. A practical approach to facilitate preliminary evaluations of active flow control with quick computational turnaround is proposed. It is shown that constant actuation can be used as proxy for predicting the effects of the unsteady actuation from synthetic jets. Its confirmation in the context of the current application has been invaluable for developing practical flow-control implementations. Aspects of integration are presented, and guidelines for preferred modes of actuation are put forth.

: As part of its Reactive Conformal Inlet Technology Enhancement (RECITE) program, AFRL is examining conformal inlet technology. Because they are flush to an air vehicle's fuselage, conformal engine air inlets cause less drag than other inlet designs. AFRL recently investigated a conformal inlet and two active flow control slot variations. Test results showed that the smaller flow control slot performed as desired and was most effective at lower wind tunnel Mach numbers and at higher active flow control mass flow rates. AFRL engineers will incorporate these valuable data into future RECITE inlet designs that require smaller amounts of active flow control air.

In this paper, the effect of an active flow control system on the onset mechanisms of rotating stall in an axial compressor is experimentally investigated. A particular emphasis is thus given on the flow patterns present in the tip region right before stall and how they evolve when active flow control is applied. The mechanisms involved during stall formation without control are carefully assessed and compared to the literature. To this purpose, the development of the first instability into rotating stall is discussed by monitoring its speed inside the compressor. This analysis also relies on instantaneous pressure measurements at the casing of the compressor.

An experimental investigation was performed on a three-dimensional, non-conformal turret with the goal of gaining a more detailed understanding of the spatial and temporal nature of the turret wake. The turret flow field was investigated using particle image velocimetry and dynamic surface pressure measurements, both with and without active flow control. Active flow control for the current work was applied at the leading edge of the turret aperture using steady suction and was matched to a configuration used for control of aero-optic distortions. The results of these experiments give insight into the complex, three-dimensional wake that develops as a result of the turret geometry as well as the significant changes that can be achieved with the application of active flow control, even when applied only to the aperture region of the cap geometry.

With a view to finding the best approach to maximize the wind energy harvesting from wind turbines, this research has focused towards assessing the effects of different innovative modifications to wind turbines at different wind speed conditions and comparing them in terms of aerodynamic performance, economy, and environmental impact. This study illustrates strategies like, active and passive flow control, incorporation of power augmentation devices (PADs) and biomimetic approaches, have huge potential to increase the output power of both the horizontal‐axis wind turbine (HAWT) and vertical‐axis wind turbine (VAWT). Literature survey reveals that boundary layer suction method from active flow control, diffusers as PADs and inspiration from flapping flight of insects from biomimetic approach can enhance the power coefficients of the turbine by 18.08%, 96%, and 35%, respectively. Furthermore, capacity factor (CF) and economic analysis showed that turbine modifications providing high‐CFs and low cost of energy (COE) will have priorities over the others, while selecting suitable turbines in a particular region. Moreover, it is evident that combining the best modifications for each strategy (active control, passive control, PADs, and biomimetic approaches) can lead to the maximum improvement in the overall performance of the turbines in terms of energy and economy.

The present study applies a Deep Reinforcement Learning (DRL) algorithm to Active Flow Control (AFC) of a two-dimensional flow around a confined square cylinder. Specifically, the Soft Actor-Critic (SAC) algorithm is employed to modulate the flow of a pair of synthetic jets placed on the upper and lower surfaces of the confined squared cylinder in flow configurations characterized by Re of 100, 200, 300, and 400. The investigation starts with an analysis of the baseline flow in the absence of active control. It is observed that at Re = 100 and Re = 200, the vortex shedding exhibits mono-frequency characteristics. Conversely, at Re = 300 and Re = 400, the vortex shedding is dominated by multiple frequencies, which is indicative of more complex flow features. With the application of the SAC algorithm, we demonstrate the capability of DRL-based control in effectively suppressing vortex shedding, while significantly diminishing drag and fluctuations in lift. Quantitatively, the data-driven active control strategy results in a drag reduction of approximately 14.4%, 26.4%, 38.9%, and 47.0% for Re = 100, 200, 300, and 400, respectively. To understand the underlying control mechanism, we also present detailed flow field comparisons, which showcase the adaptability of DRL in devising distinct control strategies tailored to the dynamic conditions at varying Re. These findings substantiate the ability of DRL to control chaotic, multi-frequency dominated vortex shedding phenomena, underscoring the robustness of DRL in complex AFC problems.

The use of Active Flow Control (AFC) technologies in aircraft design is an area of increasing interest. Understanding the missions and configurations that can maximize benefits from the use of AFC can help direct future work. In this work, particular AFC technologies are identified along with example missions and configurations, which include an Unmanned Combat Aerial Vehicle (UCAV), Intelligence Surveillance and Reconnaissance (ISR) vehicle, and Short Take-Off and Landing (STOL) vehicle. For each mission and configuration, the impact of AFC is modeled, and a particular metric of interest is identified. The effect of AFC configuration is compared to a conventional solution and the overall difference is used to determine the highest impact. The STOL configuration has the greatest impact from AFC technologies in boosting performance. A major contributing factor is the decrease in effectiveness of AFC technologies at higher dynamic pressures.

In the current paper, the active flow control for enhancing vortex induced vibration (VIV) of a circular cylinder, which can be potentially applied in ocean energy harvesting, is achieved by an artificial neural network (ANN) trained through deep reinforcement learning (DRL). The flow past a circular cylinder with and without jet control located on the cylinder is numerically investigated using OpenFOAM, and the ANN is applied to learn an active flow control strategy through experimenting with different mass flow rates of the jets. According to our results, the jets on the cylinder are able to dramatically destabilize the periodic shedding of the cylinder wake, which leads to a much larger VIV and work capability of the cylinder. Through controlling the flow rate of the jets based on the observation of the instantaneous flow field, the ANN successfully increases the drag by 30.78%, and the magnitude of the fluctuation of the drag and lift coefficient by 785.71% and 139.62%, respectively, while the energy consumption of the jets is almost negligible. Furthermore, the net energy output by VIV with jet control increases by 357.63% (case of water) compared with the uncontrolled situation. The results demonstrate that the performance of the active jet control strategy established by DRL for enhancing VIV is outstanding and promising for realizing the transformation from the ocean energy to electrical energy. Therefore, it is encouraged to perform further investigations on VIV enhancement using active flow control based on DRL.

Considering the number of applications, and the quantity of research conducted over the past few decades, it would not be an overstatement to label the piezoelectric materials as the cream of the crop of the smart materials. Among the various smart materials, the piezoelectric materials have emerged as the most researched material for practical applications. They owe it to a few key factors like low cost, large frequency bandwidth of operation, availability in many forms, and the simplicity offered in handling and implementation. For piezoelectric materials, from an application standpoint, the area of active control of vibration, noise, and flow, stands, alongside energy harvesting, as the most researched field. Over the past three decades, several authors have used piezoelectric materials as sensors and actuators, to (i) actively control structural vibrations, noise and aeroelastic flutter, (ii) actively reduce buffeting, and (iii) regulate the separation of flows. These studies are spread over several engineering disciplines—starting from large space structures, to civil structures, to helicopters and airplanes, to computer hard disk drives. This review is an attempt to concise the progress made in all these fields by exclusively highlighting the application of the piezoelectric material. The research carried out in the past five years, in the areas of modeling, and optimal positioning of piezoelectric actuators/sensors, for active vibration control (AVC), are covered. Along with this, investigations into different control algorithms, for the piezoelectric based AVC, are also reviewed. Studies reporting the use of piezoelectric modal filtering and self sensing actuators, for AVC, are also surveyed. Additionally, research on semi-AVC techniques like the synchronized switched damping (on elements like resistor, inductor, voltage source, negative capacitor) has also been covered.