Active flow control using high frequency compliant structures

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.

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.

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.

Thermal active flow control has been developed to enhance the efficiency of active flow control systems. The actuation concept is derived from gas-dynamics principles, and it is based on thermal control of the air supply. It is shown that higher air supply temperatures result in reduced mass flow rate with no degradation in active flow control performance. A computational method has been used to systematically investigate the hot air supply approach for isolated blowing actuators and fluidic oscillators. Subsequently, the thermal control concept has been computationally evaluated for airplane applications. These include enhanced control authority of a vertical tail and an airplane high-lift system, confirming the trends observed from the gas-dynamics analysis with regard to reduced actuation input as a function of supply temperature. Further confirmation of the thermal actuation concept was experimentally obtained for a bench-top actuator and a vertical-tail model in a wind-tunnel setting. The paper introduces potential approaches for system integration associated with heated supply, while highlighting the benefit of using available high-temperature sources for active 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.

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.

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.

This paper studies transition trajectory optimization for a tiltwing electrical vertical takeoff and landing (eVTOL) aircraft configuration and evaluates the benefits of equipping it with fluid injection active flow control (AFC). Using a three-degree-of-freedom model of the aircraft’s longitudinal dynamics, both hover-to-cruise (forward) and cruise-to-hover (backward) transition are investigated for a range of center-of-gravity (CG) positions to determine minimum-energy trajectories that achieve zero altitude variation while avoiding wing aerodynamic stall. It is demonstrated that constant-altitude transition can be achieved in the forward direction (with or without AFC), whereas the use of AFC in backward transition reduces the total altitude change by 24%. A comparative constant-altitude backward transition solution using a different airfoil with a higher stall limit suggests that a redesign of the tail wing of this unique tiltwing aircraft could better leverage the benefits of using AFC in this application. Additionally, it is also shown that, in the case of forward transition, AFC can be used to decrease the total transition time and increase the range of CG positions where constant-altitude transition can occur.

: 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.

Active flow control of the airflow of a ship at yaw

A novel swept-wing active flow control (AFC) technique was developed and tested on a NACA wing with 30° leading-edge sweep. A streamwise row of 20 vortex generating jets (VGJs), pitched 30° from the wing surface and skewed 60° toward the wing root, was applied at three different spanwise locations. Performance was assessed using load-cell measurements and surface oil flow visualizations at a Reynolds number of 100,000. More inboard placement of the AFC produced higher values of , but further outboard placement resulted in a higher stall angle. With a momentum coefficient of only 2%, lift was enhanced up to 15% and stall was delayed up to 12°, depending on the AFC location. The number of jets used in the row was reduced, resulting in nearly the same performance benefits with half of the momentum coefficient required for the full row of jets. The performance enhancements of this active flow control technique are comparable to a passive boundary-layer fence without the 25% increase in drag caused by the fence. The mass flow cost of the streamwise row of VGJs is 75% lower than the mass flow required for a previously studied swept-wing active flow control technique involving a streamwise slot.

An experimental investigation is conducted to reduce the unsteady stator-rotor interaction in a turbofan simulator using active flow control. The fan rotor of a 1/14-scale turbofan propulsion simulator is subjected to circumferentially periodic inlet flow distortions, generated by four stators that support a centerbody in the inlet mounted onto the simulator. These wakes are reenergized by injecting air from the trailing edge of each stator through discrete blowing holes. The flow rate through each blowing hole is controlled by an individual microelectro-mechanical system based microvalve. The microvalve actuation signal voltage is generated by a proportional-integral-derivative controller and is a function of the wake velocity defect. To determine the successful reenergizing of the wakes, far-field sound pressure level at the blade passing frequency without and with blowing is measured in an anechoic chamber. The active control experiments are performed for two simulator speeds of 29,500 and 40,000 rpm. In addition, the feasibility and advantage of active control is demonstrated by the ability of the system to respond to changes in the inlet flow velocity.