Active Flow Control System Research Articles

Experimental investigation of suction and oscillatory blowing as an alternative for a leading edge slat

High lift devices such as flaps and slats have conventionally served in multi-element wings to enhance aerodynamic performances in general, and lift in particular. However, more innovative solutions are required to address the evolving challenges the aviation industry faces to reach higher lift capabilities. Active flow control (AFC) has been studied for many years but applications have remained strictly experimental and did not reach any widespread industrial use to enhance aerodynamic performance. This experimental study investigates applying a novel AFC concept to a high-lift wing configuration to replace the slat with AFC actuators that incorporate suction and oscillatory blowing (SaOB) into the main element of a thick airfoil. The multi-element wing is equipped with an AFC system to investigate the possibility of replacing the leading edge slat with an AFC actuator. The benefits of having an AFC system acting as a virtual slat are discussed quantitatively, with experimental results, and practically as an efficient solution for designing a lighter and more capable high-lift device. This research aims to characterize the application of AFC to a wing in terms of aerodynamics and energy efficiency.

Scaling the Momentum Coefficient for Active Flow Control at Various Operating Conditions

The momentum coefficient is one of the most commonly used parameters to quantify the aerodynamic performance and system output of active flow control (AFC). Estimating the momentum coefficient accurately through experiments is challenging. Several approaches exist to estimate the momentum coefficient in laboratory conditions. However, the question arises as to whether these models hold as ambient and supply conditions change during operation in flight. Therefore, computational fluidic dynamic (CFD) simulations were carried out on two exemplary AFC actuators (i.e., a fluidic oscillator and a steady jet nozzle) in a quiescent environment with supply pressure ratios ranging from 1.5 to 4; supply temperatures at 293, 550, and 800 K; and ambient altitude conditions at sea level and 10 km. When scaled with Tsup/P∞, the mass flow for different conditions collapses as a function of the pressure ratio. Furthermore, the momentum coefficient collapses when scaled with 1/P∞. The existing models to calculate the momentum coefficient hold for changing ambient and supply conditions. Therefore, these models, in conjunction with scaling parameters, can be employed during the operation of an AFC system in flight with changing conditions in order to provide the desired momentum output. The paper discusses the details of the CFD results and the derivation of the scaling parameters.

Optimization of Co-Flow Jet Parameters for Ahmed Body Application

This study evaluates the drag reduction strategy of suction and blowing on idealize automotive vehicle, Ahmed Body. Optimization approach is adapted in order to analyse the effect of slot location, momentum coefficient and slot angle on the vehicle which experiencing drag. Despite all the efforts that have been done to reduce the Ahmed body drag using various active flow control system, most of the drag reduction were only less than 15%. A 25° Ahmed body with build in co-flow jet is modelled using a CAD software. The flow around the Ahmed body is simulated at Reynolds number based on length Re = 4.29 × 106. The governing equation were solve using an open source software package, which has been validated against experimental data. Pressure Implicit with Splitting of Operator (PISO) algorithm is applied to solve the equation. The outcome of the simulation are varies depending on the variables. Some show a decrease in drag while there are also that actually increase the drag of the system. This are caused by the suction and blowing slots that effect the surrounding air flow whether it is reducing or increasing the wake size downstream of the body. The result shows the momentum coefficient and location of both suction and blowing jet played an important role of manipulating the flow around the body and reducing the drag. The velocity contours indicated that the key to drag reduction is by using 40 m/s jet velocity, placement of suction and blowing away from each other.

Effect of Tip Gap Size on the Performance of an Axial Compressor Stage with and without Active Flow Control

The tip gap region of an axial compressor rotor is a source of complex flows, inducing losses and stability issues. Recent works have proven the ability of blowing high-speed jets in the tip region to improve the surge margin of an axial compressor stage with a narrow tip gap configuration. However, the tip gap size can evolve during the compressor lifetime, possibly affecting its performance and operability. The objective is to evaluate the performance of an active flow control system on a compressor with different tip gap sizes. The present work is based on the single-stage compressor CME2 located at the Laboratory of Fluid Mechanics of Lille and equipped with actuators blowing at the rotor tip leading edge. Configurations with two different values of the tip gap to chord ratio (0.6% and 2.4%) are experimentally tested. RANS simulations are also performed. The effect of tip gap sizes and tip blowing on the flow topology and compressor performance is evaluated (surge margin improvement of the order of 200% for the larger tip gap size).

Large-eddy simulations of the flow past a bluff-body with active flow control based on trapped vortex cells at Re = 50000

We investigate a novel flow control concept, which allows to govern bluff-body wake dynamics and the laminar–turbulent transition by suppressing vortex shedding and obtaining the mostly detached flow. Large-eddy simulations past a bluff-body (BB) with and without an active flow control (AFC) system at a Reynolds number of 50000 and zero angle of attack are presented. The BB is designed of a semicircular cylinder of the finite span-wise length with two hemispheres attached to its lateral sides. The BB has a negative lift force because of detached and attached massive recirculation zones, which respectively, originate from separation of the laminar boundary layers from its upper side and the leading edge. AFC is based on the two trapped vortex cells implemented at the upper side of BB to control the behavior of boundary layers. The chosen AFC demonstrates almost the detached flow and suppresses the vortex street. The lift-to-drag ratio of BB with integrated AFC is calculated as ≈1.5 (compared to ≈−1 obtained for the simple BB) coupled with reasonable energy losses of 10% in terms of the total drag coefficient. Finally, present findings demonstrate the potential of the concept and provide the basis for its further development.

Pressure Feedback Control of Aerodynamic Loads on a Delta Wing in Transverse Gusts

A feedback active flow control (AFC) scheme is studied for control of unsteady aerodynamic loads that act on a generic tailless delta wing during transverse gust encounters. The transverse gust and the roll angle of the wing create an unsteady roll moment. The trailing edge actuators generate unbalanced lift increments on the two sides of the model to control the roll moment. Spatially distributed pressure sensors measure surface pressure on the suction side of the wing, and the real-time aerodynamic loads are estimated through models that are identified by the Sparse Identification of Nonlinear Dynamics algorithm. The aerodynamic load estimation is then used as a surrogate to provide the AFC system with a feedback signal to alleviate the unsteady roll moment due to the gust effect. A physical interpretation of the model identification results is made to locate the source of the lift and roll moment fluctuations in gust encounters. Experimental results show that the AFC effect with a momentum coefficient input of 2% is equivalent to 26° elevon deflection in terms of the roll moment change, and the AFC doubles the control authority relative to elevons. The real-time control results show that the pressure feedback control system reduces the mean roll moment error and suppresses the fluctuations to less than 20% of the uncontrolled fluctuation level at a high angle of attack.

Dual Synthetic Jets Actuator and Its Applications—Part I: PIV Measurements and Comparison to Synthetic Jet Actuator

In order to understand the differences between dual synthetic jets (DSJs) and synthetic jets (SJs), particle image velocimetry (PIV) technology is used to capture the basic flow field characteristics of a dual synthetic jet actuator (DSJA) and a synthetic jet actuator (SJA), and then a careful comparison between them is implemented. The results indicate that a cycle of the DSJ is divided into two stages. In the near-field downstream, a pair of synthetic jets entrain fluid around them and interact with each other, making the flow field complex, and the time-periodic diaphragm dominates them. There is an unfavorable phenomenon of “self-support” between the two jets. In the far-field downstream, the two jets merge into a single, more stable SJ with a higher velocity and a double characteristic frequency. The DSJs have also shown good vectoring characteristics, with the vectoring deflection angle (VDA) changing from about −46° to 46°. The above results demonstrate that the DSJA may replace the traditional SJA in all kinds of applications and extend the applying area of the SJ to more active flow control systems, which cannot be qualified by traditional SJA.

Feed-forward Control of Vortices Using Real-Time Particle Image Velocimetry

Active flow control (AFC) is a topic with enormous potential for improving a wide array of technologies. This work presents the use and evaluation of a novel AFC system to apply feed-forward control to vortices formed in the wake of a wall-mounted spherical cap. A real-time particle image velocimetry (RT-PIV) system served as the sensor for the AFC system. The RT-PIV system produced 7.35 vector fields per second, each consisting of a two-dimensional grid of 5 × 37 velocity vectors. The actuator system consisted of a rubber surface that could deform using 16 independently controlled linear actuators. The actuators were controlled to apply wall-normal surface deformation at velocities proportional to wall-normal velocities measured by the RT-PIV system upstream of each of the 16 actuators. Analysis of the RT-PIV measurements in comparison to those from an offline PIV system run in parallel indicated that the RT-PIV system produced accurate flow measurements. As well, it was found that the RT-PIV system had a delay of approximately 0.09 seconds. The evaluation of the actuator system indicated that the actuator displacements were consistently damped by 20-25% and lagged the control signal by approximately 0.1 seconds relative to the input signal to the actuators. Therefore, the AFC system had a total delay of approximately 0.2 seconds and applied slightly weaker than specified actuations. These results were within anticipated values and, overall, the AFC system functioned adequately. The impacts of the flow control included disruption of the coherent ejection and sweep structures formed in the wake of the spherical cap and the formation of numerous smaller turbulent structures. As well, several of the control cases showed altered wall-normal velocity fluctuation variance fields in comparison to that of the unforced wake. This is a promising result for AFC using deformable surface actuators as it shows that the active control had a significant effect on an average quantity of the flow.

Data-driven enhancement of coherent structure-based models for predicting instantaneous wall turbulence

Predictions of the spatial representation of instantaneous wall-bounded flows, via coherent structure-based models, are highly sensitive to the geometry of the representative structures employed by them. In this study, we propose a methodology to extract the three-dimensional (3-D) geometry of the statistically significant eddies from multi-point wall-turbulence datasets, for direct implementation into these models to improve their predictions. The methodology is employed here for reconstructing a 3-D statistical picture of the inertial wall coherent turbulence for all canonical wall-bounded flows, across a decade of friction Reynolds number (Reτ). These structures are responsible for the Reτ-dependence of the skin-friction drag and also facilitate the inner-outer interactions, making them key targets of structure-based models. The empirical analysis brings out the geometric self-similarity of the large-scale wall-coherent motions and also suggests the hairpin packet as the representative flow structure for all wall-bounded flows, thereby aligning with the framework on which the attached eddy model (AEM) is based. The same framework is extended here to also model the very-large-scaled motions, with a consideration of their differences in internal versus external flows. Implementation of the empirically-obtained geometric scalings for these large structures into the AEM is shown to enhance the instantaneous flow predictions for all three velocity components. Finally, an active flow control system driven by the same geometric scalings is conceptualized, towards favourably altering the influence of the wall coherent motions on the skin-friction drag.

Fiber optic pressure measurement on a complex outer winglet model with active flow control actuators

The next generation of aircraft requires novel approaches on lift increase and drag reduction technologies as active flow control to counter local flow separations. Testing and monitoring its effectiveness in flight require precise pressure evaluation as a possible control input. Pressure-based measurements of local flow separation are challenging utilizing conventional electrical pressure sensors. These sensors might show degradation of their long-term performance in harsh environments, e. g., in real aircraft applications, due to their low overload protection, susceptibility to electromagnetic influences, and corrosion. Fiber optic measurements overcome those challenges and promise to be a robust sensing solution. We investigate the potential application of a novel fiber optic measurement system that monitors the active flow control system's effectiveness. Furthermore, the results are compared to conventional static pressure measurements. Due to the high-frequency capabilities of the fiber optic sensors, the dynamic behavior of the boundary layer separation and vortex generation was captured in the frequency domain. An industry-relevant outer wing was equipped with 25 fiber optic pressure sensors positioned at one wingspan section for this experiment. The experiments were conducted at a Reynolds number Re=1.1⋅106and a Mach number Ma=0.12. Wind tunnel testing results show that fiber optic pressure sensors can measure aerodynamic characteristics and, furthermore, even allow a detailed look at the dynamic behavior.

Computational Simulations of Fluidic Actuation on Rotor Blades and Their Experimental Validation

Active flow control (AFC) is conventionally used to reduce flow separation at high angles of attack. A novel alternative form of AFC for controlling rotor blade loads in attached flow is examined. The AFC system employs pulsed fluidic jets near the trailing edge of a VR-12 airfoil on the pressure side and suction side. Each jet has two operating modes: tangential blowing and normal blowing. Unsteady Reynolds-averaged Navier–Stokes simulations are performed to characterize the unsteady aerodynamic phenomena induced by actuation at low and moderate angles of attack. The simulation approach is validated using data from recent low-speed () wind-tunnel experiments. The normal blowing modes produce nearly symmetric changes in the lift and moment that are proportional to the relative jet strength. The tangential blowing modes have a minor effect on lift and moment but have a beneficial effect on drag. High-speed simulations, up to , are also performed to determine the effect of freestream Mach number on actuator performance. Finally, simulations representing rotor blade dynamics during vibrations are carried out to illustrate the effect of unsteadiness associated with blade motion. This is the first instance showing that AFC is a viable approach for potentially controlling rotor vibrations in attached flow.

Active Flow Control Utilizing an Adaptive Blade Geometry and an Extremum Seeking Algorithm at Periodically Transient Boundary Conditions

Abstract As a consequence of constant volume combustion in gas turbines, pressure waves propagating upstream the main flow into the compressor system are generated leading to incidence variations. Numerical and experimental investigations of stator vanes have shown that active flow control (AFC) by means of adaptive blade geometries is beneficial when such periodic incidence variations occur. A significant risk reduction in a compressor facing disturbances can thereby be achieved concerning stall or choke. Experimental investigations on such an AFC method with simultaneous application of a closed-loop control are missing in order to demonstrate its potential. This work investigates a linear compressor cascade that is equipped with a 3D-manufactured piezo-adaptive blade structure. The utilized actuators are piezoelectric macro-fiber-composites. A throttling device is positioned downstream the trailing edge plane to emulate an unsteady combustion process. Periodic transient throttling events with a frequency of up to 20 Hz cause incidence changes to the blade’s leading edge. Consequently, pressure fluctuations on the blade’s surface occur, having a significant impact on the pressure recovery downstream of the stator cascade. Experimental results of harmonically actuating the piezo-adaptive blade with the corresponding disturbance frequency show that the impact of disturbances can be reduced to approximately 50%. However, this is only effective if the phase shift of the harmonic actuation is adjusted correctly. Using an inadequate phase shift reverses the positive effects, causing the aforementioned stall, choke, or significant losses. In order to find the optimum phase shift, even under varying, possibly unpredictable operating conditions, an extremum seeking controller is presented. This gradient-based approach is minimizing the pressure variance over time by carefully adjusting the phase shift of the harmonic actuation of the AFC system.

An active system for controlling the wing flap flow on a model of a passenger aircraft wing section

In this work, a model of an active system for controlling the wing flap flow was designed and manufactured. The presented active flow control system operates by blowing air under pressure onto the upper surface of the flap. The system consists of a device supplying compressed air to the flap and the flap leading edge, through which the air is blown out. For experimental testing, the system was built into the existing large-scale aerodynamic model of a mechanized wing compartment.

Active flow control in an axial compressor for stability improvement: on the effect of flow control on stall inception

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.

Impact of the Precessing Vortex Core on NOx Emissions in Premixed Swirl-Stabilized Flames—An Experimental Study

Abstract The reduction of NOx emissions remains a driving factor in the design process of swirl-stabilized combustion systems, to meet legislative restrictions. In reacting swirl flows, hydrodynamic coherent structures, such as periodic large-scale vortices in the shear layer, induce zones with increased heat release rate fluctuations in connection with temperature peaks, which lead to an increase of NOx emissions. Such large-scale vortices can be induced by the helical coherent structure known as precessing vortex core (PVC), which influences the flow and flame dynamics under certain operating conditions. We developed an active flow control system, allowing for a targeted actuation of the PVC, to investigate its impact on combustion properties such as NOx emissions. In this work, a perfectly premixed flame, which slightly damps the PVC, is studied in detail. Since the PVC is slightly damped, it can be precisely excited by means of open-loop flow control. In connection with time-resolved OH*-chemiluminescence and stereoscopic particle image velocimetry (PIV) measurements, the impact of the actuated PVC on flow and flame dynamics is characterized. It turns out that the PVC rolls up the inner shear layer, which results in an interaction of PVC-induced vortices and flame. This interaction considerably influences the measured level of NOx emissions, which grows with increasing PVC amplitude in a perfectly premixed flame. Nearly, the same increase is measured for partially premixed conditions. This is in contrast to previous studies, where the PVC is assumed to reduce the NOx emissions due to vortex-enhanced mixing.

Mean field coupling mechanisms explaining the impact of the precessing vortex core on the flame transfer function

The flame transfer functions (FTF) is a key quantity to assess the thermoacoustic properties of combustion systems. It is known to depend on the hydrodynamic instabilities of the combustor flow. This work investigates how the FTF is affected by a global flow instability known as the precessing vortex core (PVC) which is often observed in swirl flames. To study the exclusive effect of the PVC on the FTF, a perfectly premixed swirl flame is considered where the PVC mode is damped with a close to zero growth rate. An active flow control system is applied in the region of high receptivity to excite the PVC at precisely controlled amplitudes. The conducted experiments show that the excited mode corresponds to the least stable global mode predicted from mean field stability analysis and that the most responsive frequency is equal to the predicted global mode frequency, which brings credibility to the control approach. FTF measurements conducted at different PVC actuation amplitudes show that the FTF gain decreases significantly with increasing PVC amplitude while the FTF phase remains unchanged. The FTF gain reduction is explained by two mechanisms: the reduction in gain of the Kelvin–Helmholtz instability through mean field modifications and the upstream movement of the flames center of mass due to enhanced coherent fluctuations at the flame root. The unchanged FTF phase is traced back to an unchanged location of the most influential heat release rate fluctuations at the flame tip. This study suggests that the control of the PVC is an effective way to avoid or mitigate thermoacoustic instabilities. The control is thereby very efficient as it exploits the natural global hydrodynamic instability of the flow.

Testing of High-Lift Common Research Model with Integrated Active Flow Control

A 10%-scale high-lift version of the Common Research Model and an active flow control (AFC) version of the model equipped with a simple-hinged flap were successfully tested. The main objective of the test was to develop an AFC system that can provide the necessary lift recovery on a simple-hinged flap high-lift system while minimizing its pneumatic power requirement. The tests were performed in the 14- by 22-Foot Subsonic Tunnel at the NASA Langley Research Center. Three types of AFC devices were examined: the double-row sweeping jets (DRSWJ), the alternating pulsed jets (APJ), and the high-efficiency low-power (HELP) actuators. The DRSWJ and the APJ actuators used two rows of unsteady jets, whereas the HELP actuators used a combination of unsteady and steady jets, to overcome strong adverse pressure gradients while minimizing the mass flow usage. Nozzle pressure ratio, mass flow consumption, and the power coefficient were used for judging the performance efficiency of the AFC devices. The current data are presented with the Transonic Wall Interference Correction System method applied. The HELP actuation concept was effective in controlling flow separation in the linear region of the curves comparing lift coefficient to mass flow rate. The HELP actuation achieved a targeted lift coefficient increment of 0.5, thus meeting the lift performance goal of the NASA Advanced Air Transport Technology project.

Plasma-Actuated Flow Control of Hypersonic Crossflow-Induced Boundary-Layer Transition

The purpose of this research was to design, fabricate, and test a plasma-based active flow-control system to accelerate and delay crossflow-induced boundary-layer transition on a cone at an angle of attack at Mach 6 under quiet-flow conditions. A model with interchangeable nosetips was designed and fabricated from stainless steel, polyether ether ketone (PEEK), and Macor®. Transition on the model was characterized using infrared thermography and Kulite pressure transducers in the Boeing/U.S. Air Force Office of Scientific Research Mach 6 Quiet Tunnel at Purdue University. The flow controllers were assessed by their impact on the transition location and wave number of the largest-amplitude hot streaks. The transition location was accelerated by critical forcing (actuator wave number equals wave number of naturally largest-amplitude waves) and delayed by subcritical forcing (actuator wave number larger than natural waves). The disturbance wave number input of the plasma actuators was observed downstream on the model for many of the plasma-on runs, demonstrating that the plasma actuators introduced discrete forcing into the flow. The precise locations of the hot streaks arising from stationary crossflow vortices varied for different nosetips, presumably due to differences in their microscale roughness.

Preliminary Design and System Considerations for an Active Hybrid Laminar Flow Control System

Hybrid laminar flow control or HLFC design is a complex and multi-disciplinary process, which demands a thorough understanding of all aspects from a global systems viewpoint. The objective of the paper is to present a preliminary design of important components of an HLFC system that helps in quick assessment of conceptual system architectures. This is important to evaluate feasibility, system performance, and overall aircraft benefits at early stages of system development. This paper also discusses the various important system requirements and issues concerning the design of active HLFC systems, and the interfaces between various disciplines are presented. It can be emphasized from the study that the future compressor design for the HLFC system should consider the thermal management aspects and additional mass flow requirements from the aerodynamics-structure design optimization and also from water drain system solutions. A method to calculate the accumulated water content inside the plenum chambers is presented, and the effect of a drain hole on the power consumption is studied. A low order thermal management study of the HLFC compressor motor shows a high temperature rise in the windings for very high speed motors for long duration operation and calls for effective cooling solutions.

Active Flow Control on Vertical Tail Models

Active flow control (AFC) subscale experiments were conducted at the Lucas Wind Tunnel of the California Institute of Technology. Tests were performed on a generic vertical tail model at low speeds. Fluidic oscillators were used at the trailing edge of the main element (vertical stabilizer) to redirect the flow over the rudder and delay or prevent flow separation. Side force increases in excess of 50% were achieved with a 2% momentum coefficient () input. The results indicated that a collective of about 1% could increase the side force by 30–50%. This result is achieved by reducing the spanwise flow on the swept back wings that contributes to early flow separation near their tips. These experiments provided the technical backdrop to test the full-scale Boeing 757 vertical tail model equipped with a fluidic oscillator system at the National Full-scale Aerodynamics Complex 40-by 80-foot Wind Tunnel, NASA Ames Research Center. The is shown to be an important parameter for scaling a fluidic oscillator AFC system from subscale to full-scale wind tunnel tests. The results of these tests provided the required rationale to use a fluidic oscillator AFC configuration for a follow-on flight test on the Boeing 757 ecoDemonstrator.