Active Flow Control Technique Research Articles

Winglets effect on the aerodynamics of aircraft wing; computational studies

Aircraft drag reduction has been of interest to aerospace industry for long time. Therefore, in the past decade different flow control techniques have been used such as active and passive flow control techniques. Winglet design has been explored in the past both experimentally and computationally. Experimental studies pose significant challenges due to the wing-span and costly equipment and facilities. Therefore, this study concerns the computational studies of winglet design using the large-eddy simulation approach. The results show that the winglet design reduces the flow separation and drag force, while the pressure and lift force increases.

A Numerical Analysis of Active Flow Control Techniques for Aerodynamic Drag Reduction in the Square-Back Ahmed Model

Aerodynamic drag reduction is required with new stringent constraints on pollutant emissions and fuel efficiency of ground vehicles. In this context, active flow control is a promising approach to achieve this target. This study focuses on applying different flow control strategies on the square-back Ahmed model to reduce aerodynamic drag. A steady blowing jet, a synthetic jet and an unsteady jet are located at the back edges of the model as flow control devices. A numerical study based on the 3D WMLES simulation was performed to evaluate the drag reduction capabilities of these devices at different operating conditions. The results showed that a maximum aerodynamic drag reduction of 26.51% was achieved when using a steady blowing jet, and 17.27% with an unsteady jet. In contrast, the effect of the synthetic jet on the aerodynamic drag of the model is solely at high control frequencies.

Geometric Optimization of Blunt Bodies with Aerodisk and Opposing Jet for Wave Drag and Heat Reduction

The use of passive, active, or hybrid flow control techniques is often investigated to reduce the acoustic signature, wave drag, and aerodynamic heating associated with the supersonic flow regime. This research explores passive and hybrid flow control techniques to achieve an optimal reduction in wave drag and aerodynamic heating on a blunt body using an aerodisk. While passive techniques use one or two aerospikes, hybrid techniques employ opposing jets and aerospikes. Numerical analysis is performed using Reynolds-Averaged Navier–Stokes (RANS) equations to analyze the bodies’ flow field. The statistical technique, Design of Experiments (DOE), is combined with Response Surface Method (RSM) to find the optimal configuration for four cases by generating design space. Two cases were considered for the optimization: single aerospike with and without opposing jet and double aerospike with and without opposing jet. Variables used for the design of the aerodisks were spike length and diameter, while the response variables were wave drag and normalized heat flux. The current study has established an optimum relationship between spike length and aerospike diameter located in front of the main blunt body for both single and double aerospikes. The study’s results suggest that a double aerodisk configuration is more beneficial for reducing drag and heat flux at supersonic speed than a single aerodisk. By incorporating an opposing jet at a pressure ratio of 0.8 from the frontal aerodisk to the spiked blunt body, it can reduce drag and heat flux by 86% and 95%, respectively. Finally, numerical verification is performed for statistically optimized designs.

Swept-Wing Active Flow Control with a Streamwise Row of Vortex Generating Jets

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.

Nacelle intake flow separation reduction at cruise condition using active flow control

Turbofan engine intakes are designed to provide separation-free flow at the fan face over a wide range of operating conditions. But at some off-design conditions, like at high flight speeds and high angles of attack (AoA), the aero engine intake may encounter flow separation. This boundary layer separation inside the nacelle inlet of an aircraft engine can lead to a large number of undesirable outcomes like reduction in fan efficiency, engine stall and high levels of stress on the fan blades. Active flow control is a promising solution to reduce inlet boundary layer separation and the associated fan-face flow distortion at such off-design conditions. By blowing pressurized air into the intake near the separation point, the boundary layer is energized and separation can be controlled. This study investigates the applicability of lip blowing, an active flow control technique, to control intake separation and flow distortion at the fan-face. First, intake separation was triggered in a 3D CFD model based on the NASA Common Research Model (CRM) using high AoA cases at cruise condition (Mach number 0.85, Mass flow capture ratio ∼0.7) and the features of separated flow were analyzed. Thereafter, active flow control was introduce to the intake in the form of two types of lip blowing, direct and pitched blowing. The efficacy of lip blowing at achieving separation control in an ultra high bypass ratio turbofan engine intake has been established through this study. The present paper also examines the significance of blowing parameters like the type of blowing, blowing pressure ratio, and blowing slot dimension, at different angles of attack to identify the critical control parameters. Our research successfully establishes proof of concept by demonstrating the feasibility of using lip blowing for separation control in aero-intakes, via numerical modelling. Furthermore, this study also provides crucial insights regarding the important variables to be considered for future experimental studies, and also for detailed studies covering a wider range of operating and blowing conditions.

The role of wake splitter deflector on performance enhancement of Savonius wind turbine

The Savonius drag-type wind turbine suffers from poor efficiency due to the adverse negative torques on the returning blades when they rotate against incoming flows. It was proven that using a suitable deflector system has the potential to improve wind turbine efficiency. However, as previously identified in studies, the common drawback of using a flat deflector is the strong wake zone behind it. The present study aims to improve the overall performance of a drag-type wind turbine by using a cylinder deflector with a wake splitter plate to manage and mitigate the detrimental effect of the wake zone behind the deflector. The wake control and suppressing vortex shedding downstream of a circular cylinder have been divided into active and passive flow control techniques. Amid the latter control methods, the adoption of splitter plates has proven to be highly effective for wake control and suppressing vortex shedding behind the cylinder. Unlike previous studies, a cylinder deflector with the wake splitter plate will be utilized instead of the conventional deflectors to improve the efficiency of the wind turbine by disturbing the wake zone downstream of the deflector. To obtain a more efficient configuration, the effect of the variations of the wake splitter length (Ls/D), attachment angle (θwsp), and the count of wake splitters on the outputs such as generated torque coefficient (Cm), power coefficient (CP), and wake zone on the performance of the Savonius rotor were investigated via Computational Fluid Dynamics simulation. The results revealed that cylinder deflectors with the wake splitter increased the Savonius rotor's performance the most for almost all Tip Speed Ratios (TSRs). The average Cm and CP raised considerably up to 15% using a deflector with two wake splitters at TSR = 0.6 compared to the case without deflectors.

Effect of Plasma Actuator on Velocity and Temperature Profiles of High Aspect Ratio Rectangular Jet

The turbulence jet centerline velocity and temperature decay intensely along the centerline flow direction. Thus, improving it could benefit engineering applications, such as air conditioners. However, active flow control techniques with high-aspect-ratio jets, especially for controlling the temperature, have not been widely investigated. This paper presents the velocity and temperature performance of a high-aspect-ratio rectangular jet controlled by two dielectric barrier discharge plasma actuators located on the longer sides of the nozzle and controlled by high-voltage and high-frequency pulse-width modulation sinusoidal waves. The scanning method was used to cover 362 cases as combinations of working parameters (modular frequency vs. duty vs. phase difference) for the velocity and temperature performances of the jets. Results show that plasma actuators can control both velocity and temperature distribution with minor input power compared with the rectangular jet’s kinetic energy and heat flux. The velocity increased up to 4% and decreased to 11%, measured at the interest position where x/h = 70 on the centerline. There were a 5% increase and a 4% decrease compared to the temperature-based case. Distinctive velocity and temperature distributions were observed under noteworthy cases, indicating the potential of the actuator to create various flow features without installing new hardware on the flow.

Numerical investigation of helicopter hover state of flight enhancement by applying active flow control over helicopter special tip airfoil

In conventional helicopter there are many operation limits and constraints, one of them is the maximum take-off weight and ability of main rotor to produce thrust during the hover mode of flight. In this study a numerical investigation is performed to show the effect of active flow control technique over the helicopter tip blade in hover mode to enhance the main rotor ability to produce lift by improving total performance of used airfoil. Active flow control applied to upper airfoil surface of blade tip close to the leading edge to control the boundary layer by adding momentum using small oscillating jet. The advantage of this technique comes from the small energy required to enrich the boundary layer which we use very small jet for excitation. Model is formed in 2D and is numerically investigated for airfoil used in main rotor blade tip with known geometry and performance parameter for clean rotor given by NASA test stand. Results shows very good impact on the hover mode in 2D model which gives enhancement in the overall performance represented by lift to drag ratio in the used tip airfoil in different case study used depending on the study plan that considered various excitation parameter. More than 60% increase in total performance represented by lift to drag ratio is obtained using suitable parameter and excitation point.

Active flow control of a wing section in stall flutter by dielectric barrier discharge plasma actuators

This paper investigates the potential of using an active flow control technique to modify stall flutter oscillations of a NACA (National Advisory Committee for Aeronautics) 0015 wing section. Wind tunnel experiments have been performed with a test-rig that provides the elastic degree of freedom in pitch. Measurements of the clean airfoil are taken at preset angles of θ0=6°−12°, and for Reynolds numbers of Rec=6.2×104−1.25×105, which reveal the dependency of the stall flutter oscillations to Rec and θ0. Then, flow control experiments are carried out at θ0=10° and Rec=1.04×105. Two dielectric barrier discharge plasma actuators have been employed simultaneously to exert dual-point excitation to the baseline flow. It is shown that during the upstroke half-cycle, plasma actuation postpones the dynamic stall of the airfoil and increases the maximum pitch angle of the stall flutter cycle. On the downstroke, dual-point excitation effectively improves the rate of pitching moment recovery and leads to flow reattachment at a larger pitch angle. Normalized excitation frequencies Fex=fex/fw (where fw is the wake mode frequency of the stalled airfoil) ranging from 0.1 up to 3 have been examined. Among the controlled cases, excitation with Fex=0.6 and Fex=2.2 provides the largest and smallest pitch amplitude, respectively, and the case of Fex=3 demonstrates the most impact on flow reattachment. Finally, it has been concluded that the employed control strategy is effectively capable of modifying the dynamic stall process and associated pitching moment. However, a more sophisticated control strategy would be required to significantly mitigate the stall flutter oscillations.

Conceptual Design of a Novel Unmanned Ground Effect Vehicle (UGEV) and Flow Control Integration Study

In this study, the conceptual design of an unmanned ground effect vehicle (UGEV), based on in-house analytical tools and CFD calculations, followed by flow control studies, is presented. Ground effect vehicles can operate, in a more efficient way, over calm closed seas, taking advantage of the aerodynamic interaction between the ground and the vehicle. The proposed UGEV features a useful payload capacity of 300 kg and a maximum range of 300 km cruising at 100 kt. Regarding the aerodynamic layout, a platform which combines the basic geometry characteristics of the blended wing body (BWB), and box wing (BXW) configurations is introduced. This hybrid layout aims to incorporate the most promising features from both configurations, while it enables the UGEV to operate under adverse flight conditions of the atmospheric boundary layer of the earth. In order to enhance the performance characteristics of the platform, both passive and active flow control techniques are studied and incorporated into the conceptual design phase of the vehicle. For the passive flow control techniques, the adaptation of tubercles and wing fences is evaluated. Regarding the active flow control techniques, a wide range of morphing technologies is investigated based on performance and integration criteria. Finally, stability studies are conducted for the proposed platform.

Three-dimensional Vortex Structure Formed by a Sweeping jet and Main flow

A sweeping jet produced by a fluidic oscillator has recently been examined from the viewpoint of flow separation control. A sweeping jet, which is an oscillating jet with high frequency and large amplitude, is generated by the internal structure of the oscillator, which eliminates the need for a drive unit. Sweeping jets have attracted attention as an active flow control device. Studies have proposed active flow control techniques for an airfoil based on sweeping jets. Most of these studies focused on the characteristics of the dynamic forces acting on the airfoil. However, the flow fields formed by the interaction between the sweeping jet and the main flow and their flow mechanisms are poorly understood, especially for the case where a sweeping jet is ejected into a main flow when a flow field is used to control flow separation. In this study, the detailed vortex structure generated by the interaction between the sweeping jet and the main flow was quantitatively examined using stereo PIV. The stream tube generated by the interaction between the sweeping jet and the main flow consists of two vortex tubes near the outlet of the sweeping jet at a location of 2.0d to 6.0d. At the wake over 10.0d, a pair of vortex tubes gradually separates.

Performance assessment of an innovative Gurney flap for straight-bladed vertical axis wind turbine

Based on the vortex trapping capability of the cavity and the lift improvement mechanism for the Gurney Flap (GF), their combined impact on aerodynamic efficiency of the straight-bladed vertical axis wind turbine (SB-VAWT) was investigated. In order to eliminate the flow resistance of the blade caused by GF in the windward zone, an active flow control technique, a pivoting GF could change its position as the blades rotates in a cycle, is proposed in this paper. A grid sensitivity analysis by Grid Convergence Index (GCI) is first carried out to examine the convergence of the numerical model. Considering different GF control strategies, Computational fluid dynamics (CFD) method was used to evaluate the potential of the movable cavity-GF (c-GF) for power augmentation of SB-VAWT with traditional fixed cavity-GF. The simulation results show that power coefficient for movable c-GF is increased by more than 20% in comparison with the clean one. An appropriate active control solution not only has excellent power extraction in the downwind revolution, but also can significantly eliminate the resistance effect in the upwind zone cause by fixed c-GF. In addition, the benefits of the movable c-GF were prominent especially at the smaller tip speed ratios which indicates that it can be used for both newly designed turbines and retrofitting solutions for existing rotors.

Aeroelastic response of a propulsive rotor blade with synthetic jets

Understanding the aeroelastic response of a propulsive rotor blade is critical to implementing active flow control techniques to mitigate blade structural vibration. In this experimental study, the coupling between rotor blade aerodynamic loading and first flap mode bending and torsion is explored. The three-bladed rotor is 2.58 m in diameter, and contains a NACA 0012 airfoil with zero pre-built twist. Each blade contains 20 high-performance synthetic jet actuators distributed along the span and are activated with a constant mean jet velocity of 66.3 m/s. The rotor was tested at rotor speeds, Ω of 250, 500, 750, and 1000 revolutions per minute (RPM) and collective blade pitch angles, θc of 2, 5, and 8 degrees. Rotor thrust and torque were measured using a high-capacity load cell, and the flow near the blade tip was measured using laser Doppler velocimetry (LDV) techniques. Blade bending and torsion was measured at three equally spaced radial stations on the blade root, middle, tip regions defined by r/R= 0.32, 0.64, and 0.96 respectively using three onboard electronic accelerometers. Synthetic jets reduce the near-wall streamwise velocity deficit and turbulence intensity in the near wall flow, improving the sectional lift coefficient of the blade and delaying flow separation which results in marginally higher thrust and lower torque loading of up to 7.4% and 4.3 % respectively. At higher rotor speeds of 750 and 1000 RPM, there is a pronounced non-linear increase in bending and torsion along blade span. In the blade tip region, the streamwise flow undergoes a significant decrease in velocity and momentum, which contributes to instabilities in the boundary layer with early-stage unsteady flow separation at θc= 8°. This is correlated to a dramatic rise in the baseline power spectral density (PSD) at the blade’s natural frequency along with an increase in root mean square (rms) of bending acceleration and blade pitch angle, where the maximum values measured are arms= 0.74 g and θrms= 1.45°.

Control of leading-edge vortices over delta wing using flow control methods: A review

The main objective of this research is to study various methods to control flow field over the delta wings. The flow control methods are of two types: active and passive flow control. Active flow control technique involves the addition of energy from an external source to the main flow. The various active flow control techniques comprise of pneumatic devices such as blowing and suction near the leading-edge, plasma actuators, steady/unsteady excitation and control surfaces. The passive flow control technique involves the modification to the delta wing geometry and does not involve any external source of energy. The passive flow control methods comprise of generation of multiple vortex arrangement and alteration of leading-edge shapes. The effect of flow control technique on the parameters such as flow reattachment, location of leading-edge vortices and breakdown are presented in this research.

Study of the influence of an external flow rate perturbation on the vortex structure and heat transfer in impinging jets

The study of physical processes dominating in submerged turbulent jets impinging on a wall is an important task because this configuration is utilized in various applications. The efficiency of heat transfer in this configuration has been a subject of a long-term study. Active flow control technique and the optimization of the control signal can be applied to exploit inherent flow properties to further improve the heat transfer from the wall in impingent jets. In this paper, IR-thermography and time-resolved PIV measurements are used for the diagnostics of wall temperature fields and large-scale vortex dynamics under external flow rate forcing control. It is found that the low-frequency forcing (for the Strouhal number St = 0.6) increases integral temperature on the wall as compared to the unforced case and the high-frequency forcing (St = 0.9).

Model-Based Analysis of Flow Separation Control in a Curved Diffuser by a Vibration Wall

Vibration wall control is an important active flow control technique studied by many researchers. Although current researches have shown that the control performance is greatly affected by the frequency and amplitude of the vibration wall, the mechanism hiding behind the phenomena is still not clear, due to the complex interaction between the vibration wall and flow separation. To reveal the control mechanism of vibration walls, we propose a simplified model to help us understand the interaction between the forced excitation (from the vibration wall) and self-excitation (from flow instability). The simplified model can explain vibration wall flow control behaviors obtained by numerical simulation, which show that the control performance will be optimized at a certain reduced vibration frequency or amplitude. Also, it is shown by the analysis of maximal Lyapunov exponents that the vibration wall is able to change the flow field from a disordered one into an ordered one. Consistent with these phenomena and bringing more physical insight, the simplified model implies that the tuned vibration frequency and amplitude will lock in the unsteady flow separation, promote momentum transfer from the main stream to the separation zone, and make the flow field more orderly and less chaotic, resulting in a reduction of flow loss.

Influence of Active Flow Control on Blunt-Edged VFE-2 Delta Wing model

This paper highlights the flow topology above blunt-edged delta wing of VFE-2 configuration when an active flow control technique called ‘blower’ is applied in the leading edge of the wing. The flow topology above blunt-edged delta wing is very complex, disorganised and unresolved compared to sharp-edged wing. For the sharp leading-edged wing, the onset of the primary vortex is fixed at the apex of the wing and develops along the entire wing towards the trailing edge. However, the onset of the primary vortex is no longer fixed at the apex of the wing for the blunt-edged case. The onset of the primary vortex develops at a certain chord-wise position and it moved upstream or downstream depending on Reynolds number, angle of attack, Mach number and the leading-edge bluntness. An active flow control namely ‘blower’ technique has been applied in the leading edge of the wing in order to investigate the upstream/downstream progression of the primary vortex. This research has been carried out in order to determine either the flow on blunt-edged delta wing would behave as the flow above sharp-edged delta wing if any active flow control is applied. The experiments were performed at Reynolds number of 0.5×106, 1.0×106 and 2.0×106 corresponding to 9 m/s, 18 m/s and 36 m/s in UTM Low Speed wind Tunnel based on the mean aerodynamic chord of the wing. The results obtained from this research have shown that the blower technique has significant effects on the flow topology above blunt-edged delta wing. The main observation from this study was that the primary vortex has been shifted 20% upstream when the blower technique is applied. Another main observation was the ability of this flow control to delay the formation of the vortex breakdown.

Drag and Heat Flux Reduction using Counterflow Jet and Spike - Analysis of their Equivalence for a Blunt Cone Geometry at Mach 8

This study aims to explore equivalence between active and passive flow control techniques in reducing the wave drag and surface heat flux over a blunt cone model kept in Mach 8 stream. Computational investigations were carried out by using finite volume-based compressible flow solver. Throughout the study, the solution of governing equations is sought by assuming two dimensional-axisymmetric nature of the flowfield. Both counter flow-stagnation point injection and forward facing-physical spike are considered to mitigate the excess drag and heat flux experienced by a blunt body representing the nose cone section of a hypersonic vehicle. Eventually, based on identified drag reductions, the present study proposes equivalence cases between these two methods. It is shown that a pointed spike of L/D=1 provides almost the same drag reduction as the counterflow injection jet with a pressure ratio of 8.25. Similarly, other equivalence cases are identified and the physics behind them is explored. The identified equivalence is expected to help the designers in effectively replacing one technique with another according to the requirement. Equivalence matrix is presented for different spike cases in terms of injection ratios of counterflow injection.

Effects of nozzle geometry and active blowing on lift enhancement for upper surface blowing configuration

Upper Surface Blowing (USB) has been regarded as a promising lift enhancement technique for short-take-off and landing (STOL) applications. The present study conducted numerical investigations to further explore the potential capability of USB technique by passive and active flow control strategies. Computations were accomplished by a Reynolds-averaged Navier-Stokes solver enclosed by the two-equation k-ω shear stress transport turbulence model. The propulsion effect of the up-mounted engine was represented by the Turbine Powered Simulator (TPS). Results show that aerodynamic lift of using USB system is significantly enhanced due to the attenuation of plume edge roll-up vortex which is beneficial to the Coanda effect. Varying nozzle geometries can augment lift by alleviating the three-dimensional edge effect and thus delaying the rollup of plume with a thinner jet sheet. Compared to the passive flow control method by nozzle geometry modification, active constant blowing flow control technique shows a more promising potential for lift enhancement for USB powered system. Within the test cases, a 50% lift increment can be achieved by using constant blowing on the USB system.

2D optimization of a Small Horizontal Axis Wind Turbine blade using flow control techniques

In this work, the optimization of the efficiency of a small horizontal axis wind turbine (SHWT) blade segment is presented. Typically, SHWTs have a radius of 1.5 to 3.5 m and a hub height of around 15 m from the ground. SHWTs operate in a relatively small Reynolds numbers range (up to 1.5x106) and are installed inside the atmospheric boundary layer. This operational environment is characterised by volatile air flow, making the flow over the blade prone to separation. In order to counter this flow behavior, a set of flow control techniques is introduced and studied. These techniques control the flow, either passively, solely by the inclusion of blade add-ons, or actively, by adding energy to the boundary layer. More specifically, two passive flow control techniques and one active flow control technique are modelled and tested on a wind turbine blade segment. The passive techniques implemented in this study are based on the use of vortex generators and tubercles. Vortex generators are small vanes attached vertically to the lifting surface and are widely used in aerospace applications with varying degrees of success. Tubercles, which is a novel flow control technique, are sinusoidal modifications of the blade’s leading edge. The original concept has been inspired from the characteristic flipper of the humpback whale (Megaptera Novaeangliae). Regarding the active flow control technique, a dielectric barrier discharge (DBD) plasma actuator (PA) is used, a technique that adds momentum on the local flow, close to the blade’s surface, by ionizing the air. The impact on the blade aerodynamic efficiency for each technique are evaluated and presented. The results from this evaluation show that flow control techniques can offer a considerable benefit to SHWT by improving the blade’s aerodynamic characteristics, i.e. by increasing the blade lift-to-drag ratio and thus, improving their critical performance efficiency factor.