Active Flow Control Technology Research Articles

Feasibility Study of a Multi-Purpose Aircraft Concept with a Leading-Edge Embedded Cross-Flow Fan

The research presented focuses on investigating the use of Cross-Flow Fan (CFF) as a high-lift device for a Short Take-off and Landing (STOL) aircraft. The wing-embedded fan performance analysis is mostly addressed from an aerodynamic perspective and focuses on using such Active Flow Control (AFC) technology in the conceptual aircraft design process. In particular, the design trade study of an aircraft featuring the fan as a high-lift device applied to a conceptual design of a medium-range multi-purpose aircraft is performed. A sensitivity analysis is employed to investigate the impact of the technology on the aircraft weight, aerodynamics, stability and control, and fight performance. The aircraft design modifications are introduced to maximize the aircraft mission performance given the fan specifications and constraints. Results indicate a reduction of the take-of field length by 18% with the payload penalty of 14%. The aircraft ferry range is also decreased by 7% compared to the baseline aircraft design. The scaling analysis of the aircraft concept is performed to determine the potential market for such technology. The results show that a light General Aviation (GA) airplane or a medium-large size Unmanned Aerial Vehicle (UAV) could benefit more from the wing-embedded CFF compared to more heavy airplanes.

Special Issue “Active Flow Control Technologies for Energy and Propulsive Systems”

Active flow control (AFC) is a fast-growing, multi-disciplinary science and technology for energy and propulsive systems [...]

Electrical and Flow Characteristics of a Double-Side Sliding Pulsed Discharge Plasma Actuator

The development of next-generation high-maneuverability aircraft requires new technologies for controlling asymmetric separated vortices at high angles of attack. Plasma flow control is an emerging type of active flow control technology that can be used to control asymmetric vortices over slender bodies by means of pulsed dielectric barrier discharge (DBD) plasma actuators. However, some problems worthy of further study remain unsolved. For example, the stimulation strength is inadequate, the region of discharge is small, and the control mechanism is unclear. This paper proposes a double-side sliding pulsed discharge (DSPD) plasma actuator that combines a sinusoidal alternating current (AC) pulsed plasma actuator with two high-voltage direct current (DC) components. This actuator can generate two groups of unsymmetrical large-scale coherent vortex structures to balance the vorticity of the asymmetric vortices over the slender forebody of an aircraft and increase the distance between the cores of the asymmetric vortices, weakening the interaction between the vortices and hindering the emergence of the asymmetric phenomenon. Focusing on the key scientific issue of DSPD plasma actuation, the discharge morphologies and electrical properties under different DC conditions are studied, and the unsteady vortex structures induced by the DSPD plasma actuator under a static atmosphere are revealed through high-speed schlieren system and particle image velocimetry (PIV) measurements. The flow control ability of the DSPD plasma actuator is verified through wind tunnel experiments, and the main rules governing its influence on the external flowfield are determined. It is speculated that this actuator will further improve the aerodynamic flow control of slender aircraft.

Comparison Between Fluidic Oscillators and Steady Jets for Separation Control

An experimental study was conducted to compare performance and efficiency between fluidic oscillators and steady jets with respect to commonly used flow control coefficients for momentum, mass flow, and energy in support of the development of active flow control technology. Different actuation modules were tested on a model of the NASA hump geometry. Time-averaged pressure measurements were conducted with high spatial resolution, and stereoscopic particle image velocimetry was performed downstream of the actuation location to investigate the performance differences in detail. Surface flow visualization in a quiescent environment was employed to assess the spreading angle of the various actuators. Results show that fluidic oscillators are almost always superior to steady jets with respect to any flow control coefficient at a given spacing. The lone outlier occurs for tightly spaced actuators in which both perform similarly with respect to the energy coefficient. This general superiority is in part due to a larger jet spreading angle, the associated spanwise coverage, and the existence of stronger coherent streamwise vortices in comparison to steady jets.

Active Control of Bluff-Body Flows Using Plasma Actuators

Actuators play an important role in modern active flow control technology. Dielectric barrier discharge plasma can be used to induce localized velocity perturbations in air, so as to accomplish modifications to the global flow field. This paper presents a selective review of applications from the published literature with emphasis on interactions between plasma-induced perturbations and original unsteady fields of bluff-body flows. First, dielectric barrier discharge (DBD)-plasma actuator characteristics, and the local disturbance fields these actuators induce into the exterior flow, are described. Then, instabilities found in separated flows around bluff bodies that controlled actuation should target at are briefly presented. Key parameters for effective control are introduced using the nominally two-dimensional flow around a circular cylinder as a paradigm. The effects of the actuator configuration and location, amplitude and frequency of excitation, input waveform, as well as the phase difference between individual actuators are illustrated through examples classified based on symmetry properties. In general, symmetric excitation at frequencies higher than approximately five times the uncontrolled frequency of vortex shedding acts destructively on regular vortex shedding and can be safely employed for reducing the mean drag and lift fluctuations. Antisymmetric and symmetric excitation at low frequencies of the order of the natural frequency can amplify the wake instability and increase the mean and fluctuating aerodynamic forces, respectively, due to vortex locking-on to the excitation frequency or its subharmonics. Results from several studies show that the geometry and arrangement of the electrodes is of utmost significance. Power consumption is typically very low, but the electromechanical efficiency can be optimized by input waveform modulation.

Impingement Cooling Performance Analysis and Improvement of Synthetic Jets for Electronic Devices

With the development of miniaturization, high power density, and a high degree of integration of electronic devices, heat dissipation has become a major factor constraining its development. As an active flow control technology, synthetic jets can significantly enhance heat transfer and cooling ability. In this paper, an unsteady numerical model of heat transfer is established to explore the impingement cooling performance of synthetic jets. The effect of nozzle-to-surface distance on the cooling performance is analyzed. The results show that the cooling ability of synthetic jets first increases and then decreases with an increasing nozzle-to-surface distance. At small nozzle-to-surface distances, there is a significant recirculation phenomenon of hot air, leading to a notable deterioration of heat transfer and cooling performance. To solve this issue at small nozzle-to-surface distances, a hybrid synthetic jet based on a fluid diode is designed. The further simulation results show that the proposed hybrid synthetic jet can effectively avoid recirculation of hot air and significantly improve the impingement cooling ability at small nozzle-to-surface distances. The temperature of the target surface under the action of the proposed hybrid synthetic jet is decreased by about 10°C in comparison with the conventional synthetic jet without fluid diodes.

Active Flow Control Computations: From a Single Actuator to a Complete Airplane

A computational method for predicting the aerodynamic performance of an airplane equipped with an active flow control system on the vertical tail has been developed and thoroughly validated. The objective of this application was to enhance airplane directional control and potentially lead to reduced fuel burn, lower emissions, and improved airplane performance. The flow control system incorporated a large array of fluidic oscillators. A systematic approach was used to assess the accuracy of the computational tool for progressively complex configurations using measured data. First, the characterization of the flow of a single actuator was established. This was followed by simulations of vertical tail models with dozens of actuators in subscale and full-scale wind-tunnel environments. Computational simulations were subsequently used to guide the implementation into a full airplane under realistic flight conditions. The simulation results were validated with the measured data obtained during a flight test of the Boeing 757 ecoDemonstrator airplane that was used to demonstrate the active flow control technology. The computational tool has also been used to assess scaling of flow control effectiveness between the wind-tunnel test and flight.

Research on forebody active disturbed flow characteristics of slender body in supersonic field

Instantaneous turn rate plays an extremely important role in the generic missile configuration with an appropriate angle of attack in supersonic air flows. Under these special supersonic flow conditions, the dominant aerodynamic coefficient of the generic missile, which was simplified to a slender baseline geometry with a blunt-cone nose, was the side force coefficient. Active disturbed flow control technology on a supersonic missile body referred to the design of micro-flow effectors and controlled the flow around the missile by actively regulating the height of the micro-flow effectors around the missile surface, which allows for active control of the missile flight attitude. This paper used the small perturbation assumptions to calculate the side force model affected by the flow effectors and used Ansys Fluent® to analyze the separated vortex around the slender body, as well as the effects of regularity of the parameters for the micro-flow effector to the side force coefficient. In this paper, we also developed methods to analyze the supersonic complex air flow zones and analyzed the air flows around the body and the side force coefficient of the slender body by changing the parameters of the micro-flow effector structure, air flow field conditions and body structural parameters. The projects concluded that the simulation analysis method and analysis of the results can be used to guide active disturbed flow control systems and actuation device designs for compact and lightweight missiles to provide the necessary reference basis.

Fluidic Flow Control Effectors for Flight Control

This paper overviews research undertaken to develop and demonstrate active flow control technologies based on the application and manipulation of Coanda flows, with the objective of developing systems having adequate control authority to trim and maneuver an aircraft without the use of conventional control surfaces. The development of circulation control and fluidic thrust vectoring concepts and the contributions of numerous researchers to the current understanding of supersonic wall jets are reviewed and placed into context with the authors’ own research. The value of supplementing fundamental experimental and numerical research with more practical system integration studies and small-scale flying demonstration activities is shown to be a valuable approach to providing focus and direction for the research. Simple technology demonstrators have also proved to be valuable motivators for the research team and provided important focus in promoting the technologies and ideas to the wider stakeholder community. Outcomes of the research are now being explored in the context of taking them to the next stage of practical maturity.

System-Level Assessment of Active Flow Control for Commercial Aircraft High-Lift Devices

The use of active flow control technology for augmentation of aircraft aerodynamic performance is of considerable interest to the aerospace industry. Research to date has included subscale and full-scale wind-tunnel experiments as well as limited flight-test demonstrations. Out of several possible applications of active flow control to commercial aircraft, this paper focuses on its application to high-lift devices, which may permit weight and drag reduction through simplification of the flap mechanism. These, however, must be traded off against the additional weight of the active flow control system, its power requirements, and redundancy considerations to assess net system-level impacts. In this work, the Integrated Subsystem Sizing and Architecture Assessment Capability is used to assess the system-level impact of multiple high-lift device active flow control architectures for a small twin-aisle aircraft. A comparative assessment of these architectures reveals a strong dependency of fuel burn performance and the identity of the best-performing architecture on the system total mass flow requirements. For a selected architecture, the impact of the system mass flow requirements on the off-design performance is investigated. Despite an increase in vehicle operating empty weight, fuel consumption still decreases for most missions in the payload–range envelope. Finally, the sensitivity of the architecture’s performance to several sources of uncertainty is ascertained through a sensitivity analysis, and it is observed that the uncertainty in estimation of flap track fairing drag has the most significant impact on the estimation of mission fuel burn.

Design and cost analysis of integration of fluidic oscillator into a flap structure

Integration of active flow control technology into civil transport aircraft is a highly desired objective due to the potential reductions in part count, weight, and recurring manufacturing costs. This study develops an optimal design for integrating a fluidic oscillator into the leading-edge of a trailing-edge flap structure on a civil transport aircraft. The design incorporates design specifications set by members of the aerospace industry, robust design methodologies, and simulation studies to create three separate designs that can be mass-produced. An analysis of the manufacturing, assembly, material, and weight reveals the cost of the design with respect to its production rate, which ranges from about $4090 per aircraft for low-production volumes to about $2600 per aircraft for high-production volumes. As a result, this study provides a basis for the design of manufacturing and assembly techniques to integrate active flow control technology into civil transport aircraft.

Wing Tip Drag Reduction at Nominal Take-Off Mach Number: An Approach to Local Active Flow Control with a Highly Robust Actuator System

This paper discusses wind tunnel test results aimed at advancing active flow control technology to increase the aerodynamic efficiency of an aircraft during take-off. A model of the outer section of a representative civil airliner wing was equipped with two-stage fluidic actuators between the slat edge and wing tip, where mechanical high-lift devices fail to integrate. The experiments were conducted at a nominal take-off Mach number of M = 0.2. At this incidence velocity, separation on the wing section, accompanied by increased drag, is triggered by the strong slat edge vortex at high angles of attack. On the basis of global force measurements and local static pressure data, the effect of pulsed blowing on the complex flow is evaluated, considering various momentum coefficients and spanwise distributions of the actuation effort. It is shown that through local intensification of forcing, a momentum coefficient of less than c μ = 0.6 % suffices to offset the stall by 2.4°, increase the maximum lift by more than 10% and reduce the drag by 37% compared to the uncontrolled flow.

Wind tunnel experiments on flow separation control of an Unmanned Air Vehicle by nanosecond discharge plasma aerodynamic actuation**Project supported by the National Natural Science Foundation of China (Grant Nos. 61503302, 51207169, and 51276197), the China Postdoctoral Science Foundation (Grant No. 2014M562446), and the Natural Science Foundation of Shaanxi Province, China (Grant No. 2015JM1001).

Plasma flow control (PFC) is a new kind of active flow control technology, which can improve the aerodynamic performances of aircrafts remarkably. The flow separation control of an unmanned air vehicle (UAV) by nanosecond discharge plasma aerodynamic actuation (NDPAA) is investigated experimentally in this paper. Experimental results show that the applied voltages for both the nanosecond discharge and the millisecond discharge are nearly the same, but the current for nanosecond discharge (30 A) is much bigger than that for millisecond discharge (0.1 A). The flow field induced by the NDPAA is similar to a shock wave upward, and has a maximal velocity of less than 0.5 m/s. Fast heating effect for nanosecond discharge induces shock waves in the quiescent air. The lasting time of the shock waves is about 80 μs and its spread velocity is nearly 380 m/s. By using the NDPAA, the flow separation on the suction side of the UAV can be totally suppressed and the critical stall angle of attack increases from 20° to 27° with a maximal lift coefficient increment of 11.24%. The flow separation can be suppressed when the discharge voltage is larger than the threshold value, and the optimum operation frequency for the NDPAA is the one which makes the Strouhal number equal one. The NDPAA is more effective than the millisecond discharge plasma aerodynamic actuation (MDPAA) in boundary layer flow control. The main mechanism for nanosecond discharge is shock effect. Shock effect is more effective in flow control than momentum effect in high speed flow control.

A CFD Benchmark for Flow Separation Control Application

This paper describes numerical studies conducted for one promising active flow control (AFC) technology for high-lift configurations, namely the pulsed blowing through slots. The selected configuration is a 2D high-lift airfoil, known as DLR-F15, and represents the state of the art for transonic turbulent airfoil for modern civil transport aircraft. The slot-actuators are applied on the suction side of the trailing edge flap to prevent local flow separation. The studies are addressed to evaluate the feasibility of unsteady RANS simulations to reproduce the trends discovered in experiments. Within the European framework of JTI-Clean Sky, numerical and experimental studies for low speed AFC are addressed. In this project, a computational study was performed at ONERA and DLR for verifying their standard approaches for AFC with available time-averaged experimental data and performing a direct comparison of computed flow unsteadiness and its implications. The results for moderate angles of attack with and with...

Validation of Numerical Simulation Method for Active Flow Control Technology

The numerical simulation method of active flow control technology was studied in this paper. The simplified mathematical model of the active flow control is established with unsteady velocity boundary condition at the specific location of model surface. The reliability of flow control model was verified by standard cases of CFDVAL2004, and the capability of capturing micro-jet flow characteristics for Spalart-Allmaras (SA) and Menter’s Shear Stress Transport (SST) turbulence model were analyzed. The results showed that the accuracy of SA turbulence model is better than the SST model, and flow control model meet the accuracy requirements for numerical simulation method.

Cavity Effect of Synthetic Jet Actuators Based on Piezoelectric Diaphragm

An active flow control technology known as synthetic jet actuator (SJA) is a zero-net mass-flux device to create pulsed jet that produces momentum to its surroundings and uses a vibrating diaphragm inside the cavity to generate an oscillatory flow through a small orifice. The performance of SJA depends on the design of an orifice and cavity, and oscillating membrane. SJA design based on piezoelectric diaphragm used in this project because of their size, lightweight, no need for external air supply, without the pipe complex, fast response time and low power consumption. This paper describes the cavity effect to SJA designs and experiments were performed to determine the air jet velocity produced through the orifice using a hot-wire anemometer at a different cavity thickness. The results demonstrate that the jet velocity increase would be better if the cavity thickness is reduced. However, more studies are needed to optimize the size of cavity and orifice for appropriate applications.

Review of actuators for high speed active flow control

Actuators are one of the key points for the development of active flow control technology. Efficient methods of high speed flow control can provide enhanced propulsive efficiency and at the same time enable safe and maneuverable high speed flight. The development of high speed flight technology promotes the emergence of novel and robust actuators. This review introduces the state of the art in the development of actuators that can be used in high speed active flow control. The classification and different operation criteria of the actuators are discussed. The specifications, mechanisms and applications of various popular actuator types including fluidic, mechanical, and plasma actuators are described. Based on the realistic need of high speed flow control and the existing results of actuators, a new actuator design method is proposed. At last, the merits and drawbacks of the actuators are summarized and some suggestions on the development of active flow control technology are put forward.

Aerodynamic drag reduction of a simplified squareback vehicle using steady blowing

A large contribution to the aerodynamic drag of a vehicle arises from the failure to fully recover pressure in the wake region, especially on squareback configurations. A degree of base pressure recovery can be achieved through careful shape optimisation, but the freedom of an automotive aerodynamicist to implement significant shape changes is limited by a variety of additional factors such styling, ergonomics and loading capacity. Active flow control technologies present the potential to create flow field modifications without the need for external shape changes and have received much attention in previous years within the aeronautical industry and, more recently, within the automotive industry. In this work the influence of steady blowing applied at a variety of angles on the roof trailing edge of a simplified ¼ scale squareback style vehicle has been investigated. Hot-wire anemometry, force balance measurements, surface pressure measurements and PIV have been used to investigate the effects of the steady blowing on the vehicle wake structures and the resulting body forces. The energy consumption of the steady jet is calculated and is used to deduce an aerodynamic drag power change. Results show that overall gains can be achieved; however, the large mass flow rate required restricts the applicability of the technique to road vehicles. Means by which the mass flow rate requirements of the jet may be reduced are discussed and suggestions for further work put forward.

Heat Dissipation using Minimum Counter Flow Jet Ejection During Spacecraft Re-Entry

Abstract A major problem of spacecraft re-entry is the extreme heat generated. Sink and ablation cooling are commonly used but the cost of building and implementing such heat protection systems is very high. The issue of heat dissipation has, therefore, been an extensive area of research. Various passive and active methods of flow control have been attempted. Unfortunately, no viable cost effective methodologies have yet been possible to develop. At the University of New South Wales, various methods of active flow control technologies for application to subsonic and supersonic flows with a bias towards higher efficiency and lower energy usage are been explored. In this paper, a new approach of deploying minimum counter flow jet ejection as an active flow control method for heat dissipation is attempted. The concept relies on imparting sufficient momentum on the strong bow shock wave formed to induce oscillation but low enough to avoid instability during the flow interaction. To test the concept, an Apollo command module has been employed. It has been subjected to a free stream Mach number of 3 and a two-dimensional numerical flow simulation using ANSYS as the computational fluid dynamics tool has been performed for several cases of counter flow jet injection imparted on the bow shock wave formed and the interaction with the oncoming flow through the frontal stagnation point of the test module and compared with no injection case. Results obtained are highly promising. It appears that substantial heat reduction on the body is possible using the new approach. This finding may be of high practical significance and open up the possibility of developing a new thermal protective system using active flow control of dissipating and reducing heat during spacecraft re-entry with minimum energy input.

An Investigation into the Wake Structure of Square Back Vehicles and the Effect of Structure Modification on Resultant Vehicle Forces

A large contribution to the aerodynamic drag of a vehicle (30%(1) or more depending on vehicle shape) arises from the low base pressure in the wake region, especially on square-back configurations. A degree of base pressure recovery can be achieved through careful shape optimization, but the flow structures and mechanisms within the wake that cause these base pressure changes are not well understood. A more complete understanding of these mechanisms may provide opportunities for further drag reductions from both passive shape changes and in the future through the use of active flow control technologies.