Application of Active Flow Control Technique for Gust Load Alleviation

A new gust load alleviation technique is presented in this paper based on active flow control. Numerical studies are conducted to investigate the beneficial effects on the aerodynamic characteristics of the quasi ＂Global Hawk＂ airfoil using arrays of jets during the gust process. Based on unsteady Navier-Stokes equations, the grid-velocity method is introduced to simulate the gust influence, and dynamic response in vertical gust flow perturbation is investigated for the airfoil as well. An unsteady surface transpiration boundary condition is enforced over a user specified portion of the airfoil’s surface to emulate the time dependent velocity boundary conditions. Firstly, after applying this method to simulate typical NACA0006 airfoil gust response to a step change in the angle of attack, it shows that the indicial responses of the airfoil make good agreement with the exact theoretical values and the calculated values in references. Furthermore, gust response characteristic for the quasi ＂Global Hawk＂ airfoil is analyzed. Five kinds of flow control techniques are introduced as steady blowing, steady suction, unsteady blowing, unsteady suction and synthetic jets. The physical analysis of the influence on the effects of gust load alleviation is proposed to provide some guidelines for practice. Numerical results have indicated that active flow control technique,as a new technology of gust load alleviation, can affect and suppress the fluid disturbances caused by gust so as to achieve the purpose of gust load alleviation.

Active flow control using machine learning:A brief review

Nowadays the rapidly developing artificial intelligence has become a key solution for problems of diverse disciplines,especially those involving big data.Successes in these areas also attract researchers from the community of fluid mechanics,especially in the field of active flow control(AFC).This article surveys recent successful applications of machine learning in AFC,highlights general ideas,and aims at offering a basic outline for those who are interested in this specific topic.In this short review,we focus on two methodologies,i.e.,genetic programming(GP)and deep reinforcement learning(DRL),both having been proven effective,efficient,and robust in certain AFC problems,and outline some future prospects that might shed some light for relevant studies.

Ferrofluid moving thin films for active flow control

Ferrofluid moving thin films and their possible significance with regard to active flow control for lift and attack angle enhancement are discussed.In this strategy,a very thin film of ferrofluid is strongly attached at the wall of the wing by a normal magnetic field from below and pumped tangentially along the wing.Utilizing a simplified physical model and from the available experimental data on moving walls,the expected lift enhancement and effect on the attack angle were assessed.Additional research and design is required in order to explore the possibilities in the use of ferrofluid moving thin films.

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 faceover a wide range of operating conditions. But at some off-design conditions, like at high flightspeeds 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 largenumber of undesirable outcomes like reduction in fan efficiency, engine stall and high levels ofstress on the fan blades. Active flow control is a promising solution to reduce inlet boundarylayer separation and the associated fan-face flow distortion at such off-design conditions. Byblowing pressurized air into the intake near the separation point, the boundary layer is ener-gized 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 CommonResearch Model (CRM) using high AoA cases at cruise condition (Mach number 0.85, Massflow capture ratio w0.7) and the features of separated flow were analyzed. Thereafter, activeflow control was introduce to the intake in the form of two types of lip blowing, direct andpitched blowing. The efficacy of lip blowing at achieving separation control in an ultra highbypass ratio turbofan engine intake has been established through this study. The present paperalso examines the significance of blowing parameters like the type of blowing, blowing pres-sure ratio, and blowing slot dimension, at different angles of attack to identify the critical con-trol 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 beconsidered for future experimental studies, and also for detailed studies covering a wider rangeof operating and blowing conditions.

PASSIVE-ACTIVE CONTROL OF A FLEXIBLE ISOLATI0N SYSTEM

Passive-active control of a flexible isolation system is investigated from the viewpoint ofpower flow. Dynamic transfer equations of the system are deduced based on a matrix method whichuses mobility or impedance representations of three substrictures: the source of vibration, the receiverand the mounting system which connects the source to the receiver The cancellation of axial inputforces to the receiver is considered as the active control strategy and its effects are discussed. Theresults of the study show that the strategy adopted herein can effectively reduce the power transmittedto the receiver.

The law of anti-VCAM-1 targeted microbubbles adhesion to activated endothelial cells under controlled shear flow

Microbubbles can enhance the detection in noninvasive ultrasound imaging.Recently,targeted microbubbles have been developed to selectively adhere to specific and overexpressed p molecules in endothelial cells in some pathologic conditions.However,the law of

DOUBLE LOOP ACTIVE VIBRATION CONTROL OF PNEUMATIC ISOLATOR WITH TWO SEPARATE CHAMBERS

A newly designed pneumatic spring with two separate chambers is promoted and double-loop active control is introduced to overcome the following drawbacks of passive pneumatic isolation： ① The low frequency resonances introduced into the system; ② Conflict between lower isolation frequency and stiffness high enough to limit quasi-static stroke;③ Inconsistent isolation level with different force load. The design of two separate chambers is for the purpose of tuning support frequency and force independently and each chamber is controlled by a different valve. The inner one of double-loop structure is pressure control, and in order to obtain good performance, nonlinearities compensation and motion flow rate compensation （MFRC） are added besides the basic cascade compensation, and the influence of tube length is studied. The outer loop has two functions： one is to eliminate the resonance caused by isolation support and to broaden the isolation frequency band by payload velocity feedback and base velocity feed forward, and the other is to tune support force and support stiffness simultaneously and independently, which means the support force will have no effect on support stiffness. Theoretical analysis and experiment results show that the three drawbacks are overcome simultaneously.

Microjet flow control in an ultra-compact serpentine inlet

Microjets are used to control the internal flow to improve the performance of an ultra-compact serpentine inlet. A highly offset serpentine inlet with length-to-diameter ratio of 2.5 is designed and static tests are conducted to analyze the internal flow characteristics in terms of pressure recovery, distortion and flow separation. Flow separation is encountered in the second S-turn, and two strong counter-rotating vortices are formed at the aerodynamic interthce plane （AIP） face which occupy a quarter of the outlet area and result in severe pressure loss and distortion. A flow control model employing a row of microjets in the second turn is designed based on the internal flow characteristics and simplified CFD simulations. Flow control tests are conducted to verify the control effectiveness and understand the characteristics as a function of inlet throat Mach number, injection mass flow ratio, jet Mach number and momentum coefficient. At all test Mach numbers, microjet flow control （MFC） effectively improves the recovery and reduces the distortion intensity. Between inlet throat Mach number 0.2 and 0.5, the strong flow separation in the second S-turn is suppressed at an optimum jet flow ratio of less than 0.65%, resulting in a maximum improvement of 4% for pressure recovery coefficient and a maximum decrease of 75% for circumferential distortion intensity at cruise. However, in order to suppress the flow separation, the injection rate should retain in an effective range. When the injection rate is higher than this range, the flow is degraded and the distortion contour is changed from 90° circumferential distortion pattern to 180° circumferential distortion pattern. Detailed data analysis shows that this optimum flow ratio depends on inlet throat Mach number and the monlcntunl coefficient affects the control effectiveness in a dual stepping manner.

Surrogate-based modeling and dimension reduction techniques for multi-scale mechanics problems

Successful modeling and/or design of engineering systems often requires one to address the impact of multiple ＂design variables＂ on the prescribed outcome.There are often multiple,competing objectives based on which we assess the outcome of optimization.Since accurate,high fidelity models are typically time consuming and computationally expensive,comprehensive evaluations can be conducted only if an efficient framework is available.Furthermore,informed decisions of the model/hardware＇s overall performance rely on an adequate understanding of the global,not local,sensitivity of the individual design variables on the objectives.The surrogate-based approach,which involves approximating the objectives as continuous functions of design variables from limited data,offers a rational framework to reduce the number of important input variables,i.e.,the dimension of a design or modeling space.In this paper,we review the fundamental issues that arise in surrogate-based analysis and optimization,highlighting concepts,methods,techniques,as well as modeling implications for mechanics problems.To aid the discussions of the issues involved,we summarize recent efforts in investigating cryogenic cavitating flows,active flow control based on dielectric barrier discharge concepts,and lithium（Li）-ion batteries.It is also stressed that many multi-scale mechanics problems can naturally benefit from the surrogate approach for ＂scale bridging.＂

Experimental study on airfoil flow separation control via an air-supplement plasma synthetic jet

An air-supplement plasma synthetic jet(PSJ)actuator increases the air supplemental volume in the recovery stage and improves the jet energy by attaching a check valve to the chamber of a conventional actuator.To explore the flow control effect and mechanism of the air-supplement actuator,via particle image velocimetry experiments in a low-speed wind tunnel,the flow field and boundary layer characteristics of a two-dimensional airfoil surface under different actuation states were compared for different attack angles and jet orifices.The experimental results show that,compared with the conventional actuation state,the jet energy of the air-supplement PSJ is higher and the indirect mixing effect of the counter-vortex sequence produced by the jet-mainstream interaction is stronger.Furthermore,the boundary layer mixing effect is better,which can further suppress flow separation and improve the critical flow separation attack angle.Moreover,increasing the jet momentum coefficient can enhance the flow control effect.The findings of this study could provide guidance for the flow control application of air-supplement PSJs.

Circular cylinder wakes and their control under the influence of oscillatory flows: A numerical study

Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering.The wake formation and vortex shedding behind a variety of ocean structures such as spars,are subjected to fatigue failure limiting their life span.The additional influences due to ocean waves and currents further exacerbate these effects.In the present study,flow past an isolated circu-lar cylindrical structure subjected to an oscillatory upstream are numerically investigated.These studies involve high resolution simulations over the low Reynolds number range(100-200).Although the prac-tical range of interest is in high Reynolds number range of 103-105,the flow physics and a number of qualitative and quantitative aspects are similar to the low Reynolds number flows.In the high Reynolds number range,statistical averaging tools in conjunction with suitable closure models would be neces-sary.The control of wake vortices is achieved with the aid of two small rotors located in the aft of the main cylinder.A control algorithm was coupled to determine the quantum of actuation to the rotating elements.Although control of wake vortices was observed for harmonic in-let forcing,residual vortical structures were found to persist at higher amplitudes of oscillation.To study the efficacy of this control,numerical simulations were further extended,when the circular cylinder was flexibly mounted.The con-trol of flow induced vibrations was observed to be reasonably effective in controlling the wake generated behind the main cylinder due to oscillatory upstream.

Virtual flight test of pitch and roll attitude control based on circulation control of tailless flying wing aircraft without rudders

Circulation Control(CC) realizes rudderless flight control by driving compressed air jet to generate a virtual rudder surface, which significantly improves low detectability. The layout plan of combined control rudder surface is proposed based on the tailless flying wing aircraft. The closed-loop jet actuator system and stepless rudder surface switching control strategy are used to quantitatively study the control characteristics of circulation actuator for pitch and roll attitude through 3-DOF virtual flight test in a wind tunnel with a powered model at wind speed of 40 m/s. The results show that the combined use of circulation actuators can achieve bidirectional continuous and stable control of the aircraft’s pitch and roll attitude, with the maximum pitch rate of 12.3(°)/s and the maximum roll rate of 21.5(°)/s;the response time of attitude angular rate varying with the jet pressure ratio is less than 0.02 s, which can satisfy the control response requirements of aircraft motion stability for the control system;the jet rudder surface has a strong moment control ability, and the pitch moment of the jet elevator with a pressure ratio of 1.28 is the same as that of the mechanical elevator with 28° rudder deflection, which can expand the flight control boundary.

Numerical investigation of co-flow jet airfoil with parabolic flap

Both the Active Flow Control(AFC)and the variable-camber technology are considered as efficient ways to enhance the aerodynamic performance of an aircraft.The present study investigated the feasibility of the combination of a Co-Flow Jet(CFJ)airfoil and a parabolic flap,where the Reynolds Average Navier-Stokes(RANS)equations and the Spalart-Allmaras(S-A)turbulence model were exploited for the numerical simulation.Several significant geometric parameters,including the injection slot location,the suction slot location,the injection slot angle,the suction slot angle and the airfoil Suction Surface Translation(SST),were selected to study their effects on the aerodynamics of the proposed configuration.Then,an optimized design was created and compared with the baseline airfoil.The results show that the CFJ airfoil combined with the parabolic flap is more beneficial to the aerodynamic performance enhancement at small angles of attack.It is preferable to locate the injection slot at a 2%chord-wise location and the suction slot at a 75%chord-wise location.Both the decrease of the injection slot angle and the augmentation of the suction slot angle could reduce the drag.Furthermore,the SST of 0.5%chord is selected due to its high gain in the corrected aerodynamic efficiency at small angles of attack.Compared with the baseline,the optimized design could increase the lift coefficient and the corrected lift-to-drag ratio by 32.1%and 93.8%respectively at the angle of attack a=4°.

Control strategies for aircraft airframe noise reduction

With the development of low-noise aircraft engine, airframe noise now represents a major noise source during the commercial aircraft＇s approach to landing phase. Noise control efforts have therefore been extensively focused on the airframe noise problems in order to further reduce aircraft overall noise. In this review, various control methods explored in the last decades for noise reduction on airframe components including high-lift devices and landing gears are summarized. We introduce recent major achievements in airframe noise reduction with passive control methods such as fairings, deceleration plates, splitter plates, acoustic liners, slat cove cover and side-edge replacements, and then discuss the potential and control mechanism of some promising active flow control strategies for airframe noise reduction, such as plasma technique and air blowing/suction devices. Based on the knowledge gained throughout the extensively noise control testing, a few design concepts on the landing gear, high-lift devices and whole aircraft are provided for advanced aircraft low-noise design. Finally, discussions and suggestions are given for future research on airframe noise reduction.

Numerical Study of Dual Sweeping Jet Actuators for Corner Separation Control in Compressor Cascade

Unsteady behaviors are important issues in flow control of turbomachinery.Pulsed excitation or suction is widely investigated in compressor cascades.This paper presents a discussion on the unsteady flow control realized by dual sweeping jet actuator(SJA)located on the blade suction surface.The unsteady numerical simulations were utilized to study the effect of applying dual SJAs on controlling the corner separation.With the numerical results,the following conclusions could be drawn with current compressor cascade.A maximum total pressure loss coefficient reduction of 6.8%was obtained.The analysis of the flow field pointed out that the regulation mechanisms of the corner separation were different with each SJA.The SJA ahead achieved an interruption of the suction side boundary layer development and the rear SJA enhanced the interaction and entrainment between the excitation stream and the secondary flows.Meanwhile,the different unsteadiness structures of the flow field frequency spectrum compared with single SJA cases were identified.The first peak frequency corresponded to the difference of the two SJAs and the rest frequencies could be regulated to a base frequency and its harmonic frequencies.

Metamodeling-based parametric optimization of DBD plasma actuation to suppress flow separation over a wind turbine airfoil model

While dielectric-barrier-discharge(DBD)based plasma actuation systems have been successfully demonstrated to suppress massive flow separation over wind turbine blades to reduce the transient aerodynamic loadings acting on the turbine blades,it is still a non-trivial task to establish a best combination of various operating parameters for a DBD plasma actuation system to achieve the optimized flow control effectiveness.In the present study,a regression Kriging based metamodeling technique is developed to optimize the operating parameters of a DBD plasma actuation system for suppressing deep stall over the surface of a wind turbine blade section/airfoil model.The data points were experimentally obtained by embedding a nanosecond-pulsed DBD(NS-DBD)plasma actuator at the leading edge of the airfoil model.The applied voltage and frequency for the NS-DBD plasma actuation were used as the design variables to demonstrate the optimization procedure.The highest possible lift coefficient of the turbine airfoil model at deep stalled angles of attack(i.e.,α?=?22°and 24°)were selected as the objective function for the optimization.It was found that,while the metamodeling-based procedure could accurately predict the objective function within the bounds of the design variables with an uncertainty~?2%,a global accuracy level of~?97%was achieved within the whole design space.

Aerodynamic characteristics of co-flow jet wing with simple high-lift devices

Numerical investigations are conducted to explore the aerodynamic characteristics of three-dimensional Co-Flow Jet(CFJ) wing with simple high-lift devices during low-speed takeoff and landing. Effects of three crucial parameters of CFJ wing, i.e., angle of attack, jet momentum and swept angle, are comprehensively examined. Additionally, the aerodynamic characteristics of two CFJ configurations, i.e., using open and discrete slots for injection, are compared. The results show that applying CFJ technique to a wing with simple high-lift device is able to generate more lift,reduce drag and enlarge stall margin with lower energy expenditure due to the super-circulation effect. Increasing the jet intensity can reduce the drag significantly, which is mainly contributed by the reaction jet force. The Oswald efficiency factor is, in some circumstances, larger than one,which indicates the potential of CFJ in reducing induced drag. Compared with clean wing configuration, using CFJ technique allows the aerodynamic force variation less sensitive to the swept angle, and such phenomenon is better observed for small swept angle region. Eventually, it is interesting to know that the discrete slotted CFJ configuration demonstrates a promising enhancement in aerodynamic performance in terms of high lift, low drag and efficiency.

Experimental investigations for parametric effects of dual synthetic jets on delaying stall of a thick airfoil

A promising strategy of synthetic jet arrays （SJA） control for NACA0021 airfoil in preventing flow separation and delaying stall is investigated. Through aerodynamic forces, flowfield and velocity profiles measurements, it indicates that the synthetic jet （S J） could enlarge the mixing of the shear layer and then enhance the stability of boundary layer, resulting in scope reduction of the flow separation zone. Furthermore, the control effects of dual jet arrays positioned at 15%c （Actuator 1） and 40%c （Actuator 2） respectively are systematically investigated with different jet parameters, such as two typical relative phase angles and various incline angles of the jet. The jet closer to the leading edge of airfoil is more advantageous in delaying the stall of airfoil, and overall, the flow control performances of jet arrays are better than those of single actuator. At the angle of attack （AoA） just approaching and larger than the stall AoA, jet array with 180° phase difference could increase the lift coefficient more significantly and prevent flow separation. When momentum coefficient of the jet arrays is small, a larger jet angle of Actuator 2 is more effective in improving the maximum lift coefficient of airfoil. With a larger momentum coefficient of jet array, a smaller jet angle of Actuator 2 is more effective.

Novel roll effector based on zero-mass-flux dual synthetic jets and its flight test

The autonomous and controllable Dual Synthetic Jet Actuator(DSJA)is firstly integrated into the Unmanned Aerial Vehicle(UAV),and flight tests without the deflection of rudders are carried out to verify the viability of DSJA to control the attitudes of UAV during cruising.DSJA is improved into an actuator with two diaphragms and three cavities,which has higher energy levels.Actuators,differentially distributed on both sides of the wings,are installed on the trailing edge close to the wing tips.Flight tests,containing Differential Circulation Control(DCC)using double-side actuators,Positive Circulation Control(PCC)using left-side actuators and Negative Circulation Control(NCC)using right-side actuators,are implemented at cruising speed of 25 m/s.Results show that roll attitude control without rudders could be realized by DSJAs.DCC and NCC can generate the rightward roll and yaw angular velocity,prompting UAV to turn right.The stronger controlling ability can be achieved by DCC,with the maximum roll angular velocity of 15.62(°)/s.PCC can generate a rightward roll moment,but a leftward yaw moment will be produced at the same time.Leftward yaw induces the leftward rolling moment,which weakens the roll control effect,making UAV keep to yaw to the left with a small slope.

Aerodynamic Performance Improvement of a Highly Loaded Compressor Airfoil with Coanda Jet Flap

Coanda jet flap is an effective flow control technique,which offers pressurized high streamwise velocity to eliminate the boundary layer flow separation and increase the aerodynamic loading of compressor blades.Traditionally,there is only single-jet flap on the blade suction side.A novel Coanda double-jet flap configuration combining the front-jet slot near the blade leading edge and the rear-jet slot near the blade trailing edge is proposed and investigated in this paper.The reference highly loaded compressor profile is the Zierke&Deutsch double-circular-arc airfoil with the diffusion factor of 0.66.Firstly,three types of Coanda jet flap configurations including front-jet,rear-jet and the novel double-jet flaps are designed based on the 2D flow fields in the highly loaded compressor blade passage.The Back Propagation Neural Network(BPNN)combined with the genetic algorithm(GA)is adopted to obtain the optimal geometry for each type of Coanda jet flap configuration.Numerical simulations are then performed to understand the effects of the three optimal Coanda jet flaps on the compressor airfoil performance.Results indicate all the three types of Coanda jet flaps effectively improve the aerodynamic performance of the highly loaded airfoil,and the Coanda double-jet flap behaves best in controlling the boundary layer flow separation.At the inlet flow condition with incidence angle of 5°,the total pressure loss coefficient is reduced by 52.5%and the static pressure rise coefficient is increased by 25.7%with Coanda double-jet flap when the normalized jet mass flow ratio of the front jet and the rear jet is equal to 1.5%and 0.5%,respectively.The impacts of geometric parameters and jet mass flow ratios on the airfoil aerodynamic performance are further analyzed.It is observed that the geometric design parameters of Coanda double-jet flap determine airfoil thickness and jet slot position,which plays the key role in supressing flow separation on the airfoil suction side.Furthermore,there exists an optimal combination of front-jet and rear-jet mass flow ratios to achieve the minimum flow loss at each incidence angle of incoming flow.These results indicate that Coanda double-jet flap combining the adjust of jet mass flow rate varying with the incidence angle of incoming flow would be a promising adaptive flow control technique.