Active control of flow separation over an airfoil using synthetic jets

We perform large-eddy simulation of turbulent flow separation over an airfoil and evaluate the effectiveness of synthetic jets as a separation control technique. The flow configuration consists of flow over an NACA 0015 airfoil at Reynolds number of 896,000 based on the airfoil chord length and freestream velocity. A small slot across the entire span connected to a cavity inside the airfoil is employed to produce oscillatory synthetic jets. Detailed flow structures inside the synthetic-jet actuator and the synthetic-jet/cross-flow interaction are simulated using an unstructured-grid finite-volume large-eddy simulation solver. Simulation results are compared with the 2005 experimental data of Gilarranz et al., and qualitative and quantitative agreements are obtained for both uncontrolled and controlled cases. As in the experiment, the present large-eddy simulation confirms that synthetic-jet actuation effectively delays the onset of flow separation and causes a significant increase in the lift coefficient. Modification of the blade boundary layer due to oscillatory blowing and suction and its role in separation control is discussed.     

Transient Characterization of the Reattachment of a Massively Separated Turbulent Boundary Layer Under Flow Control

The transient dynamics of a high Reynolds number separated flow over a two-dimensional ramp submitted to pulsed fluidic control is investigated. A spanwise array of 22 round jets, located upstream of the flap leading edge, is used as actuator to generate co-rotating vortical structures. Simultaneous measurements of wall friction using hot-film anemometry and phase-averaged velocity using 2D2C PIV are conducted. The PIV plane encompasses the incoming boundary layer upstream the flap leading edge, the separation bubble and the natural reattachment region. The dynamics of the separated flow is studied for successive sequences of pulsed actuation. Pockets of turbulence are periodically generated by the separation process and pushed downstream. After the transition period, the controlled flow shows large amplitude oscillations around a steady mean, particularly for the separation area. The transient dynamics of the flow at the actuation activation is also studied. The separated flow is strongly modified by the actuation from the first pulse. Characteristic times of the transient dynamics can be determined by fitting a first-order model with delay on the data. For the reattachment, the dimensionless characteristic rising times defined as \(\tau _{r}^{+} = \tau _{r} ~ U_{0} ~/~ L_{sep}\) of 11.7 for the friction gain, 4.8 for the separation length and 4.1 for the first mode of a Conditional Proper Orthogonal Decomposition analysis of the phase-averaged velocity fields were found. These values are in good agreement with previous studies and are of particular interest for modeling the transients and for further closed-loop control applications.     

Numerical Investigation of Active Flow Control Around a Generic Truck A-Pillar

Large Eddy Simulations (LES) are conducted to study the actuated flow field around a bluff body. The model is a simplified section of a truck. The aim of the work is to model the separation of the flow acting at the front rounded corners, the so called A-pillars, and to minimize the separation of the flow by means of Zero Net Mass Flux synthetic jets. LES data show the interaction of the flow main structures, the separation mechanism and the effects of the actuation on the flow field. The flow is post processed using modal and frequency decompositions. Relevant results in terms of drag reduction were observed for the actuated flow. The principle flow mechanisms are discussed and an optimal actuation frequency, in terms of induced fluctuations and drag reduction, is identified.     

An experimental study of a radial wall jet formed by the normal impingement of a round synthetic jet

The flow field of a radial wall jet created by the impingement of a round synthetic jet normal to a flat surface was characterized using hot-wire anemometry. In the synthetic wall jets the width of the outer layer was observed to increase linearly with the radial distance along the wall, while the local maximum velocity varied inversely. The synthetic wall jet exhibits self-similar behavior as distinguished by the collapse of the mean and rms velocity profiles when normalized by the outer layer scaling variables. Increasing the actuator driving amplitude at a fixed frequency (i) increased the growth rate of the outer layer, and (ii) decreased the decay rate of the local velocity maximum. The flow field of the synthetic wall jet was dominated by vortical structures associated with the actuator driving frequency, and harmonics connected with the interaction of the produced vortex structures. For the actuator conditions investigated, neither the classical laminar nor fully turbulent analytical solutions for continuous wall jets were amenable to modeling the synthetic wall jet profile due to the transitional and unsteady nature of the synthetic wall jet.     

A PLIF and PIV study of liquid mixing enhanced by a lateral synthetic jet pair

In this paper, enhancement of mixing between two water streams of the same flow rate in a planar channel by means of a lateral synthetic jet pair is studied at a net flow Reynolds number of 83 using PLIF and PIV. The synthetic jet pair is operated 180° out-of-phase at a range of actuation frequencies and displacements, with the latter being characterized by the dimensionless stroke length. The extent of mixing is evaluated using PLIF data at a location further downstream in the mixing channel. It is found that at a fixed actuation frequency a higher dimensionless stroke length produces a better mixing, and as the actuation frequency increases a lower dimensionless stroke length is required to achieve a given mixing degree. At a sufficiently high frequency or dimensionless stroke length, a nearly homogenous mixing with a mixing degree greater than 0.9 can be obtained. A functional relationship between actuation frequency and dimensionless stroke length is also obtained by best fitting the experimental data, which can be used for selecting the synthetic jet operating conditions to ensure a good mixing. Furthermore, both PLIF and PIV results show that each synthetic jet actuation cycle produce two opposing vortex pairs, which play an important role in prompting mixing between the two fluid streams. The excellent mixing obtained at a high frequency or a high dimensionless stroke length is found to be largely caused by a strong interaction between these opposing vortex pairs.     

Experimental investigation of the interaction between a plane wall jet and a parallel offset jet

The dual-jet flow generated by a plane wall jet and a parallel offset jet at an offset ratio of d/w = 1.0 has been investigated using Particle Image Velocimetry (PIV). The particle images are captured, processed, and subsequently used to characterize the flow in terms of the 2D velocity and vorticity distributions. Statistical characteristics of the flow are obtained through ensemble averaging of 360 instantaneous velocity fields. Also presented is a time series of instantaneous flow fields to illustrate the dynamic interaction between the two jets. Results reveal that the near field of the flow is characterized by a periodic large-scale Karman-like vortex shedding similar to what would be expected in the wake of a bluff body. The existence of the Karman-like vortices results in periodic interactions between the two jets; in addition, these vortices produce noticeable impact on the jet outer layers, i.e., the free shear layer of the offset jet and the wall boundary layer of the wall jet. A schematic of vortex/shear layer interaction is proposed to illustrate the flow pattern.     

The Effect of Wall Normal Actuation on a Turbulent Boundary Layer

In this work, a series of direct numerical simulations are conducted to study the effect of wall normal spanwise homogeneous wall actuation on a turbulent boundary layer. The moving boundary is represented by a boundary data immersion technique. A parametric study was performed, varying the actuator length, the wall normal actuation amplitude and the actuation frequency. It was found that localized actuation, relying only on wall motion instead of requiring a plenum in the case of synthetic jets, generated a net momentum flux jet affecting the flow not only in the immediate vicinity of the actuator but also for a significant distance downstream. The cases with an actuator velocity of \( u^{+}_{act}=?20.1 \) showed a particularly pronounced effect on the boundary layer and resulted in a recirculation region.     

Control of flow distortion in offset diffusers using trapped vorticity

Controlled concentrations of trapped vorticity within an offset, subsonic (M_AIP ≤ 0.7) diffuser are explored for active suppression of flow distortion in joint experimental and numerical investigations. The coupling between trapped vorticity, used to model boundary-layer separation, and secondary-flow vortices is manipulated using an array of fluidic oscillating jets, which are spanwise distributed just upstream of the trapped vortex. Actuation energizes the separated shear layer, reducing the size of separation and effecting an earlier reattachment of the boundary layer, which favorably effects the flow field downstream of reattachment. It is shown that optimal interactions between actuation and the trapped vortex fully suppress the central vortex pair, and redistributes the residual vorticity around the diffuser's circumference. This results in a 68% reduction in circumferential distortion at the Aerodynamic Interface Plane (AIP), using an actuation mass flow rate that is only 0.25% of the diffuser mass flow rate.     

Towards the Design of Synthetic-jet Actuators for Full-scale Flight Conditions

Despite the proven capability of synthetic-jet actuators in delaying boundary-layer separation in laboratory experiments, a capability that allows the geometry and operating conditions of these devices to be designed and selected for maximum flow-control effectiveness in full-scale flight conditions has yet to be developed. In this two-part paper, the key results obtained during a 3-year research programme aiming at establishing such a capability based on a better understanding of the fluid mechanics of synthetic jets and an improved modelling capacity are reported. In Part 1 of this paper, the experimental studies of the behaviour of synthetic jets in both quiescent flow and a boundary layer are described. The work has led to an improved understanding of the dimensionless parameters that determine the formation and development of vortex rollup and how the strength of rollup can be enhanced by optimizing the geometry and operating condition. Based on the study of the nature of vortical structures produced as the result of the interaction with a boundary layer and their impact in the near-wall region where flow control is desired, the conditions for producing effective vortical structures for delaying flow separation were established. The finding from this work forms the basis of a number of criteria used for designing synthetic jet actuators for full-scale flight condition to be presented in Part 2.     

Numerical and experimental study of two turbulent opposed plane jets

The turbulent interaction between two opposed plane jets separated by a distance H is experimentally studied by using a PIV (Particle Image Velocimetry) method and numerically investigated by means of a finite volume code. Two turbulence models have been tested: the standard k-ɛ model and a second-order model. The validation of the numerical study was performed by comparing the results with experimental data obtained for the case of two interacting opposed jets at ambient temperature (isothermal case). The effect of the angle of inclination of the jets is studied. Conclusions of the validation are then used to study the interaction between two jets, one being maintained at ambient temperature whereas the other is heated. Results show that the stagnation point moves towards the heated jet. It is shown that the heating induces a stabilizing effect on the flow.     

等离子体激励气动力学探索与展望

等离子体激励气动力学是研究等离子体激励与流动相互作用下, 绕流物体受力和流动特性以及管道内部流动规律的科学, 属于空气动力学、气体动力学与等离子体动力学交叉前沿领域. 等离子体激励是等离子体在电磁场力作用下运动或气体放电产生的压力、温度、物性变化, 对气流施加的一种可控扰动. 局域、非定常等离子体激励作用下, 气流运动状态会发生显著变化, 进而实现气动性能的提升. 国际上对介质阻挡放电等离子体激励、等离子体合成射流激励及其调控附面层、分离流动、含激波流动等开展了大量研究. 等离子体激励调控气流呈现显著的频率耦合效应, 等离子体冲击流动控制是提升调控效果的重要途径. 发展高效能等离子体激励方法, 通过等离子体激励与气流耦合, 激发和利用气流不稳定性, 揭示耦合机理、提升调控效果, 是等离子体激励气动力学未来的发展方向.      

Numerical study of spatial supersonic flow of a perfect gas with transverse injection of jets

Three-dimensional supersonic turbulent flow in the presence of symmetric transverse injection of round jets through slots in the walls is studied numerically. The simulation is based on Favre averaged Navier-Stokes equations solved using the Beam-Warming method. The influence of the ratio of the pressure in the jet to that in the flow (pressure ratio) on the spatial interaction of the injected jet with the oncoming flow is studied. Experimental pressure distributions on the wall near the jet approximated by curvilinear closed ellipses are reproduced numerically. The mechanism of the formation of two symmetric vortices in the mixing layer between the jet and the oncoming flow is studied. The results of the calculations are found to be in satisfactory agreement with the experimental dependence of the length of the separation zone on the pressure ratio of the jet to the flow.     

Space–time correlation measurements of high-speed axisymmetric jets using optical deflectometry

An optical deflectometry system is used to provide unique space–time correlation measurements at two positions separated by varying axial distances within a high-speed jet shear layer. The measurements were made for both pure air and for helium/air mixture jets at Mach numbers M=0.9 and M=1.5. The jets issue from round nozzles and the sensing volumes at the two measurement positions consist of small light filaments along spanwise lines that are tangential to the annular jet shear layer. Applying this technique to obtain measurements detailing the level of correlation, spectral content, and convection velocity for jet flows in these flow regimes near the end of the potential core is particularly important in the understanding and prediction of jet noise. Measurements near the end of the potential core along the jet lip line exhibit distinct cross-correlation curves for the pure air jet cases. However, helium/air mixture jets display much lower levels of correlation and little evidence of large-scale structure in the measured spectra. It is believed that the thick visual density gradients dominated by smaller scales throughout the shear layer of the helium/air mixture jets effectively mask the large-scale structure, thus, reflecting a limitation of this optical deflectometer. Finally, a decrease in normalized convection velocity with helium addition is observed.     

Wavepacket models for subsonic twin jets using 3D parabolized stability equations

An extension of the classical parabolized stability equations to flows strongly dependent on the two cross-stream spatial directions and weakly dependent on the streamwise one is applied to model the large-scale structures present in twin-jet configurations. The existence of these unsteady flow structures, usually referred to as wavepackets, has been demonstrated in the literature for both subsonic and supersonic round jets, along with their relation to the generation of highly directional noise emitted in the aft direction. The present study considers twin-jet configurations with different separations at high Reynolds number and subsonic conditions. The existing instability modes for the twin-jet mean flow, their dependence on the separation of the two jets, and the interaction between the wavepackets originating from the two jets is investigated here. Arising from the axisymmetric mode for single round jets, two dominant modes are found for twin jets: a varicose one, relatively insensitive to jets' proximity, but likely to be efficient in radiating noise; a sinuous one, whose amplification is strongly dependent on the jets' distance, and which can be expected to produce weaker acoustic signatures.     

热激励器对超声速圆管射流的控制机理

采用大涡模拟方法研究了热激励器对马赫1.3 超声速圆管射流涡结构的影响, 采用加入净热源的方法模拟了热激励器的热效应, 分析了不同激励模态(m =±1 和m =±4) 的降噪和增加射流掺混的效果. 研究发现: (1)射流在受到激励作用时, 产生了更大的径向和周向速度扰动, 这有利于流向涡的形成, m =±1 激励模态下流向涡的发展更快, 特别是在摆动面上会有更大的增长速度; (2) 气动噪声分布的频带很宽, 激励作用对抑制高频气动噪声有较强的作用; (3) 对于在喷管壁面处产生的固壁噪声, m =±4 模态下的激励作用一定程度上加强了固壁噪声.      

Active flow control applications with a jet and vortex actuator in a laminar cross flow

In this study the effect of an oscillatory, zero-net-mass flux device called a Jet and Vortex Actuator (JaVA) on the laminar boundary layer is investigated. The JaVA can be utilized to energize the boundary layer over a flat plate by creating jets and vortices thus it can delay or prevent boundary layer separation if it is used properly.It appears to produce qualitatively different flow regimes depending on its actuation parameters, e.g. frequency and amplitude or subtle changes in geometry such as the position of the actuator plate with respect to the JaVA-cavity. This latter effect was only discovered recently. Since little is known about the underlying fluid dynamics and because of a complete lack of unsteady data, a device is designed and built for experiments in water. Unsteady flow fields have been recorded for visualization and furthermore quantitative evaluation by means of Particle Image Velocimetry (PIV) has been carried out in addition to numerical simulations. Results show that the JaVA-induced vortices ejected into the flat plate boundary layer significantly enhance the velocity profiles and its characteristics such as the displacement thickness and the momentum thickness if the plate is oscillating at high frequencies as it is flush-mounted or inside the cavity. But if the plate is extracted out of the cavity then there is no improvement in the flow fields hence separation can be delayed or prevented for long downstream distances only if the actuation parameters and plate positions are selected properly.     

Drag reduction on the 25° slant angle Ahmed reference body using pulsed jets

This paper highlights steady and unsteady measurements and flow control results obtained on an Ahmed model with slant angle of 25° in wind tunnel. On this high-drag configuration characterized by a large separation bubble along with energetic streamwise vortices, time-averaged and time-dependent results without control are first presented. The influence of rear-end periodic forcing on the drag coefficient is then investigated using electrically operated magnetic valves in an open-loop control scheme. Four distinct configurations of flow control have been tested: rectangular pulsed jets aligned with the spanwise direction or in winglets configuration on the roof end and rectangular jets or a large open slot at the top of the rear slant. For each configuration, the influence of the forcing parameters (non-dimensional frequency, injected momentum) on the drag coefficient has been studied, along with their impact on the static pressure on both the rear slant and vertical base of the model. Depending on the type and location of pulsed jets actuation, the maximum drag reduction is obtained for increasing injected momentum or well-defined optimal pulsation frequencies.     

Genetic-algorithm-based artificial intelligence control of a turbulent boundary layer

An artificial intelligence (AI) open-loop control system is developed to manipulate a turbulent boundary layer (TBL) over a flat plate, with a view to reducing friction drag. The system comprises six synthetic jets, two wall-wire sensors, and genetic algorithm for the unsupervised learning of optimal control law. Each of the synthetic jets through rectangular streamwise slits can be independently controlled in terms of its exit velocity, frequency and actuation phase. Experiments are conducted at a momentum-thickness-based Reynolds number Re_θ of 1450. The local drag reduction downstream of the synthetic jets may reach 48% under conventional open-loop control. This local drag reduction rises to 60%, with an extended effective drag reduction area, under the AI control that finds optimized non-uniform forcing. The results point to the significant potential of AI in the control of a TBL given distributed actuation.

Graphical abstract

     

Fluidic control of wake-flow behind a two-dimensional square back bluff body

This experimental study deals with wake-flow fluidic control behind a two-dimensional square back geometry positioned close to the ground. The fluidic control system is made of pulsed jets positioned at the upper edge of the model base. The objective of the fluidic action is to modify the wake-flow development, and as a consequence the static pressure distribution over the model base and hence the pressure drag. The main concern of this study is to determine to what extent the presence of a flow confined between the model and the floor influences the effectiveness of the control. Static pressure measurements at the model base and wake-flow characteristics derived from PIV measurements at a high acquisition frequency indicate global similarities between a case where an underbody flow exists and a case where this underbody flow is absent. For low actuation frequencies, discrepancies in the way the coherent structures due to the control develop in the shear layer appear.     

Modified flapping jet for increased jet spreading using synthetic jets

The present paper is an experimental investigation, using a PIV system, on modified rectangular jet flow co-flowing with a pair of synthetic jets placed symmetrically with respect to the geometric centerline of the main flow. The objective was to determine the optimal forcing conditions that would result in jet spreading beyond what would be obtained in a simple flapped jet. The main jet had an exit Re_h = 36,000, based on the slot height, h. The synthetic jets were operated in a periodic manner with a periodic momentum coefficient of about 3.3% and at a frequency of the main jet preferred mode. A short, wide angle diffuser of half angle of about 45° was attached to the main jet. Generally for the vectored jet, much of the flow features found here resembled those reported in the literature except that the deflection angle in this study increased with downstream distances inside the diffuser and then remained roughly unchanged thereafter. Larger jet spreading was achieved when the main jet was subjected to simultaneous actuation of the synthetic jets but the flow did not achieve the initial jet spreading that was observed in the vectored jet. Further jet spreading was achieved when the synthetic jets were alternately actuated in which each synthetic jet was actuated for a number of cycles before switching. This technique allowed the jet to flap across the flow between transverse positions larger than what would be obtained in a simple flip-flop jet. Under the present flow geometry and Reynolds number, it was found that when the ratio f_s/f_al, where f_s is the synthetic actuation frequency and f_al is the alternating frequency, was larger than 10, the mean streamwise velocity of the main jet had two peaks symmetrically placed with respect to the jet axis and the jet had the appearance of flowing into two streams each moving nearly parallel to the diffuser wall. For a value of f_s/f_al of about 10, the optimal value in this study, the desired flow properties were achieved in that, the mean velocity was nearly uniform with an increase in the jet width compared to the simultaneous actuations, and the jet flapping was more effective in redistributing and homogenizing the turbulent kinetic energy across the main jet.