基于MEMS的流动主动控制技术及其研究进展

MEMS技术与流动控制技术的结合, 使得流动主动控制技术的实际应用逐步成为现实, 极大地推动了流动主动控制技术的发展. 简述了流动主动控制技术的基本原理、关键技术, 以及应用MEMS技术实现流动主动控制的机理和途径. 介绍了几家国外研究机构近年来在流动主动控制技术领域基于MEMS技术的微传感、微控制和微执行技术及其集成技术的研究进展, 以及在三角翼前沿涡控制、减阻研究、发动机喷流控制、细长体背风面分离流控制等方面的应用情况.      

正弦交流介质阻挡放电等离子体激励器诱导流场研究的进展与展望

正弦交流介质阻挡放电等离子体流动控制技术是基于等离子体激励的主动流动控制技术,具有响应时间短、结构简单、能耗低、不需要额外气源装置等优点,在飞行器增升减阻、抑振降噪、助燃防冰等方面具有广阔的应用前景.针对“激励器消耗的大部分能量尚未被挖掘利用、诱导流场的完整演化过程尚未完全掌握、诱导流场的演化机制尚不明确”这三方面问题,本文首先从激励器诱导流场的空间结构、时空演化过程、演化机制三个方面回顾总结了激励器诱导流场的研究进展.在诱导流场空间结构方面,发现了高电压激励下诱导射流的湍流特性,辨析了壁面拟序结构与无量纲激励参数之间的关联机制;从激励器诱导声能方面挖掘出了激励器潜在的能量,发现了“等离子体诱导超声波与诱导声流”的新现象,提出了声激励机制;在时空演化过程方面,阐明了激励器诱导流场从薄型壁射流发展为“拱形”射流、再演变为启动涡,最终形成准定常射流的完整演化过程;在演化机制方面,结合声学特性提出了以“升推”为主的诱导流场演化机制.其次,围绕激励器诱导流场,进一步凝练出下一步研究重点,为突破等离子体流动控制技术瓶颈,打通“概念创新—技术突破—演示验证”的创新链路,实现工程应用提供支撑.     

多段翼型吹气流动分离控制研究

基于雷诺平均Navier-Stokes粘性流动方程,采用数值模拟方法,分析了吹气控制对多段翼型气动性能的影响,阐述了吹气改善多段翼型流动的机理。采用有限体积法对雷诺平均Navier-Stokes方程进行空间离散,时间方向推进采用二阶迎风格式,湍流模型采用SST k-ω模型。结果表明:在多段翼型基础上采取吹气控制可以获得很好的气动增升效果,三段翼型的最大升力系数可达4.98;吹气可改善多段翼型表面流动,减小其流动分离,增加升力;在同样的吹气口几何参数条件下,在一定范围内增大吹气动量系数可以提高多段翼型的升力系数;在多段翼型主翼后段和襟翼同时施加吹气流动控制可以获得更好的效果,升力系数比基本三段翼型(基本构型A)增加30.05%。     

机翼尺度效应对等离子体分离流动控制特性的影响

等离子体流动控制技术是一种以等离子体气动激励为控制手段的主动流动控制技术. 为了进一步提高等离子体激励器可控机翼尺度, 以超临界机翼SC(2)-0714大迎角分离流为研究对象, 以对称布局介质阻挡放电等离子体为控制方式, 以测力、粒子图像测速仪为研究手段, 从等离子体激励器特性研究出发, 深入开展了机翼尺度效应对等离子体控制的影响研究, 提出了适用于分离流控制的能效比系数, 探索了分离流等离子体控制机理, 掌握了机翼尺度对分离流控制的影响规律. 结果表明: (1)随着机翼尺度的增大, 布置到机翼上的激励器电极长度会相应增加; 在本文的参数研究范围内, 激励器的平均消耗功率不会随电极长度的增加而线性增大; 当电极长度达到一定阈值时, 激励器的平均消耗功率趋于定值; (2)在固定雷诺数的情况下, 随着机翼尺度的增大, 等离子体的控制效果并未降低, 激励器能效比系数提高; (3)等离子体在主流区诱导的大尺度展向涡与在壁面附近产生的一系列拟序结构成为分离流控制的关键. 研究结果为实现真实飞机的等离子体分离流控制, 推动等离子体流动控制技术工程化应用提供了技术支撑.      

电磁力控制翼型绕流分离的增升减阻效率研究

电磁力可有效对流体流动进行控制,增升减阻,抑制流动分离,制约其推广应用的瓶颈为控制效率问题.为提高其控制效率,基于翼型绕流的电磁力控制,对电磁力增升减阻的控制效率问题进行数值研究. 根据能量守恒定律,推导电磁力控制能耗的比,基于升力和阻力计算节省能量. 定义电磁力的控制效率为能量节省与电磁力控制所需能耗的比值,研究不同工况下电磁力增升减阻的控制效率. 发现在控制开始阶段,电磁力能量主要消耗在增加边界层流体的动能上,电磁力控制效率非常低,但电磁力控制效率会随着电磁力工作时间的增长而增加;电磁力控制效率随着来流速度的增加呈指数下降;通过增加电磁力激活板的输入能量可增强电磁力的控制效果,但无法明显增加其控制效率.      

局部弹性翼型非定常分离的动力学特性

对在低雷诺数下局部弹性翼型绕流中, 局部弹性导致的自激振动所产生的复杂非定常流动分离现象和描述方法进行了分析. 采用ALE-CBS方法数值模拟了具有可动边界的绕流流场问题, 同时采用Galerkin方法求解局部弹性结构的控制方程. 着重研究了翼型的局部弹性对流动分离和翼型性能的影响, 并分别从Eulerian和Lagrangian的角度分析了局部弹性结构导致的不同非定常分离现象, 其中Lagrangian角度可以方便地揭示出局部弹性翼型大幅度提高升力的机理和流动中的能量迁移. 结果表明翼型的局部弹性对非定常分离和分离泡的演化过程有着明显的影响, 可以使得流体质点由主流获取动量实现再附, 并且在一定的攻角下可以将固定分离转变为移动分离, 从而明显地提高了翼型的升力.      

风力机翼型动态失速等离子体流动控制数值研究

针对动态失速引起的风力机翼型气动性能恶化的问题,本文基于动网格和滑移网格技术, 开展了大涡模拟数值计算研究,探索了非定常脉冲等离子体的动态流动控制机理. 结果表明,等离子体气动激励能够有效控制翼型动态失速, 改善平均和瞬态气动力,减小力矩负峰值和迟滞环面积. 压力分布在等离子体施加范围内出现了负压"凸起",上翼面吸力峰值明显增大.脉冲频率和占空比这两个非定常控制参数对流动控制影响显著,无因次脉冲频率为1.5时等离子体控制效果较好,占空比为0.8时即可接近连续工作模式下的气动收益. 翼型深失速状态,等离子体促使流动分离位置明显向后缘移动, 抵抗了大尺度动态失速涡的发生,分离涡结构破碎耗散、重新附着, 涡流影响范围减小; 浅失速状态,等离子体激励具有较强的剪切层操纵能力, 诱导了翼型边界层提前转捩,促进了与主流的动量掺混. 等离子体气动激励诱导出前缘附近贴体翼面"涡簇",起到了虚拟气动外形的作用.不同尺度、频域的动态涡结构与等离子体气动激励的非线性、强耦合作用导致了气动力/力矩的谐波振荡.      

合成射流技术及其在流动控制中应用的进展

流动控制是流体技术最主要的研究领域,21世纪的空气动力学将在流动控制领域取得重大突破; 合成射流是一种基于旋涡运动的零质量射流,是流动控制领域近10年来最热门活跃的流动主动控制技术.首先介绍了合成射流激励器及激励器的基本类型,并概括了激励器的主要发展方向,即宽频域、高动量、``强壮'的合成射流激励器和微小型激励器.尔后,对合成射流激励器工作原理、合成射流结构以及合成射流独特的流场特征和合成射流技术的特点进行了综述.最后着重对合成射流技术主要和潜在应用,如流动分离及气动力控制、射流矢量控制、增强掺混及加强传热和传质、抑制噪声、微流体控制、飞行控制以及粒子的散布控制、合成射流陀螺仪技术等进行了介绍和综述,同时对其在各应用领域的控制机理进行了归纳总结.      

飞翼布局无人机流动分离控制及机理分析

以小展弦比飞翼式无人机为对象,开展了基于零质量射流的主动流动控制数值模拟研究. 比较分析了应用零质量射流前后飞翼式无人机纵向气动特性的改善效果,并通过流场特征的分析探讨了流动控制技术产生气动增益的原因. 研究结果表明在模型中等迎角、大迎角范围,零质量射流技术可以显著增加升力系数,最大幅值达25%,并且拓宽了纵向力矩的线性范围. 机理分析表明,零质量射流扰动通过提高模型绕流场的边界层掺混,增强附面层内外的动量输运,使得附面层有足够的能量克服逆压梯度和黏性损耗,从而达到减缓流动分离甚至使分离流再附的目的.      

UNSTEADY SEPARATION OF FLOW AROUND AIRFOIL WITH LOCAL ELASTIC STRUCTURE

对在低雷诺数下局部弹性翼型绕流中,局部弹性导致的自激振动所产生的复杂非定常流动分离现象和描述方法进行了分析．采用ALE—CBS方法数值模拟了具有可动边界的绕流流场问题,同时采用Galerkin方法求解局部弹性结构的控制方程．着重研究了翼型的局部弹性对流动分离和翼型性能的影响,并分别从Eulerian和Lagrangian的角度分析了局部弹性结构导致的不同非定常分离现象,其中Lagrangian角度可以方便地揭示出局部弹性翼型大幅度提高升力的机理和流动中的能量迁移．结果表明翼型的局部弹性对非定常分离和分离泡的演化过程有着明显的影响,可以使得流体质点由主流获取动量实现再附,并且在一定的攻角下可以将固定分离转变为移动分离,从而明显地提高了翼型的升力．     

Effects of Periodic Excitation on the Flow Around a D-Shaped Cylinder at Low Reynolds Numbers

The baseline and forced flow around a bluff body with semi-elliptical D-shape was investigated by solving the 2D Navier–Stokes equations at low Reynolds numbers. A D-shape rather than the canonic circular-cylinder was selected due to the fixed separation points in the latter, enabling to study a pure wake rather than boundary-layer control. The correlation between Strouhal and Reynolds numbers, the mean drag, the lift and drag oscillations vs. the Reynolds number and wake structure were investigated and compared to experimental and numerical data. Effects of open-loop forcing, resulting from the influence of zero-mass-flux actuators located at the fixed separation points, were studied at a Reynolds number of 150. Fluidic rather than body motion or volume forcing was selected due to applicability considerations. The motivation for the study was to quantify the changes in the flow field features, as captured by Proper Orthogonal Decomposition (POD) analysis, due to open-loop forcing, inside and outside the “lock-in” regime. This is done in order to evaluate the suitability of low-order-models based on POD modes of this changing flow field, for future feed-back flow control studies. The evolution of the natural and the excited vortices in the Kármán wake were also investigated. The formation and convection regions of the vortex evolution were documented. It was found that the forcing causes an earlier detachment of the vortices from the boundary-layers, but does not affect their circulation or convection speeds. The results of the POD analysis of the near-wake flow show that the influence of the bluff body shape (“D”-shaped versus circular cylinder) on the baseline POD wake modes is small. It was found that the eigenfunctions (mode-shapes) of the POD velocity modes are less sensitive to slot excitation than the vorticity modes. As a result of the open-loop excitation, two types of mode-shape-change were observed: a mode can be exchanged with a lower-energy mode or shifted to a low energy level. In the latter case, the most energetic mode becomes the “actuator” mode. The evolution of one-slot excitation on still fluid (“Synthetic jet”) was studied and compared to published data and to “actuator” modes with external flow present. Based on the current findings, it is hypothesized that the cross-flow velocity POD modes are suitable for feedback control of wake flow using periodic excitation, due to their low sensitivity to the excitation as compared to the streamwise velocity or vorticity modes.     

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.     

二维扩压叶栅非定常分离流控制途径探索

二维扩压叶栅非定常黏性数值模拟结果表明,在一定攻角范围内,叶片前缘点附近的周期性吹吸气激励能有效控制混乱的非定常分离流．详细研究了非定常激励频率、幅值、位置对流场的影响．满足一定条件的非定常激励能够使流动由无序变为有序,时均气动性能提高。     

The Influence of Periodic Excitation on a Turbulent Separation Bubble

Turbulent separation limits the performance in many engineering applications, for example creating pressure losses in diffuser like flows or stall on aircraft wings. In the present study the turbulent boundary layer flow over a flat plate separating due to an adverse pressure gradient is studied as a model problem and the effect of periodic excitation in both time and space is investigated through direct numerical simulations. Linear stability analysis is used to analyse the sensitivity of the flow with respect to time-periodic excitations. The dependence on position, amplitude and frequency of the forcing is investigated. For a certain frequency range at sufficiently high amplitudes, it is possible to eliminate the separated region. Furthermore, three-dimensional effects are studied by applying a steady spanwise forcing as well as a both time-dependent and spanwise varying forcing. A forcing varying in spanwise direction is shown to be the most effective in eliminating the separated region, whereas two-dimensional time-periodic excitation was not as efficient as it was expected.     

分离流动激振控制的实验研究

综述了二维后向台阶、翼型以及轴墩称钝体等典型分离流动激振控制的实验研究．分析了激振控制分离流动的作用机理,得到了一个具有普适性的最佳激振频率范围．另外,介绍了目前常用的激振手段,并对其发展方向进行了展望      

The application of the gradient-based adjoint multi-point optimization of single and double shock control bumps for transonic airfoils

A shock control bump (SCB) is a flow control method that uses local small deformations in a flexible wing surface to considerably reduce the strength of shock waves and the resulting wave drag in transonic flows. Most of the reported research is devoted to optimization in a single flow condition. Here, we have used a multi-point adjoint optimization scheme to optimize shape and location of the SCB. Practically, this introduces transonic airfoils equipped with the SCB that are simultaneously optimized for different off-design transonic flight conditions. Here, we use this optimization algorithm to enhance and optimize the performance of SCBs in two benchmark airfoils, i.e., RAE-2822 and NACA-64-A010, over a wide range of off-design Mach numbers. All results are compared with the usual single-point optimization. We use numerical simulation of the turbulent viscous flow and a gradient-based adjoint algorithm to find the optimum location and shape of the SCB. We show that the application of SCBs may increase the aerodynamic performance of an RAE-2822 airfoil by 21.9 and by 22.8 % for a NACA-64-A010 airfoil compared to the no-bump design in a particular flight condition. We have also investigated the simultaneous usage of two bumps for the upper and the lower surfaces of the airfoil. This has resulted in a 26.1 % improvement for the RAE-2822 compared to the clean airfoil in one flight condition.     

Resonant phenomena in a transonic flow around an airfoil with pulsed periodic energy supply

Interaction of a pulsed periodic source of energy with a closing shock wave arising near airfoils in transonic flight is studied. The evolution of the shock-wave structure of the flow around a symmetric airfoil is examined by solving two-dimensional unsteady gas-dynamic equations, and a resonant mechanism of interaction is found, which leads to considerable (by an order of magnitude) reduction of the wave drag of the airfoil.     

SBLI control for wings and inlets

Flow control can be applied to shock wave/boundary layer interactions to achieve two different goals;the delay of shock-induced separation and/or the reduction ofstagnation pressure losses, which cause wave drag or inletinefficiencies. This paper introduces the principles and maintechniques for both approaches and assesses their relativesuitability for practical applications. While boundary layersuction is already in wide use for separation control, themost promising novel device is the micro-vortex generator,which can deliver similar benefits to traditional vortex generatorsat much reduced device drag. Shock control is notyet used on practical applications for a number of reasons,but recent research has focused on three-dimensional deviceswhich promise to deliver flow control with improved offdesignbehaviour. Furthermore, there are some indicationsthat a new generation of control devices may be able to combinethe benefits of shock and boundary layer control andreduce shock-induced stagnation pressure losses as well asdelay shock-induced separation.     

On the cylinder wake subjected to in-phase and out-of-phase periodic forcing

Experiments were conducted in a wind tunnel to investigate the effect of periodic forcing on the wake of a circular cylinder specially designed for independent control of the phase of forcing from slits located on opposite sides. Tests were conducted at Reynolds number (Re) of 12,000, based on diameter (D). Wake was forced at subharmonic and harmonic frequencies at constant blowing coefficient. Measurements consisted of wake pitot surveys, hotwire anemometry, and Particle Image Velocimetry. A 20% reduction in drag was achieved when the two slits operated in-phase at third harmonic. The drag reduction, however, was 18% when the two slits operated in an out-of-phase mode for the same frequency. Wake lock-in was observed for the out-of-phase forcing case. Wake power spectrum indicated the effectiveness of both types of forcing in suppressing the primary instability. This was also evident from the results of the Proper Orthogonal Decomposition of the PIV data and turbulence statistics.     

多喷口高效能厚翼的研究

提出了以下高效能翼型的思想：用多喷口小速度切向吹气控制厚翼上的流动分离,使流动接近于理想流状况,以产生大升力,小阻力;因多喷口小速度吹气耗能小,故翼型的有效升阻比可以很大．基于雷诺平均N－S方程进行了数值模拟实验．主要结果表明：对于厚度为0.4的儒氏翼型,在升力系数高达3．5时,有效升阻比可达约50（单喷口吹气约为23）;对于厚度为0．4的"升力体"翼型,在升力系数达2．2时,有效升阻比可达40（喷口吹气约为10）．