涡流发生器对细长体侧向力的控制研究

细长体在大攻角情况下会出现很大的侧向力. 在10°半顶角细长旋成体头部安置涡流发生器（微型三角翼）,通过调节涡流发生器相对模型轴线的滚转安装角,实现了对大迎角状态下旋成体侧向力的近似比例控制.研究了涡流发生器半展长、后掠角和攻角等参数对侧向力控制效果的影响. 研究发现半展长为6mm、后掠角为45°的涡流发生器具有较好的控制效果,在一系列攻角下均能够实现侧向力的近似比例控制. 由于该机构非常简单,在工程中具有一定的应用前景.      

Development, modeling and deflection analysis of hybrid micro actuator with integrated thermal and piezoelectric actuation

Micro actuators are irreplaceable part of motion control in minimized systems. The current study presents an analytical model for a new Hybrid Thermo Piezoelectric micro actuator based on the combination of piezoelectric and thermal actuation mechanisms. The micro actuator structure is a double PZT cantilever beam consisting of two arms with different lengths. The presented micro actuator uses the structure of electrothermal micro actuator in which polysilicon material is replaced by PZT. Also the voltage and poling directions are considered in the lengthwise of PZT beams. As a result, the piezoelectric actuation mechanism is based on d ₃₃ strain coefficient. The tip deflection of micro actuator is obtained using Timoshenko beam theory. Analytical results are compared with FEM results along with other reported results in the literature. The effects of geometrical parameters and PZT material constants on actuator tip deflection are studied to provide an efficient optimization of HTP micro actuator.     

Prediction of asymmetric vortical flows around slender bodies using Navier-Stokes equations

Steady and unsteady asymmetric vortical flows around slender bodies at high angles of attack are solved using the unsteady, compressible, this-layer Navier-Stokes equations. An implicit, upwind-biased, flux-difference splitting, finite-volume scheme is used for the numerical computations. For supersonic flows past point cones, the locally conical flow assumption has been used for efficient computational studies of this phenomenon. Asymmetric flows past a 5° semiapex-angle circular cone at different angles of attack, free-stream Mach numbers, and Reynolds numbers has been studied in responses to different sources of disturbances. The effects of grid fineness and computational domain size have also been investigated. Next, the responses of three-dimensional supersonic asymmetric flow around a 5° circular cone at different angles of attack and Reynolds numbers to short-duration sideslip disturbances are presented. The results show that flow asymmetry becomes stronger as the Reynolds number and angles of attack are increased. The asymmetric solutions show spatial vortex shedding which is qualitatively similar to the temporal vortex shedding of the unsteady locally conical flow. A cylindrical afterbody is also added to the same cone to study the effect of a cylindrical part on the flow asymmetry. One of the cases of flow over a cone-cylinder configuration is validated fairly well by experimental data.     

Experimental damping of boundary-layer oscillations using DBD plasma actuators

In the present work artificially excited Tollmien–Schlichting waves were cancelled using plasma actuators operated both in continuous and pulsed modes. To achieve this a vibrating surface, driven by an electromagnetic turbulator, was flush-mounted in a flat plate to excite the TS waves. These were amplified by an adverse pressure gradient induced by an insert on the upper wall of the test section. Control plasma actuators positioned downstream of the excitation actuator attenuate the waves by imparting a steady or unsteady force into the boundary-layer. In the case with steady actuation the two actuators change the velocity profile of the laminar boundary-layer, which then attenuates the waves by itself. In the case of pulsed actuation the actuator creates an unsteady body force to counteract directly the oscillation. As a result the amplitude of the velocity fluctuations at the excitation frequency is reduced significantly in both cases. The principles and the results of the two sets of experiments are presented and discussed.     

Exploration of plasma-based control for low-Reynolds number airfoil/gust interaction

Large-eddy simulation (LES) is employed to investigate the use of plasma-based actuation for the control of a vortical gust interacting with a wing section at a low Reynolds number. Flow about the SD7003 airfoil section at 4° angle of attack and a chord-based Reynolds number of 60,000 is considered in the simulation, which typifies micro air vehicle (MAV) applications. Solutions are obtained to the Navier–Stokes equations that were augmented by source terms used to represent body forces imparted by the plasma actuator on the fluid. A simple phenomenological model provided these body forces resulting from the electric field generated by the plasma. The numerical method is based upon a high-fidelity time-implicit scheme and an implicit LES approach which are used to obtain solutions on a locally refined overset mesh system. A Taylor-like vortex model is employed to represent a gust impinging upon the wing surface, which causes a substantial disruption to the undisturbed flow. It is shown that the fundamental impact of the gust on unsteady aerodynamic forces is due to an inviscid process, corresponding to variation in the effective angle of attack, which is not easily overcome. Plasma control is utilised to mitigate adverse effects of the interaction and improve aerodynamic performance. Physical characteristics of the interaction are described, and several aspects of the control strategy are explored. Among these are uniform and non-uniform spanwise variations of the control configuration, co-flow and counter-flow orientations of the directed force, pulsed and continuous operations of the actuator and strength of the plasma field. Results of the control situations are compared with regard to their effect upon aerodynamic forces. It was found that disturbances to the moment coefficient produced by the gust can be greatly reduced, which may be significant for stability and handling of MAV operations.     

不同网格拓扑对细长体绕流计算的影响研究

分别运用扇形(Fan)、阶梯形(Ladder)、交界面形(Interface)网格对细长体小攻角对称、大攻角对称、大攻角非对称绕流流场进行了数值研究．通过涡核位置、涡簇显示、物面压力分布、轴向力分布等的计算结果比较了三种网格的计算精度．数值实验表明：细长体分离涡流场对边界层网格非常敏感,应严格控制边界层网格的正交性;随着攻角增大,流场对网格特性的敏感性有增高的趋势;阶梯形网格可能会对流场带入非物理性扰动,交界面网格对流场捕捉有不连续现象;将三种网格得到的物面压力、侧向力、流动分离位置与实验值进行对比,发现扇形网格误差最小、交界面网格误差最大;大攻角非对称流动时,扇形网格计算的侧向力有整体向细长体头部压缩的趋势,涡脱落位置靠前,第二个及第三个极值更大,说明非对称现象有向尾部发展的趋势．     

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.     

Experimental flow study over a blunt-nosed axisymmetric body at incidence

The flowfield over a blunt-nosed cylinder was examined experimentally at a low subsonic speed for Re=1.88×10⁵ and angles of attack up to 40°. Velocity measurements were carried out (employing a seven-hole Pitot tube) as well as wall static pressure and wall shear-stress measurements. Surface flow visualization was applied using liquid crystals and a mixture of oil–TiO₂. For all the examined cases no flow asymmetries were found. For high angles of attack (20° and above) a separation “bubble” appears at the leeside of the nose area (streamwise flow separation). The basic feature of the circumferential pressure distribution at the after body area for these angles of attack is a plateau close to the suction peak and a fast recovery next to it. One streamwise vortex on each side of the symmetry plane is formed as well as a separation bubble about 90° far from this plane, where the cross-flow primary separation line is located. Each cross-flow primary separation line starts at the leeside nose area and moves towards the windward side along the cylindrical after body. The space between the two primary separation lines close to the wall is characterized by high flow fluctuations on the leeside, compared to the low fluctuations of the windward side.     

On the Vortex Dynamic of Airflow Reattachment Forced by a Single Non-thermal Plasma Discharge Actuator

Commercial and military aircrafts or miniature aerial vehicles can suffer from massive flow separation when high angles of attack are required. Single dielectric barrier discharge (DBD) actuators have demonstrated their capability of controlling such a separated flow at low external velocity. However, the processes resulting in the improvement of the flight performances remain unclear. In the present study, the reattachment process along the suction side of a NACA 0015 placed at an angle of attack of 16° is experimentally investigated for an external velocity of 20 m/s (Re = 260,000). A single DBD actuator is mounted at the leading edge of the model. The velocity fields above the suction side of the airfoil are measured by a high-speed acquisition system (3 kHz). The results indicate that the baseline flow presents shed vortices that form at the leading edge and linearly grow along the free shear layer axis. This vortex shedding is organized and exhibits a specific frequency of about 90 Hz. The continuous actuation produces a partial flow reattachment up to 70% of the chord length. Temporal cross-correlation function indicates the presence of a vortex shedding at the trailing edge of the controlled flow. Finally, the temporal analysis demonstrates that the reattachment process requires 50 ms to reach a stabilized attached flow. The time-resolved analysis of the reattachment suggests that the actuation by plasma discharge acts as a catalyser by reinforcing one of the coherent flow structures already existing in the natural flow.     

Modeling and simulation of a novel vertical actuator based on electrowetting on dielectric

A novel vertical actuator based on electrowetting on dielectric （EWOD） was designed, analyzed and simulated. Modeling results indicated that the vertical driving force of the actuator obeyed a second order polynomial of applied voltage, which was verified by Covent_ware 2006. As a resuit, the vertical driving force of the EWOD actuator with a 1.1 nL droplet and a 1.75 μm thick polymer was about 0.5 μN under an applied voltage 100V which was comparable to that of the electrostatic actuators. Moreover, the noise from plane forces we analyzed and simulated was very low. Therefore, we made a conclusion that the EWOD actuator can be used in MEMS transducer.     

流体边界层上电磁力的控制效应研究

利用作用于流体边界层上的电磁体积力改变流体边界层的结构，研究电磁力对流场的控制 作用效果. 电极与磁极交替分布的电磁场激活板包覆在圆柱体表面置于流动的电解质溶液 中，产生的电磁力沿圆柱体表面分布，可以改变流体边界层的结构，从而实现对流场的控制. 用电磁屏蔽和时域控制的方法调整电磁力的时空分布参数，圆柱绕流分离点可以在前驻点和 后驻点之间变动，产生不同的控制效果. 流体边界层上的电磁力能连续控制圆柱绕流、尾流 涡街的形态. 正向电磁力具有较好的消涡、减震和减阻控制效应. 反向电磁力具有明显的增 涡控制效应，具有较强的制动控制效应，此时圆柱体表面涡量分布的对称性和稳定性被破坏.     

基于可逆畴变的压电圆环多层致动器应变响应研究

压电致动器在现代工业中发挥着非常重要的作用。然而，目前应用的压电致动器均是基于线性压电效应，最大应变一般只有0.1-0.15%，实现大的致动应变一直是该领域学者追求的目标。本文中，我们提出了两种经过特殊设计的基于可逆非180°电畴翻转的PZT圆环多层致动器，一种是径向极化、部分电极（RPPE）的4层圆环，另一种是周期性正交极化（POP）的4层圆环，以期能够实现大的致动应变，而且圆环构型层数增加时也不容易发生失稳等问题。实验结果表明，在相同的驱动电场（2kV/mm，0.1Hz）下，4层RPPE最大致动应变为0.27%，约为普通PZT圆环的2倍，但表面变形很不均匀。相比之下，4层的POP圆环致动器的最大输出应变为0.36%，是普通PZT圆环的2.7倍。这两种致动器的致动应变都是随着频率的增加而减小， RPPE致动器在超过1Hz后稳定在0.19%， POP致动器在超过5Hz后稳定在0.2%。而且， POP圆环致动器重复性能很好，经过2万次致动循环后致动应变几乎不变。这种POP PZT多层圆环致动器具有结构稳定、输出应变大等优点，在致动领域具有很好的应用前景。     

基于可逆畴变的压电圆环多层致动器应变响应研究

压电致动器在现代工业中发挥着非常重要的作用。然而,目前应用的压电致动器均是基于线性压电效应,最大应变一般只有0.1-0.15%,实现大的致动应变一直是该领域学者追求的目标。本文中,我们提出了两种经过特殊设计的基于可逆非180°电畴翻转的PZT圆环多层致动器,一种是径向极化、部分电极（RPPE）的4层圆环,另一种是周期性正交极化（POP）的4层圆环,以期能够实现大的致动应变,而且圆环构型层数增加时也不容易发生失稳等问题。实验结果表明,在相同的驱动电场（2kV/mm,0.1Hz）下,4层RPPE最大致动应变为0.27%,约为普通PZT圆环的2倍,但表面变形很不均匀。相比之下,4层的POP圆环致动器的最大输出应变为0.36%,是普通PZT圆环的2.7倍。这两种致动器的致动应变都是随着频率的增加而减小, RPPE致动器在超过1Hz后稳定在0.19%, POP致动器在超过5Hz后稳定在0.2%。而且, POP圆环致动器重复性能很好,经过2万次致动循环后致动应变几乎不变。这种POP PZT多层圆环致动器具有结构稳定、输出应变大等优点,在致动领域具有很好的应用前景。     

Dynamic stability of electrostatically actuated initially curved shallow micro beams

Micro and nano devices incorporating bistable structural elements have functional advantages including the existence of several stable configurations at the same actuation force, extended working range, and tunable resonant frequencies. In this work, after a short review of operational principles of bistable micro devices, results of a theoretical and numerical investigation of the transient dynamics of an initially curved, shallow, double-clamped micro beam, actuated by distributed electrostatic and inertial forces are presented. Due to the unique combination of mechanical and electrostatic nonlinearities, typically not encountered in large scale structures, the device exhibits sequential snap-through and electrostatic (pull-in) instabilities. A phase plane analysis, performed using a consistently derived lumped model along with the numerical results, indicate that critical voltages corresponding to the dynamic snap-through and pull-in instabilities are lower than their static counterparts, while the minimal curvature required for the appearance of the dynamic snap-through is higher than in the static case. The boundaries of the bistability region of a quasi-statically loaded beam are found in terms of the geometrical and loading parameters and are shown to be bounded from above by the dynamic pull-in instability. Some of the post-buckling states cannot be reached under suddenly applied or quasi-statically increasing voltages: specially tailored loading schemes are suggested for realization of these configurations often beneficial in applications.     

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.      

Nonlinear vibration analysis of piezoelectric bending actuators: Theoretical and experimental studies

Piezoelectric bimorph actuators are used in a variety of applications, including micro positioning, vibration control, and micro robotics. The nature of the aforementioned applications calls for the dynamic characteristics identification of actuator at the embodiment design stage. For decades, many linear models have been presented to describe the dynamic behavior of this type of actuators; however, in many situations, such as resonant actuation, the piezoelectric actuators exhibit a softening nonlinear behavior; hence, an accurate dynamic model is demanded to properly predict the nonlinearity. In this study, first, the nonlinear stress–strain relationship of a piezoelectric material at high frequencies is modified. Then, based on the obtained constitutive equations and Euler–Bernoulli beam theory, a continuous nonlinear dynamic model for a piezoelectric bending actuator is presented. Next, the method of multiple scales is used to solve the discretized nonlinear differential equations. Finally, the results are compared with the ones obtained experimentally and nonlinear parameters are identified considering frequency response and phase response simultaneously. Also, in order to evaluate the accuracy of the proposed model, it is tested out of the identification range as well.     

圆柱表面包覆电磁场消涡与增涡实验研究

利用电磁体积力改变流体边界层的结构，用作用于流体边界层上的电磁力进行消涡与增涡控制。交替分布的电极和磁极包覆在圆柱体的表面置于电介质溶液中，简单调整电磁力的时间与空间分布可以方便地控制圆柱绕流的形态。通过理论分析和数值模拟确定了实验控制的关键参数，实现了电磁力消涡和增涡的连续控制，电磁力作用下的圆柱绕流的分离点可以在前驻点和后驻点之间变动。     

Experimental study on aerodynamic coefficients of yawed cylinders

This paper presents wind tunnel tests on a stationary cylinder inclined with the flow. The cylinder was positioned at different sets of yaw and vertical angles. The flow regime of the tests remained in the subcritical state. Two load cells were designed and installed to measure the aerodynamic forces, with enough sensitivity to measure vortex shedding frequencies. In this paper, the three aerodynamic force coefficients are normalized using the free stream velocity instead of its normal component. The results show that the drag coefficient and the resultant of the lift and side forces coefficients can be described by an empirical function of the incidence angle. The lift and side force coefficients remain however functions of both the horizontal yaw and vertical angles and cannot be expressed as functions of the incidence angle only. The Independence Principle was observed to become inaccurate for yaw angles larger than 40°. However, the measured Strouhal numbers indicate that the vortex shedding frequencies of a yawed cylinder can be predicted using the Independence Principle.     

Electromagnetic control of seawater flow around circular cylinders

We investigate numerically the electromagnetic control of seawater flows over an infinitely long circular cylinder. Stripes of electrodes and magnets, wrapped around the cylinder surface, produce a tangential body force (Lorentz force) that stabilizes the flow. This mechanism delays flow separation, reduces drag and lift, and finally suppresses the von Kármán vortex street. Results from two-dimensional simulations of the Navier–Stokes equations in a range 10<Re<300 and Lorentz force calculations are presented. Emphasis is placed on the disclosure of physical phenomena as well as a quantitative detection of the flow field and forces. It is shown that the drag strongly depends on the geometry of the electromagnetic actuator and on its location at the cylinder surface. The effect of flow control increases with larger Reynolds numbers, since the boundary layer thickness and the penetration depth of the Lorentz force are closely connected.     

Two-phase stratified flow splitting at a T-jun

The objective of this study is to investigate, experimentally and theoretically, two-phase splitting under stratified wavy flow conditions at a regular horizontal T -junction with an inclined branch arm.

Experimental data have been acquired with the side arm at horizontal conditions, downward inclination angles of −5, −10, −25, −40 and −60°, and upward inclinations of 1, 5, 10, 20, and 35° from the horizontal. The data reveal that gravity forces have a significant effect on the flow splitting. For downward inclination of the side arm more liquid is diverted into the branch arm, as compared to the case in which the side arm was horizontal. All the liquid was found to be diverted into the branch arm when the branch arm inclination was increased to −60°. For upward inclination angles a significant amount of the inlet gas has to be diverted into the side arm in order to get any liquid to flow into that arm. However, once liquid has started flowing, not much more additional gas has to be diverted into the side arm to get all of the liquid to flow into the branch. At 35° almost all the gas has to be diverted for any liquid flow into the branch.

A mechanistic model has been developed for the prediction of the splitting phenomenon for both the horizontal and the downward orientations of the side arm. The model is based on the momentum equations applied for the separation streamlines of the gas phase and the liquid phase. Very good agreement is observed between the prediction of the model and the data acquired for all the cases.