A novel MAV with treadmill motion of wing

The Magnus effect is well known phenomena for producing high lift values from spinning symmetrical geometries such as cylinders, spheres, or disks. But, the Magnus force may also be produced by treadmill motion of aerodynamic bodies. To accomplish this, the skin of aerodynamic bodies may circulate with a constant circumferential speed. Here, a novel wing with treadmill motion of skin is introduced which may generate lift at zero air speeds. The new wing may lead to micro aerial vehicle configurations for vertical take-off or landing. To prove the concept, the NACA0015 aerofoil section with circulating skin is computationally investigated. Two cases of stationary air and moving air are studied. It is observed that lift can be generated in stationary air although drag force is also high. For moving air, the lift and drag forces may be adopted between the incidence angles 20° to 25° where lift can posses high values and drag can remain moderate.     

On the influence of near-wall forces in particle-laden channel flows

Computation of a turbulent dilute gas–solid channel flow has been undertaken to study the influence of using wall-corrected drag coefficients and of the lift force on the dispersed phase characteristics. The incompressible Navier–Stokes equations governing the carrier flow were solved by using a direct numerical simulation approach and coupled with a Lagrangian particle tracking. Calculations were performed at Reynolds number based on the wall-shear velocity and channel half-width, Re_τ ≈ 184, and for three different sets of solid particles. For each particle set, two cases were examined, in the first one the particle motion was governed by both drag and lift wall-corrected forces, whereas in the other one, the standard drag force (not corrected) was solely acting. The lift force model used represents the most accurate available expression since it accounts for weak and strong shear as well as for wall effects. For this study, we considered elastic collisions for particles contacting the walls and that no external forces were acting. Present results indicate that the use of the lift force and of the drag corrections does not lead to significant changes in the statistical properties of the solid phase, at the exception of some statistics for the high inertia particles.     

Aerodynamic mechanism of forces generated by twisting model-wing in bat flapping flight

The aerodynamic mechanism of the bat wing membrane Mong the lateral border of its body is studied. The twist-morphing that alters the angle of attack （AOA） along the span-wise direction is observed widely during bat flapping flight. An assumption is made that the linearly distributed AOA is along the span-wise direction. The plate with the aspect ratio of 3 is used to model a bat wing. A three-dimensional （3D） unsteady panel method is used to predict the aerodynamic forces generated by the flapping plate with leading edge separation. It is found that, relative to the rigid wing flapping, twisting motion can increase the averaged lift by as much as 25% and produce thrust instead of drag. Furthermore, the aerodynamic forces （lift/drag） generated by a twisting plate-wing are similar to those of a pitching rigid-wing, meaning that the twisting in bat flight has the same function as the supination/pronation motion in insect flight.     

钝后缘风力机翼型的环量控制研究

钝后缘风力机翼型具有结构强度高、对表面污染不敏感等优点,但其较大的阻力系数使得翼型的整体气动特性不够理想. 利用环量控制方法对钝后缘风力机翼型进行了流动控制,以改善钝后缘风力机翼型的气动特性,减弱尾迹区脱体涡强度. 通过对钝后缘风力机翼型环量控制方法进行相关的数值模拟,对比研究了环量控制方法的增升减阻效果, 研究了环量控制下翼型升阻力特性随射流动量系数的变化规律,并对不同射流动量系数下环量控制方法的气动品质因子和控制效率进行了分析. 研究结果表明:环量控制方法能够大幅提升钝后缘风力机翼型的升力系数,同时有效地降低翼型的阻力系数; 翼型的升力系数随射流动量系数的增大而增大,表现出很明显的分离控制阶段和超环量控制阶段的变化规律; 射流能耗的功率系数随射流动量系数的增大而增大,且增长速率逐渐增大;实施环量控制方法后叶片的输出功率同样随射流动量系数增大而增大,但增长速率逐渐降低. 总体来说,环量控制方法可以有效地改善钝后缘风力机翼型的气动特性以及功率输出特性,在大型风力机流动控制中具有很好的应用前景.      

Rolling/sliding of a particle on a flat wall in a linear shear flow at finite Re

Recently Lee and Balachandar proposed analytically-based expressions for drag and lift coefficients for a spherical particle moving on a flat wall in a linear shear flow at finite Reynolds number. In order to evaluate the accuracy of these expressions, we have conducted direct numerical simulations of a rolling particle for shear Reynolds number up to 100. We assume that the particle rolls on a horizontal flat wall with a small gap separating the particle from the wall (L = 0.505) and thus avoiding the logarithmic singularity. The influence of the shear Reynolds number and the translational velocity of the particle on the hydrodynamic forces of the particle was investigated under both transient and the final drag-free and torque-free steady state. It is observed that the quasi-steady drag and lift expressions of Lee and Balachandar provide good approximation for the terminal state of the particle motion ranging from perfect sliding to perfect rolling. With regards to transient particle motion in a wall-bounded shear flow it is observed that the above validated quasi-steady drag and lift forces must be supplemented with appropriate wall-corrected added-mass and history forces in order to accurately predict the time-dependent approach to the terminal steady state. Quantitative comparison with the actual particle motion computed in the numerical simulations shows that the theoretical models quite effective in predicting rolling/sliding motion of a particle in a wall-bounded shear flow at moderate Re.     

Drag and lift reduction of a 3D bluff-body using active vortex generators

In this study, a passive flow control experiment on a 3D bluff-body using vortex generators (VGs) is presented. The bluff-body is a modified Ahmed body (Ahmed in J Fluids Eng 105:429–434 1983) with a curved rear part, instead of a slanted one, so that the location of the flow separation is no longer forced by the geometry. The influence of a line of non-conventional trapezoïdal VGs on the aerodynamic forces (drag and lift) induced on the bluff-body is investigated. The high sensitivity to many geometric (angle between the trapezoïdal element and the wall, spanwise spacing between the VGs, longitudinal location on the curved surface) and physical (freestream velocity) parameters is clearly demonstrated. The maximum drag reduction is ?12%, while the maximum global lift reduction can reach more than ?60%, with a strong dependency on the freestream velocity. For some configurations, the lift on the rear axle of the model can be inverted (?104%). It is also shown that the VGs are still efficient even downstream of the natural separation line. Finally, a dynamic parameter is chosen and a new set-up with motorized vortex generators is proposed. Thanks to this active device. The optimal configurations depending on two parameters are found more easily, and a significant drag and lift reduction (up to ?14% drag reduction) can be reached for different freestream velocities. These results are then analyzed through wall pressure and velocity measurements in the near-wake of the bluff-body with and without control. It appears that the largest drag and lift reduction is clearly associated to a strong increase of the size of the recirculation bubble over the rear slant. Investigation of the velocity field in a cross-section downstream the model reveals that, in the same time, the intensity of the longitudinal trailing vortices is strongly reduced, suggesting that the drag reduction is due to the breakdown of the balance between the separation bubble and the longitudinal vortices. It demonstrates that for low aspect ratio 3D bluff-bodies, like road vehicles, the flow control strategy is much different from the one used on airfoils: an early separation of the boundary layer can lead to a significant drag reduction if the circulation of the trailing vortices is reduced.     

On the interaction between two fixed spherical particles

The variation of the drag (C_D) and lift coefficients (C_L) of two fixed solid spherical particles placed at different positions relative each other is studied. Simulations are carried out for particle Reynolds numbers of 50, 100 and 200 and the particle position is defined by the angle between the line connecting the centers of the particles and the free-stream direction (α) and the separation distance (d₀) between the particles. The flow around the particles is simulated using two different methods; the Lattice Boltzmann Method (LBM), using two different computational codes, and a conventional finite difference approach, where the Volume of Solid Method (VOS) is used to represent the particles. Comparisons with available numerical and experimental data show that both methods can be used to accurately resolve the flow field around particles and calculate the forces the particles are subjected to. Independent of the Reynolds number, the largest change in drag, as compared to the single particle case, occurs for particles placed in tandem formation. Compared to a single particle, the drag reduction for the secondary particle in tandem arrangement is as high as 60%, 70% and 80% for Re = 50, 100 and 200, respectively. The development of the recirculation zone is found to have a significant influence on the drag force. Depending on the flow situation in-between the particles for various particle arrangements, attraction and repulsion forces are detected due to low and high pressure regions, respectively. The results show that the inter-particle forces are not negligible even under very dilute conditions.     

关于颗粒悬浮机理和悬浮动的讨论

从分析气体分子的悬浮和静水中Ｂｒｏｗｎ微粒的悬浮之机理出发，论述了重力场中粒子（分子、微粒等）的悬浮不一定需要其它外力，粒子本身的任何形式的无规则运动，达到一定强度后都能使粒子弥散悬浮．河流中的泥沙颗粒和气（水）力输送管道中的颗粒的悬浮也主要靠颗粒物的无规则运动．作用于颗粒的升力和其它力可改变颗粒悬浮沿高度的分布，但仅用这些力（若无任何无规则运动）无法解释颗粒的弥散悬浮状态．讨论了颗粒对流动阻力的双重作用：支持颗粒悬浮的湍流脉动因引入颗粒而削弱，这是颗粒的减阻作用；颗粒增阻的一个主要机制是，流体给予颗粒的水平动量在颗粒一壁面碰撞中不断地损失．用悬浮动概念解释颗粒引起的增阻是不正确的．     

Mean and fluctuating components of drag and lift forces on an isolated finite-sized particle in turbulence

We perform fully resolved direct numerical simulations of an isolated particle subjected to free-stream turbulence in order to investigate the effect of turbulence on the drag and lift forces at the level of a single particle, following Bagchi and Balachandar’s work (Bagchi and Balachandar in Phys Fluids 15:3496–3513, 2003). The particle Reynolds numbers based on the mean relative particle velocity and the particle diameter are Re?=?100, 250 and 350, which covers three different regimes of wake evolution in a uniform flow: steady axisymmetric wake, steady planar symmetric wake, and unsteady planar symmetric vortex shedding. At each particle Reynolds number, the turbulent intensity is 5–10% of the mean relative particle velocity, and the corresponding diameter of the particle is comparable to or larger than the Kolmogorov scale. The simulation results show that standard drag values determined from uniform flow simulations can accurately predict the drag force if the turbulence intensity is sufficiently weak (5% or less compared to the mean relative velocity). However, it is shown that for finite-sized particles, flow non-uniformity, which is usually neglected in the case of the small particles, can play an important role in determining the forces as the relative turbulence intensity becomes large. The influence of flow non-uniformity on drag force could be qualitatively similar to the Faxen correction. In addition, finite-sized particles at sufficient Reynolds number are inherently subjected to stochastic forces arising from their self-induced vortex shedding in addition to lift force arising from the local ambient flow properties (vorticity and strain rate). The effect of rotational and strain rate of the ambient turbulence seen by the particle on the lift force is explored based on the conditional averaging using the generalized representation of the quasi-steady force proposed by Bagchi and Balachandar (J Fluid Mech 481:105–148, 2003). From the present study, it is shown that at Re?=?100, the lift force is mainly influenced by the surrounding turbulence, but at Re = 250 and 350, the lift force is affected by the wake structure as well as the surrounding turbulence. Thus, for a finite-sized particle of sufficient Reynolds number supporting self-induced vortex shedding, the lift force will not be completely correlated with the ambient flow. Therefore, it appears that in order to reliably predict the motion of a finite-sized particle in turbulence, it is important to incorporate both a deterministic component and a stochastic component in the force model. The best deterministic contribution is given by the conditional average. The influence of ambient turbulence at the scale of the particle, which are not accounted for in the deterministic contribution, can be considered in stochastic manner. In the modeling of lift force, additional stochastic contribution arising from self-induced vortex shedding must also be included.     

Effect of Shape on Inertial Particle Dynamics in a Channel Flow

Particle dynamics in a channel flow are investigated using large eddy simulation and a Lagrangian particle tracking technique. Following validation of single-phase flow predictions against DNS results, fluid velocities are subsequently used to study the behaviour of particles of differing shape assuming one-way coupling between the fluid and the particles. The influence of shape- and orientation-dependent drag and lift forces on both the translational and rotational motion of the particles is accounted for to ensure accurate representation of the flow dynamics of non-spherical particles. The size of the particles studied was obtained based on an equivalent-volume sphere, and differing shapes were modelled using super-quadratic ellipsoid forms by varying their aspect ratio, with their orientation predicted using the incidence angle between the particle relative velocity and the particle principal axis. Results are presented for spherical, needle- and platelet-like particles at a number of different boundary layer locations along the wall-normal direction within the channel. The time evolution and probability density function of selected particle translational and rotational properties show a clear distinction between the behaviour of the various particles types, and indicate the significance of particle shape when modelling many practically relevant flows.     

Supersonic pulsed jets for material sorting

Air ejectors are used for sorting materials by displacing selected items through the action of aerodynamic drag. Subsonic units are common in the marketplace. This paper explores the potential for the use of an overexpanded supersonic jet because of it having reduced lateral spread and giving significantly higher particle drag. Experiments have been conducted to explore the flow field associated with a pulsed supersonic jet impacting particles in different positions relative to the jet exit, and of various shapes. The forces acting on a typical particle are measured and schlieren photography is used to visualize the flow.     

Drag of a flat wedge in a two-phase flow

The drag of a flat wedge in a subsonic two-phase flow is investigated. In contrast to earlier work of Balanin and Zlobin [1] particular attention is devoted to the influence of the particle size. Detailed investigations are made of the dependences of the forces and aerodynamic coefficients on the concentration of the solid phase, the opening angle of the wedge, and the particle size. It is established that the drag coefficients depend on the particle size only for particles with diameters less than 30 um.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 177–180, March–April, 1982.     

Migration of settling particles in a horizontal viscous flow through a vertical slot with porous walls

Particle migration in a horizontal flow of dilute suspension through a vertical slot with porous walls is studied using the two-continua approach. The lateral migration is induced by two opposite effects: an inertial lift force due to particle settling and directed toward the slot centre-line, and a drag due to leak-off entraining particles toward the walls. An expression for the inertial lift on a settling particle in a horizontal channel flow found recently is generalized to the case of a low leak-off velocity. The evolution of an initial uniform particle concentration profile is studied within the full Lagrangian approach. Four migration regimes are found differing by the direction of particle migration and numbers of equilibrium positions. Conditions of the regime change and a critical value of dimensionless leak-off velocity for particle deposition on the walls are obtained analytically. Suspension flows with zones where the particle concentration is zero or increases infinitely, are studied numerically.     

Energy harvesting of a heaving and forward pitching wing with a passively actuated trailing edge

This study experimentally investigates the energy harvesting capabilities of an oscillating wing with a passively actuated trailing edge. The oscillation kinematics are composed of a combined heaving and forward pitching motions, where the pitching axis is well behind the wing center of mass. Passive actuation is attained by connecting the trailing edge with the wing body using a torsion rod. The degree of flexibility of the trailing edge is represented by the Strouhal number based on the trailing edge natural frequency. The trailing edge passive response is studied for oscillation Strouhal numbers of 0.017, 0.025 and 0.033. Instantaneous aerodynamic forces are measured in a closed loop wind tunnel at a Reynolds number of 40 000, based on the free stream velocity and the wing chord length. Measured results include the effective angle of attack induced by the trailing edge actuation as well as the lift and moment during the oscillation cycle. For the imposed kinematics in this study, the pitching motion has a positive contribution to the mean power output whereas the heaving motion has a relatively small but negative contribution. Additionally, by decreasing the natural frequency of the trailing edge closer to that of the imposed oscillation frequency, the magnitude of the lift and moment forces and hence the mean power output, increases. It is found that there exists a strong correlation between mean power output and the effective angle of attack, shown through the passive trailing edge response, resulting in an increase in energy harvesting potential.     

可侵蚀地表上方含尘大气运动的数值模型

在稀相气固两相流的双流体模型框架下，导出含尘大气运动的基本方程，其中的源项考虑相间双向耦合作用；通过对基本方程进行无量纲化，求得控制气相和弥散相动力学行为的相似准则，包括弗洛德数、罗斯贝数、颗粒雷诺数、颗粒惯性参数、颗粒剪切横移参数、颗粒旋转横移参数和沙尘质量载荷率等. 作为一个模型问题，研究可侵蚀地表上方充分发展湍流大气边界层流动对土壤颗粒的气动卷扬作用，其中计及气动阻力、萨弗曼升力和重力等因素的影响. 为了克服由于沙尘跃移运动轨迹交叉导致流动参数多值性所造成的困难，引进拉格朗日坐标下弥散相连续方程. 对两种不同风速和4种不同粒径条件下的沙尘运动进行数值模拟，给出相应情况中沙尘运动特性和密度分布剖面并讨论风速和粒径等参数的影响，还细致分析了含尘大气边界层中沙尘与气流之间的能量传递过程. 所得的方程、准则和方法可以为土壤风蚀和沙尘暴等自然灾害的预报提供理论基础.     

Effect of vortex shedding in unsteady aerodynamic forces for a low Reynolds number stationary wing at low angle of attack

This study elucidates the relation between wake vortex shedding and aerodynamic force fluctuations for a low Reynolds number wing from time resolved particle image velocimetry (TR-PIV) experimental measurements. The results reveal a periodic lift and drag variation within the shedding cycle and resolve the frequencies of those fluctuations from a proper orthogonal decomposition (POD) and power spectral density (PSD) analysis. To show the effect of vortex shedding on the body force fluctuations, the evolution of instantaneous aerodynamic forces is compared to the pressure field of the fluid flow and to the vortical structures in the wake of the airfoil. A six step model describing the vortex-force relation is proposed. It shows that changes in lift such as maximum lift and minimum lift are associated with the detachment of a vortex. It also shows that the minimum or local minimum drag value is obtained at the onset formation of a vortex on the airfoil wake. Similarly, the maximum or local maximum drag is obtained at the onset formation of the saddle on the airfoil wake. The model further explains the asymmetry observed in the unsteady drag force evolution. The model can be used to optimize flow control and fluid-structure interaction applications.     

Transverse motions of a single sphere in hanging or swing configurations under a longitudinal flow

As a result of the reporting of casual observations of the oscillation or rotation of the beacons in transmission line guard cables, some attention has been paid to the stability of the guard cables with beacons.The relatively more frequent observation of these motions has been explained in recent papers dealing with the elastic part of the problem as a consequence of the increasing number of resonant frequencies (one for each additional beacon) that can be excited by appropriate aerodynamic loads. But a model that could explain the aerodynamic forces that can give rise to this motion is still lacking.In this paper we consider the transverse motions of a single sphere in two simplified configurations, (1) hanging (tethered at one point), and (2) swing (tethered at two points) under a longitudinal flow, performing small amplitude swinging oscillations or circular-orbit autorotation about an axis parallel to the main flow direction. The dynamic model here presented is based on the motion equations, which also include a model for the aerodynamic lift and drag forces on the sphere in transverse motion, which considers the effect of changes of flow around the sphere due to the cable interference. These forces are contained in the symmetry plane of the flow relative to the sphere, and, when projected on the lateral direction, give rise to a lateral force, which can explain the existence of the azimuthal motion even at a large reduced velocity, outside the vortex induced vibration (VIV) range The conditions for stable small oscillation motion and circular-orbit autorotation of a sphere in a swing configuration are given.The results for the aerodynamic loads in transverse motion have also been applied to the case of a circular-orbit autorotation of a hanging sphere (spherical pendulum) under a vertical flow. The angular rotation speed and the orbit radius (or cable angle) have been determined as a function of aerodynamic coefficients and configuration parameters.     

Measurement of Thrust and Lift Forces Associated With Drag of Compliant Flapping Wing for Micro Air Vehicles Using a New Test Stand Design

Compliant wing designs have the potential of improving flapping wing Micro-Air Vehicles (MAVs). Designing compliant wings requires a detailed understanding of the effect of compliance on the generation of thrust and lift forces. The low force and high-frequency measurements associated with these forces necessitated a new versatile test stand design that uses a 250 g load cell along with a rigid linear air bearing to minimize friction and the dynamic behavior of the test stand while isolating only the stationary thrust or lift force associated with drag generated by the wing. Moreover, this stand is relatively inexpensive and hence can be easily utilized by wing designers to optimize the wing compliance and shape. The frequency response of the wing is accurately resolved, along with wing compliance on the thrust and lift profiles. The effects of the thrust and lift force generated as a function of flapping frequency were also determined. A semi-empirical aerodynamic model of the thrust and lift generated by the flapping wing MAV on the new test stand was developed and used to evaluate the measurements. This model accounted for the drag force and the effects of the wing compliance. There was good correlation between the model predictions and experimental measurements. Also, the increase in average thrust due to increased wing compliance was experimentally quantified for the first time using the new test stand. Thus, our measurements for the first time reveal the detrimental influence of excessive compliance on drag forces during high frequency operation. In addition, we were also able to observe the useful effect of compliance on the generation of extra thrust at the beginning and end of upstrokes and downstrokes of the flapping motion.     

Measurement of the aerodynamic forces on a small particle attached to a wall

Using operating principles similar to that applied in atomic force microscopes, we have developed a novel measuring method to study the aerodynamic forces, in particular the lift and drag force, acting on a small particle attached to a wall and immersed in a linear shear flow. Results thus far have shown that the system is capable of measuring both the minute aerodynamic lift and drag forces that a particle experiences as a result of the flow.C. Muthanna has also published under the name C. M. Kolera

---

顺桨风力机塔叶干涉翼型非稳气动力时频特性

由于风力机叶片与塔筒流场相互干涉,实际气动力与理想情况存在较大差异,这种干涉作用造成的气动力差异给叶片与塔筒结构可靠性带来不可忽视的影响.以翼型DU91-W2-250为研究对象,采用瞬态数值分析与本征正交分解方法,考虑叶片和塔筒流场相互干涉作用,分析顺桨工况翼型非稳气动力时频特性及其影响规律,量化不同雷诺数下塔叶相对位置及几何参数对气动力均值、波动幅度和频率的影响程度,通过流场模态能量分布形态分析,揭示流场干涉对气动力的影响机制.结果表明,翼型气动中心至塔筒几何中心的垂直距离、水平距离以及塔筒直径相对于翼型弦长的无量纲参数y*,x*和D*对气动力均有不同程度影响,其中y*对升阻力系数均值影响最大,对频率无明显影响,y*绝对值越大,C_l均值越接近单翼型C_l值,y*绝对值越小升阻力系数波动幅度越大,y*从-12增大到12,升力系数均值最小值为-0.48,最大值为1.16;x*减小和D*增大,反向阻力均值增大,波动幅度增大,波动频率略有下降,当x*小于临界值5时,带塔翼型阻力均值反向;在计算范围内,带塔翼型升力系数均值相对于单翼型升力系数最大偏差为...