Comparative aerodynamic performance of flapping flight in two bat species using time-resolved wake visualization

Bats are unique among extant actively flying animals in having very flexible wings, controlled by multi-jointed fingers. This gives the potential for fine-tuned active control to optimize aerodynamic performance throughout the wingbeat and thus a more efficient flight. But how bat wing performance scales with size, morphology and ecology is not yet known. Here, we present time-resolved fluid wake data of two species of bats flying freely across a range of flight speeds using stereoscopic digital particle image velocimetry in a wind tunnel. From these data, we construct an average wake for each bat species and speed combination, which is used to estimate the flight forces throughout the wingbeat and resulting flight performance properties such as lift-to-drag ratio (L/D). The results show that the wake dynamics and flight performance of both bat species are similar, as was expected since both species operate at similar Reynolds numbers (Re) and Strouhal numbers (St). However, maximum L/D is achieved at a significant higher flight speed for the larger, highly mobile and migratory bat species than for the smaller non-migratory species. Although the flight performance of these bats may depend on a range of morphological and ecological factors, the differences in optimal flight speeds between the species could at least partly be explained by differences in their movement ecology.     

Time-varying span efficiency through the wingbeat of desert locusts

The flight performance of animals depends greatly on the efficacy with which they generate aerodynamic forces. Accordingly, maximum range, load-lifting capacity and peak accelerations during manoeuvres are all constrained by the efficiency of momentum transfer to the wake. Here, we use high-speed particle image velocimetry (1 kHz) to record flow velocities in the near wake of desert locusts (Schistocerca gregaria, Forskål). We use the measured flow fields to calculate time-varying span efficiency throughout the wing stroke cycle. The locusts are found to operate at a maximum span efficiency of 79 per cent, typically at a plateau of about 60 per cent for the majority of the downstroke, but at lower values during the upstroke. Moreover, the calculated span efficiencies are highest when the largest lift forces are being generated (90% of the total lift is generated during the plateau of span efficiency) suggesting that the combination of wing kinematics and morphology in locust flight perform most efficiently when doing the most work.     

The aerodynamic cost of flight in the short-tailed fruit bat (Carollia perspicillata): comparing theory with measurement

Aerodynamic theory has long been used to predict the power required for animal flight, but widely used models contain many simplifications. It has been difficult to ascertain how closely biological reality matches model predictions, largely because of the technical challenges of accurately measuring the power expended when an animal flies. We designed a study to measure flight speed-dependent aerodynamic power directly from the kinetic energy contained in the wake of bats flying in a wind tunnel. We compared these measurements with two theoretical predictions that have been used for several decades in diverse fields of vertebrate biology and to metabolic measurements from a previous study using the same individuals. A high-accuracy displaced laser sheet stereo particle image velocimetry experimental design measured the wake velocities in the Trefftz plane behind four bats flying over a range of speeds (3–7 m s⁻¹). We computed the aerodynamic power contained in the wake using a novel interpolation method and compared these results with the power predicted by Pennycuick''s and Rayner''s models. The measured aerodynamic power falls between the two theoretical predictions, demonstrating that the models effectively predict the appropriate range of flight power, but the models do not accurately predict minimum power or maximum range speeds. Mechanical efficiency—the ratio of aerodynamic power output to metabolic power input—varied from 5.9% to 9.8% for the same individuals, changing with flight speed.     

Phasing of dragonfly wings can improve aerodynamic efficiency by removing swirl

Dragonflies are dramatic, successful aerial predators, notable for their flight agility and endurance. Further, they are highly capable of low-speed, hovering and even backwards flight. While insects have repeatedly modified or reduced one pair of wings, or mechanically coupled their fore and hind wings, dragonflies and damselflies have maintained their distinctive, independently controllable, four-winged form for over 300Myr. Despite efforts at understanding the implications of flapping flight with two pairs of wings, previous studies have generally painted a rather disappointing picture: interaction between fore and hind wings reduces the lift compared with two pairs of wings operating in isolation. Here, we demonstrate with a mechanical model dragonfly that, despite presenting no advantage in terms of lift, flying with two pairs of wings can be highly effective at improving aerodynamic efficiency. This is achieved by recovering energy from the wake wasted as swirl in a manner analogous to coaxial contra-rotating helicopter rotors. With the appropriate fore–hind wing phasing, aerodynamic power requirements can be reduced up to 22 per cent compared with a single pair of wings, indicating one advantage of four-winged flying that may apply to both dragonflies and, in the future, biomimetic micro air vehicles.     

Time-resolved vortex wake of a common swift flying over a range of flight speeds

The wake of a freely flying common swift (Apus apus L.) is examined in a wind tunnel at three different flight speeds, 5.7, 7.7 and 9.9 m s⁻¹. The wake of the bird is visualized using high-speed stereo digital particle image velocimetry (DPIV). Wake images are recorded in the transverse plane, perpendicular to the airflow. The wake of a swift has been studied previously using DPIV and recording wake images in the longitudinal plane, parallel to the airflow. The high-speed DPIV system allows for time-resolved wake sampling and the result shows features that were not discovered in the previous study, but there was approximately a 40 per cent vertical force deficit. As the earlier study also revealed, a pair of wingtip vortices are trailing behind the wingtips, but in addition, a pair of tail vortices and a pair of ‘wing root vortices’ are found that appear to originate from the wing/body junction. The existence of wing root vortices suggests that the two wings are not acting as a single wing, but are to some extent aerodynamically detached from each other. It is proposed that this is due to the body disrupting the lift distribution over the wing by generating less lift than the wings.     

Wake structure and wingbeat kinematics of a house-martin Delichon urbica.

The wingbeat kinematics and wake structure of a trained house martin in free, steady flight in a wind tunnel have been studied over a range of flight speeds, and compared and contrasted with similar measurements for a thrush nightingale and a pair of robins. The house martin has a higher aspect ratio (more slender) wing, and is a more obviously agile and aerobatic flyer, catching insects on the wing. The wingbeat is notable for the presence at higher flight speeds of a characteristic pause in the upstroke. The essential characteristics of the wing motions can be reconstructed with a simple two-frequency model derived from Fourier analysis. At slow speeds, the distribution of wake vorticity is more simple than for the other previously measured birds, and the upstroke does not contribute to weight support. The upstroke becomes gradually more significant as the flight speed increases, and although the vortex wake shows a signature of the pause phase, the global circulation measurements are otherwise in good agreement with surprisingly simple aerodynamic models, and with predictions across the different species, implying quite similar aerodynamic performance of the wing sections. The local Reynolds numbers of the wing sections are sufficiently low that the well-known instabilities of attached laminar flows over lifting surfaces, which are known to occur at two to three times this value, may not develop.     

建筑物周围风致静压场的分布及振动特性研究

对均匀流及湍流中风洞内有和无建筑物时的静压状况进行了研究,针对目前无法精确测量静压的情况,提出了一种无量纲化方法,给出了静压场的分布规律,运用幅值域及频域方法详细分析了建筑物周围各区域的流场特性,之后给出了静压场主要方向的分布函数型式,并推断了静压场的静力及动力边界。研究结果显示:在风洞中进行风压测试时,有可能激发风洞气柱自振模态并对测试结果产生误导;建筑物迎风区脉动主要取决于来流脉动;背风区脉动则与湍流来流脉动较为相像,但能引起更多非高斯特性;地面粗糙度由小变大时,建筑物侧风区脉动由小振幅周期振动向大振幅随机振动转变;地面粗糙度越大,静压场的静力作用越小而动力作用越大。所提静压场分布函数可对风工程实测参考静压的测试及建筑群体的风致干扰问题提供参考。     

The complex aerodynamic footprint of desert locusts revealed by large-volume tomographic particle image velocimetry

Particle image velocimetry has been the preferred experimental technique with which to study the aerodynamics of animal flight for over a decade. In that time, hardware has become more accessible and the software has progressed from the acquisition of planes through the flow field to the reconstruction of small volumetric measurements. Until now, it has not been possible to capture large volumes that incorporate the full wavelength of the aerodynamic track left behind during a complete wingbeat cycle. Here, we use a unique apparatus to acquire the first instantaneous wake volume of a flying animal''s entire wingbeat. We confirm the presence of wake deformation behind desert locusts and quantify the effect of that deformation on estimates of aerodynamic force and the efficiency of lift generation. We present previously undescribed vortex wake phenomena, including entrainment around the wing-tip vortices of a set of secondary vortices borne of Kelvin–Helmholtz instability in the shear layer behind the flapping wings.     

Factorial Modeling for Transient Solid Particle Velocity in a Fluidized Bed Combustor Cold Model

Transient hydrodynamics phenomena in the fluidized bed combustor (FBC) freeboard have been critical in the past two decades. Within a 152 mm ID FBC cold model, solid particle transient velocities were measured and analyzed with the assistance of advanced laser-based particle image velocimetry (PIV) instrumentation. Two layers of swirling secondary air were injected into the cold model. The PIV system was applied to the FBC cold model to visualize transient solid particle velocity. A series of transient solid particle velocity profiles were generated for the factorial analysis. In each profile, the solid particle velocity vectors (Vx and Vy) for 10 × 10 grids were generated. Analysis of variance (ANOVA) was used to determine the significant factors that affect transient solid particle velocities, time, and position coordinates. Then, the 10¹⁰ factorial design method was used to develop a specific empirical model of transient solid particle velocity in the FBC freeboard, which was in the shape of V_x = f₁(t, x, y) and V_y = f₂(t, x, y).

This unique factorial analysis method proved to be a very effective and practical method to evaluate experimental conditions and analyze experimental results in the FBC systems.     

Factorial Modeling for Transient Solid Particle Velocity in a Fluidized Bed Combustor Cold Model

ABSTRACT

Transient hydrodynamics phenomena in the fluidized bed combustor (FBC) freeboard have been critical in the past two decades. Within a 152 mm ID FBC cold model, solid particle transient velocities were measured and analyzed with the assistance of advanced laser-based particle image velocimetry (PIV) instrumentation. Two layers of swirling secondary air were injected into the cold model. The PIV system was applied to the FBC cold model to visualize transient solid particle velocity. A series of transient solid particle velocity profiles were generated for the factorial analysis. In each profile, the solid particle velocity vectors (Vx and Vy) for 10 × 10 grids were generated. Analysis of variance (ANOVA) was used to determine the significant factors that affect transient solid particle velocities, time, and position coordinates. Then, the 10¹⁰ factorial design method was used to develop a specific empirical model of transient solid particle velocity in the FBC freeboard, which was in the shape of V_x = f₁(t, x, y) and V_y = f₂(t, x, y).

This unique factorial analysis method proved to be a very effective and practical method to evaluate experimental conditions and analyze experimental results in the FBC systems.     

Flight efficiency is a key to diverse wing morphologies in small insects

Insect wings are hybrid structures that are typically composed of veins and solid membranes. In some of the smallest flying insects, however, the wing membrane is replaced by hair-like bristles attached to a solid root. Bristles and membranous wing surfaces coexist in small but not in large insect species. There is no satisfying explanation for this finding as aerodynamic force production is always smaller in bristled than solid wings. This computational study suggests that the diversity of wing structure in small insects results from aerodynamic efficiency rather than from the requirements to produce elevated forces for flight. The tested wings vary from fully membranous to sparsely bristled and were flapped around a wing root with lift- and drag-based wing kinematic patterns and at different Reynolds numbers (Re). The results show that the decrease in aerodynamic efficiency with decreasing surface solidity is significantly smaller at Re = 4 than Re = 57. A replacement of wing membrane by bristles thus causes less change in energetic costs for flight in small compared to large insects. As a consequence, small insects may fly with bristled and solid wing surfaces at similar efficacy, while larger insects must use membranous wings for an efficient production of flight forces. The above findings are significant for the biological fitness and dispersal of insects that fly at elevated energy expenditures.     

Computational investigation of cicada aerodynamics in forward flight

Free forward flight of cicadas is investigated through high-speed photogrammetry, three-dimensional surface reconstruction and computational fluid dynamics simulations. We report two new vortices generated by the cicada''s wide body. One is the thorax-generated vortex, which helps the downwash flow, indicating a new phenomenon of lift enhancement. Another is the cicada posterior body vortex, which entangles with the vortex ring composed of wing tip, trailing edge and wing root vortices. Some other vortex features include: independently developed left- and right-hand side leading edge vortex (LEV), dual-core LEV structure at the mid-wing region and near-wake two-vortex-ring structure. In the cicada forward flight, approximately 79% of the total lift is generated during the downstroke. Cicada wings experience drag in the downstroke, and generate thrust during the upstroke. Energetics study shows that the cicada in free forward flight consumes much more power in the downstroke than in the upstroke, to provide enough lift to support the weight and to overcome drag to move forward.     

New optical tools for fluid mechanics

Optical flow measurement is the most important non-intrusive technique in fluid mechanics. A second advantage of some new optical measurement techniques is the high resolution in space and time. As a third advantage, flow visualisation also has a special importance in fluid mechanics. The unsteady three-dimensional flow fields need a visual display of experimental as well as theoretical results. Optical visualisation techniques often lead to completely new insights in flow phenomena especially in experiments. Some discoveries which have been made this way are mentioned here.     

An integrated platform to facilitate the calculation,validation and visualization of optical flow velocities in biological images

Optical flow algorithms have seen poor adoption in the biological community compared with particle image velocimetry for quantifying cellular dynamics because of the lack of proper validation and an intuitive user interface. To address these challenges, we present OpFlowLab, an integrated platform that integrates our motion estimation workflow. Using routines in our workflow, we demonstrate that optical flow algorithms are more accurate than PIV in simulated images of the movement of nuclei. Qualitative assessment with actual nucleus images further supported this finding. Additionally, we show that refinement of the optical flow velocities is possible with a simple object-matching procedure, opening up the possibility of obtaining reasonable velocity estimates under less ideal imaging conditions. To visualize velocity fields, we employ artificial tracers to allow for the drawing of pathlines. Through the adoption of OpFlowLab, we are confident that optical flow algorithms will allow for the exploration of dynamic biological systems in greater accuracy and detail.     

缆索承重桥并列索平均气动性能的雷诺数效应

近距离并列吊索和并列斜拉索在大跨度缆索承重桥中有广泛的应用,但目前在计算并列索静力风荷载时只考虑顺风向阻力的影响,没有考虑因尾流干扰所产生的横风向升力的作用。通过风洞试验,在雷诺数Re= 0.9×10⁵~4.6×10⁵（涉及亚临界雷诺数区和临界雷诺数区）研究了多种不同相对位置的并列双圆柱的平均阻力、平均升力和斯特罗哈数随雷诺数的变化规律。研究结果表明：并列圆柱的平均气动性能有强烈的雷诺数效应,临界雷诺数下的气动力系数与亚临界区有很大的差异;在临界雷诺数区域,下游圆柱的平均升力系数会随雷诺数的增大发生不连续突变,这种气动力的突变很可能与双圆柱绕流场的流态结构的演变有关;受上游圆柱的尾流干扰,在临界雷诺数区域内下游圆柱的平均升力系数可达到1.25以上,远大于下游圆柱的平均阻力系数,因而在计算并列索静力风荷载时有必要考虑横风向平均升力的作用。     

Modeling of the Transient Particle Velocity Distribution in the Fluidized Bed Combustor (FBC) Riser

ABSTRACT

The geometrical distributions of transient particle velocity in a fluidized bed combustor (FBC) riser are critical to FBC design and manufacturing. Particle image velocimetry (PIV) instrumentation was applied to visualize the particle transient movement in the area of interest (AOI) of an experimental cold model (152 mm ID × 610 mm height) of an FBC. Sixteen (16) PIV particle velocity profiles were generated for the AOI in a 1.6-second time period. A ten (columns) × five (rows) grid was set for each profile. Three levels of fluidizing air velocity were set for the experiments at 2.477 m/s, 2.677 m/s, and 2.823 m/s respectively. A knowledge-based regression method was applied to generate the empirical model of the two-dimensional particle transient velocity with the consideration of four independent variables, x (x coordinate), y (y coordinate), V_g (fluidizing air velocity), and t (time). This model had a data fitting accuracy of 88%. In addition, this model had a very good validation performance with the data obtained from other tests at different experimental setups.

The modeling method is applied for the first time to the particle movement in FBC risers. The research work proved that the design of the experiment and regression analysis are very effective and practical for evaluating experimental conditions and analyzing experimental results in FBC systems.     

Elytra boost lift,but reduce aerodynamic efficiency in flying beetles

Flying insects typically possess two pairs of wings. In beetles, the front pair has evolved into short, hardened structures, the elytra, which protect the second pair of wings and the abdomen. This allows beetles to exploit habitats that would otherwise cause damage to the wings and body. Many beetles fly with the elytra extended, suggesting that they influence aerodynamic performance, but little is known about their role in flight. Using quantitative measurements of the beetle''s wake, we show that the presence of the elytra increases vertical force production by approximately 40 per cent, indicating that they contribute to weight support. The wing-elytra combination creates a complex wake compared with previously studied animal wakes. At mid-downstroke, multiple vortices are visible behind each wing. These include a wingtip and an elytron vortex with the same sense of rotation, a body vortex and an additional vortex of the opposite sense of rotation. This latter vortex reflects a negative interaction between the wing and the elytron, resulting in a single wing span efficiency of approximately 0.77 at mid downstroke. This is lower than that found in birds and bats, suggesting that the extra weight support of the elytra comes at the price of reduced efficiency.     

Modeling of the Transient Particle Velocity Distribution in the Fluidized Bed Combustor (FBC) Riser

The geometrical distributions of transient particle velocity in a fluidized bed combustor (FBC) riser are critical to FBC design and manufacturing. Particle image velocimetry (PIV) instrumentation was applied to visualize the particle transient movement in the area of interest (AOI) of an experimental cold model (152 mm ID × 610 mm height) of an FBC. Sixteen (16) PIV particle velocity profiles were generated for the AOI in a 1.6-second time period. A ten (columns) × five (rows) grid was set for each profile. Three levels of fluidizing air velocity were set for the experiments at 2.477 m/s, 2.677 m/s, and 2.823 m/s respectively. A knowledge-based regression method was applied to generate the empirical model of the two-dimensional particle transient velocity with the consideration of four independent variables, x (x coordinate), y (y coordinate), V_g (fluidizing air velocity), and t (time). This model had a data fitting accuracy of 88%. In addition, this model had a very good validation performance with the data obtained from other tests at different experimental setups.

The modeling method is applied for the first time to the particle movement in FBC risers. The research work proved that the design of the experiment and regression analysis are very effective and practical for evaluating experimental conditions and analyzing experimental results in FBC systems.     

基于PIV的大豆蛋白液双枪喷涂速度场的研究

目的 为了在保证喷涂雾化表征均匀性的前提下实现宽幅覆膜,提高覆膜的效率,以期得到均匀一致的大豆蛋白覆膜。方法 利用粒子图像速度场仪（Particle Image Velocimetry,PIV）,对不同干涉程度下的大豆蛋白液双枪喷涂雾化场进行拍摄,利用PIV和Origin软件处理图像,得到速度数据并对比。结果 随着双枪间偏转角的增大,基线上的平均速度减小;偏转角为0°时,两喷雾粒子流在干涉线处碰撞,少有透过干涉线的粒子;在不同偏转角下,基线上的速度峰值从大到小为偏转角7°时的速度峰值、偏转角0°时的速度峰值、偏转角15°时的速度峰值;当偏转角为0°时,随着液压的增大,基线上的第2个速度谷值会右移,并且会增大直至消去。结论 当偏转角为0°、液压为0.24 MPa时,在基线上干涉区域内的速度最均匀。