Distortion cancellation of frequency converted pulses with simple linear signal processing and application to frequency modulation to amplitude modulation conversion in high power lasers

It is known that a linear filter may be easily compensated with its inverse transfer function. However, it was shown that this approach could also be valid even for such a complex nonlinear system as frequency conversion. As a matter of fact, it is possible to at least partly precompensate for distortions occurring within, or even downstream from, frequency conversion crystals with a simple linear optical filter set upstream. In this paper, we give the theoretical background and derive the optimum precompensation filter from simple analytical formulas even in the case of saturation. We first show the relevance of our approach for Gaussian pulses: the pulse may be short or not and chirped or not, and the same linear precompensation filter may be used as long as saturation is not reached. We then study the case of phase-modulated pulses, as can be found on high power lasers such as lasers for fusion. We show that previous experimental results are in perfect agreement with these calculations. Finally, justified by our simple analytical formulas, we present a rigorous parametrical study giving the distortion reduction for any second and third harmonic generation system in the case of phase-modulated pulses.     

The role of passive avian head stabilization in flapping flight

Birds improve vision by stabilizing head position relative to their surroundings, while their body is forced up and down during flapping flight. Stabilization is facilitated by compensatory motion of the sophisticated avian head–neck system. While relative head motion has been studied in stationary and walking birds, little is known about how birds accomplish head stabilization during flapping flight. To unravel this, we approximate the avian neck with a linear mass–spring–damper system for vertical displacements, analogous to proven head stabilization models for walking humans. We corroborate the model''s dimensionless natural frequency and damping ratios from high-speed video recordings of whooper swans (Cygnus cygnus) flying over a lake. The data show that flap-induced body oscillations can be passively attenuated through the neck. We find that the passive model robustly attenuates large body oscillations, even in response to head mass and gust perturbations. Our proof of principle shows that bird-inspired drones with flapping wings could record better images with a swan-inspired passive camera suspension.     

From daily movements to population distributions: weather affects competitive ability in a guild of soaring birds

The ability of many animals to access and exploit food is dependent on the ability to move. In the case of scavenging birds, which use soaring flight to locate and exploit ephemeral resources, the cost and speed of movement vary with meteorological factors. These factors are likely to modify the nature of interspecific interactions, as well as individual movement capacity, although the former are less well understood. We used aeronautical models to examine how soaring performance varies with weather within a guild of scavenging birds and the consequences this has for access to a common resource. Birds could be divided broadly into those with low wing loading that are more competitive in conditions with weak updraughts and low winds (black vultures and caracaras), and those with high wing loading that are well adapted for soaring in strong updraughts and moderate to high winds (Andean condors). Spatial trends in meteorological factors seem to confine scavengers with high wing loading to the mountains where they out-compete other birds; a trend that is borne out in worldwide distributions of the largest species. However, model predictions and carcass observations suggest that the competitive ability of these and other birds varies with meteorological conditions in areas where distributions overlap. This challenges the view that scavenging guilds are structured by fixed patterns of dominance and suggests that competitive ability varies across spatial and temporal scales, which may ultimately be a mechanism promoting diversity among aerial scavengers.     

Short time measurement of frequency and amplitude in the presenceof noise

A method for the measurement of amplitude and frequency of sinusoidal signals in the presence of noise during a few periods is proposed. The method presented gives one the possibility to make measurement of frequency and amplitude of sinusoidal signals without noise during a very short time (less than a half period) and in the case of signals corrupted by noise (during only two periods) with sufficiently acceptable results. The theoretical basis, several measurement algorithms and selected results of computer simulation are presented. The performance is compared to that of known methods     

On the quasi-steady aerodynamics of normal hovering flight part II: model implementation and evaluation

This paper introduces a generic, transparent and compact model for the evaluation of the aerodynamic performance of insect-like flapping wings in hovering flight. The model is generic in that it can be applied to wings of arbitrary morphology and kinematics without the use of experimental data, is transparent in that the aerodynamic components of the model are linked directly to morphology and kinematics via physical relationships and is compact in the sense that it can be efficiently evaluated for use within a design optimization environment. An important aspect of the model is the method by which translational force coefficients for the aerodynamic model are obtained from first principles; however important insights are also provided for the morphological and kinematic treatments that improve the clarity and efficiency of the overall model. A thorough analysis of the leading-edge suction analogy model is provided and comparison of the aerodynamic model with results from application of the leading-edge suction analogy shows good agreement. The full model is evaluated against experimental data for revolving wings and good agreement is obtained for lift and drag up to 90° incidence. Comparison of the model output with data from computational fluid dynamics studies on a range of different insect species also shows good agreement with predicted weight support ratio and specific power. The validated model is used to evaluate the relative impact of different contributors to the induced power factor for the hoverfly and fruitfly. It is shown that the assumption of an ideal induced power factor (k = 1) for a normal hovering hoverfly leads to a 23% overestimation of the generated force owing to flapping.     

The design and flight performance of advanced carrier projectiles at hypervelocity

The ability to destroy the key assets of an enemy is an important goal of an attacking force and requires an agile and adaptable military capability effective against a number of very different threats. In ballistics, this contributes to an interest in moving to higher projectile velocities to benefit from the potential to increase the range of a projectile. It also generates an interest in the study of the benefits of increased kinetic energy that can be delivered to a target. The paper describes the work to produce a projectile design solution under certain constraints but with an impact velocity of about 2 km s⁻¹, good in-bore and in-flight stability and acceptable retardation at the target. The paper shows that a projectile with these characteristics was achieved with a projectile mass of 8.1 kg. The second phase of the research programme was concerned with building a hollow aluminium carrier shell. The paper describes the research to assess the integrity and stability of the projectile under gun launch conditions, to demonstrate acceptable dispersion and impact yaw at the target and to demonstrate a minimum impact velocity of 2.1 km s⁻¹.     

非线性振动系统极限环振幅与频率的控制

研究非线性振动系统极限环振幅与频率的控制.利用多尺度法,计算了动力系统Hopf分岔点附近的正规型,获得极限环振幅及频率与反馈系数的近似解析关系.通过选择适当的反馈系数,可对极限环的振幅与频率进行控制.选取一类三维系统作为算例,讨论并比较了几类线性、非线性反馈控制器的控制效果,所得的解析结果与数值模拟的结果基本吻合.     

The effects of frequency and amplitude on the powder coating of fluidizing particles in a vibro-fluidized bed

Glass beads were coated in a vibro-fluidized bed by atomizing a fine silica powder and polyvinyl alcohol solution. The coating efficiency and weight fraction of the agglomerated particles were measured under various experimental conditions, and their dependencies on the amplitude and frequency of vibration were investigated. When the humidity of the outlet gas was high, high coating efficiency and agglomeration of core particles were observed. The efficiency of the powder coating decreased as the amplitude and the frequency increased, while the agglomeration was prevented by the addition of vibration. It was confirmed that high-quality and high-efficiency coating, where few agglomerates were produced and silica powder was utilized efficiently, was possible using the vibro- fluidized bed with adequate amplitude and frequency.     

The role of flight experiments in the development of cryogenic fluid management technologies

This paper reviews the history of cryogenic fluid management technology development and infusion into both the Saturn and Centaur vehicles. Ground testing and analysis proved inadequate to demonstrate full scale performance. As a consequence flight demonstration with full scale vehicle was required by both the Saturn and Centaur programs to build confidence that problems were addressed. However; the flight vehicles were highly limited on flight instrumentation and the flight demonstration “locked-in” the design without challenging the function of design elements. Projects reviewed include: the Aerobee Sounding Rocket Cryogenic Fluid Management (CFM) tests which served as a valuable stepping stone to flight demonstration and built confidence in the ability to handle hydrogen in low gravity; the Saturn IVB Fluid Management Qualification flight test; the Atlas Centaur demonstration flights to develop two burn capability; and finally the Titan Centaur two post mission flight tests.     

电力系统低频振荡中的非线性奇异现象

低频振荡是影响电力系统稳定运行的一个重要方面.阐述了低频振荡产生的机理和分析方法,利用Hopf分歧理论,针对一个四阶模型的单机无穷大系统,分析了在临界点附近时可能出现的非线性奇异现象.研究表明:由于分歧的存在,导致系统稳定性在临界点附近出现奇异,从而影响了电力系统的稳定域.     

The role of LNG in energy supply

This paper reviews the current status of base-load LNG plants involved in international trade. In the historical review of the development of of the LNG industry brief references are made to peak-shaving LNG plants. The paper also describes the complex co-operative effort required to organize international LNG projects, and presents technical information concerning the refrigeration aspects of base-load LNG projects. The information includes brief reviews of liquefaction cycles, storage tanks, types of ships, receiving terminals, and the potential for utilizing the refrigeration available in revaporized LNG.     

The status quo and prospect of biomass power generation in China

Facing the pressure of fossil energy exhaustion and environment pollution, people begin to search for clean and renewable energy to partly substitute fossil energy and realize sustainable development. As the fourth type of energy, biomass energy has many advantages: wide distribution, large quantity, being renewable, clean, can be stored and transported and so on. By adopting thermo-chemical and biochemical technologies, biomass energy can be converted to high quality solid, gaseous and liquid energy products, and provide convenient heat and power energy for human being’s production and daily life. In January 2006, China Renewable Energy Law began to be implemented, biomass energy power generation industry has been developing rapidly in China. This paper will present the status quo of biomass power generation industry in China and also introduce briefly the future development models.     

循环荷载频率和幅值对有砟道床动力行为的影响

为研究有砟道床在长期反复列车荷载作用下的动态响应及劣化规律,利用离散元分析软件PFC并考虑道砟的真实外形,建立可考虑道砟破碎劣化的有砟道床离散元计算模型,研究分析了不同频率、不同幅值循环荷载作用下有砟道床的振动响应、道砟受力以及道砟摩擦耗能、破碎行为。结果表明:道砟的振动加速度随动荷载频率和幅值的提高而非线性增长;道砟所受的接触力主要受荷载幅值影响,受荷载频率影响较小,荷载幅值提高,道砟接触力增大;提高荷载频率和幅值均会增加道砟的摩擦耗能,在荷载作用初期,道砟的摩擦耗能有一个突增过程;动荷载的幅值对道砟的破碎行为占主导因素,荷载幅值较低时道砟稳定在较低破碎水平,荷载幅值较高时道砟破碎现象加剧并受荷载频率影响,此时荷载频率越高道砟破碎越严重。     

Objectively identifying landmark use and predicting flight trajectories of the homing pigeon using Gaussian processes

Pigeons home along idiosyncratic habitual routes from familiar locations. It has been suggested that memorized visual landmarks underpin this route learning. However, the inability to experimentally alter the landscape on large scales has hindered the discovery of the particular features to which birds attend. Here, we present a method for objectively classifying the most informative regions of animal paths. We apply this method to flight trajectories from homing pigeons to identify probable locations of salient visual landmarks. We construct and apply a Gaussian process model of flight trajectory generation for pigeons trained to home from specific release sites. The model shows increasing predictive power as the birds become familiar with the sites, mirroring the animal''s learning process. We subsequently find that the most informative elements of the flight trajectories coincide with landscape features that have previously been suggested as important components of the homing task.     

空调水系统变频节能改造全年节电率预测方法探讨

中央空调变频节能改造是合同能源管理产业重要的技术方向之一。变频节能改造全年节电率预测的准确性关系到改造项目的可行性、业主的积极性、节能服务公司的投资风险及回收年限,因此探讨研究科学合理的节电率预测方法十分必要。本文分析变流量情况下冷冻水泵、冷却水泵的节电率及变流量对冷水机组COP的影响,并在此基础上,对现行变频节能改造全年节电率预测方法进行改进,最后通过实例论证这一节电率预测方法的准确性。     

The wake of hovering flight in bats

Hovering means stationary flight at zero net forward speed, which can be achieved by animals through muscle powered flapping flight. Small bats capable of hovering typically do so with a downstroke in an inclined stroke plane, and with an aerodynamically active outer wing during the upstroke. The magnitude and time history of aerodynamic forces should be reflected by vorticity shed into the wake. We thus expect hovering bats to generate a characteristic wake, but this has until now never been studied. Here we trained nectar-feeding bats, Leptonycteris yerbabuenae, to hover at a feeder and using time-resolved stereoscopic particle image velocimetry in conjunction with high-speed kinematic analysis we show that hovering nectar-feeding bats produce a series of bilateral stacked vortex loops. Vortex visualizations suggest that the downstroke produces the majority of the weight support, but that the upstroke contributes positively to the lift production. However, the relative contributions from downstroke and upstroke could not be determined on the basis of the wake, because wake elements from down- and upstroke mix and interact. We also use a modified actuator disc model to estimate lift force, power and flap efficiency. Based on our quantitative wake-induced velocities, the model accounts for weight support well (108%). Estimates of aerodynamic efficiency suggest hovering flight is less efficient than forward flapping flight, while the overall energy conversion efficiency (mechanical power output/metabolic power) was estimated at 13%.