Analytical model for instantaneous lift and shape deformation of an insect-scale flapping wing in hover

In the analysis of flexible flapping wings of insects, the aerodynamic outcome depends on the combined structural dynamics and unsteady fluid physics. Because the wing shape and hence the resulting effective angle of attack are a priori unknown, predicting aerodynamic performance is challenging. Here, we show that a coupled aerodynamics/structural dynamics model can be established for hovering, based on a linear beam equation with the Morison equation to account for both added mass and aerodynamic damping effects. Lift strongly depends on the instantaneous angle of attack, resulting from passive pitch associated with wing deformation. We show that both instantaneous wing deformation and lift can be predicted in a much simplified framework. Moreover, our analysis suggests that resulting wing kinematics can be explained by the interplay between acceleration-related and aerodynamic damping forces. Interestingly, while both forces combine to create a high angle of attack resulting in high lift around the midstroke, they offset each other for phase control at the end of the stroke.     

基于翼尖支撑和螺旋桨独立支撑的螺旋桨滑流影响实验研究

针对倾转旋翼机,开展了悬停状态气动干扰风洞试验和数值模拟研究。试验中,测量了悬停状态下的旋翼升力、扭矩以及半模机翼的气动力。同时,采用运动嵌套网格方法,通过求解N-S方程对机翼倾角0°和90°两种状态进行数值模拟,开展了数值模拟与风洞试验的相关性分析研究,验证了该数值模拟方法的有效性。结果表明：不考虑机身气动力时,孤立旋翼、机翼攻角0°和机翼攻角90°三种状态下旋翼气动特性差异不明显;考虑机身气动力时,机翼攻角0°时,机身产生约18.2%向下载荷,单片桨叶和机身出现强烈非定常气动特性,其中桨叶升力系数动态值与平均值比为9.8%,机身升力系数动态值与平均值比为18.38%。

     

Cost reduction techniques for the design of non‐linear flapping wing structures

This work details a computational framework for gradient‐based optimization of a non‐linear flapping wing structure with a large number of design variables, where analytical sensitivities of the unsteady finite element system are computed using the adjoint method. Two techniques are used to reduce the large computational cost of this structural design process. The first projects the finite element system onto a reduced basis of POD modes. The second uses a monolithic time formulation with spectral elements, and can be used to compute only the desired time‐periodic response. Results are given in terms of the trade‐off between accuracy and computational efficiency of these methods for both system response and adjoint computations, for a variety of mesh/time step refinements, degrees of non‐linearity (i.e. weakly or strongly non‐linear), and harmonic content. The work concludes with the structural design of a flapping wing: the elastic deformation at the wingtip is minimized through the flapping stroke by varying the thickness of each finite element. Significant improvements in computational cost are obtained at little expense to the accuracy of the results obtained via design optimization. Published in 2011 by John Wiley & Sons, Ltd.     

Current trends in modelling research for turbulent aerodynamic flows

The engineering tools of choice for the computation of practical engineering flows have begun to migrate from those based on the traditional Reynolds-averaged Navier-Stokes approach to methodologies capable, in theory if not in practice, of accurately predicting some instantaneous scales of motion in the flow. The migration has largely been driven by both the success of Reynolds-averaged methods over a wide variety of flows and the inherent limitations of the method itself. Practitioners, emboldened by their ability to predict a wide variety of statistically steady equilibrium turbulent flows, have now turned their attention to flow control and non-equilibrium flows, i.e. separation control. This review gives some current priorities in traditional Reynolds-averaged modelling research as well as some methodologies being applied to a new class of turbulent flow control problem.     

Distributed and non-steady-state modelling of an air cooler

The refrigerant flow inside the coils of a dry expansion plate-finned air cooler can be distinguished into two completely different types: two-phase flow and single-phase flow. The most difficult part of non-steady-state modelling of an air cooler is to describe the liquid and vapour mass transport phenomena occurring in the two-phase flow region, as this determines the boundary position between the two regions and then the superheat temperature, which is in turn the feedback signal of the thermostatic expansion valve. In fact, the mass transport is mainly governed by the momentum exchange between refrigerant liquid and vapour, which is usually called slip-effect. Because the momentum or force equilibrium is so fast compared to the thermal equilibrium, the slip-effect can be considered as a steady-state phenomenon. With this assumption, the mass transport in an air cooler can be described by using a simple propagation equation. The steady-state slip-effect, however, is found by solving the momentum equations for one-dimensional two-phase flow using advanced computer packages such as . This paper presents the derivation of the equations in non-steady-state modelling of an air cooler as well as the results obtained from the model. Because the model is purely distributed, it is applicable to various kinds of tube circuit arrangements of air coolers. The purpose of the model is studying and optimization of non-steady-state behaviour of refrigerating systems with capacity control.     

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.     

Agent-based organizational modelling for analysis of safety culture at an air navigation service provider

Assessment of safety culture is done predominantly by questionnaire-based studies, which tend to reveal attitudes on immaterial characteristics (values, beliefs, norms). There is a need for a better understanding of the implications of the material aspects of an organization (structures, processes, etc.) for safety culture and their interactions with the immaterial characteristics. This paper presents a new agent-based organizational modelling approach for integrated and systematic evaluation of material and immaterial characteristics of socio-technical organizations in safety culture analysis. It uniquely considers both the formal organization and the value- and belief-driven behaviour of individuals in the organization. Results are presented of a model for safety occurrence reporting at an air navigation service provider. Model predictions consistent with questionnaire-based results are achieved. A sensitivity analysis provides insight in organizational factors that strongly influence safety culture indicators. The modelling approach can be used in combination with attitude-focused safety culture research, towards an integrated evaluation of material and immaterial characteristics of socio-technical organizations. By using this approach an organization is able to gain a deeper understanding of causes of diverse problems and inefficiencies both in the formal organization and in the behaviour of organizational agents, and to systematically identify and evaluate improvement options.     

A profile of aerodynamic research in VSSC with application to satellite launch vehicles

Some of the aerodynamic and fluid dynamic problems associated with the satellite launch vehicles of the Indian Space programme are discussed in this paper. Taking into account the aerodynamic flight profile of a launch vehicle and also the configurations of interest, the topics considered are vehicle lift-off fluid dynamics, strap-on aerodynamics, strap-on separation problem, stage separation aerodynamics, boat-tail aerodynamics, hypersonic flow problems and some special fluid dynamic problems of interest like nozzle flows and shock wave-boundary layer interaction. The topics are mostly related to external fluid dynamics though certain aspects of internal ballistics are also touched upon. More emphasis is placed on the results and their applications rather than on the methodology used. Also, the main features of two special wind tunnel facilities developed at the Vikram Sarabhai Space Centre (VSSC) — the hypersonic wind tunnel and the heat transfer facility — are discussed briefly. It is hoped that this paper will give an idea of the profile of aerodynamic research being conducted inVSSC with application to satellite launch vehicles in view.     

Control of the aerodynamic characteristics of wing airfoils by nonstationary energy supply in transonic flow

The possibility of controlling the aerodynamic characteristics of wing airfoils in transonic regimes of flight using one-sided pulse-periodic energy supply has been studied. The flow structure near the symmetric airfoil at different angles of attack and its aerodynamic characteristics as functions of the value of energy in its nonsymmetric (about the airfoil) supply have been determined by numerical solution of two-dimensional nonstationary gasdynamic equations. A comparison of the obtained results and the data of calculation of flow past the airfoil at different angles of attack without energy supply has been made. It has been established that a prescribed lift can be obtained, using energy supply, with a much higher fineness ratio of the airfoil than that in the case of flow past it at an angle of attack. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 1, pp. 18–22, January–February, 2009.     

On modelling an iceberg embedded in shore-fast sea ice

The effect of long ice-coupled waves impinging on a tabular iceberg, an ice island or a thick sea ice floe trapped within a thin veneer of shore fast sea ice of substantial extent is considered. The waves most likely originate as ocean waves in the open sea beyond the fast ice boundary, from where they propagate into the sea ice. There their character is altered because of the flexural properties of the ice. The geophysical/engineering problem posed is solved by a Green's function method that redevelops, for a different surface boundary condition, an earlier study concerned with a freely floating ice floe. Reflection and transmission coefficients for the berg are found to depend strongly on its thickness and length. Amongst other things, the work relates to the operational safety of natural and artificially thickened Arctic ice platforms located in a contiguous ice sheet.     

Using an air cycle heat pump system with a turbocharger to supply heating for full electric vehicles

Air cycle heat pump has large potentials in heating applications. However, a key challenge faced nowadays is the matching problem between its expander and compressor. This paper presents the performance evaluation of an air cycle heat pump system integrated with a turbocharger, a blower and a regenerated heat exchanger. A thermodynamic model for this system is first developed and the relationships between the system performance and the operating parameters are investigated. Then, the performance of three different air cycle heat pumps with a blower installed before the compressor, and a blower installed before the turbine, and with an expander, are numerically simulated. The results indicated that the blower installed before the compressor can achieve a higher heating capacity and thus a higher COP. Finally, the heating power consumption of air cycle heat pump was compared with the PTC and the vapor compression heat pump of the full electric vehicle.     

浅述微模块数据中心制冷空调方案的选择

简述了微模块数据中心概念及发展现状,分别介绍了单柜微模块数据中心、单列微模块数据中心的系统组成及空调方案。假设一个组级微模块样本,以列间机房空调方案的数值为基准值,按照单机柜散热量为3 kW^10 kW的不同情况,分别计算出1~10个不同模块数量时下送风机房空调方案的面积比及运行功率比。应依据不同应用场景及机柜散热量选择合理的空调机组方案。     

一种车用膜式空气弹簧有效面积的预测方法

空气弹簧帘线-橡胶复合材料结构具有刚度可变、轻量化、高度可调、隔振效果好等优势,在汽车“新四化”的发展趋势下,车用空气弹簧力学成为学术和工程研究热点。但其受力的“有效面积”这一重要参数还未建立完善理论模型。结合复合材料力学特性与几何学特征,提出一种车用膜式空气弹簧有效面积理论分析与预测方法。给出了空气弹簧有效面积理论预测表达式,体现了空气弹簧气囊内压强、空气弹簧高度等因素对有效面积的综合影响。利用力学综合实验台架设计实验,对某型号空气弹簧进行有效面积测量,实验结果与理论分析的对比显示在实验测量的范围内,理论预测的有效面积误差在1%以内,表明了这种理论分析方法的合理性。这种方法对进行有效面积的预测、空气弹簧的准确建模及进一步进行高精度的车高控制具有一定的指导意义。      

Finite element modelling of high air pressure forming processes for polymer sheets

 In this paper we describe the mathematical modelling and computational simulation of the high air pressure (HAP) thermoforming process which is used in the creation of thin walled polymeric structures. This involves, using data from material tests, an elastic-plastic constitutive equation valid for large deformations and a constrained deformation in which there is frictional contact between the polymeric sheet and a constraining surface (the mould surface). Despite a number of simplifying assumptions and some uncertainities in the mathematical model the finite element computations presented predict quite well the actual shape and thickness distribution which are found on sample products. RID="*" ID="*" The work for this paper was supported in part by the EPSRC under grant GR/M38070 for the Materials Processing for Engineering Applications (MaPEA) programme. Dedicated to the memory of Prof. Mike Crisfield, for his cheerfulness and cooperation as a colleague and friend over many years. The authors would like to acknowledge the help of the Department of Mechanical Engineering at the University of Leeds in allowing testing equipment to be used and also for the contribution of A. M. Wiese to this testing. Further testing was undertaken at the IRC in Polymer Science and Technology, University of Bradford. The contribution of Drs. P. Caton-Rose and J. Sweeney to this testing is also gratefully acknowledged.     

燃料电池车用超高速空压机临界转速分析与提高措施

针对空压机进气增压可提高大功率燃料电池发动机性能和功率密度,但空压机超高速电机转子-轴承系统存在共振失稳乃至断轴的实际工程问题.本文采用有限元法建立空压机高速电机轴承-转子系统动力学模型,基于ANSYS软件对某超高速永磁电机转子进行动力学仿真,分析某燃料电池空压机高速电机转子-轴承系统的临界转速,揭示轴承刚度、轴承位置以及转轴质量等因素对临界转速的影响规律,并提出改善措施.结果显示,增大轴承刚度、适当减小轴承跨距以及减轻转轴的质量可以有效地增加转子轴承系统的临界转速.     

Performance influences on metamodelling for aerodynamic surrogate-based optimization of an aerofoil

It is now common in aerospace design to build and adapt a surrogate model to use during optimization. This process starts with a training stage based on an initial database of high-fidelity results. However, it is difficult to have a priori knowledge of the database size necessary to achieve a particular level of accuracy. Thus, many optimization processes, such as aerodynamic optimization, start with a large number of initial cases, which usually implies long computational times on high-performance computers. This article identifies and analyses key variables which influence the surrogate performance, as applied to the aerodynamic optimization of an aerofoil, such as the database size, the number of geometric design parameters, variation from the baseline geometry, Reynolds number and angle of attack. Their relationship is explored by means of sensitivity analysis and a multivariable polynomial model, and general guidelines for the selection of the most suitable database size are presented.     

Feedback control of vibrations in an extendible cantilever wing

The vibrations of a cantilever wing which may extend or contract during flight are studied. A mathematical model for the coupled bending and torsional vibrations of an extendible wing is developed. It is found that the extending or contracting motion of the wing has a stabilizing or destabilizing effect on the wing vibrations respectively. A Galerkin-type approximation based on an appropriate time-dependent basis for the solution space is used to obtain an approximate finite-dimensional model for simulation studies. A simple implementable nonlinear feedback control for damping the motion-induced vibrations is derived by considering the time rate-of-change of the total vibrational energy of the wing. Simulation studies show that the proposed control law is effective in reducing the peak amplitude of the motion-induced vibrations.     

电动汽车热泵空调冷凝器空气侧性能模拟与结构优化

电动汽车热泵空调是用于解决电动汽车冬季无法制热的问题,本文设计一种用于电动汽车热泵空调的新型平行流换热器。通过对换热器的冷凝工况进行数值模拟,得到不同入口速度条件下的换热及阻力特性,并与经验公式进行比较,验证数值模拟的正确性。分析不同的翅片倾斜角度及翅片间距下换热及阻力特性变化,得到优化的结构参数,为电动汽车热泵空调换热器设计提供依据。