Validation of finite-element models using full-field experimental data: Levelling finite-element analysis data through a digital image correlation engine

Full-field data from digital image correlation (DIC) provide rich information for finite-element analysis (FEA) validation. However, there are several inherent inconsistencies between FEA and DIC data that must be rectified before meaningful, quantitative comparisons can be made, including strain formulations, coordinate systems, data locations, strain calculation algorithms, spatial resolutions and data filtering. In this paper, we investigate two full-field validation approaches: (1) the direct interpolation approach, which addresses the first three inconsistencies by interpolating the quantity of interest from one mesh to the other, and (2) the proposed DIC-levelling approach, which addresses all six inconsistencies simultaneously by processing the FEA data through a stereo-DIC simulator to ‘level' the FEA data to the DIC data in a regularisation sense. Synthetic ‘experimental' DIC data were generated based on a reference FEA of an exemplar test specimen. The direct interpolation approach was applied, and significant strain errors were computed, even though there was no model form error, because the filtering effect of the DIC engine was neglected. In contrast, the levelling approach provided accurate validation results, with no strain error when no model form error was present. Next, model form error was purposefully introduced via a mismatch of boundary conditions. With the direct interpolation approach, the mismatch in boundary conditions was completely obfuscated, while with the levelling approach, it was clearly observed. Finally, the ‘experimental' DIC data were purposefully misaligned slightly from the FEA data. Both validation techniques suffered from the misalignment, thus motivating continued efforts to develop a robust alignment process. In summary, direct interpolation is insufficient, and the proposed levelling approach is required to ensure that the FEA and the DIC data have the same spatial resolution and data filtering. Only after the FEA data have been ‘levelled' to the DIC data can meaningful, quantitative error maps be computed.     

重力对大型环形可展天线展开动力学的影响研究

受限于火箭尺寸,大型星载环形天线在发射时收拢,入轨后在零重力环境下展至工作态。然而在地面实验重力环境下,克服重力的悬吊卸载系统并不能完全抵消重力场的全部影响,导致地面实验难以准确预测在轨展开动力学行为。因此,有必要借助仿真深入对比分析地面上重力加卸载展开与太空中无重力展开的动力学异同。该文基于柔性多体动力学的仿真方法,建立了天线在零重力和重力加卸载系统两种工况下的全尺寸展开动力学模型。仿真结果不但复现了地面实验的非同步展开现象,而且系统地给出了天线在整个展开历程中所有杆件载荷和驱动力的变化规律。并进一步基于能量分析,半解析地研究了重力对驱动力的影响。该文的研究不但有助于理解大型网状天线的展开动力学,为天线的机构设计、优化和地面实验设计提供技术支撑,而且提出的建模方法和得到的认识也可用于研究和理解其他大型网状天线。     

Fast Calculation of Field Currents and Reactances for Doubly Fed Generators With Rotor Duct Pieces

Doubly fed generators have been used as adjustable-speed pumped-storage generator motors and wind turbine generators. Accurate determination of field currents and reactances is important for the design of these machines. We propose a calculation method to obtain the field currents and reactances of machines with rotor duct pieces under any steady-state balanced load condition. The method links two-dimensional static finite-element analysis (FEA) with an approximate calculation to consider three-dimensional (3-D) skin effect in the duct pieces. Its advantage is that the computational time is much smaller than 3-D transient FEA when the slip frequency is not zero. The method will contribute to improvement of the design of doubly fed generators with rotor duct pieces. It was applied to a 395 MVA machine, and the calculated field currents agreed well with the measurements. Variation in the reactances due to saturation is also discussed     

Nanomechanical characterization of red blood cells using optical tweezers

Deformation behaviours of red blood cells (RBCs) have been studied by applying stretching forces via optical tweezers. Combined with finite-element analyses (FEA), the RBCs’ mechanical properties are determined quantitatively based on a best fitting between the experimental deformed geometries and the simulated counterparts. Experimentally, a silica beads attached erythrocyte is optical-mechanically stretched to different lengths. On the theoretical front, a large deformation model with Mooney-Rivlin constitutive equations has been simulated by using FEA to predict the cell deformation geometries. The numerically simulated transverse and longitudinal strains which are in a good agreement with the experimental measurements facilitate the determination of elastic constants of the cells.     

Matrix crack detection of CFRP using electrical resistance change with integrated surface probes

For the cryogenic tanks of next generation reusable launching vehicles, the laminated composite tank is one of the key technologies. For composite fuel tanks made from laminated carbon fibre reinforced polymers (CFRP), matrix cracking is a significant problem that may cause fuel leakage. In the present paper, an electrical resistance change method with integrated probes on a single side of the surface of a CFRP composite structure is adopted to detect the matrix cracking of the laminated composites. For a fuel tank structure made of a CFRP laminate, we cannot mount electrical probes on the end of structure or on the inside of the tank structure. We have to mount all probes only on the outside surface. The present method used finite element analyses (FEA) to search for the best placement of probes for matrix crack detection using a rectangular plate. To simulate the tank structure, all probes are placed on a single surface of the CFRP plate specimen. The present study adopted a four-probe method for measuring the electrical resistance change. The FEA revealed that the electrical resistance increases linearly with increase in the number of matrix cracks inside of the probes. By means of thin CFRP cross-ply laminate, the method was experimentally confirmed to be useful for detecting matrix crack density between the probes. Residual electrical resistance at the completely unloaded condition increased with increase in matrix crack density. Measurements of the residual electrical resistance enabled us to detect the matrix crack density without loading.     

Evaluation on mass sensitivity of SAW sensors for different piezoelectric materials using finite-element analysis

The mass sensitivity of the piezoelectric surface acoustic wave (SAW) sensors is an important factor in the selection of the best gravimetric sensors for different applications. To determine this value without facing the practical problems and the long theoretical calculation time, we have shown that the mass sensitivity of SAW sensors can be calculated by a simple three-dimensional (3-D) finite-element analysis (FEA) using a commercial finite-element platform. The FEA data are used to calculate the wave propagation speed, surface particle displacements, and wave energy distribution on different cuts of various piezoelectric materials. The results are used to provide a simple method for evaluation of their mass sensitivities. Meanwhile, to calculate more accurate results from FEA data, surface and bulk wave reflection problems are considered in the analyses. In this research, different cuts of lithium niobate, quartz, lithium tantalate, and langasite piezoelectric materials are applied to investigate their acoustic wave properties. Our analyses results for these materials have a good agreement with other researchers' results. Also, the mass sensitivity value for the novel cut of langasite was calculated through these analyses. It was found that its mass sensitivity is higher than that of the conventional Rayleigh mode quartz sensor.     

Fast Analytical Determination of Aligned and Unaligned Flux Linkage in Switched Reluctance Motors Based on a Magnetic Circuit Model

The main advantages of the switched reluctance motor are high torque, wide speed range, simple structure and fault tolerance. Because a switched reluctance motor has inherently nonlinear magnetic characteristics and a doubly salient pole structure, a finite-element analysis approach (FEA) is often adopted to obtain accurate magnetic representation. However, the solution time can be large for a FEA simulation if the mesh is detailed and/or many simulations are required. We propose a rapid analytical solution for determining the aligned and unaligned flux linkage using a magnetic circuit model. We present a simple method for obtaining the air-gap permeance for unaligned linkage. The results of our method agree well with FEA solutions.      

What makes an accurate and reliable subject-specific finite element model? A case study of an elephant femur

Finite element modelling is well entrenched in comparative vertebrate biomechanics as a tool to assess the mechanical design of skeletal structures and to better comprehend the complex interaction of their form–function relationships. But what makes a reliable subject-specific finite element model? To approach this question, we here present a set of convergence and sensitivity analyses and a validation study as an example, for finite element analysis (FEA) in general, of ways to ensure a reliable model. We detail how choices of element size, type and material properties in FEA influence the results of simulations. We also present an empirical model for estimating heterogeneous material properties throughout an elephant femur (but of broad applicability to FEA). We then use an ex vivo experimental validation test of a cadaveric femur to check our FEA results and find that the heterogeneous model matches the experimental results extremely well, and far better than the homogeneous model. We emphasize how considering heterogeneous material properties in FEA may be critical, so this should become standard practice in comparative FEA studies along with convergence analyses, consideration of element size, type and experimental validation. These steps may be required to obtain accurate models and derive reliable conclusions from them.     

Effect of Gravity During Condensation of R134a in a Circular Minichannel

A number of steady-state simulations of condensation of R134a inside a 1 mm i.d. circular minichannel at two far different mass flux values are proposed. The VOF method is used to track the vapour-liquid interface. The first simulations are run at G?=?100 kg m^???2 s^???1 and G?=?800 kg m^???2 s^???1 assuming that the channel displays horizontal orientation. The effects of interfacial shear stress, gravity and surface tension are all taken into account in this case and the results are validated by means of experimental data already available. As a further step, the same simulations have been run under normal gravity conditions but vertical downflow and finally assuming zero-gravity conditions. The condensation process is found to be gravity dominated at low mass flux, and thus very different results are obtained when neglecting gravity at this mass flux. An opposite result is achieved at high mass flux, as expected from the increased relative importance of interfacial shear stress in this case. The present results also allow to verify the influence of the surface tension effect during condensation in the circular cross section minichannel.     

Modeling automotive gas-exchange solenoid valve actuators

We develop a finite-element analysis (FEA) model to describe transient and static operation of gas-exchange valves. Such valves, directly controlled by solenoids, are a promising method for enhancing automotive engine efficiency. The FEA model is validated by experimental testing on an actual automotive prototype valve. We show that a nonlinear lumped-parameter model that uses FEA results also closely matches experimental data. The lumped-parameter model is suitable for optimization of design and can be readily used for closed-loop simulation. We present a simplified lumped-parameter model to facilitate controller design. Finally, we compare a dynamic open-loop simulation with experimental results.     

Field weakening for radial force reduction in brushless permanent-magnet DC motors

In brushless permanent-magnet dc (BLDC) machines, the attraction between the rotor permanent magnets and the stator iron causes radial stator forces that excite the stator structural response and radiate unwanted acoustic noise. In this paper, we develop an analytical model that predicts rotor torque and radial force ripple as functions of the stator currents. The model shows that field weakening of sinusoidally commutated BLDC machines can reduce radial forces but requires higher currents to maintain the desired torque. We confirmed the analytical results numerically on a BLDC motor using ANSYS finite-element analysis and found a 30% reduction in force ripple at no load.     

Axially laminated reluctance motor: analytical and finite-elementmethods for magnetic analysis

This paper describes two methods - one numerical, the other analytical for analyzing an axially laminated reluctance (ALAREL) motor. The numerical method, finite-element analysis (FEA), is generally used for estimation of the motor magnetic loading and capability for given stator currents. The accuracy of the FEA prediction is confirmed by a comparison between computed and measured magnetic quantities. The analytical model allows a rapid evaluation of the flux density in different parts of the motor, as well as of the capability for given stator current. It uses a distributed parameter network. The paper compares the results obtained from this network with those obtained from FEA. The methods agree well. Finally, the paper uses the two methods to investigate the effect of the quality of the magnetic material as well as the effect on motor performance of a nonuniform distribution of the laminations in the rotor. It shows that, with grain-oriented steel and nonuniformly distributed laminations, a higher flux linkage can be obtained. Consequently, lower current is required to develop the same motor torque     

Zero-gravity experiments on superfluid helium aboard a rocket

Superfluid experiments in the zero-gravity state were performed with a cryostat aboard the rocket S-520. Observation of the dynamic behaviour of liquid helium (He ll), verification of a porous-plug phase separator with a controlling heater, investigation of thermal behaviour of superfluid thin films and a superfluid heat pipe, and the noise test of a cooled FET pre-amplifier for i.r. detectors were performed. The experiments showed that the initial bulk fluid motions were rather quickly damped and a stable liquid containment in the cryostat was achieved. Very good temperature uniformity was attained throughout the vessel. The porous-plug was found to work as well as predicted in the zero-gravity state. The adsorbed film became thicker in the reduced gravity state than on the ground.     

Die Teildispersion von Gläsern Teil II

The measurement of displacement vectors in holographic interferometry requires a knowledge of the coordinates of the corresponding points on the object surface, and in some cases a knowledge of the direction of illumination as well. Usually these quantities are determined by geometrical measurements which can be complicated and prone to error. It is shown how these geometrical quantities can also be determined by interferometric evaluation—by the same kind of measurement as the subsequent deformation analysis.     

Free and forced vibration analysis of thin, laminated, cylindrically curved panels

A comprehensive vibration study of thin, laminated, cylindrically curved shell panels (based on the shell theory of Love with a modification by Arnold and Warburton) is conducted by using the h-p version of the finite-element method (FEM). Polynomially enriched stiffness and mass matrices are derived from classical shell theory using Symbolic Computing, and then stored in algebraic form for a single, generic element. A number of such elements may then be combined to form the global stiffness and mass matrices for a more general co-axial and/or co-circumferential assembly. Any of the classical edge conditions, or point corner supports, may be accommodated in the analysis; forcing may be applied through one or more point forces acting normal to the shell surface. Excellent agreement has been found with the work of other investigators, and some new results are presented for a multiply supported curved panel made from the aluminium-glass-fibre hybrid GLARE. The h-p method is shown, by example, to offer an efficient means of conducting typical repetitive sensitivity analyses, such as varying the fibre orientation and the stacking sequences of a given panel.     

Design of a Halbach Magnet Array Based on Optimization Techniques

We have designed a Halbach magnet array by using a numerical optimization method based on finite-element analysis. The magnetization direction of each element is defined as the design variable. The optimal magnet arrays composed of two and three linear magnet layers can then be investigated to increase the attractive, repulsive, and tangential magnetic forces between magnet layers. We have applied a magnet array maximizing the tangential force to a torsional spring composed of two- and three-magnet rings. The two-dimensional finite-element analysis incorporates optimization techniques such as the sequential linear programming and the adjoint variable method.      

A novel switched reluctance motor with C-core stators

This paper presents a novel switched reluctance motor (SRM) design in which the stator is simply formed from C-cores. Unlike conventional SRMs, the windings of the new motor can be individually wound into the stator cores without complex winding equipment. Because of the inherent axial field distribution, this type of SRM requires a three-dimensional (3-D) finite-element analysis (FEA) model for detailed flux analysis. This paper proposes an approximated two-dimensional FEA model to speed up computational time. In addition, since the proper current that ensures operation in the saturated region (to maximize torque and efficiency) is often hard to determine systemically, the paper proposes a simple method to determine the optimum operating current so that one can easily decide the rated current and also obtain the maximum motor efficiency. Finally, the paper compares some characteristics of a traditional SRM with those of the proposed SRM. The comparison shows that the proposed SRM performs well in terms of torque and efficiency, and provides a higher degree of flexibility in winding design.     

Modelling cell motility and chemotaxis with evolving surface finite elements

We present a mathematical and a computational framework for the modelling of cell motility. The cell membrane is represented by an evolving surface, with the movement of the cell determined by the interaction of various forces that act normal to the surface. We consider external forces such as those that may arise owing to inhomogeneities in the medium and a pressure that constrains the enclosed volume, as well as internal forces that arise from the reaction of the cells'' surface to stretching and bending. We also consider a protrusive force associated with a reaction–diffusion system (RDS) posed on the cell membrane, with cell polarization modelled by this surface RDS. The computational method is based on an evolving surface finite-element method. The general method can account for the large deformations that arise in cell motility and allows the simulation of cell migration in three dimensions. We illustrate applications of the proposed modelling framework and numerical method by reporting on numerical simulations of a model for eukaryotic chemotaxis and a model for the persistent movement of keratocytes in two and three space dimensions. Movies of the simulated cells can be obtained from http://homepages.warwick.ac.uk/∼maskae/CV_Warwick/Chemotaxis.html.     

Design of Emergency Aircraft Repairs Using Composite Patches

The increasing demand for fatigue life extension of both military and civilian aircraft has led to advances in repair technology for cracked metallic structures. Conventional structural repairs may significantly degrade the aircraft fatigue life and lower its aerodynamic performance. Adhesively bonded composite reinforcement is a new technology of great importance due to the remarkable advantages obtained, such as mechanical efficiency and repair time and cost reduction. In this article, bonded composite patch repairs were designed for quick application to aircraft under emergency conditions, such as aircraft battle damage repair (ABDR). A formulated method was developed, to be applied when damage has to be restored quickly, without restrictions to safety of flight. Different damage cases were investigated using finite-element analysis (FEA), taking into account specific parameters of the structure under repair. Based on the FEA results, a quick design procedure using composite patch repairs for the most frequent damage cases is proposed.     

Analysis of large synchronous machines with axial skew, part 1: flux density and open-circuit voltage harmonics

An approach to predict the air-gap flux density and voltage harmonics in large synchronous machines is presented. Combining a small number of solutions of magnetostatic finite-element analysis (FEA) with harmonic analysis, the approach allows designers to rapidly investigate the source of flux density harmonic predictions. The method provides the ability to trace flux density predictions to individual mmf-permeance combinations and applies to machines with both integral and fractional slot windings, with arbitrary skew. Predicted results are compared with that of measured values of open- and short-circuit air-gap flux density in one machine. Open-circuit voltage predictions for five machines are compared with measurements and predictions using commercial FEA.