A broadbanded pressure differential wave energy converter based on dielectric elastomer generators

Nonlinear Dynamics - Dielectric elastomer generators (DEGs) are a promising option for the implementation of affordable and reliable sea wave energy converters (WECs), as they show considerable...     

Control of an oscillating water column wave energy converter based on dielectric elastomer generator

This paper introduces a model-based control strategy for a wave energy converter (WEC) based on dielectric elastomer generators (DEGs), i.e. a device that can convert the energy of ocean waves into electricity by employing deformable elastomeric transducers with variable capacitance. The analysed system combines the concept of oscillating water column WEC with an inflated circular diaphragm DEG (ICD-DEG). The device features strongly nonlinear dynamics due to the ICD-DEG electro-hyperelastic response and the compressibility of the air volume comprised between the water column and the ICD-DEG, while the hydrodynamic loads can be approximated as linear. The optimal control solution that maximises the power extraction of the device is numerically investigated in the case of monochromatic waves over the typical frequency and amplitude ranges of sea waves. The more realistic case of panchromatic waves is also analysed through the implementation, in simulation environment, of a real-time controller. This regulator is based on a simple sub-optimal control logic that is deduced from the monochromatic case. The performance of the proposed control strategy is illustrated in comparison with unoptimised algorithms.     

垂荡双气室振荡水柱波能装置水动力特性研究

振荡水柱(OWC)波能转换装置因其结构简单、便于安装维护等特点, 被公认为最具应用前景的波能转换技术. 本研究以垂荡式双气室OWC波能转换装置为研究对象, 借助开源代码平台OpenFOAM及基于interFoam求解器开发的造/消波工具箱waves2Foam, 采用流体体积法(VOF)捕捉自由面和六自由度(6DOF)动网格求解器模拟垂荡运动响应, 数值研究在不同入射规则波作用下, 前后气室相对宽度、弹簧弹性系数对装置捕能宽度比及水动力特性的影响规律. 通过与已有的固定情况下的双气室OWC装置结果对比, 并通过对比自由衰减运动响应验证动网格技术, 揭示了本研究中数值模型的合理性和有效性. 计算结果表明, 较宽的后气室结构布置有利于双气室振荡水柱装置在垂荡状态下的波能提取; 前后气室宽度比为1/2时, 垂荡式双气室OWC装置在测试波频段具有最优的捕能宽度比; 相较于固定状态, 垂荡装置的后气室在中高波频段有着更高的捕能宽度比; 装置前后气室内水柱与OWC装置垂荡运动间存在的相位差使得气室内水面相对振幅和相对压强在测试波频段存在多峰值现象, 进一步发现弹装置通过垂向弹簧进行相位控制, 可显著拓宽高效频谱带, 实现较大的捕能宽度比.      

Recent advances in wave energy converters based on nonlinear stiffness mechanisms

Wave energy is one of the most abundant renewable clean energy sources, and has been widely studied because of its advantages of continuity and low seasonal variation. However, its low capture efficiency and narrow capture frequency bandwidth are still technical bottlenecks that restrict the commercial application of wave energy converters (WECs). In recent years, using a nonlinear stiffness mechanism (NSM) for passive control has provided a new way to solve these technical bottlenecks. This literature review focuses on the research performed on the use of nonlinear mechanisms in wave energy device utilization, including the conceptual design of a mechanism, hydrodynamic models, dynamic characteristics, response mechanisms, and some examples of experimental verification. Finally, future research directions are discussed and recommended.

     

Wave flume experiments on two-dimensional oscillating water column wave energy devices

Experiments were performed in a wave flume with regular waves of very small amplitude-to-wavelength ratio (less than 0.01). Their purpose was mainly the validation of the oscillating surface pressure theory of Sarmento and Falcão (1985) applied to wave energy absorption by oscillating water column (OWC) devices. Experimental and theoretical curves for the efficiency and for the reflection and transmission coefficients were obtained and compared. The test also included the validation of the two-wave-gauge experimental procedure used to decouple direct and reflected wave trains, as well as the effects of increasing the depth of immersion of the OWC overhang on the efficiency-wave period curves.     

Spall Characterization in Epoxy Via Laser Spallation

Background: The strength of materials under extreme dynamic loading conditions, such as in the case of shock wave loading, is assessed from their spallation characteristics. Under laboratory conditions, flyer plate impact, or sometimes laser-induced stress waves, is employed to instigate spall in a material. These methods are often combined with velocity interferometer system for any reflector (VISAR) technique for performing transient measurements. Although the VISAR can record the velocity of extremely fast-moving surfaces, it requires a complex optical setup and a specialized data reduction technique. Objective: In this study, a simpler approach is adopted by extending laser spallation method to determine the spall strength of epoxy, while performing in situ interferometric measurements, directly on top of thick epoxy films. Methods: The glass/epoxy test samples are prepared by transferring an aluminum coating on top of epoxy layers with different thicknesses. Laser-induced stress waves transmit across the substrate/film interface and induce subsurface failure in the epoxy at sufficiently high incident laser energy. The nature and magnitude of the waves are deciphered from the out-of-plane displacement histories of the top reflective sample surfaces, which are recorded by using a Michelson interferometer. Results: The interferometric data reveal the development of two (temporally) well-separated stress waves: an ablation-induced high-amplitude short-duration longitudinal pulse, which is referred to as the primary wave, and a secondary wave, which travels at a comparatively slower speed. The complex constructive interaction of the two waves develops a high-magnitude tensile stress region in the epoxy layer. The spall strength is quantified by superimposing the two stress wave histories associated with the critical energy fluence. Conclusions: The spall depths predicted from spatiotemporal wave travel analyses are in excellent agreement with the experimental observations. The newly adopted methodology estimates the spall strength of epoxy as 260?±?20 MPa.

     

Measurement of electrical charges carried by airborne bacteria laboratory-generated using a single-pass bubbling aerosolizer

Widely used bioaerosol generators like Collison nebulizer probably produce electrostatically charged particles, but the electrical charges carried by laboratory-generated airborne microorganisms using bubbling aerosolizers are poorly understood. In this study, we measured the fraction of neutral particles and number of elementary charges per particle as a function of the aerodynamic diameter of airborne bacteria (Escherichia coli and Enterococcus hirae). Bioaerosols were produced by a liquid sparging aerosolizer-type bubbling generator, with particle sizes ranging from roughly 0.6 to 2 μm. The experimental setup included an electrostatic precipitator and real-time devices including an electrometer, aerodynamic particle sizer, and electrical low-pressure impactor. Experimental results obtained for various operating conditions showed that aerosols produced with a higher bubbling airflow contained a larger proportion of neutral particles (from around 30% to 50%) and that bacteria carried a greater average absolute number of elementary charges (from around –10 to –60 elementary units) than those under lower airflow. Under the investigated conditions, a neutralization step is unnecessary because it may have a negative effect on the viability of sensitive microorganisms. Our results suggest that the neutral fraction can be used downstream of an electrostatic precipitator, and that this setup may have advantages over bipolar neutralizers.     

Displacement Uncertainty Quantifications in T3-Stereocorrelation

Background Uncertainty quantifications are required for any measurement result to be meaningful.

Objective The present work aims at deriving and comparing a priori estimates of displacement uncertainties in T3-stereocorrelation for a setup to perform high temperature tests.

Methods Images acquired prior to the actual experiment (i.e.,at room temperature) were registered using 3-noded triangular elements (T3-stereocorrelation) to determine displacement uncertainties for different positions of the experimental setup.

Results The displacement uncertainties were then compared to their a priori estimates.

Conclusions For the analyzed experiment, it is shown that noise floor estimates only differed by a factor 2 when compared to a posteriori measurements of standard displacement uncertainties.

     

Earthmoving in minature

This paper has described a similitude approach to earthmoving, using small-scale models based on a dimensional analysis with certain simplifying assumptions. An earthmoving mechanics laboratory has been developed, with facilities for testing two sizes of models.

Despite the variability of real soils, and the complexity of the soil action during earthmoving processes, the somewhat idealized model tests are useful and exceedingly realistic. Models yield design information that is not available by any other means. Full-scale testing is still needed for final evaluation of an actual machine.     

Constitutive modelling of the amplitude and frequency dependency of filled elastomers utilizing a modified Boundary Surface Model

A phenomenological uniaxial model is derived for implementation in the time domain, which captures the amplitude and frequency dependency of filled elastomers. Motivated by the experimental observation that the frequency dependency is stronger for smaller strain amplitudes than for large ones, a novel material model is presented. It utilizes a split of deformation between a generalized Maxwell chain in series with a bounding surface plasticity model with a vanishing elastic region. Many attempts to capture the behaviour of filled elastomers are found in the literature, which often utilize an additive split between an elastic and a history dependent element, in parallel. Even though some models capture the storage and loss modulus during sinusoidal excitations, they often fail to do so for more complex load histories. Simulations with the derived model are compared to measurements in simple shear on a compound of carbon black filled natural rubber used in driveline isolators in the heavy truck industry. The storage and loss modulus from simulations agree very well with measurements, using only 7 material parameters to capture 2 decades of strain (0.5–50% shear strain) and frequency (0.2–20 Hz). More importantly, with material parameters extracted from the measured storage and loss modulus, measurements of a dual sine excitation are well replicated. This enables realistic operating conditions to be simulated early in the development process, before an actual prototype is available for testing, since the loads in real life operating conditions frequently are a combination of many harmonics.     

小波变换在随机海浪及相关课题中的应用与前景

扼要介绍了小波变换的基本原理及其信号处理功能,综述了应用这一技术处理非平稳过程随机海浪,确定波群和异常波特性,判断波浪破碎,进行入反射分离,分析海洋和湖泊流场以及研究岸滩演变的时空变化过程与气象、海象的相关关系等方面的研究成果.讨论了这一技术在波浪动力特性认识、随机海浪的实验室模拟、波浪与建筑物相互作用研究方面的应用前景.      

Development of a PVDF low-cost shock-wave hydrophone

During a few past years a series of shock-wave generators for lithotripsy and/or tissue destruction studies have been developed in our laboratory. Based on the experiences in shock wave measurements and the drawbacks in existing hydrophones, we have developed a very low-cost, wideband, reproducible shock-wave hydrophone. The key element of this device is the rapidly mounting, disposable PVDF membrane. This is a commercially available PVDF shock gauge which is poled by a patented cyclic poling technique. To obtain the widest possible bandwidth, we have adopted a special coplanar membrane design. The PVDF film is sandwiched between the surfaces of a P.V.C. and a metallic plate of brass which the latter is in contact with the surrounding medium. On the other hand, the active lead is isolated from medium and it is in contact with an isolating liquid (degassed petroleum) held in a cylindrical chamber over the membrane. By the incorporation of this design, the hydrophone can be used for shock wave measurements even in conductive media like different physiological liquids, with a negligible change of sensitivity.     

Numerical simulations of Z-pinch experiments to create supersonic differentially-rotating plasma flows

The aim of this work is to produce and study a high energy density laboratory plasma relevant to astrophysical accretion disks. To this end, an experimental setup based on a modified cylindrical wire array was devised, which employs a cusp magnetic field to introduce angular momentum into the system. The setup was studied numerically with the three-dimensional, resistive magneto-hydrodynamic code GORGON. Simulations show that a differentially-rotating flow is formed, with typical rotation velocity and Mach number values of 60 km/s and M_φ ～ 5 respectively. The plasma is radiatively cooled and presents a Reynolds number higher than 10⁷. In addition, the magnetic Reynolds number and the plasma β are >1. Such a plasma is of interest for the study of hydrodynamic and magneto-hydrodynamic instabilities, and turbulence generation in differentially-rotating plasma flows.     

Generalized and exact solutions for oblique shock waves of real gases with application to real air

Calculation of the oblique shock wave of real gases is a difficult and time consuming problem because it involves numerical solution of a set of 10 equations, two of which (i.e., the equation of state and enthalpy function)—if available—are of a very complicated algebraic form. The present work presents a generalized method for calculating oblique shock waves of real gases, based on the Redlich-Kwong equation of state. Also described is an exact method applicable when the exact equation of state and enthalpy function of a real gas are available. Application of the generalized and the exact methods in the case of real air showed that the former is very accurate and at least twenty times faster than the latter. An additional contribution of the study is the derivation of real gas oblique shock wave equations, which are of the same algebraic form as the well known ideal gas normal shock wave relations.     

Experimental study on interaction of aerodynamics with flexible wings of flapping vehicles in hovering and cruise flight

Flapping wings are promising lift and thrust generators, especially for very low Reynolds numbers. To investigate aeroelastic effects of flexible wings (specifically, wing’s twisting stiffness) on hovering and cruising aerodynamic performance, a flapping-wing system and an experimental setup were designed and built. This system measures the unsteady aerodynamic and inertial forces, power usage, and angular speed of the flapping wing motion for different flapping frequencies and for various wings with different chordwise flexibility. Aerodynamic performance of the vehicle for both no wind (hovering) and cruise condition was investigated. Results show how elastic deformations caused by interaction of inertial and aerodynamic forces with the flexible structure may affect specific power consumption. This information was used here to find a more suitable structural design. The best selected design in our tests performs up to 30% better than others (i.e., less energy consumption for the same lift or thrust generation). This measured aerodynamic information could also be used as a benchmarking data for unsteady flow solvers.     

A novel method for predicting the power outputs of wave energy converters

This paper focuses on realistically predicting the power outputs of wave energy converters operating in shallow water nonlinear waves. A heaving two-body point absorber is utilized as a specific calculation example, and the generated power of the point absorber has been predicted by using a novel method(a nonlinear simulation method) that incorporates a second order random wave model into a nonlinear dynamic filter. It is demonstrated that the second order random wave model in this article can be utilized to generate irregular waves with realistic crest–trough asymmetries, and consequently, more accurate generated power can be predicted by subsequently solving the nonlinear dynamic filter equation with the nonlinearly simulated second order waves as inputs. The research findings demonstrate that the novel nonlinear simulation method in this article can be utilized as a robust tool for ocean engineers in their design, analysis and optimization of wave energy converters.     

Migrating sea ripples

Ripple formation under sea waves is investigated by means of a linear stability analysis of a flat sandy bottom subject to the viscous flow which is present in the boundary layer at the bottom of propagating sea waves. Nonlinear terms in the momentum equation are retained to account for the presence of a steady drift. Hence the work by Blondeaux is extended by considering steeper waves and/or less deep waters. Second order effects in the sea wave steepness are found to cause neither destabilizing nor stabilizing effects on the process of ripple formation. However, because of the presence of a steady velocity component in the direction of wave propagation, ripples are found to migrate at a constant rate which is predicted as function of sediment and wave characteristics. The analysis assumes the flow regime in the bottom boundary layer to be laminar and the results are significant for ripples at the initial stage of their formation or for mature ripples of small amplitude (rolling-grain ripples). A comparison of the theoretical findings with laboratory experiments supports the reliability of the approach and of the theoretical results.     

Rotary motion of the parametric and planar pendulum under stochastic wave excitation

In this paper a rotary motion of a pendulum subjected to a parametric and planar excitation of its pivot mimicking random nature of sea waves has been studied. The vertical motion of the sea surface has been modelled and simulated as a stochastic process, based on the Shinozuka approach and using the spectral representation of the sea state proposed by Pierson–Moskowitz model. It has been investigated how the number of wave frequency components used in the simulation can be reduced without the loss of accuracy and how the model relates to the real data. The generated stochastic wave has been used as an excitation to the pendulum system in numerical and experimental studies. For the first time, the rotary response of a pendulum under stochastic wave excitation has been studied. The rotational number has been used for statistical analysis of the results in the numerical and experimental studies. It has been demonstrated how the forcing arrangement affects the probability of rotation of the parametric pendulum.     

Experimental and theoretical investigation of superharmonic resonances in a planar oscillator under angular base excitation

This paper explores the complicated dynamic behavior of a mechanical oscillator under harmonic angular excitation. The motivation behind this work comes from the nature of the actuation produced by high-performance dither motors. A lumped-mass model, which captures the primary and the 1 : 2 superharmonic resonances observed on an analogous experimental test setup, is put forward. The equations of motion governing the dynamics of the model are derived and are found to comprise both parametric and direct forcing terms. The governing equations are solved analytically using the generalized harmonic balance method and numerical integration. The method of multiple scales is utilized to obtain closed-form expressions that relate the system parameters to the oscillation amplitudes in the vicinity of the direct and the 1 : 2 superharmonic resonances. It is found that eccentricity plays a vital role in the occurrence of the resonances. Besides, the relationship between the excitation amplitudes and the resulting oscillations for the direct and the superharmonic resonances are dissimilar. A few salient differences between classical (rectilinear) and angular base excitation mechanisms are pointed out.

     

Remote displacement analysis using multimode fiber-optic bundles

An improved method for holographically recording displacement through fiber optics is presented. The object is illuminated by focusing the exit end of a multimode fiber bundle on the test surface. The real image of the object is transmitted through a second coherent bundle to a photographic plate where it is recorded using a reference wave front. The resulting image-plane hologram is reconstructed in white light. Experimental results agree well with theory and demonstrate the potential of measuring displacement on surfaces far removed from the basic holographic setup; or, in remote areas of a structure where optical access is limited.