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In this research, a camber changing mechanism feasible for use in a flapping hydrofoil tidal stream turbine is suggested to generate high lift force and subsequently a large flapping angle as a dynamic response. The hydrodynamic characteristics of rigid and variable-camber hydrofoils, which mainly affect the response, were numerically obtained using XFOIL. The flapping responses of the hydrofoils were estimated by a validated nonlinear dynamic model with the hydrodynamic characteristics. The estimated responses of a variable-camber hydrofoil were compared with the measured responses of the rigid hydrofoil from a previous study as well as the estimated responses of a rigid hydrofoil. It was shown from these comparisons that the variable-camber hydrofoil produced a much higher flapping angle amplitude with the same pitch angle relative to that of the rigid wing due to the cambered shape of the former. Moreover, the variable-camber hydrofoil could generate considerable flapping responses compared to experimental data, even when a relative small input pitch angle was applied in the estimation.

In this study, the visualization of the flow inside a Taylor cone formed during an electrohydrodynamic (EHD) spraying is conducted to analyze its stability among five liquid candidates. A micro-PIV with a micro-nozzle is used for the visualization, and the physical properties as well as measured values are utilized in the analysis. First, in forming the Taylor cone, the electrohydrodynamic force is required to be sufficiently large in order to overcome the surface tension of the liquid. Thus, among the five liquids tested here, three, in this case IPA, EtOH, and MeOH, form a Taylor cone due to the relatively low surface tension levels as compared to the others. Once electrohydrodynamic jetting occurs, the average and maximum velocities become monotonically proportional to the average current. As the velocities are the smallest in using IPA, the circulation flow becomes superior to the extrusive flow, which yields the stable formation of a Taylor cone. Also, low fluctuation of the instantaneous currents supports the stable formation of IPA. Consequently, IPA shows the most stable formation of the Taylor cone in our condition due to the lowest average current and low-level surface tension. Eventually, micro-PIV would be a good tool in choosing an optimal fluid for stable EHD spraying.

This work focuses on the hydrodynamic performance analyses of three different types of flapping hydrofoils, in this case the pitch-heave (PH), left-swing (LS), and right-swing (RS) types, in their power extraction regimes via two dimensional CFD simulations. The power extraction performance is presented in an isocontour parametric map and is investigated from a comparison of the kinematics parameters and the development of unsteady vortices among the three types. It is found from the parametric analysis and comparison that the LS-type of flapping hydrofoil outperforms the PH and RS-types due to relatively high forces as well as the good synchronization of the forces and moment with the translational velocity and pitch angular rate, respectively upon the different aspects of the vortex development. Consequently, the power extraction performance improves from the LS to the PH and then to the RS-types. It is also recognized from another isocontour parametric map that the maximum power-extraction efficiency is achieved at a similar maximum effective angle of attack, at the maximum effective angle of attack rate, and at the maximum pitch angle rate for the LS and RS types, while the range of the maximum effective angle of attack rate is lower in the PH-type hydrofoil.

In recent years, nonlinear dynamic models have been developed for flapping-type energy harvesting systems with a rigid wing, but not for those with a flexible wing. Thus, in this study, flexible wing designs of NACA0012 section are proposed and measurements of the forces of rigid cambered wings, which are used to estimate the performance of the designed wings, are conducted. Polar curves from the measured lift and drag coefficients show that JavaFoil estimation is much closer to the measured values than Eppler over the entire given range of angles of attack. As the camber of the rigid cambered wings is increased, both the lift and drag coefficients increase, in turn increasing the resultant forces. Moreover, the maximum resultant forces for all rigid cambered wings are achieved at the same angle of attack as the maximum lift coefficient, meaning that the lift coefficient is dominant in representations of the wing characteristics. Keywords: Flapping-type energy harvesting system, Flexible wing, Cambered rigid wing, Wing characteristics, NACA0012

Recently, an alginate hydrogel containing amphiphilic liposomes was suggested for application to an antifouling coating. In this study, we investigate the shaping characteristics of the alginate hydrogel by a spraying method for application to the coating. Sodium alginate droplets from the spraying nozzle are known to be differently shaped by several external forces during the encapsulation process when such a droplet impacts onto the surfaces of liquid containing calcium ions. We adopted a two-fluid spraying method for fast alginate droplets with a scalable size suitable for mass production. Various shapes, such as mushroom, petal, sphere, and thin-sheet alginate hydrogel shapes containing liposomes, were obtained via this experimental approach. Supported by a theoretical analysis, we determined regimes for the various shapes depending on the droplet size and the concentration of the sodium alginate solution. Alginate hydrogel of different shapes based on the regimes will be used for the coating throughout a different post-processing.

In this work, we report on the structural design of a 5-m-long composite blade intended for use in a horizontal-axis tidal turbine. The blade geometry is constructed through an optimization process to obtain the maximum power coefficient at the desired tip speed ratio of 4.5 by applying the blade element-momentum theory (BEMT). The blade is primarily designed using a NACA 63-424 hydrofoil. The blade structure is designed by using the BEMT to compute the loading conditions at various inflow velocities. Two parallel spars were chosen to produce the blade structure grid, and the preliminary lay-up structure of the composite blade was determined according to the thickness distribution identified using the twisted beam theory under the assumption that the two spars plus the upper and lower skins mostly contribute to the flap-wise bending stiffness while withstanding an external load. Then, high-strength unidirectional and double-bias fiber glass/epoxy materials were chosen to fabricate the blade. The final blade structure was then analyzed in ANSYS Workbench using the finite element method. The results show that the blade structure can withstand the applied load with failure indices

In this study, we investigated the wetting transition of a moving meniscus in a grooved microchannel through a detailed parametric study based on measurement by an optical tool and μ-particle image velocimetry (PIV) to avoid the transition in designing the microchannel. The parameters investigated were pitch, flow rate, and height of a microchannel. The contact angle, contact speed, and interfacial pressure difference were analyzed according to the parameters. We found that the pitch is most effective, the flow rate is moderately effective, and the height is least effective on that. The height even does not affect the contact angle, because the solid–fluid interaction at the groove edge is stronger than the fluid–air interaction. As the critical correlation, the contact angle, which is dependent on the pitch and the flow rate, and the height affect the air pressure between the grooves, which governs the air penetration flux and mainly determines the wetting transition. Therefore, a powerful way to delay the wetting transition is to reduce the degree of air pressure variation, specifically with a low pitch and a tall height with a low flow rate. Eventually, understanding dominant input parameters in relation to the wetting transition will be very useful in the design stage of microfluidic applications.

In this paper, a linear dynamic model of flapping-type tidal turbine (FTT) is proposed. First, a nonlinear dynamic model is derived to predict the equation of motion of the system. Then, the nonlinear dynamic model was analyzed and validated using simulation and experimental data. Second, a linear dynamic model is derived from the nonlinear dynamic model using approximation method. To validate this method, the proposed linear dynamic model is analyzed and validated using simulation and experimental results. This dynamic model is also compared to another linearization method. The result showed that the proposed linear dynamic model could represent the characteristic of the system as in the nonlinear dynamic model.

This paper describes the control design of the oscillation motion of hydroplane in dual system of lab-scale flapping type tidal energy harvester (FT-TEH).In dual system, two identical single systems of lab-scale FT-TEH are arranged cascade in the distance of a chord length of the hydroplane, wherein the flapping arm of each system is facing inward to each other. However, the pitch noses of hydroplanes are facing the same direction against the water flow. The controller is designed to control the motion of hydroplane such that each system is moving in synchronized motion to each other. Here, the system is using automatic gain controller with PID as the control feedback. Simulation and experiment results indicate that the controller of pitch oscillation for dual system of lab-scale FT-TEH is well implemented.

A lab-scale flapping-type turbine with a semi-passive activation mode has been designed and implemented. A non-linear dynamic model, developed in our previous work, is validated by a series of experiments along with computational fluid dynamics (CFD) simulations. Previously, the dynamic model was used only to estimate the dynamic response of a flapping-type turbine. In this work, the applicability of the dynamic model is extended to estimate the hydrodynamic forces, extracted power, and efficiency. It was demonstrated from a comparison of the CFD results and measured values that the dynamic model based on a quasi-steady approach estimates the aforementioned performance parameters of measurements well in cases particularly with a low effective angle of attack, thus demonstrating the usefulness of the dynamic model for a flapping-type turbine at an early stage.

In this study, we investigate the effect of hydrofoil flexibility on the power extraction of a flapping tidal stream generator with hydrofoils down-scaled for a water channel in an experiment with a typical Strouhal number and frequency. The described deformations in the chord and spanwise directions are imposed onto the surfaces of the hydrofoil to analyze the flexibility effect. In a two-dimensional (2D) simulation, parameter studies of the chordwise flexure are conducted and a 30% improvement in the rate of the power-extraction efficiency is then achieved when the chordwise flexure is 20% of the chord length. In a three-dimensional (3D) simulation, the chordwise flexure of 20% achieves a 15% improvement in the rate of the power-extraction efficiency for the hydrofoil with an aspect ratio (AR) of 5, which is less than that in the 2D simulation due to 3D effects such as tip loss and a spanwise vortex. Meanwhile, the effect of the spanwise flexure on the power extraction is minor as compared to that of the chordwise flexure. It was also found throughout the parametric study of the AR variation that the 3D effect of the chordwise flexible hydrofoil is slightly stronger than that of the rigid hydrofoil.

The aerodynamic role of the elytra during a beetle's flapping motion is not well-elucidated, although it is well-recognized that the evolution of elytra has been a key in the success of coleopteran insects due to their protective function. An experimental study on wing kinematics reveals that for almost concurrent flapping with the hind wings, the flapping angle of the elytra is 5 times smaller than that of the hind wings. Then, we explore the aerodynamic forces on elytra in free forward flight with and without an effect of elytron-hind wing interaction by three-dimensional numerical simulation. The numerical results show that vertical force generated by the elytra without interaction is not sufficient to support even its own weight. However, the elytron-hind wing interaction improves the vertical force on the elytra up to 80%; thus, the total vertical force could fully support its own weight. The interaction slightly increases the vertical force on the hind wind by 6% as well.

본 연구에서는, 블레이드 요소-모멘텀 이론을 바탕으로, 최대 출력계수를 갖는 직경 80 cm의 실험실용 수평축 조류 터빈의 형상을 제시하고, 블레이드 피치각이 변할 때 출력계수의 변화 경향을 조사하였다. 또한 ANSYS-Fluent를 이용한전산유체해석을 실시하여, 주어진 블레이드 피치각에 대하여 블레이드 요소-모멘텀 이론으로 계산한 출력계수를 검증하였다. 전산유체해석에는 계산 영역의 직경과 길이를 조류 터빈 반경의 15배로 하였고, 계산 영역의 경계에는 열린 경계조건을 인가하였다. 블레이드 요소-모멘텀 이론과 전산유체해석으로 계산한 조류 터빈의 최대 출력계수 약 48%로 서로 잘 일치하였다. 블레이드 피치각을 증가한 경우에는 두 방법으로 산출한 출력계수가 모두 감소하는 경향을 보였고,그 값들도 서로 유사하였다. 이로부터, 블레이드 요소-모멘텀 이론을 기반으로 설계한 조류 터빈 형상 및 다양한 조건에서 대한 출력계수의 신뢰성을 확인하였다. Abstract − In this work, based on the blade element-momentum theory (BEMT), we proposed the geometry of alab-scale horizontal axis tidal turbine with a diameter of 80cm, which can demonstrate the maximum power coeffi-cient, and investigated the effect of blade pitch angle increase on the power coefficient. For validation of the com-puted power coefficients by the BEMT, we also computed the power coefficient using the computational fluiddynamics (CFD) for each case. For the CFD, 15 times of the turbine radius was used for the length and diameter ofthe computational domain, and the open boundary condition was prescribed at the boundary of the computationaldomain. The maximum power coefficients of the turbine acquired by the BEMT and CFD were about 48%, show-ing a good agreement. Both of the power coefficients computed by the BEMT and CFD tended to decrease whenthe blade pitch angle increases. The two power coefficients for a given tip-speed ratio were in good agreement.Through the present study, we have confirmed that we can trust the proposed geometry and the computed powercoefficients based on the BEMT. Keywords: Blade Element Momentum Theory, BEMT(블레이드 요소-모멘텀 이론), Horizontal Tidal Turbine(수평축 조류 터빈), Turbine Blade(터빈 블레이드), Pitch Angle(피치각)

This paper presents a mathematical model to predict the dynamic response of a lab-scale flapping-type tidal energy harvester. In the model, the forces produced by the hydroplane were derived based on a quasi-steady-state assumption, and no approximation was applied to the terms of the force equation. Therefore, the equation of motion becomes nonlinear; it was solved using the Runge–Kutta method. The mathematical dynamic model was validated through a series of experiments. Thereafter, the flapping speed of the system was estimated and compared with the measured speed. The comparison proved that the dynamic model reasonably estimates the flapping response, flapping speed, and generated forces. The dynamic model can be used for performance prediction of the lab-scale flapping tidal energy harvester and optimization of the scaled-up model in the future.

In this study, a genetic algorithm (GA) with an analytic model is adopted to conduct multi-objective optimization for design of an underwater chain trencher. The optimization problem is defined as minimizing a product of the chain power and weight subject to the uniaxial compressive strength, coefficient of traction, bar length (L), nose radius (R) and ratio of the chipping depth over the spacing (l/S), of which the ranges are determined based on the specifications of commercial trenchers satisfying established performance requirements and previous parametric studies. It is found that an optimal design of the GA was obtained with L and l/S close to their low bound and R far from its low bound while that of a simple parametric analysis was acquired with the three parameters close to their low bounds. Moreover, in the most severe soft rock and traction conditions, the power and weight in the optimal design obtained by the GA are turn to be within the feasible ranges of targeted commercial trenchers.

Aerodynamic characteristic of the beetle, Trypoxylus dichotomus, which has a pair of elytra (forewings) and hind wings, is numerically investigated. Based on the experimental results of wing kinematics, two-dimensional (2D) and three-dimensional (3D) computational fluid dynamic simulations were carried out to reveal aerodynamic performance of the hind wing. The roles of the spiral Leading Edge Vortex (LEV) and the spanwise flow were clarified by comparing 2D and 3D simulations. Mainly due to pitching down of chord line during downstroke in highly inclined stroke plane, relatively high averaged thrust was produced in the free forward flight of the beetle. The effects of the local corrugation and the camber variation were also investigated for the beetle's hind wings. Our results show that the camber variation plays a significant role in improving both lift and thrust in the flapping. On the other hand, the local corrugation pattern has no significant effect on the aerodynamic force due to large angle of attack during flapping.

In this paper, a simple but effective design for implementing a negative wing twist in a beetle-mimicking wing system is presented. The effectiveness of the design in terms of force generation and power consumption is confirmed by both experiment and calculation. An unsteady blade-element-theory model is used to estimate the aerodynamic forces produced by two different wing kinematics of a flapping-wing system. The model was first validated with the measurement data and two three-dimensional computational-fluid-dynamics results from the literature. The difference between the estimated average lift and the measured lift is 5.6%, which proves that the unsteady blade-element-theory model provides reasonable aerodynamic force estimation. The time history of the current estimation is also close to the measured data and is in between the two computational-fluid-dynamics results. The forces generated by the flapping wings with and without wing twist are estimated using the unsteady blade-element theory to invest...

In this work, a dual flapping tidal current generator is developed and then validated in indoor and in-situ experiments. It is recognized from the indoor experiments that the dual flapping generation system is optimal at the reduced frequency, 0.125 in terms of power extraction. In the consecutive in-situ experiments, system power, 6.4kW and system efficiency 36.5 % are obtained. The dual power transmission, which is also developed for transferring the swing motion of the flappers to the rotational output shaft, is validated in the both experiments and amended into a simple and different arrangement.

In our study, a closed-type penetration unit for standard penetration test (SPT) equipment was developed in order to operate in an underwater environment. This type causes energy dissipation, mainly due to the small gap between an airtight case and moving hammer. The dissipation was estimated through a CFD analysis. The computed dissipated energy was less than 1.2% compared to the potential energy of the hammer with the given gap. Subsequently, the impact energy of the underwater SPT equipment was within 1.2% of that for the SPT equipment on land.

Molecular dynamics (MD) simulation is frequently used in exploring the mechanical behaviors of nanowires, which have strong dependency on parameters such as size, strain-rate, temperature, and so on. However, the MD simulations require heavy computation power. For efficient parametric study on the mechanical behaviors, the use of a snapshot-based proper orthogonal decomposition (POD) is proposed. Per-atom virial stress and atom arrangement, which are chosen as the snapshots in this study, are expressed by the linear combinations of POD basis. As a next step, the moving least square method approximates the corresponding POD coefficients. The mechanical behaviors are then predicted by these POD expressions in the parametric ranges of interest. It is demonstrated from the parametric study that the proposed method not only obtains the snapshots but also predicts the abrupt drop-down of stress at the onset of inelastic deformation more efficiently than the original tedious repeated MD simulations.

Inspired by the fact that a high flexible wing in nature generates high aerodynamic performance, we investigated the aerodynamic performance of the flapping wing with different chord flexures. The unsteady, incompressible, and viscous flow over airfoil NACA0012 in a plunge motion was analyzed by using Navier-Stokes equation. Grid deformation, in which finite element and interpolation ideas are mixed, was introduced for computing large grid deformation caused by the chord flexures. We explored the optimal phase angle for thrust force and propulsive efficiency by varying the chord flexure from 0.05 to 0.7 when reduced frequency and plunge amplitude were fixed. Throughout parametric study on the phase angle and chord flexure amplitude, the maximum thrust force is achieved near at 0° in all given conditions, meanwhile, it is found that the optimal phase angle has dependency of chord flexure amplitude, which achieves higher aerodynamic performance compared to previous studies. These findings will provide a useful guideline for determining wing flexibility in design of a bio-mimetic air vehicle.

Mid-frequency response analysis often faces computational difficulties when a conventional modal approach is used for a finite element linear system. In this paper, the computational burden is relieved by frequency sweep algorithm or mode acceleration method for a reduced-order system constructed by algebraic substructuring, a variant of model order reduction. The two methods are compared with the help of numerical experiments and their computational complexity. As demonstrated by the finite element simulations, in which proportional damping is assumed, of a turbo-prop aircraft and a ring resonator, the frequency sweep algorithm for reduced-order systems shows the best performance among all considered numerical solutions, including the conventional approach.

Array forms of MEMS resonator that uses a specific mid-frequency normal mode have been introduced for acquiring a wider bandwidth of frequency response function (FRF). A conventional frequency response solver based on a modal approach faces computational difficulties in obtaining the FRF of these array forms because of the increase in the order of a linear dynamic model and the number of retained normal modes. The computational difficulties can be resolved by using a substructuring-based model order reduction and a frequency sweep algorithm, which requires a smaller number of retained modes of a reduced dynamic system than the conventional solver. In computing the FRF of a single resonator and its array forms, the presented method shows much better efficiency than the conventional solution by ANSYS as the number of resonators increases. In addition, the effects of multiple resonators in the array forms on filter performance are discussed compared with experimental data.

One of important factors in designing MEMS resonators for RF filters is obtaining a desired frequency response function (FRF) within a specific frequency range of interest. Because various array-type MEMS resonators have been recently introduced to improve the filter characteristics such as bandwidth, pass-band, and shape factor, the degrees of freedom (DOF) of finite elements for their FRF calculation dramatically increases and therefore raises computational difficulties. In this paper the Krylov subspace-based model order reduction using moment-matching with non-zero expansion points is represented as a numerical solution to perform the frequency response analyses of those array-type MEMS resonators in an efficient way. By matching moments at a frequency around the specific operation range of the array-type resonators, the required FRF can be efficiently calculated regardless of their operating frequency from significantly reduced systems. In addition, because of the characteristics of the moment-matching method, a minimal order of reduced system with a prearranged accuracy can be determined through an error indicator using successive reduced models, which is very useful to automate the order reduction process and FRF calculation for structural optimization iterations. We also found out that the presented method could obtain the FRF of a 6×6 array-type resonator within a seventieth of the computational time necessary for the direct method and in addition FRF calculation by the mode superposition method could not even be completed because of a data overflow with a half after calculation of 9,722 eigenmodes.

Nature-inspired flapping foils have attracted interest for their high thrust efficiency, but the large motions of their boundaries need to be considered. It is challenging to develop robust, efficient grid deformation algorithms appropriate for the large motions in three dimensions. In this paper, a volume grid deformation code is developed based on finite macro-element and transfinite interpolation, which successfully interfaces to a structured multi-block Navier-Stokes code. A suitable condition that generates the macro-elements with efficiency and improves the robustness of grid regularity is presented as well. As demonstrated by an airfoil with various motions related to flapping, the numerical results of aerodynamic forces by the developed method are shown to be in good agreement with those of an experimental data or a previous numerical solution.

A simple way to make the closed forms stiffness matrix of macro triangle elements is proposed through the individual element test and the computational algebra system in this paper. The newly developed element consists of three constant strain triangles, which only satisfy the convergence conditions according to the individual element test. Computational algebra system MAPLE and the free formulation framework are used to combine the stiffness matrix with the free parameters in terms of the algebraic symbols. The assembly of three constant strain triangles produces the linear strain modes in the resulted macro elements. The rigid body modes and constant strain modes of elements and the newly produced linear strain modes of the macro element are systematically assembled in the basic stiffness and the higher order stiffness in the free formulation procedure. The parameter of the decomposed higher order stiffness was tuned through the bending test. The results of numerical examples show the advantages of the proposed macro element in the distorted mesh and elements with the high aspect ratio.

Finite macro-element-based grid deformation has been developed to improve efficiency and robustness of a finite element-based grid deformation. However, conventional stiffness determination based on distance between macro-element and moving boundary is not sufficient for a large moving boundary with a solid deformation included. The Jacobian option that uses element size is applied and a first macro-element layer (FML) is introduced based on the idea of solid-extension mesh moving for a finite macro-element model. Heave, pitch, chord flexure, and span flexure are employed for the verification of grid quality by using some sophisticated measures. Through the verification, the Jacobian option with FML improves the grid quality as compared with the conventional way, and its parameters can be chosen considering the grid quality near the moving boundary and the robustness of overall grid quality. Copyright © 2009 John Wiley & Sons, Ltd.

Biological tiny structures have been observed on many kinds of surfaces such as lotus leaves, which have an effect on the coloration of Morpho butterflies and enhance the hydrophobicity of natural surfaces. We investigated the micro-scale and nano-scale structures on the wing surfaces of insects and found that the hierarchical multiple roughness structures help in enhancing the hydrophobicity. After examining 10 orders and 24 species of flying Pterygotan insects, we found that micro-scale and nano-scale structures typically exist on both the upper and lower wing surfaces of flying insects. The tiny structures such as denticle or setae on the insect wings enhance the hydrophobicity, thereby enabling the wings to be cleaned more easily. And the hydrophobic insect wings undergo a transition from Cassie to Wenzel states at pitch/size ratio of about 20. In order to examine the wetting characteristics on a rough surface, a biomimetic surface with micro-scale pillars is fabricated on a silicon wafer, which exhibits the same behavior as the insect wing, with the Cassie-Wenzel transition occurring consistently around a pitch/width value of 20.

In this work, the structural response of thin-walled composite beams with pretwist angle is investigated by using a mixed beam approach that combines the stiffness and flexibility methods in a unified manner. The Reissner's semi-complimentary energy functional is used to derive the stiffness matrix that approximates the beam in an Euler–Bernoulli level for extension and bending and Vlasov level for torsion. The bending and torsion-related warpings induced by the pretwist effects are derived in a closed form. The developed theory is validated with available literature and detailed finite element structural analysis results using the MSC/NASTRAN. Pretwisted composite beams with rectangular solid and thin-walled box sections are illustrated to validate the current approach. Acceptable correlation has been achieved for cases considered in this study. The effects of pretwist and fiber orientation angles on the static behavior of pretwisted composite beams are also studied.

An aggregation multigrid method (AMM) is a leading iterative solver in solid mechanics. Recently, AMM is applied for solving Schur Complement system in the FE analysis of shell structures. In this work, an extended application of AMM for solving Schur Complement system in the FE analysis of continuum elements is presented. Further, the performance of the proposed AMM in multiple load cases, which is a challenging problem for an iterative solver, is studied. The proposed method is developed by combining the substructuring and the multigrid methods. The substructuring method avoids factorizing the full-size matrix of an original system and the multigrid method gives near-optimal convergence. This method is demonstrated for the FE analysis of several elastostatic problems. The numerical results show better performance by the proposed method as compared to the preconditioned conjugate gradient method. The smaller computational cost for the iterative procedure of the proposed method gives a good alternative to a direct solver in large systems with multiple load cases.

High-frequency response analysis (Hi-FRA) of large-scale dynamical systems is critical to predict the resonant behavior of modern micro-devices and systems operated over MHz or GHz frequency range. Algebraic substructuring (AS) is a powerful technique to extract a large number of natural frequencies. In this work, we extend the AS technique for FRA between two specified cutoff frequencies ωmin and ωmax. The technique is referred to as ASFRA. ASFRA can be efficiently applied to Hi-FRA, as demonstrated by two examples of microelectromechanical sensors operated at 1–2 and 200–250 MHz ranges. To some extent ASFRA generalizes the underlying numerical algorithm and functionality of commercially viable automated multi-level substructuring (AMLS) technique. AMLS is designed for FRA up to a specific frequency ωmax, starting from the lowest, and is inefficient for Hi-FRA. Copyright © 2008 John Wiley & Sons, Ltd.

Algebraic substructuring (AS) is a state-of-the-art method in eigenvalue computations, especially for large-sized problems, but originally it was designed to calculate only the smallest eigenvalues. Recently, an updated version of AS has been introduced to calculate the interior eigenvalues over a specified range by using a shift concept that is referred to as the shifted AS. In this work, we propose a combined method of both AS and the shifted AS by using multiple shifts for solving a considerable number of eigensolutions in a large-sized problem, which is an emerging computational issue of noise or vibration analysis in vehicle design. In addition, we investigated the accuracy of the shifted AS by presenting an error criterion. The proposed method has been applied to the FE model of an automobile body. The combined method yielded a higher efficiency without loss of accuracy in comparison to the original AS.

In this study, we conduct experimental and consecutive numerical analyses of a flapping tidal current generator with a mirror-type dual configuration with front-swing and rear-swing flappers. An experimental analysis of a small-scale prototype is conducted in a towing tank, and a numerical analysis is conducted by means of two-dimensional computational fluid dynamics simulations with an in-house code. An experimental study with a controller to determine the target arm angle shows that the resultant arm angle is dependent on the input arm angle, the frequency, and the applied load, while a high pitch is obtained simply with a high input arm angle. Through a parametric analysis conducted while varying these factors, a high applied load and a high input arm angle were found to be advantageous. Moreover, the optimal reduced frequency was found to be 0.125 in terms of the power extraction. In consecutive numerical investigations with the kinematics selected from the experiments, it was found that a rear-swing flapper contributes to the total amount of power more than a front-swing flapper with a distance of two times the chord length and with a 90° phase difference between the two. The high contribution stems from the high power generated by the rear-swing flapper, which mimics the tail fin movement of a dolphin along a flow, compared to a plunge system or a front-swing system, which mimics the tail fin movement of a dolphin against a flow. It is also due to the fact that the shed vorticities of the front-swing flapper slightly affect negatively or even positively the power performance of the rear-swing system at a given distance and phase angle.

So far, most tidal stream generators have been developed in three types: a horizontal axis turbine, a vertical axis turbine, and a flapping-type hydrofoil. The first two are operated by one-way rotation, but the last one is oscillated. The flapping system inspired by nature has attracted researchers due to its eco-friendly characteristics, applicability to shallow water, and nonconventional oscillating movement.

An aggregation multigrid method is utilized in constructing a preconditioner for a Schur complement system of automatically partitioned, non-overlapping subdomains. Preserving the relationship of the partitioned subdomains, we apply a rigid body based aggregation method, which employ geometric data, as a coarsening procedure. And then, we derive a new Schur complement coarse grid matrix by an approach of a condensation after the coarsening procedure. Therefore, we generate a multi-level preconditioner of a Krylov subspace method for Schur complement matrices using the coarse grid matrix. Through numerical experiments, the proposed preconditioner shows efficient performance and robust convergences irrespect of the size of elements and subdomains. It also shows better performance than the preconditioned conjugate gradient method (PCG) for the partitioned system and the aggregation multigrid method for the original domain in shell problems of structural mechanics.