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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 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, flow-driven rotor simulations with a given load are conducted to analyze the operational characteristics of a vertical-axis Darrieus turbine, specifically its self-starting capability and fluctuations in its torque as well as the RPM. These characteristics are typically observed in experiments, though they cannot be acquired in simulations with a given tip speed ratio (TSR). First, it is shown that a flow-driven rotor simulation with a two-dimensional (2D) turbine model obtains power coefficients with curves similar to those obtained in a simulation with a given TSR. 3D flow-driven rotor simulations with an optimal geometry then show that a helical-bladed turbine has the following prominent advantages over a straight-bladed turbine of the same size: an improvement of its self-starting capabilities and reduced fluctuations in its torque and RPM curves as well as an increase in its power coefficient from 33% to 42%. Therefore, it is clear that a flow-driven rotor simulation provides more information for the design of a Darrieus turbine than a simulation with a given TSR before experiments.

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.

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.

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.

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.

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.

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.

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.

A flapping-type turbine extracts power from the kinetic energy of water stream by heaving up and down. A nose-up pitch angle will create an upstroke motion of the flapping arm and vice versa for nose-down pitch. These upstroke and downstroke motion will then be converted into a continuous rotary motion of an output shaft by a transmission system. In this work, we describe our current progress on the development of design and prototype implementation of a flapping-type tidal energy harvester and introduce the dynamic model and its experimental validation together with power and efficiency.

Inspired by nature, flapping-type tidal stream generators have been introduced in recent years. The improvement in their power generation ability is known to be a critical factor in the success of these generators. So far, corrugation and camber observed in flying insects and swimming animals are known to enhance the performance of a flapping-type propulsive system. In this study, we explore the effect of corrugation and camber in a system that mimics a scallop shell in terms of its ability to extract flow energy through a two-dimensional Navier–Stokes simulation. The simulations show that the size and the activity of the leading edge vortex are strongly affected by the morphological factors of the mimicked foils, the effects of which are then advantageous in terms of the power efficiency of the flapping-type tidal stream generator. Eventually, an optimal mimicked foil, as suggested based on the morphological effects, would be a good alternative type of foil with a typical section with regard to the hydrodynamic performance and structural properties of tidal stream generators.

Various offshore structures such as long-span bridge, offshore wind farm, undersea tunnel and manmade island, etc are now being constructed all around the world. Geotechnical investigations for offshore sites are necessary to the economical and efficient design of the structures. Offshore geotechnical investigations for a site with sea depth lower than 20m are usually accomplished by using SEP-barge and onshore equipment. However, it is very difficult to utilize SEP-barge for a site with sea depth higher than 30m because of severe circumstances including deep sea, high wave and tidal current etc. In this study, seabed type unmanned automated site investigation equipment, which can operate in deep sea conditions (sea depth: 100m, maximum boring depth: 50m), was newly introduced. The underwater equipment is designed to include the underwater boring module, common platform module, remote control module as well as SPT (Standard Penetration Test) module, which is widely utilized at onshore sites. The penetration part in Standard Penetration Test (SPT) equipment has been developed in a closed type in order to adjust underwater environment in this study. This type causes energy dissipated due to the small gap between a closed case and a hammer. This paper deals with the conceptual design of the equipment and specific mechanism of automated SPT system. Also, the dissipation is estimated through CFD analysis. The dissipated energy is computed by less than 2% compared to the potential energy of the hammer with the given gap, subsequently the impact energy of the underwater SPT equipment is under 2% difference as compared to the SPT equipment on land.

This paper presents numerical quasistatic simulation results of the air entrainment phenomenon between a web and a spirally grooved roller. The numerical results show that during one complete rotation of the spirally grooved roller, the traction coefficient between the web and the roller changes with time due to the changing shape of the groove in the contact region, and the average traction coefficient of the circumferentially grooved roller is higher than that of the spirally grooved roller. Using a laser sensor, web deflection is measured and compared with the numerical results at the inlet. Furthermore, a smoke wire experiment shows a clear view of the air entrainment phenomenon at the entrance, between the web and the roller.

We investigate the flight characteristics of the beetle (Allomyrina dichotoma), which has hind wings and elytra, based on the numerical simulation. Flapping kinematics of both the hind wings and the elytra are obtained by visualization in a wind tunnel. One can observe veins and membrane on the hind wings, which allow corrugated patterns on the cross section of the wing along streamwise direction. Therefore, we investigate details of the vein and membrane structures, which is able to be used for modelling the wing for the numerical simulation. The veins thickness ranges from 100 µm to 1000 µm, meanwhile, the thickness of the membrane is around 5 µm. Numerical results show that the corrugated cross section wing allows higher aerodynamic performance than the elliptic wing for the same flapping kinematics. The elytra have the positive contribution of vertical force during forward flight, while its effect is not significant when there is no income flow.

High-frequency response analysis (Hi-FRA) is required to predict the resonant behavior of modern microsystems operated over a high frequency range. Algebraic substructuring (AS) method is a powerful numerical technique for FRA. However, the existing AS method is developed for low-FRA, say over the range 1Hz---2KHz. In this work, we extend the AS method for FRA over a given frequency range [? min ,? max ]. Therefore, it can be efficiently applied to systems operated at high frequency, say over the range 1MHz---2MHz. The success of the proposed method is demonstrated by Hi-FRA of a microgyroscope.

In this work, the aerodynamic performance of beetle wing in free-forward flight was explored by a three-dimensional computational fluid dynamics (CFDs) simulation with measured wing kinematics. It is shown from the CFD results that twist and camber variation, which represent the wing flexibility, are most important when determining the aerodynamic performance. Twisting wing significantly increased the mean lift and camber variation enhanced the mean thrust while the required power was lower than the case when neither was considered. Thus, in a comparison of the power economy among rigid, twisting and flexible models, the flexible model showed the best performance. When the positive effect of wing interaction was added to that of wing flexibility, we found that the elytron created enough lift to support its weight, and the total lift (48.4 mN) generated from the simulation exceeded the gravity force of the beetle (47.5 mN) during forward flight.

The affiliation details for author Jongin Hong are: Department of Chemistry, Chung-Ang University, Korea.

Clamping loss caused by elastic wave radiation to a support is one of the most dominant loss mechanisms in predicting the quality factor of very or ultra high frequency nanocantilever resonators. To consider nanosurface effects and different scales of the resonator and support, we propose a multiscale model combining a scale-bridging model for the resonator and a perfectly matched layer to estimate the loss in the support. The proposed model is used to investigate the surface effect on the nanoresonator and also shows the tradeoff between the resonant frequency and the quality factor with or without the surface effects.