• Ocean and Polar Research
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• v.42 no.4
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• pp.293-301
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• 2020

Blades of tidal stream turbines have to sustain many different loads during operation in the underwater environment, so securing their structural safety is a key issue. In this study, we focused on periodic loads due to wave orbital motion and propose a load reduction method with a blade design. The flap of an airplane wing is a well-known structure designed to increase lift, and it can also change the load distribution on the wing through deflection. For this reason, we adopted a passive flap structure for the load reduction and investigated its effectiveness by an analytical method based on the blade element moment theory. Flap torsional stiffness required for the design of the passive flap can be obtained by calculating the flap moment based on the analytic method. Comparison between a flapped and a fixed blade showed the effect of the flap on load reduction in a high amplitude wave condition.

• Journal of the Korean Society of Laryngology, Phoniatrics and Logopedics
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• v.32 no.1
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• pp.1-8
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• 2021

Human vocal cords vibrate as quickly as 100-250 times per second, so it is impossible to observe them with normal endoscopic diagnostic equipment. High-speed videolaryngoscopy (HSV) allows the visualization of non-periodic vibratory motion of vocal fold beyond the limitation of videostroboscopy. New developed post-processing methods that converts HSV to two-dimensional videokymography (2D VKG) using U-medical image-processing software can provide quantitative information on vocal fold mucosa vibration. Multifunctional laryngeal examination system is composed of 3 kinds of examinations such as HSV, 2D scanning digital kymography (2D DKG) and line scanning digital kymography (DKG). Evaluation of entire vocal cord vibratory pattern in each cord is possible using 2D DKG and a faster and more reliable quantitative information can be obtained. As this system is used in clinical and research, it is expected to bring much advances to the diagnosis of voice disorders. In this review, I will introduce the principles and advantages on examination of the vocal fold vibration, which is in the spotlight recently, and proceed with the literature review.

• International Journal of High-Rise Buildings
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• v.10 no.2
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• pp.149-164
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• 2021

Most of high-rise buildings in Japan^*1 are structure with damping systems recently. The design procedure is performance-based design (PBD), which is based on the nonlinear response history procedure (NRHP) using 2 or 3-dimentional frame model. In addition, hysteretic property of steel plates or velocity-dependent property of viscous dampers are common practice for the damping system. However, for the selection of damping system, the easy dynamic analysis of recent date may lead the most of engineers to focus attention on the maximum response only without thinking how it shakes. By nature, the seismic design shall be to figure out the action of inertia forces by complex & dynamic loads including periodic and pulse-like characteristics, what we call seismic ground motion. And it shall be done under the dynamic condition. On the contrary, we engineers engineers have constructed the easy-to-use static loads and devoted ourselves to handle them. The structures with damping system shall be designed considering how the stiffness & damping to be applied to the structures against the inertia forces with the viewpoint of dynamic aspect. In this paper we reconsider the role of damping in vibration and give much thought to the basic of shake with damping from a standpoint of structural design. Then, we present some design examples based on them.

• International Journal of Naval Architecture and Ocean Engineering
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• v.13 no.1
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• pp.746-757
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• 2021

To investigate the mechanism of friction-induced vibration and noise of ship water lubricated stern bearings, a two-degree-of-freedom (2-DOF) nonlinear self-excited vibration model is established. The novelty of this work lies in the detailed analysis of influence of different parameters on the stability and nonlinear vibration characteristics of the system, which provides a theoretical basis for the various friction vibration and noise phenomenon and has a very important directive meaning for low noise design of water lubricated stern bearings. The results reveal that the change of any parameter, such as rotating speed of shaft, contact pressure, friction coefficient, system damping and stiffness, has an important influence on the stability and nonlinear response of the system. The vibration amplitudes of the system increase as (a) rotating speed of shaft, contact pressure, and the ratio of static friction coefficient to dynamic friction coefficient increase and (b) the transmission damping between motor and shaft decreases. The frequency spectrum of the system is modulated by the first mode natural frequency, which is continuous multi-harmonics of the first mode natural frequency. The response of the system presents a quasi-periodic motion.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.21 no.5
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• pp.16-21
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• 2022

This study investigates the effect of the deformation on the sensitivity of a flexible polydimethylsiloxane (PDMS) membrane sensor. A PDMS membrane sensor was developed to measure the impact force of a water droplet using a silver nanowire (AgNW). The initial deformation of the membrane was confirmed with the application of a tensile force (i.e., tension) and fixing force (i.e., compressive force) at the gripers, which affects the sensitivity. The experimental results show that as the tension applied to the membrane increased, the sensitivity of the sensor decreased. The initial electrical resistance increased as the fixing force increased, while the sensitivity of the sensor decreased as the initial resistance increased. The movement of the membrane due to the impact force of the water droplet was observed with a high-speed camera, and was correlated with the measured sensor signal. The analysis of the motion of the membrane and droplets after collision confirmed the periodic movement of not only the membrane but also the change in the height of the droplet.

• The Journal of the Korea institute of electronic communication sciences
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• v.18 no.4
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• pp.731-736
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• 2023

Walking is a periodic motion that contains specific phases and is a basic movement method for humans. Through gait analysis, various musculoskeletal health conditions can be identified. In this study, we propose a calf wearable sensor system that can perform gait analysis without space limitations. Using a ToF(: Time-of-Flight) sensor that measures distance and an IMU(: Inertial Measurement Unit) sensor that measures inclination the heel trajectory of walking was derived by proposed method. In case of abnormal gait with risk of fall, gait is evaluated by analyzing the change pattern of the heel trajectory.

• Structural Engineering and Mechanics
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• v.86 no.2
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• pp.221-235
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• 2023

To calculate the vibrations of a tout cable subjected to axial support excitations, a nonlinear relationship of cable force and the support displacement under static situations are employed to depict the quasi-static vibration of the cable. The dynamic components of quasi-static vibration are inputted as "direct loads" to cause the parametric vibrations on the cable. Both the governing equations of motion and deformation compatibility for parametric vibrations are then derived, which indicates the high coupling of cable parametric force and deformation. Numerical solutions, based on the finite difference method, are put forward for the parametric vibrations, which is validated by the finite element method under periodic axial support excitations. For the quasi-static response, the shorter cables are more sensitive to support excitations than longer ones at small cable force. The quasi-static cable force makes the greatest contribution to the total cable force, but the parametric cable force is responsible for the occurrence of cable loosening at large excitation amplitudes. Moreover, this study also revealed that the traditional approach, assuming a linear relationship between quasi-static cable force and axial support displacement, would result in some great error of the cable parametric responses.

• Ocean Systems Engineering
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• v.13 no.2
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• pp.147-172
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• 2023

This study investigates the influence of loading and inflow conditions on tidal turbine performance from a hydrodynamic and hydroacoustic point of view. A boundary element method is utilized for the former to investigate turbine performance at various loading conditions under zero/non-zero yaw inflow. The boundary element method is selected as it has been selected, tested, and validated to be computationally efficient and accurate for marine hydrodynamic problems. Once the hydrodynamic solutions are obtained, such as the time-dependent surface pressures and periodic motion of the turbine blade, they are taken as the known noise sources for the subsequence hydroacoustic analysis based on the Ffowcs Williams-Hawkings formulation given in a form proposed by Farassat. This formulation is coupled with the boundary element method to fully consider the three-dimensional shape of the turbine and the speed of sound in the acoustic analysis. For validations, a model turbine is taken from a reference paper, and the comparison between numerical predictions and experimental data reveals satisfactory agreement in hydrodynamic performance. Importantly, this study shows that the noise patterns and sound pressure levels at both the near- and far-field are affected by different loading conditions and sensitive to the inclination imposed in the incoming flow.

• Journal of the Korean Society of Marine Environment & Safety
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• v.24 no.2
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• pp.140-145
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• 2018

This paper deals with the dynamical analysis of a two-point mooring vessel under surge excitations. The characteristics of nonlinear behaviors are investigated completely including bifurcation and limit cycle according to particular input parameter changes. The strong nonlinearity of the mooring system is mainly caused by linear and cubic terms of restoring force. The numerical simulation is performed based on the fourth order Runge-Kutta algorithm. The bifurcation diagram and several instability phenomena are observed clearly by varying amplitudes as well as frequencies of surge excitations. Stable periodic solutions, called the periodic windows, can be obtained in succession between chaotic clouds of dots in case of frequency ${\omega}=0.4rad/s$. In addition, the chaotic region is unexpectedly increased when external forcing amplitude exceeds 1.0 with the angular frequency of ${\omega}=0.7rad/s$. Compared to the cases for ${\omega}=0.4$, 0.7rad/s, the region of chaotic behavior becomes more fragile than in the case of ${\omega}=1.0rad/s$. Finally, various types of steady states including sub-harmonic motion, limit cycle, and symmetry breaking phenomenon are observed in the two-point mooring system at each parameter value.

• Journal of The Korean Astronomical Society
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• v.36 no.3
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• pp.81-87
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• 2003

It is suggested that a flying-by star in a hot accretion disk may cool the hot accretion disk by the Comptonization of the stellar emission. Such a stellar cooling can be observed in the radio frequency regime since synchrotron luminosity depends strongly on the electron temperature of the accretion flow. If a bright star orbiting around the supermassive black hole cools the hot disk, one should expect a quasi-periodic modulation in radio, or even possible an anti-correlation of luminosities in radio and X-rays. Recently, the unprecedentedly accurate infrared imaging of the Sagittarius A$\ast$ for about ten years enables us to resolve stars around it and thus determine orbital parameters of the currently closest star S2. We explore the possibility of using such kind of observation to distinguish two quite different physical models for the central engine of the Sagittarius A$\ast$, that is, a hot accretion disk model and a jet model. We have attempted to estimate the observables using the observed parameters of the star S2. The relative difference in the electron temperature is a few parts of a thousand at the epoch when the star S2 is near at the pericenter. The relative radio luminosity difference with and without the stellar cooling is also small of order $10^{-4}$, particularly even when the star S2 is near at the pericenter. On the basis of our findings we tentatively conclude that even the currently closest pass of the star S2 is insufficiently close enough to meaningfully constrain the nature of the Sagittarius A$\ast$ and distinguish two competing models. This implies that even though Bower et al. (2002)have found no periodic radio flux variations in their data set from 1981 to 1998, which is naturally expected from the presence of a hot disk, a hot disk model cannot be conclusively ruled out. This is simply because the energy bands they have studied are too high to observe the effect of the star S2 even if it indeed interacts with the hot disk. In other words, even if there is a hot accretion disk the star like S2 has imprints in the frequency range at v $\le$ 100 MHz.