• 대한조선학회논문집
• /
• 제46권6호
• /
• pp.667-674
• /
• 2009

The researches on the development of composite material underwater vehicle propeller have been actively attempted for the reduction of radiation noise with outstanding damping effects. Composite material propellers have almost been designed and produced by the foreign experts, and it is difficult to obtain the related informations about their flow, vibration, material characteristics because they are treated as the secrets with close relationship to the military technology, especially in the case of underwater vehicles. For the security of domestic manufacture of composite material propeller and the comparison and examination of its performance and radiation noise characteristics with those of German CONTUR composite material propeller, two propellers were self-produced according to the fiber weaving and array using compressible molding process and their self performances and radiation noise characteristics were measured. The mean fluctuations of blade tip of self-produced composite material propeller were increased and the radiation noises in the low frequency band were reduced compared to those of CONTUR, which could be estimated as the change of material characteristics and also be thought to be used for the future research informations.

• Current Optics and Photonics
• /
• 제1권1호
• /
• pp.23-28
• /
• 2017

The laser has been regarded as the potential illumination source for the next generation of projectors. However, currently the major issues in applying the laser as an illumination source for projectors are beam shaping and laser speckle. We present a compact solution for both issues by using a vibrating diffractive optical element (DOE). The DOE is designed and fabricated, and it successfully transforms the circular Gaussian laser beam to a low speckle contrast uniform rectangular pattern. Under a vibration frequency of 150 Hz and amplitude of $200{\mu}m$, the speckle contrast value is reduced from 67.67% to 13.78%, and the ANSI uniformity is improved from 24.36% to 85.54%. The experimental results demonstrate the feasibility and potential of the proposed scheme, and the proposed method is a feasible approach to the miniaturization of laser projection display illumination systems.

• Corrosion Science and Technology
• /
• 제20권1호
• /
• pp.26-36
• /
• 2021

Pipes operating in the seawater environment faces cavitation degradation and corrosion of the metallic component, as well as a negative synergistic effect. Cavitation degradation shows the mechanism by which materials deteriorate by causing rapid change of pressure or high-frequency vibration in the solution, and introducing the formation and explosion of bubbles. In order to rate the cavitation resistance of materials, constant conditions have been used. However, while a dynamic cavitation condition can be generated in a real system, there has been little reported on the effect of ultrasonic amplitude on the cavitation resistance and mechanism of composites. In this work, 3 kinds of epoxy coatings were used, and the cavitation resistance of the epoxy coatings was evaluated in 3.5% NaCl at 15 ℃ using an indirect ultrasonic cavitation method. Eleven kinds of mechanical properties were obtained, namely compressive strength, flexural strength and modulus, tensile strength and elongation, Shore D hardness, water absorptivity, impact test, wear test for coating only and pull-off strength for epoxy coating/carbon steel or epoxy coating/rubber/carbon steel. The cavitation erosion mechanism of epoxy coatings was discussed on the basis of the mechanical properties and the effect of ultrasonic amplitude on the degradation of coatings.

• Earthquakes and Structures
• /
• 제17권2호
• /
• pp.175-189
• /
• 2019

The aim of the study is to show the results of complex shaking table experimental investigation focused on the response of two models of cylindrical steel tanks under mining tremors and moderate earthquakes, including the aspects of diagnosis of structural damage. Firstly, the impact and the sweep-sine tests have been carried out, so as to determine the dynamic properties of models filled with different levels of liquid. Then, the models have been subjected to seismic and paraseismic excitations. Finally, one fully filled structure has been tested after introducing two different types of damages, so as to verify the method of damage diagnosis. The results of the impact and the sweep-sine tests show that filling the models with liquid leads to substantial reduction in natural frequencies, due to gradually increasing overall mass. Moreover, the results of sweep-sine tests clearly indicate that the increase in the liquid level results in significant increase in the damping structural ratio, which is the effect of damping properties of liquid due to its sloshing. The results of seismic and paraseismic tests indicate that filling the tank with liquid leads initially to considerable reduction in values of acceleration (damping effect of liquid sloshing); however, beyond a certain level of water filling, this regularity is inverted and acceleration values increase (effect of increasing total mass of the structure). Moreover, comparison of the responses under mining tremors and moderate earthquakes indicate that the power amplification factor of the mining tremors may be larger than the seismic power amplification factor. Finally, the results of damage diagnosis of fully filled steel tank model indicate that the forms of the Fourier spectra, together with the frequency and power spectral density values, can be directly related to the specific type of structural damage. They show a decrease in the natural frequencies for the model with unscrewed support bolts (global type of damage), while cutting the welds (local type of damage) has resulted in significant increase in values of the power spectral density for higher vibration modes.

• 한국전산구조공학회논문집
• /
• 제32권1호
• /
• pp.29-35
• /
• 2019

본 논문에서는 건축물 진동저감장치에 적용되는 액체감쇠기 내 유체 자유표면의 동적 거동 계측을 위해 저가형 RGB-depth 센서인 Microsoft사 $Kinect^{(R)}$ v2의 활용과 계측시스템을 구축하는 방법을 제안하였다. $Kinect^{(R)}$ v2의 성능검토 및 실효성 확인, SDK(software development kit)를 사용한 실시간 모니터링, 3D 공간상에서 유체의 표면 정보 취득, 기존 비디오 센싱기법과의 비교를 통해 본 연구에서 제안한 유체의 동적 거동 계측 시스템의 정확성과 우수성을 검증하였다. 제안된 계측시스템을 활용하여 소형 수조 내 액체에 대한 동적 거동 정밀계측을 수행하였으며, 이를 바탕으로 광범위한 가진입력에 대한 유체 자유표면의 동적 거동 특징을 확인하였다. 본 연구의 결과를 바탕으로 RGB-depth센서의 건축물 진동저감 적용을 통해 정밀한 모니터링 시스템을 구축하고 최적화된 액체감쇠기의 설계 및 운용을 기대할 수 있다.

• Advances in nano research
• /
• 제7권3호
• /
• pp.191-208
• /
• 2019

In the present work the dynamic analysis of the functionally graded rectangular nanoplates is studied. The theory of nonlocal elasticity based on the quasi 3D high shear deformation theory (quasi 3D HSDT) has been employed to determine the natural frequencies of the nanosize FG plate. In HSDT a cubic function is employed in terms of thickness coordinate to introduce the influence of transverse shear deformation and stretching thickness. The theory of nonlocal elasticity is utilized to examine the impact of the small scale on the natural frequency of the FG rectangular nanoplate. The equations of motion are deduced by implementing Hamilton's principle. To demonstrate the accuracy of the proposed method, the calculated results in specific cases are compared and examined with available results in the literature and a good agreement is observed. Finally, the influence of the various parameters such as the nonlocal coefficient, the material indexes, the aspect ratio, and the thickness to length ratio on the dynamic properties of the FG nanoplates is illustrated and discussed in detail.

• 한국공간구조학회논문집
• /
• 제19권1호
• /
• pp.117-126
• /
• 2019

Micro-Electro-Mechanical Systems (MEMS) sensors have been widely used in Structural Health Monitoring due to their convenience and lower costs in comparison to conventional sensors. Triggered measurements are relevant in events such as earthquakes because unlike continuous measurements, they only record the structural response once an event happens. This is more cost effective and it makes the data more manageable because only the required measurements from the event are recorded. The most common method of triggering is amplitude triggering. However, lower input amplitudes (less than 0.1g) cannot be triggered by using this method. In this paper, sound triggering was introduced to allow triggered measurements for lower input amplitude values. The performance of the sound triggering and amplitude triggering were compared by a series of shaking-table tests. It was seen that sound-triggering method has a wider frequency (0.5~10Hz) and amplitude (0.01~1.0g) range of measurements. In addition, the sound triggering method performs better than the amplitude triggering method at lower amplitudes. The performance of the amplitude triggering, in terms of the triggering being simultaneous improves at higher input amplitudes.

• Physical Therapy Rehabilitation Science
• /
• 제8권1호
• /
• pp.32-39
• /
• 2019

Objective: The aim of this study is to investigate the effects of whole body vibratory stimulation on muscle strength, balance, and fall efficacy among super-aged women. Design: Randomized controlled trial. Methods: Twenty-eight super-aged women over 80 years of age were assigned to either the experimental group (n=14) and control group (n=14). The experimental group received an exercise program that used the whole body vibratory stimulation with a frequency of 30 Hz and amplitude of 3 mm, and the control group received an exercise program without vibratory stimulation. Intervention was provided for 4 weeks, 3 sessions per week, and 30 minutes per session. In order to measure lower extremity muscle strength the 30-second chair stand test (CST) was used. The Berg Balance Scale (BBS) was used to measure dynamic balance. Static balance was measured by tracking the path length, velocity, and area of the center of pressure (CoP). The Falls Efficacy Scale (FES) was used to measure the subjects' fear of falling. Results: Both the experimental and control group demonstrated statistically significant increase in muscle strength, dynamic balance, and fall efficacy (p<0.05). Only the experimental group showed significant improvements in static balance before and after the intervention (p<0.05). The experimental group showed significantly greater improvements in CST, BBS, and CoP (path length, velocity) than control group (p<0.05). Conclusions: Whole body vibratory stimulation exercise is shown to be a safe and appropriate physical therapy intervention method to enhance muscle strength, balance, and fall efficacy of super-aged women.

• Journal of Dental Anesthesia and Pain Medicine
• /
• 제20권6호
• /
• pp.397-402
• /
• 2020

Background: Inferior alveolar nerve block (IANB) is the most common, painful, and anxiety-provoking procedure involving needle insertion for anesthetic solution deposition. DentalVibeⓇ (DV) delivers vibration at a sustained frequency as a counter-stimulation to the site of injection, thereby alleviating pain. The aim of this study was to evaluate and compare the effectiveness of DV and lignocaine hydrochloride 2% gel (Lox 2% jelly) in pain reduction during IANB in children. Methods: A split-mouth randomized clinical trial was designed with a sample of 60 children (age, 6 to 12 years) requiring bilateral IANB for various dental procedures; DV was used while administering IANB and Lox 2% jelly was used as the topical anesthetic before administering IANB at subsequent appointments. During both appointments, pain perception was measured using the sound, eye, motor (SEM) scale and Wong-Baker faces pain rating scale (WBFPRS); oxygen saturation (SpO2) and pulse rate were measured using a pulse oximeter before, during, and after the IANB procedure. The obtained values were tabulated and subjected to statistical analysis. Wilcoxon test was used for intergroup comparison, and Friedman test, for intragroup comparison of measured variables at different treatment phases. Results: The medians and interquartile ranges of the WBFPRS scores recorded during the IANB procedure for DV and Lox 2% jelly were 2 (2-4) and 2 (0-2), respectively (P < 0.05). The SEM scale scores, mean SpO2, and pulse rate did not show any significant differences during the IANB procedure between both treatments. Conclusion: Both DV and Lox 2% jelly were found to be effective in pain reduction during IANB in children.

• Steel and Composite Structures
• /
• 제39권1호
• /
• pp.95-107
• /
• 2021

In this work, the dynamic response of functionally graded beams on variable elastic foundations is studied using a novel higher-order shear deformation theory (HSDT). Unlike the conventional HSDT, the present one has a new displacement field which introduces undetermined integral variables. The FG beams were assumed to be supported on Winkler-Pasternak type foundations in which the Winkler modulus is supposed to be variable in the length of the beam. The variable rigidity of the elastic foundation is assumed to be linear, parabolic and sinusoidal along the length of the beam. The material properties of the FG porous beam vary according to a power law distribution in terms of the volume fraction of the constituents. The equations of motion are determined using the virtual working principle. For the analytical solution, Navier method is used to solve the governing equations for simply supported porous FG beams. Numerical results of the present theory for the free vibration of FG beams resting on elastic foundations are presented and compared to existing solutions in the literature. A parametric study will be detailed to investigate the effects of several parameters such as gradient index, thickness ratio, porosity factor and foundation parameters on the frequency response of porous FG beams.