• Wind and Structures
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• 제26권2호
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• pp.57-68
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• 2018

Wind tunnel tests are conducted to investigate wake-induced vibrations of two circular cylinders with a center-to-center spacing of 4 diameters and attack angle varying from $0^{\circ}$ to $20^{\circ}$ for Reynolds numbers between 18,000 and 168,800. Effects of structural damping, Reynolds number, attack angle and reduced velocity on dynamic responses are examined. Results show that wake-induced vortex vibrations of the downstream cylinder occur in a wider range of the reduced velocity and have higher amplitudes in comparison to the vortex-induced vibration of a single circular cylinder. Two types of wake-induced instability phenomena with distinct dynamic characteristics are observed, which may be due to different generation mechanisms. For small attack angles like $5^{\circ}$ and $10^{\circ}$, the instability of the downstream cylinder characterizes a one-degree-of-freedom (1-DOF) oscillation moving in the across-wind direction. For a large attack angle like $20^{\circ}$, the instability characterizes a two-degree-of-freedom (2-DOF) oscillation with elliptical trajectories. For an attack angle of $15^{\circ}$, the instability can transform from the 1-DOF pattern to the 2-DOF one with the increase of the Reynolds number. Furthermore, the two instabilities show different sensitivity to the structural damping. The 1-DOF instability can be either completely suppressed or reduced to an unsteady oscillation, while the 2-DOF one is relatively less sensitive to the damping level. Reynolds number has important effects on the wake-induced instabilities.

• PNF and Movement
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• 제18권2호
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• pp.173-182
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• 2020

Purpose: The purpose of the study was to investigate the effects of visual feedback exercise (VFE), sandal exercise (SE), and thera-band exercise (TE) on people with functional ankle instability. Methods: The 45 study participants were divided into three groups of 15. In the VFE group, participants performed short-foot exercises using visual feedback; the SE group performed sandal exercises, and the TE group performed thera-band exercises. Over a period of 8 consecutive weeks, the experimental groups exercised for 15 minutes per session, four times per week. Balance was measured using the Biodex Balance System for dynamic balance ability (overall balance index [OBI], mediolateral balance index [MBI], and anterioposterior balance index [ABI]). Vibration threshold sense (VTS) was assessed using a vibratory sensory analyzer. Ankle muscular strength was measured using the Biodex 4 Pro. The main outcome measures were assessed at baseline and after 8 weeks of training. Results: Significant improvements in dynamic balance were noted in all three groups (p < 0.05). Significant improvements in VTS and muscular strength were also noted in the VFE and TE groups but not in the SE group (p < 0.05). Conclusion: Appropriate environmental and situational mediation may be of great clinical significance for patients suffering from chronic ankle instability. It is hoped that the findings will inform a program for prevention of ankle instability.

• 한국윤활학회:학술대회논문집
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• 한국윤활학회 2000년도 제31회 춘계학술대회
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• pp.153-158
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• 2000

The purpose of present paper is to prove an improvement on stability of plane Journal bearing due to the leaf spring damper(LSD) experimentally A flexible rotor system is designed and manufackuea in order to generate oil whip instability of journal bearing at relative lower rotating speed. Vibration amplitude and instability onset speeds are investigated for a conventional plane journal bearing and plane journal bearing with LSD. To investigate the damping effects of LSD on stability of bearing, experiments are also conducted on the leaf spring dampers with and without working oil. It is found that the leaf spring damper can considerably increase the instability onset speed of a plane journal bearing.

• 산업기술연구
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• 제22권A호
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• pp.137-144
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• 2002

A series of wind tunnel tests was conducted to investigate the existence of the galloping instability of inclined dry cables and also to Identify the influence of some parameters on it. These parameters are the structural damping and cable surface roughness, which may have significant impact on the vibration characteristics. The test results showed both the divergent type of galloping instability and the limited amplitude high wind speed vortex shedding excitation. Galloping instability was observed in only one case. Parametric study shows that the vortex shedding oscillation can be easily suppressed with an increase of structural damping. It was also shown that the instability criterion indicated by earlier research was too conservative compared to the results obtained from the present study.

• Tribology and Lubricants
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• 제17권3호
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• pp.185-190
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• 2001

The purpose of present paper is to prove an improvement on stability of plane journal bearing due to the leaf spring damper (LSD) experimentally. A flexible rotor system is designed and manufactured, in order to generate oil whip instability of journal bearing at relative lower rotating speed. Vibration amplitude and instability onset speeds are investigated for a conventional plane journal bearing and plane journal bearing with LSD. To investigate the damping effects of LSD on stability of bearing, experiments are also conducted on the leaf spring dampers with and without working oil. It is found that the leaf spring damper can considerably increase the instability onset speed of a plane Journal bearing.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2524-2529
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• 2008

The thermal system like a combustion chamber is believed to experience a significant instability problem with vibration in case that the thermal energy or the acoustic energy are transformed into a different form through a relevant path. This study deals with a numerically- predicted, Thermoacoustic instability in a Rijke tube by using a non-linear model for a heat source. The heating part where the energy transformation occurs actively is modeled after simulating two-dimensional cylinder case with constant surface temperature, and a nonlinear model that accounts for the transfer function of magnitude- and phase-characteristics is properly implemented so as to be dependent on the pulsation strength in the tube. The heat source model is observed to result in equivalent Thermoacoustic instabilities in the Rijke tube except low flow-rate cases in which the natural convection is dominant.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2017년도 제48회 춘계학술대회논문집
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• pp.61-64
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• 2017

한국형발사체에 적용되는 액체로켓엔진은 가스발생기 개방형 엔진 사이클을 채택한다. 가스발생기에서 연소 불안정이 발생하면 연소 성능이 변하고 진동, 소음 등의 문제점을 야기하거나 하드웨어의 손상을 초래할 수 있다. 본 연구에서는 액체로켓엔진의 가스발생기를 개발하면서 나타난 연소불안정 경험 사례를 소개하고자 한다.

• Steel and Composite Structures
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• 제31권3호
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• pp.277-290
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• 2019

This paper investigates the parametric instability (PI) of multilayered composite conical shells (MLCCSs) under axial load periodically varying the time, using the first order shear deformation theory (FOSDT). The basic equations for the MLCCSs are derived and then the Galerkin method is used to obtain the ordinary differential equation of the motion. The equation of motion converted to the Mathieu-Hill type differential equation, in which the DI is examined employing the Bolotin's method. The expressions for left and right limits of dimensionless parametric instability regions (PIRs) of MLCCSs based on the FOSDT are obtained. Finally, the influence of various parameters; lay-up, shear deformations (SDs), aspect ratio, as well as loading factors on the borders of the PIRs are examined.

• Nuclear Engineering and Technology
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• 제55권3호
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• pp.1118-1124
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• 2023

Experimental study on the fluidelastic instability (FEI) of a curved tube bundle in single phase downward cross flow is investigated for the design qualification and analysis input preparation of helical coiled steam generator tubing. A 6×9 normal square curved tube array with equal and different vertical/horizontal pitch-to-diameter ratio was under-tested up to 6 m/s in term of gap flow velocity to measure the critical velocity for FEI. The critical velocity for FEI was measured at the turning point from the vibration amplitude plot along the gap flow velocity. Our test results were compared with straight tube results and published data in the design guideline. The applicability of the current design guidelines to a curved tube bundle is also assessed. We found that introducing frequency difference in a curved tube array increases the critical velocity for fluidelastic instability.

• Steel and Composite Structures
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• 제31권2호
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• pp.187-199
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• 2019

This paper presents the semi-analytical development of the dynamic instability behavior and the dynamic response of functionally graded (FG) cylindrical shallow shell panel subjected to different type of periodic axial compression. First, in prebuckling analysis, the stresses distribution within the panels are determined for respective loading type and these stresses are used to study the dynamic instability behavior and the dynamic response. The prebuckling stresses within the shell panel are the same as applied in-plane edge loading for the case of uniform and linearly varying loadings. However, this is not true for the case of parabolic loadings. The parabolic edge loading produces all the stresses (${\sigma}_{xx}$, ${\sigma}_{yy}$ and ${\tau}_{xy}$) within the FG cylindrical panel. These stresses are evaluated by minimizing the membrane energy via Ritz method. Using these stresses the partial differential equations of FG cylindrical panel are formulated by applying Hamilton's principal assuming higher order shear deformation theory (HSDT) and von-$K{\acute{a}}rm{\acute{a}}n$ non-linearity. The non-linear governing partial differential equations are converted into a set of Mathieu-Hill equations via Galerkin's method. Bolotin method is adopted to trace the boundaries of instability regions. The linear and non-linear dynamic responses in stable and unstable region are plotted to know the characteristics of instability regions of FG cylindrical panel. Moreover, the non-linear frequency-amplitude responses are obtained using Incremental Harmonic Balance (IHB) method.