• International Journal of Naval Architecture and Ocean Engineering
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• v.8 no.1
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• pp.13-21
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• 2016

Parametric pitch instability of a Deep Draft Semi-submersible platform (DDS) is investigated in irregular waves. Parametric pitch is a form of parametric instability, which occurs when parameters of a system vary with time and the variation satisfies a certain condition. In previous studies, analyzing of parametric instability is mainly limited to regular waves, whereas the realistic sea conditions are irregular waves. Besides, parametric instability also occurs in irregular waves in some experiments. This study predicts parametric pitch of a Deep Draft Semi-submersible platform in irregular waves. Heave motion of DDS is simulated by wave spectrum and response amplitude operator (RAO). Then Hill equation for DDS pitch motion in irregular waves is derived based on linear-wave theory. By using Bubnov-Galerkin approach to solve Hill equation, the corresponding stability chart is obtained. The differences between regular-waves stability chart and irregular-waves stability chart are compared. Then the sensitivity of wave parameters on DDS parametric pitch in irregular waves is discussed. Based on the discussion, some suggestions for the DDS design are proposed to avoid parametric pitch by choosing appropriate parameters. The results indicate that it's important and necessary to predict DDS parametric pitch in irregular waves during design process.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.2 s.140
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• pp.124-128
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• 2005

This paper describes motion stability of a spar platform with and without a damping plate in regular waves. The heave and pitch motion equation is derived in terms of Mathieu equation and the stability diagram is obtained. It is shown that the spar platform with damping plate has smaller unstable region than that without damping plate in the stability diagram. Model tests are carried out to verify the mathematical analysis. Under the condition that the pitch natural period is approximately double the heave natural period and the heave motion is amplified at heave resonance, unstable pitch motions are evoked. However the unstable motion is stabilized in cases of spar platform with damping plate. Therefore the damping plate is an effective device to stabilize the motion of spar platform.

• Journal of the Korean Society of Marine Environment & Safety
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• v.23 no.3
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• pp.320-329
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• 2017

In this research, we have calculated characteristics of wave-piercing high-speed planing hull, by using a RANS solver and overset grid method, for comparing with experimental measurements of that and simulating with several appendages, since the computed results of commercial CFD code look reasonable for the prediction of the performances of planing hulls on calm water in planing conditions. As a result, it is confirmed that the dynamic instability phenomena in pitch and heave motions (porpoising) occurred after a certain $Fn_V$, and effectively suppressed using some of appendages, especially the 0.5L spray rail is suppressed to 24-55 % in the pitch motion and 33-55 % in the heave motion. In spray phenomenon, 1L hard chine suppress spray effectively and it is effective to set the angle of appendages to be less than $0^{\circ}$ in order to suppress wave.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2002.10a
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• pp.185-192
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• 2002

In this study, we choose the sinusoidal shaped arch with pin-ends subjected to sinusoidal distributed excitation to investigate the fundamental mechanism of the dynamic instability. We derive the nonlinear equations of motion to investigate the instability phenomenon of arch structures and Identify the buckling characteristics of sinusoidal shaped arch structures through the nonlinear eigenvalue analysis with discreted equations of motion by Galerkin's method. We examine that phenomenons which direct snapping and indirect snapping with backbone curves to understand occurrence paths of the dynamic buckling.

• Tribology and Lubricants
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• v.17 no.4
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• pp.283-289
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• 2001

The paper presents the stability characteristics of a rotor-bearing system supported by actively controlled hydrodynamic journal bearing. The proportional and derivative controls including coupled motion are adopted for the control algorithm to control the hydrodynamic journal bearing with a circumferentially groove. Also, the cavitation algorithm implementing the Jakobsson-Floberg-Olsson boundary condition is adopted to predict cavitation regions in the fluid film more accurately than a conventional analysis which uses the Reynolds condition. The stability characteristics of a rotor-bearing system supported by actively controlled hydrodynamic journal bearing are investigated for various control gains with the Routh-Hurwitz criteria using the linear dynamic coefficients which are obtained from the perturbation method. It is found that the speed at onset of the instability is increased for both proportional and derivative control of the bearing. It is also found that the proportional and derivative control of the coupled motion is more effective than that of the uncoupled motion.

• Structural Engineering and Mechanics
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• v.48 no.1
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• pp.103-124
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• 2013

This paper studies the static and dynamic characteristics of composite plates subjected to an arbitrary periodic load in hygrothermal environments. The material properties of composite plates are depended on the temperature and moisture. The governing equations of motion of Mathieu-type are established by using the Galerkin method with reduced eigenfunction transforms. A periodic load is taken to be a combination of axial pulsating load and bending stress in the example problem. The regions of dynamic instability of laminated composite plates are determined by solving the eigenvalue problems based on Bolotin's method. The effects of temperature rise and moisture concentration on the dynamic instability of laminated composite plates are investigated and discussed. The influences of various parameters on the instability region and dynamic instability index are also investigated. The numerical results reveal that the influences of hygrothermal effect on the dynamic instability of laminated plates are significant.

• Journal of the Korean Applied Science and Technology
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• v.40 no.6
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• pp.1259-1267
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• 2023

The purpose of this study was to investigate the effects of a acute ankle mobility exercise (AE) program on the range of motion and pain levels associated with chronic ankle instability (CAI). Twenty adult women with an average score of 24 or less on both sides in the ankle instability questionnaire were selected and divided into two groups: the Ankle Mobility Exercise Program group (AE, n=10) and the control group (CON, n=10). The AE treatment involved a one-time, 20-minute exercise session for each ankle, conducted separately on the left and right ankles, to examine the response to a acute exercise. Measures of ankle instability, pain, and ankle range of motion were evaluated before and after the treatment. In the AE group, which underwent the AE program, a significant decrease in pain was observed post-AE compared to pre-AE (p<.01). In addition, an increase in dorsiflexion was observed in the AE group post-AE compared to pre-AE following the AE program(p<.05). These results suggest that a acute AE program can effectively alleviate some aspects of ankle instability by improving ankle pain and range of motion in adult women with chronic ankle instability (CAI).

• The Journal of Korean Academy of Orthopedic Manual Physical Therapy
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• v.23 no.1
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• pp.7-13
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• 2017

Background: To evaluate the effect of Kinesio taping applied on the ankle instability, joint range of motion and balance. Methods: The participants included in this study were male and female, 20~30 ages, who experiencing an ankle sprain or had chronic pain, did not exercise during the intervention, and did not experience severe exercise at least 3 weeks before. A total of twenty-four participants were divided into two groups: Kinesio taping applied group (n=12) and control group (n=12). The experiment was conducted for a three days. Measurements were taken for ankle joint range of motion using goniometer, and measurements were taken for balance using good balance system. Pre-test measurements were conducted on before Kinesio taping apply, and 24 hours after, 48 hours after, 72 hours after measurements were conducted. Statistical analysis was done using a independent samples t-test and repeated measure ANOVA. Results: There were significant differences to the duration of intervention in ankle joint range of motion and balance within the both group. However, there was a significant differences Kinesio taping group when comparing the groups. Conclusions: According to the results of this study, applying Kinesio taping to ankle instability is more effective on ankle joint range of motion and to recover balance.

• Journal of Korean Foot and Ankle Society
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• v.6 no.2
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• pp.251-255
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• 2002

Purpose: Chronic subtalar instability is not common and similar to chronic ankle instability and the incidence and cause chronic subtalar instability are not well known. Recently we have experienced chronic subtalar instability without chronic ankle instability which was treated with modified Brostrom procedures. Materials and Methods: The patient is 46 year old man who has suffered from left ankle sprain for 30 years and recently aggravated more than twice a day. On subtalar stress view, 14 degree angulation of subtalar joint was noted and on anterior drawer view, 8 mm anterior displacement of left ankle was seen. Results: In operation, there was no anterior talofibular ligament abnormility but calcaneofibular ligament loosening was found. Ligament reconstruction was performed using modified Brostrom procedure. At 12 months after operation, the patient complains no pain and no limit of motion and no instability. Conclusion: We experienced chronic subtalar instability without ankle instability treated with modified Brostrom procedures. No instability was found after treatment without complication.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.12
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• pp.2038-2046
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• 1997

In this paper the instability of the system which has a disk and a mass-spring-damper system interacting through a medium having stiffness and damping is analyzed. To solve the equations of motion of this systme, it is assumed that the solution consists of the eigenfunctions which are the products of the Bessel functions and sine or cosine functions. The former represents the radial characteristics of the disk and the latter represents the circumferential characteristics. Using this assumed solution and the orthogonality of the eigenfunctions, the equations of motion can be transformed into a set of equations of motion with variables dependent only on the time. After this set is changed to the state equation, the eigenvalue problem can be made. Once the eigenvalues are calculated according to the angular velocity of the disk, the dynamic characteristics ofthis system is obtained. Because the thickness of the disk and the element characteristics of the mass-spring-damper system have important effects on the stability of the system, it will be understood how these factors affect the system and then a method to ameliorate the stability of the system with a disk will be presented.