• Bulletin of the Society of Naval Architects of Korea
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• v.23 no.2
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• pp.1-13
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• 1986

The nonlinear hydrodynamic forces acting on a two-dimensional circular cylinder, oscillating with large amplitude in the free surface, are calculated by using the Semi-Lagrangian Time-Step-ping Method used by O.M. Faltinsen. In present calculation the position and the potential value of free surface are calculated using the exact kinematic and dynamic free surface boundary condition. At each time step an integral equation is solved to obtain the value of potential and normal velocity along the boundaries, consisting of both the body surface and the free surface. Some effort was devoted to the elimination of instability arising in the range of high frequency. Numerical simulations were performed up to the 3rd or 4th period which seems to be enough for the transient effect to die out. Each harmonic component and time-mean force are obtained by the Fourier transform of forces in time domain. The results are compared with others' experimental and theoretical results. Particularly, the calculation shows the tendency that the acceleration-phase 1st-harmonic component(added mass) increases as the motion amplitude increases and a reverse tendency in the velocity-phase 1st-harmonic component(damping coefficient). The Yamashita's experimental result also shows the same tendency. In general, the present result show relatively good agreement with the Yamashita's experimental result except for the time-mean force.

• Structural Engineering and Mechanics
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• v.36 no.5
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• pp.561-573
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• 2010

The potential of the liquid column vibration absorber (LCVA) as a seismic vibration control device for structures has been explored in this paper. In this work, the structure has been modeled as a linear, viscously damped single-degree-of-freedom (SDOF) system. The governing differential equations of motion for the damper liquid and for the coupled structure-LCVA system have been derived from dynamic equilibrium. The nonlinear orifice damping in the LCVA has been linearized by a stochastic equivalent linearization technique. A transfer function formulation for the structure-LCVA system has been presented. The design parameters of the LCVA have been identified and by applying the transfer function formulation the optimum combination of these parameters has been determined to obtain the most efficient control performance of the LCVA in terms of the reduction in the root-mean-square (r.m.s.) displacement response of the structure. The study has been carried out for an example structure subjected to base input characterized by a white noise power spectral density function (PSDF). The sensitivity of the performance of the LCVA to the coefficient of head loss and to the tuning ratio have also been examined and compared with that of the liquid column damper (LCD). Finally, a simulation study has been carried out with a recorded accelerogram, to demonstrate the effectiveness of the LCVA.

• Structural Engineering and Mechanics
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• v.84 no.4
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• pp.503-515
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• 2022

Based on the sliding isolation concrete LSS (liquid-storage structure), the specific seismic vulnerability is analyzed according to the general failure mode. In this study, 12 seismic inputs with different characteristics are used, and their acceleration peak values are modulated. By inputting these waves to the sliding isolation concrete storage structure, the finite-element models of different concrete rectangular LSSs are obtained and analyzed, and the failure probabilities are obtained according to the IDA (incremental dynamic analysis) curves of the structure. The results show that when the seismic acceleration peak value gradually increases from 0.1 g to 1.0 g, the failure probability of LSS gradually increases with the increase in friction coefficient. However, the failure probability of a sliding isolation LSS is less than 100% and far less than the failure probability of a non-isolated rectangular LSS, which shows that an isolated liquid storage structure continues working under a big earthquake. Thus, the sliding isolation for the concrete LSS has a significant damping effect.

• Journal of Korean Tunnelling and Underground Space Association
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• v.19 no.1
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• pp.1-9
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• 2017

In this study, a test-bed system simulated a tunnel and concrete track is tested on cite and invested an allowed limit of multi-layered sound absorbing panel for reducing noise reflected on the concrete track in train tunnel considering the criteria and limitation on the theoretical back ground. The studied results are an effective evaluating system of the sound absorbing coefficient influenced fluid effects depending on the vehicle speed in the urban train tunnel and measuring not only structural behaviors of maximum displacement and acceleration of the panel but also dynamic characteristics of damping ratio and natural frequency.

• Steel and Composite Structures
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• v.45 no.3
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• pp.409-423
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• 2022

Nowadays, there is a high demand for great structural implementation and multifunctionality with excellent mechanical properties. The porous structures reinforced by graphene platelets (GPLs) having valuable properties, such as heat resistance, lightweight, and excellent energy absorption, have been considerably used in different engineering implementations. However, stiffness of porous structures reduces significantly, due to the internal cavities, by adding GPLs into porous medium, effective mechanical properties of the porous structure considerably enhance. This paper is relating to vibration analysis of fluidconveying cantilever porous graphene platelet reinforced (GPLR) pipe with fractional viscoelastic model resting on foundations. A dynamical model of cantilever porous GPLR pipes conveying fluid and resting on a foundation is proposed, and the vibration, natural frequencies and primary resonant of such a system are explored. The pipe body is considered to be composed of GPLR viscoelastic polymeric pipe with porosity in which Halpin-Tsai scheme in conjunction with the fractional viscoelastic model is used to govern the construction relation of nanocomposite pipe. Three different porosity distributions through the pipe thickness are introduced. The harmonic concentrated force is also applied to the pipe and the excitation frequency is close to the first natural frequency. The governing equation for transverse motions of the pipe is derived by the Hamilton principle and then discretized by the Galerkin procedure. In order to obtain the frequency-response equation, the differential equation is solved with the assumption of small displacement, damping coefficient, and excitation amplitude by the multiple scale method. A parametric sensitivity analysis is carried out to reveal the influence of different parameters, such as nanocomposite pipe properties, fluid velocity and nonlinear viscoelastic foundation coefficients, on the primary resonance and linear natural frequency. Results indicate that the GPLs weight fraction porosity coefficient, fractional derivative order and the retardation time have substantial influences on the dynamic response of the system.

• Korean Journal of Agricultural Science
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• v.50 no.4
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• pp.773-784
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• 2023

In this study, we developed a dynamic model and steering controller model for an autonomous tractor and evaluated their performance. The traction force was measured using a 6-component load cell, and the rotational speed of the wheels was monitored using proximity sensors installed on the axles. Torque sensors were employed to measure the axle torque. The PI (proportional integral) controller's coefficients were determined using the trial-error method. The coefficient of the P varied in the range of 0.1 - 0.5 and the I coefficient was determined in 3 increments of 0.01, 0.05, and 0.1. To validate the simulation model, we conducted RMS (root mean square) comparisons between the measured data of axle torque and the simulation results. The performance of the steering controller model was evaluated by analyzing the damping ratio calculated with the first and second overshoots. The average front and rear axle torque ranged from 3.29 - 3.44 and 6.98 - 7.41 kNm, respectively. The average rotational speed of the wheel ranged from 29.21 - 30.55 rpm at the front, and from 21.46 - 21.63 rpm at the rear. The steering controller model exhibited the most stable control performance when the coefficients of P and I were set at 0.5 and 0.01, respectively. The RMS analysis of the axle torque results indicated that the left and right wheel errors were approximately 1.52% and 2.61% (at front) and 7.45% and 7.28% (at rear), respectively.

• Steel and Composite Structures
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• v.50 no.2
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• pp.201-216
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• 2024

This paper is motivated by the lack of studies relating to vibration and nonlinear resonance of fluid-conveying cantilever porous GPLR pipes with fractional viscoelastic model resting on nonlinear foundations. A dynamical model of cantilever porous Graphene Platelet Reinforced (GPLR) pipes conveying fluid and resting on nonlinear foundation is proposed, and the vibration, natural frequencies and primary resonant of such system are explored. The pipe body is considered to be composed of GPLR viscoelastic polymeric pipe with porosity in which Halpin-Tsai scheme in conjunction with fractional viscoelastic model is used to govern the construction relation of the nanocomposite pipe. Three different porosity distributions through the pipe thickness are introduced. The harmonic concentrated force is also applied on pipe and excitation frequency is close to the first natural frequency. The governing equation for transverse motion of the pipe is derived by the Hamilton principle and then discretized by the Galerkin procedure. In order to obtain the frequency-response equation, the differential equation is solved with the assumption of small displacement, damping coefficient, and excitation amplitude by the multiple scale method. A parametric sensitivity analysis is carried out to reveal the influence of different parameters, such as nanocomposite pipe properties, fluid velocity and nonlinear viscoelastic foundation coefficients, on the primary resonance and linear natural frequency. Results indicate that the GPLs weight fraction porosity coefficient, fractional derivative order and the retardation time have substantial influences on the dynamic response of the system.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.11
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• pp.561-567
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• 2018

This study was carried out to improve the ride quality of high-speed electric multiple unit. Through dynamic analysis of the HEMU-430X, the range of the equivalent conicity with a critical speed of 300 km/h was between 0.05 and 0.25. The initial adopted wheel profile of HEMU-430X was S1002. The equivalent conicity of S1002 with the mileage of more than 40,000 km was about 0.033 and it was confirmed that XP55 is more suitable for stable operation because XP55 has the equivalent conicity of over 0.061. In order to improve ride quality of high-speed electric multiple unit, the change of installation angle of the yaw damper was suggested from $7.35^{\circ}$ to $0^{\circ}$. From sensitivity analysis and optimization, the air spring lateral and vertical stiffness was suggested to be reduced by 30% and the secondary vertical and lateral damper damping coefficient was increased by 50%. By applying this, it was expected that the car body acceleration could be improved by about 20% on average. The HEMU-430X's yaw damper installation angle was changed to $0^{\circ}$ and the damping coefficient of the lateral damper was increased by 30%. When the test run was carried out at the speed of 300 km/h on the Kyungbu high-speed line, the vehicle lateral acceleration had improved by 34.3%. The effect of additional improvement measures proposed in this paper will be tested in the on track test. The riding quality improvement process used in this study can be used to solve ride quality problems that can occur in commercial operation of high-speed electric multiple unit in the future.

• Smart Structures and Systems
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• v.16 no.5
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• pp.891-918
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• 2015

In many of microdevices a part of a microbeam is covered by a piezoelectric layer. Depend on the application a DC or AC voltage is applied between upper and lower side of the piezoelectric layer. A common method in many of previous works for evaluating the response of these structures is discretizing by Galerkin method. In these works often single mode shape of a uniform microbeam i.e. the microbeam without piezoelectric layer has been used as comparison function, and so the convergence of the solution has not been verified. In this paper the Galerkin method is used for discretization, and a comprehensive analysis on the convergence of solution of equation that is discretized using this comparison function is studied for both clamped-clamped and clamped-free microbeams. The static and dynamic solution resulted from Galerkin method is compared to the modal expansion solution. In addition the static solution is compared to an exact solution. It is denoted that the required numbers of uniform microbeam mode shapes for convergence of static solution due to DC voltage depends on the position and thickness of deposited piezoelectric layer. It is shown that when the clamped-clamped microbeam is coated symmetrically by piezoelectric layer, then the convergence for static solution may be obtained using only first mode. This result is valid for clamped-free case when it is covered by piezoelectric layer from left clamped side to the right. It is shown that when voltage is AC then the number of required uniform microbeam shape mode for convergence is much more than the number of required mode in modal expansion due to the dynamic effect of piezoelectric layer. This difference increases by increasing the piezoelectric thickness, the closeness of the excitation frequency to natural frequency and decreasing the damping coefficient. This condition is often indefeasible in microresonator system. It is concluded that discreitizing the equation of motion using one mode shape of uniform microbeam as comparison function in many of previous works causes considerable errors.

• Geomechanics and Engineering
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• v.36 no.4
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• pp.367-380
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• 2024

Investigations has shown that the correct estimation of the effective amplification period is as important as the amplification value itself. It gets more important in 2D basins. This study presents a quantitative coefficient for consideration of the nonlinearity effect in terms of amplification value and the shift in its period which is missing or ineffectively considered in the previous studies. To attain this goal, by the application of a time domain fully nonlinear method, the deviation of the more common equivalent linear results from the basin nonlinear behavior under strong ground motions is investigated quantitatively. Also, despite the increase in the damping ratio, the possibility of the increase in the amplification due to the increase in motion strength is shown. To make the results useful in engineering practice, by introducing nonlinearity ratio, the effect of the nonlinearity is quantitatively estimated for two soft and stiff clayey basins with three different depths under a set of motions scaled to two target spectrum. Results show that at the 100 m depth soft clayey basin, while the nonlinearity ratio shows a 35% deviation at the basin edge part under DD1 motion level, its effect moves to the central part with 20% effect under DD3 motion level. By the increase in depth to 150 m, the results show a decrease in the overall effect of the nonlinear behavior for both clay types. At this depth, the nonlinearity ratio gives a 30% and 17% difference on a limited distance from outcrop at the soft clayey basin under DD1 and DD3 motion levels, respectively. At the 30 m depth basins, the nonlinearity ratio shows up to 25% difference for different cases. The presented ratio would be introduced as nonlinearity coefficients for consideration of the nonlinearity effects in the codes. The presented quantitative margins will help the designer to have a better understanding of the amplification period change because of nonlinearity over 2D basin surface.