• Structural Engineering and Mechanics
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• v.27 no.3
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• pp.333-345
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• 2007

In this study, the nonlinear vibrations of stepped beams having different boundary conditions were investigated. The equations of motions were obtained using Hamilton's principle and made non dimensional. The stretching effect induced non-linear terms to the equations. Forcing and damping terms were also included in the equations. The dimensionless equations were solved for six different set of boundary conditions. A perturbation method was applied to the equations of motions. The first terms of the perturbation series lead to the linear problem. Natural frequencies for the linear problem were calculated exactly for different boundary conditions. Second order non-linear terms of the perturbation series behave as corrections to the linear problem. Amplitude and phase modulation equations were obtained. Non-linear free and forced vibrations were investigated in detail. The effects of the position and magnitude of the step, as well as effects of different boundary conditions on the vibrations, were determined.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.2 s.95
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• pp.161-168
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• 2005

The absorbing material is mostly used to changing the acoustic energy to the heat energy in the passive control, and that consists of the porous media. That controls an air borne noise while the stiffened plates, damping material and additional mass control a structure borne noise. The additional mass can decrease the sound by mass effect and shift of natural frequency, and damping material can decrease the sound by damping effect. The passive acoustic control using these kinds of control materials has an advantage that is possible to control the acoustic in the wide frequency band and the whole space at a price as compared with the active control using the various electronic circuit and actuator. But the space efficiency decreased and the control ability isn't up to the active control. So it is necessary to maximize the control ability in the specific frequency to raise the capacity of passive control minimizing the diminution of space efficiency such an active control. Therefore, the characteristics of control materials and the optimum layout of control materials that attached to the boundary of structure-acoustic coupled cavity were studied using sequential optimization on this study.

• Steel and Composite Structures
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• v.26 no.6
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• pp.771-791
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• 2018

This paper deals with the low velocity impact response and dynamic stresses of composite sandwich truncated conical shells (STCS) with compressible or incompressible core. Impacts are assumed to occur normally over the top face-sheet and the interaction between the impactor and the structure is simulated using a new equivalent three-degree-of-freedom (TDOF) spring-mass-damper (SMD) model. The displacement fields of core and face sheets are considered by higher order and first order shear deformation theory (FSDT), respectively. Considering continuity boundary conditions between the layers, the motion equations are derived based on Hamilton's principal incorporating the curvature, in-plane stress of the core and the structural damping effects based on Kelvin-Voigt model. In order to obtain the contact force, the displacement histories and the dynamic stresses, the differential quadrature method (DQM) is used. The effects of different parameters such as number of the layers of the face sheets, boundary conditions, semi vertex angle of the cone, impact velocity of impactor, trapezoidal shape and in-plane stresses of the core are examined on the low velocity impact response of STCS. Comparison of the present results with those reported by other researchers, confirms the accuracy of the present method. Numerical results show that increasing the impact velocity of the impactor yields to increases in the maximum contact force and deflection, while the contact duration is decreased. In addition, the normal stresses induced in top layer are higher than bottom layer since the top layer is subjected to impact load. Furthermore, with considering structural damping, the contact force and dynamic deflection decrees.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 1998.04a
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• pp.163-170
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• 1998

The influence of the dam-reservoir interaction on the seismic response of dams is studied. The impounded water is assumed to be inviscid and compressible ideal fluid. Material damping is introduce to simulate the energy loss of wave propagation in the water. The irregular region of the impounded water adjacent to the dam is modeled by boundary element method. The regular region extending to infinity is modeled by the transmitting boundary. The dam body is assumed to behave elastically and modeled by finite element method. The coupled equation of motion is obtained by substructure method.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2009.04a
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• pp.403-407
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• 2009

It is reported that the heavy weight floor impact noise of a slab system is very sensitive to the location of microphone and impact load. In addition, it is known that the aspect raio, thickness and boundary condition of a slab also have great effect on the noise induced by impact load. However, the effect has been mainly evaluated by experimental test and numerical analysis is nearly performed to verify the effect quantitatively. In this study, the effect of the aspect ratio, thickness and boundary condition on the heavy weight floor impact noise is examined through numerical analysis for simple rectangular slab system. The results show that the thickness and boundary condition have a strong correlation with the noise of the slab, on the contrary, the aspect ratio has little relation with the noise.

• International Journal of Naval Architecture and Ocean Engineering
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• v.7 no.4
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• pp.670-690
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• 2015

In this paper, numerical investigations for tank sloshing, based on commercial CFD package FLUENT, are performed to study effects of boundary layer grid, liquid viscosity and compressible air on sloshing pressure, wave height and rising time of impact pressure. Also, sloshing experiments for liquids of different viscosity are carried out to validate the numerical results. Through comparison of numerical and experimental results, a computational model including boundary layer grid can predict the sloshing pressure more accurately. Energy dissipation due to viscous friction leads to reduction of sloshing pressure and wave elevation. Sloshing pressure is also reduced because of cushion effect of compressible air. Due to high viscosity damping effect and compressible air effect, the rising time of impact pressure becomes longer. It is also found that liquid viscosity and compressible air influence distribution of dynamic pressure along the vertical tank wall.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.11 no.3
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• pp.135-140
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• 1999

To simulate the wave transformation by an energy dissipation region, a numerical model is suggested by discretizing the elliptic mild-slope equation. Generalized conjugate gradient method is used as solution algorithm to apply parabolic approximation to open boundary condition. To demonstrate the applicabil-ity of the numerical procedure suggested, the wave scattering by a circular damping region is examined. The feature of reflection in front of the damping region is captured clearly by the numerical solution. The effect of the size of dissipation coefficient is examined for a rectangular damping region. The recovery of wave height by diffraction occurs very slowly with distance behind the damping region.

• Advances in aircraft and spacecraft science
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• v.2 no.1
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• pp.57-76
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• 2015

We investigated complex damped modes in beams in the presence of a viscoelastic layer sandwiched between two elastic layers. The problem was solved using two approaches, (1) Rayleigh beam theory and analyzed using the Ritz method, and (2) by using 2D plane stress elasticity based finite-element method. The damping in the layers was modeled using the complex modulus. Simply-supported, cantilever, and viscously supported boundary conditions were considered in this study. Simple trigonometric functions were used as admissible functions in the Ritz method. The key idea behind sandwich structure is to increase damping in a beam as affected by the presence of a highly-damped core layer vibrating mainly in shear. Different assumptions are utilized in the literature, to model shear deformation in the core layer. In this manuscript, we used FEM without any kinematic assumptions for the transverse shear in both the core and elastic layers. Moreover, numerical examples were studied, where the base and constraining layers were also damped. The loss factor was calculated by modal strain energy method, and by solving a complex eigenvalue problem. The efficiency of the modal strain energy method was tested for different loss factors in the core layer. Complex mode shapes of the beam were also examined in the study, and a comparison was made between viscoelastically and viscously damped structures. The numerical results were compared with those available in the literature, and the results were found to be satisfactory.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2012.10a
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• pp.818-823
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• 2012

Assessment of the dynamics properties, like damping, dynamic stiffness and resonance sharpness is essential for the development of a robust system, specifically for the reduction of a traction motor noise. A practical method for identifying dynamic characteristics of a traction motor hosing for an electric vehicle is proposed. Assembling using interference fit of the components of the motor is attributed to the main cause of strong nonlinearity. It is well known that nonlinearity of a structure makes it difficult to assess damping properties or dynamic characteristics of the system. This research presents a practical damping or dynamic stiffness identifying procedures for a nonlinear system according to the boundary condition between assembled components. Based on the simple idea that impact forces of modal tests are highly affected on the condition of the hammer tip, Auto Power Spectrum of the impact forces are used to assess the assembling condition and dynamic characteristics of the system, especially, damping of the system.

• Journal of Navigation and Port Research
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• v.36 no.9
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• pp.729-735
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• 2012

The primary wave energy conversion by a three-dimensional bottom-mounted oscillating water column (OWC) wave power device in regular waves has been studied. The linear potential boundary value problem has been solved following the boundary matching method. The optimum shape parameters such as the chamber length and the depth of the front skirt of the OWC chamber obtained through two-dimensional numerical tests in the frequency domain have been applied in the design of the present OWC chamber. Time-mean wave power converted by the OWC device and the time-mean second-order wave forces on the OWC chamber structure have been presented for different wave incidence angles in the frequency-domain. It has been shown that the peak period of $P_m$ for the optimum damping parameter coincides with the peak period of the time.mean wave drift force when ${\gamma}=0$.