• Geomechanics and Engineering
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• v.13 no.5
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• pp.775-791
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• 2017

Surface wave techniques are widely used as non-invasive method for geotechnical site characterization. Field surface wave data are collected and analyzed using different processing techniques to generate the dispersion curves, which are further used to extract the shear wave velocity profile by inverse problem solution. Characteristics of a dispersion curve depend on the subsurface layering information of a vertically heterogeneous medium. Sometimes soft layer can be found between two stiff layers in the vertically heterogeneous media, and it can affect the wave propagation dramatically. Now most of the surface wave techniques use the fundamental mode Rayleigh wave propagation during the inversion, but this may not be the actual scenario when a soft layer is present in a vertically layered medium. This paper presents a detailed and comprehensive study using finite element method to examine the effect of soft layers which sometimes get trapped between two high velocity layers. Determination of the presence of a soft layer is quite important for proper mechanical characterization of a soil deposit. Present analysis shows that the thickness and position of the trapped soft layer highly influence the dispersion of Rayleigh waves while the higher modes also contribute in the resulting wave propagation.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.2B
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• pp.171-178
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• 2006

The finite element model based on the 2-D advection-dispersion equation incorporating the dispersion tensor that is calculated using velocity field data was developed in order to analyze more accurately 2-D mixing of pollutants for meandering streams. The proposed model was tested using the straight channel that inclined at 45o in the Cartesian coordinate system. The simulation results showed that dispersion tensor model using velocity field data gives an accurate solution. The suitability of the proposed model in analyzing actual pollutant mixing in meandering channels was demonstrated by comparing the simulation results with experimental data obtained from the tracer tests in the laboratory flume. Comparison results showed that the proposed model with dispersion tensor can represents more accurately the mixing phenomena of the pollutants in the meandering channels in which the direction of the primary flow is varying periodically along the channel.

• Journal of Ocean Engineering and Technology
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• v.12 no.2 s.28
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• pp.97-110
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• 1998

The numerical damping and dispersion error characteristics associated with difference schemes and a panel shift method used for the calculation of steady free surface flows by a panel method are an analysed in this paper. First, 12 finite difference operators used for the double model flow by Letcher are applied to a two dimensional cylinder with the Kelvin free surface condition and the numerical errors with these schemes are compared with those by the panel shift method. Then, 3-D waves due to a submerged source are calculated by the difference schemes, the panel shift method and also by a higher order boundary element method(HOBEM). Finally, the waves and wave resistance for Wigley's hull are calculated with these three schemes. It is shown that the panel shift method is free of numerical damping and dispersion error and performs better than the difference schemes. However, it can be concluded that the HOBEM also free of the numerical damping and dispersion error is the most stable, accurate and efficient.

• Water Engineering Research
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• v.4 no.1
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• pp.45-58
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• 2003

Two-dimensional depth-averaged advection-dispersion equation was simulated using FEM. In the straight rectangular channel, the advection-dispersion processes are simulated so that these results can be compared with analyti-cal solutions for the transverse line injection and the point injection. In the straight domain the standard Galerkin method with the linear basis function is found to be inadequate to the advection-dispersion analysis compared to the upwind finite element scheme. The experimental data in the S-curved channel were compared with the result by the numerical model using SUPG(Streamline upwind Petrov-Galerkin) method.

• Steel and Composite Structures
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• v.43 no.1
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• pp.31-54
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• 2022

This paper is concerned with the buckling behavior of functionally graded graphene reinforced porous nanocomposite beams based on the finite element method (FEM) using two variables trigonometric shear deformation theory. Both Young's modulus and material density of the FGP beam element are simultaneously considered as grading through the thickness of the beam. The finite element approach is developed using a nonlocal strain gradient theory. The governing equations derived here are solved introducing a 3-nodes beam element, and then the critical buckling load is calculated with different porosity distributions and GPL dispersion patterns. After a convergence and validation study to verify the accuracy of the present model, a comprehensive parametric study is carried out, with a particular focus on the effects of weight fraction, distribution pattern of GPL reinforcements on the Buckling behavior of the nanocomposite beam. The effects of various structural parameters such as the dispersion patterns for the graphene and porosity, thickness ratio, boundary conditions, and nonlocal and strain gradient parameters are brought out. The results indicate that porosity distribution and GPL pattern have significant effects on the response of the nanocomposite beams, and the results allows to identify the most effective way to achieve improved buckling behavior of the porous nanocomposite beam.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.11
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• pp.1065-1075
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• 2017

In this study, high velocity impact tests along with modeling of material behavior and numerical analyses were conducted to predict the dispersion behavior of the debris resulting from a high velocity impact fracture. For the impact tests, two different materials were employed for both the projectile and the target plate - the first setup employed aluminum alloy while the second employed steel. The projectile impacts the target plate with a velocity of approximately 1 km/s were enforced to generate the impact damages in the aluminum witness plate through the fracture debris. It was confirmed that, depending on the material employed, the debris dispersion behavior as well as the dispersion radii on the witness plate varied. A numerical analysis was conducted for the same impact test conditions. The smoothed particle hydrodynamics (SPH)-finite element (FE) coupled technique was then applied to model the fracture and damage upon the debris. The experimental and numerical results for the diameters of the perforation holes in the target plate and the debris dispersion radii on the witness plate were in agreement within a 5％ error. In addition, the impact test using steel was found to be more threatening as proven by the larger debris dispersion radius.

• Smart Structures and Systems
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• v.21 no.2
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• pp.195-205
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• 2018

Research on Lamb wave-based damage identification in plate-like structures depends on precise knowledge of dispersive wave velocity. However, boundary reflections with the same frequency of interest and greater amplitude contaminate direct waves and thus compromise measurement of Lamb wave dispersion in different materials. In this study, non-reflecting boundaries were proposed in both numerical and experimental cases to facilitate time-frequency characterization of Lamb wave dispersion. First, the Lamb wave equations in isotropic and laminated materials were analytically solved. Second, the non-reflecting boundaries were used as a series of frames with gradually increased damping coefficients in finite element models to absorb waves at boundaries while avoiding wave reflections due to abrupt property changes of each frame. Third, damping clay was sealed at plate edges to reduce the boundary reflection in experimental test. Finally, the direct waves were subjected to the slant-stack and short-time Fourier transformations to calculate the dispersion curves of phase and group velocities, respectively. Both the numerical and experimental results suggest that the boundary reflections are effectively alleviated, and the dispersion curves generated by the time-frequency analysis are consistent with the analytical solutions, demonstrating that the combination of non-reflecting boundary and time-frequency analysis is a feasible and reliable scheme for characterizing Lamb wave dispersion in plate-like structures.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.6
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• pp.512-519
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• 2013

It is important to understand the vibrating behavior of plate structures for its many engineering applications. In this study, the vibration characteristics of strip plates that have finite width and infinite length are investigated theoretically and numerically. The waveguide finite element(WFE) approach, which is an effective tool for studying waveguide structures, is used in this study. The WFE method requires only a cross-sectional finite element model, and uses theoretical harmonic solutions to assess wave propagation along the longitudinal direction. First, WFE results for a simple strip plate are compared with the theoretical results(i.e., dispersion diagrams and point mobilities) to validate the numerical model. Then, in the numerical analysis, different numbers of longitudinal stiffeners are included in the plate model to investigate the effects of stiffeners in terms of the dispersion curves and mobilities. Finally, the dispersion curves of a stiffened double plate are obtained to examine the characteristics of its wave propagation.

• The Journal of the Acoustical Society of Korea
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• v.34 no.3
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• pp.210-218
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• 2015

When the driving voltage of an ultrasound transducer is increased to improve the quality of ultrasound images, heat is generated inside the transducer that can cause patient's skin burn and degradation of transducer performance. Hence, in this paper, a method to disperse the heat of the transducer has been studied. The phased array transducer having 3 MHz center frequency and 32 channels was selected for analyses of the thermal dispersion. First, mechanism of the heat generation was investigated in relation to the transducer operation through theoretical analysis, and material damping and sound pressure amplitude were confirmed to be influential on the heat generation. Further, we investigated the effects of the properties of the materials constituting the transducer on the thermal dispersion through finite element analysis. Based on the analysis results, we determined the thermal properties of the constituent materials that could facilitate the thermal dispersion inside the transducer. The determined thermal properties were applied to the finite element model, and the results showed that the maximum temperature at an acoustic lens contacting with a patient was decreased to 51 % of its initial value.

• Journal of the Korean Society for Nondestructive Testing
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• v.25 no.3
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• pp.222-227
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• 2005

An efficient technique fur the calculation of guided wave dispersion curves in composite pipes is presented. The technique uses a forward-calculating variational calculus approach rather than the guess and iterate process required when using the more traditional partial wave superposition technique. The formulation of each method is outlined and compared. The forward-calculating formulation is used to develop finite element software for dispersion curve calculation. Finally, the technique is used to calculate dispersion curves for several structures, including an isotropic bar, two multi-layer composite bars, and a composite pipe.