Browsing by Author "C. C. David Tung, Committee Member"

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Dynamics of Saturated Porous Media: Wave Induced Response and Instability of Seabed
(2010-01-07) Ulker, Mehmet Baris Can; M. Shamimur Rahman, Committee Chair; Murthy N. Guddati, Committee Co-Chair; Mohammed A. Gabr, Committee Member; C. C. David Tung, Committee Member
Problems in fields ranging from geomechanics to biomechanics require response of saturated porous media subjected to dynamic loading. An engineering problem requiring the true behavior of saturated porous medium should consider the coupling of both fluid and solid phases yielding the simultaneous analysis of flow of pore fluid and deformation of solid skeleton. Depending on the nature of loading vis-ÃƒÂ -vis the characteristics of the media, different formulations; fully-dynamic (FD), partially-dynamic (PD), quasi-static (QS) are possible. In this study, analytical solutions and numerical models are developed for the response of plane strain saturated porous media, and wave-induced response of seabed in free field and around a breakwater under pulsating/breaking waves. For each formulation, the results are presented with pore-pressure, shear and normal stress distributions within porous medium. The response is studied for various conditions and regions of applicability of formulations are identified in non-dimensional and actual parametric spaces. This can be used for a specific case with known loading and medium characteristics and may help engineers identify the necessary formulation to be used in a given problem. Effect of seabed-wave parameters and inertial terms on standing/breaking wave-induced pulsating/impact response of seabed-caisson system were investigated. The selection of the adequate formulation is decided depending upon the response variable and ranges of physical parameters. While FD formulation yields the minimum response in cyclic wave, for impacting wave it yields variable distributions in between the other two formulations. The areas of instantaneous liquefaction were identified inside the domain through contours of mean effective stress for both types of waves. Liquefied regions are concentrated at the front toe of rubble under cyclic wave which can initiate a vertical-horizontal movement and rotation towards the seabed causing structural failure. Liquefied areas in case of breaking waves are much larger compared to cyclic waves. Additional analyses made introducing a constitutive model for the inelastic behavior of soil to evaluate the nonlinear dynamic response of seabed reveal the importance of the inclusion of material nonlinearity effects.
Novel Methods for Acoustic and Elastic Wave-Based Subsurface Imaging
(2005-04-29) Heidari, Amir Homayoun; M. Shamimur Rahman, Committee Member; C. C. David Tung, Committee Member; Murthy N. Guddati, Committee Chair; Richard Y. Kim, Committee Member
Novel, accurate and computationally efficient methods for wave-based subsurface imaging in acoustic and elastic media are developed. The methods are based on Arbitrarily Wide-Angle Wave Equations (AWWE), which are highly-accurate space-domain one-way wave equations, formulated in terms of displacement components. Main contributions of this research are as follows. (I) Acoustic-AWWE Imaging, a new time-domain migration technique that is highly accurate for imaging steep dips in heterogeneous media. Similar in form to conventional 15° equation, the acoustic AWWE is implemented using an efficient double-marching explicit finite-difference scheme. Its accuracy and efficiency is studied both analytically and through numerical experiments. The method is able to achieve highly accurate images with only a few times the computational cost of the conventional low-order methods. (II) A new class of highly-accurate Absorbing Boundary Conditions (ABCs) for modeling and imaging with high-order one-way wave equations and parabolic equations. These ABCs, are developed using special imaginary-length finite elements. They effectively absorb the incident wave front and generate artifact-free images with as few as three absorbing layers. They are essential tools in imaging in truncated domains and underwater acoustics. (III) Elastic-AWWE imaging: The first high-order space-domain displacement-based elastic imaging method is developed in this research. The method, which is applicable to complex elastic media, is implemented using a unique downward continuation technique. At each depth step, a half-space is attached to the physical layer to simulate one-way propagation. The half-space is effectively approximated using special imaginary-length finite elements. The method is eventually implemented in frequency-space domain using a finite difference method. Numerical instabilities due to improper mapping of complex wave modes are suppressed by rotating the AWWE parameters in complex wavenumber plane thus adjusting its mapping properties. Effectiveness of the method is illustrated through analytical studies and numerical experiments in homogeneous and heterogeneous elastic media.
A Numerical Investigation of the Effects of Loading Conditions on Soil Response
(2009-03-27) Zhao, Xueliang; T. Matthew Evans, Committee Chair; Roy H. Borden, Committee Co-Chair; Mohammed A. Gabr, Committee Member; M. Shamimur Rahman, Committee Member; Murthy N. Guddati, Committee Member; C. C. David Tung, Committee Member
All three principal stresses play a part in the stress-strain-strength response and volumetric behavior of solids and granular materials. In geotechnical engineering, conventional triaxial compression (CTC), plane strain (PS), and direct shear (DS) are the three most commonly used laboratory tests to simulate the field conditions. It is natural to assume that specimens subjected to different loading conditions will show different responses and behaviors. In reality, many soil problems involving shear strength approximate to PS loading conditions in the field (e.g., earth dam, embankment, and retaining wall). However, CTC or DS test is typically used to measure the stress-strain-strength parameters for design because of their simplicity and versatility compared with the complexity and difficulty of the PS test, even though they might not closely mimic the field condition. The current research focuses on the numerical analysis of effects of different loading conditions (e.g., CTC, PS, and DS) on the macro- and micro-behaviors of granular materials using discrete element method (DEM). Analytical, statistical, and stereological approaches are employed. It is the first work to compare the results under the three most common loading conditions (PS, CTC, and DS) in DEM modeling. Models of the CTC, PS, and DS tests are developed. A new method to simulate the membrane behavior is proposed. Parametric analyses to qualitatively assess the effects of the specific parameter on the macroscale response of the specimen are performed. Macroscale responses of sets of simulations of assemblies under PS, CTC, and DS loading conditions are studied. Small-strain responses, shear strengths, and volumetric behaviors of the assemblies under different loading conditions are investigated. Microscale analyses on the assembly behaviors (e.g., void ratio and coordination number) and particle behaviors (e.g., particle rotation and displacement) are conducted. Particle orientation and contact properties (e.g., contact normal and contact force) are investigated using statistical analysis method. An algorithm to generate numerical slicing images which is to simulate the way in laboratory experiments is proposed. The local void ratio distribution analysis and particle orientation distribution analysis are performed using stereological method. Integrating macro-, micro-, and stereological methods, some issues such as strain localization, critical state, and principal stress direction rotation of DS test are investigated.