Conferences, Lectures, & Seminars
Events for June
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Oral Dissertation Defense
Mon, Jun 11, 2012 @ 01:00 PM - 03:00 PM
Sonny Astani Department of Civil and Environmental Engineering
Conferences, Lectures, & Seminars
Speaker: Songyoung Son, CE Ph.D, Candidate
Talk Title: Wave Induced Hydrodynamic Complexity and Transport in the Nearshore
Abstract: In the coastal area, defined as the region between the shoreline and some offshore limit where the depth can no longer influence the waves, complex behavior of waves is expected due to various physical effects such as turbulence, wave-structure interaction, wave-current interaction, wave breaking and fluid-density variations.
In this study, depth-integrated numerical models used in long wave simulation are developed for better understanding of complicated hydrodynamics at the nearshore.
First, a non-dispersive shallow water model and dispersive Boussinesq model are two-way coupled to develop a seamless model for long wave evolution from deep to shallow water with fine scale resolution, without the loss of locally important physics.
Second, a set of depth-integrated equations describing combined wave-current flows are derived mathematically and discretized numerically. To account for the effect of turbulent interaction between waves and underlying currents with arbitrary profile, new additional stresses are introduced, which represent radiation stress of waves over the ambient current field.
Finally, numerical model for gravity waves propagating over variable density fluids is developed by allowing horizontal and vertical variation of fluid density. Throughout the derivation, density change effects appear as correction terms while the internal wave effects on the free surface waves in a two-layer system are accounted for through direct inclusion of internal wave velocity component.
For each of the studied topics, numerical tests are performed to support its accuracy and applicability. Consequently, we developed a comprehensive tool for numerical simulation of complex nearshore hydrodynamics.
Advisor: Dr. Patrick Lynett
Location: Kaprielian Hall (KAP) - 209
Audiences: Everyone Is Invited
Contact: Evangeline Reyes
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Oral Dissertation Defense
Fri, Jun 15, 2012 @ 02:00 PM - 04:00 PM
Sonny Astani Department of Civil and Environmental Engineering
Conferences, Lectures, & Seminars
Speaker: Mehrdad Bozorgnia, CE Ph.D, Candidate
Talk Title: Computational Fluid Dynamics Analysis of Highway Bridges Exposed to Violent Waves
Abstract:
The vulnerability of coastal bridges to damage during Hurricane induced storm surge has been illustrated along the U.S. Gulf Coast in several hurricane events. The total losses during Hurricane Katrina, considering all direct and indirect losses (e.g., job losses), are estimated to exceed 100 billion dollar. Bridges revealed to be the most vulnerable critical components of the transportation system, suffering damage during hurricane induced storm surges and wave loads, and costing over 1 billion dollar to repair or replace TCLEE (2006).
The objective of this study was to calculate the hydrodynamic forces on bridge superstructure via Computational Fluid Dynamic (CFD). Three dimensional numerical wave-load model based on two-phase Navier-Stokes equations is used to evaluate dynamic wave forces exerted on the bridge deck. In order to accurately capture the complex interaction of waves with bridge deck, several millions of mesh cells are used in the simulation domain and simulations are ran on High Performance Computing and Communication (HPCC) cluster at University of Southern California.
First, CFD software was validated by simulating interaction of a solitary wave with a flat plate. The simulation results for pressure under the plate and velocities inside water were in good agreement with experimental data available from French (1970).
Second, Validated numerical model was applied to a 1:5 scale Escambia Bay bridge which was heavily damaged during Hurricane Ivan. Compared to simple flat plate problem, Highway bridge superstructures pose a unique challenge due to their complex geometries, bluff profile, and their relatively large width-to-wavelength ratio. When the added complexities of trapped air, turbulence, and structural response are incorporated, analytical solutions become impractical and the available empirical solutions based on small-scale experiments may be biased by scale effects. Simulation results were compared to experimental data available from the O.H. Hinsdale Wave Research Laboratory at Oregon State University. Influence of modeling (2D vs 3D), time step and grid refinement have been investigated. It has been determined that the two phase Navier Stokes equations are very sensitive to the choice of mesh size and time step. Some guidelines based on simulation results are developed for optimum choice of mesh size and time step for similar wave structure interaction problems.
Third, In order to evaluate scale effects in the wave bridge interaction problem, a bridge prototype with exact Escambia Bay Bridge dimensions is setup. Equivalent wave heights and period are calculated using Froude similitude laws from the wave heights and periods used in model simulations. The forces obtained from CFD simulations for prototype bridge are compared to forces calculated using Froude similitude law from model bridge simulations.
Forth, CFD simulation results for model and prototype bridge was compared with recently published AASHTO guidelines. Recently published AASHTO guidelines for coastal bridges vulnerable to storms provide a series of equations to estimate maximum quasi-steady and slamming forces due to wave impact. These guidelines are developed based on experiments conducted at University of Florida on 1:8 scale Escambia Bay bridge.
Fifth, since air entrapped between bridge girders and diaphragm was determined to be the main reason behind highway bridge failures during recent Hurricanes and
Tsunamis, two retrofitting options are evaluated in terms of their efficacy in reducing hydrodynamics forces applied to bridge superstructure. These two options were using airvents in bridge deck and using airvents in bridge diaphragms.
The ability of CFD to model a complex flow such as described in this dissertation would provide a powerful tool to predict the hydrodynamic forces under various conditions and furthermore to devise effective disaster prevention plan against bridge failure.
Advisor: Prof. J.J. Lee
Location: Kaprielian Hall (KAP) - 460
Audiences: Everyone Is Invited
Contact: Evangeline Reyes
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Oral Dissertation Defense
Tue, Jun 26, 2012 @ 09:00 AM - 11:00 AM
Sonny Astani Department of Civil and Environmental Engineering
Conferences, Lectures, & Seminars
Speaker: Fabian Rojas, CE Ph.D, Candidate
Talk Title: Development of a Nonlinear Quadrilateral Layered Membrane Element with Drilling Degrees of Freedom and a Nonlinear Quadrilateral Thin Flat Layered Shell Element for the Modeling of Reinforced Concrete Walls
Abstract:
The primary thrusts of this dissertation are to develop and test a new quadrilateral layered membrane element with drilling degrees of freedom (DOF) and a quadrilateral thin flat layered shell element for the nonlinear analysis of reinforced concrete walls. The drilling degrees of freedom refers to the incorporation of the in-plane rotation as a degree of freedom at each node of the element. The membrane element consists of a quadrilateral element with a total of 12 DOF, 3 per node, 2 displacements and 1 in-plane rotation, and uses a blended field interpolation for the displacements over the element. This formulation is an extension of the one developed by Xia et al. in 2009. The shell element is created by the combination of the membrane element developed in this dissertation and a Discrete Kirchhoff Quadrilateral Element (DKQ, 12 DOF), formulated by Batoz and Tahar in 1982, to model the out of plane bending behavior of the element. The modeling of the section of the membrane and the shell element consists of a layered system of fully bonded, smeared steel reinforcement and smeared orthotropic concrete material with the rotating angle formulation. The layered section for the shell includes the coupling membrane and bending effects. These elements are implemented on a finite element framework using the object oriented programing language under MATLAB. The framework or MATLAB toolbox for Finite Elements developed for this dissertation allows to incorporate, develop and test new elements, materials, sections and analysis algorithms in a easy and quick manner. The proposed elements are evaluated using experimental results that are available in the literature. It is shown that the new elements are in excellent agreement with the experimental results for the different load configuration, monotonic and cyclic loading, and they are able to predict the failure modes for the different wall configurations analyzed in this dissertation.
Advisor: Prof. James C. Anderson
Location: Kaprielian Hall (KAP) - 209 Conference Room
Audiences: Everyone Is Invited
Contact: Evangeline Reyes
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Masters Thesis Defense
Thu, Jun 28, 2012 @ 11:00 AM - 12:30 PM
Sonny Astani Department of Civil and Environmental Engineering
Conferences, Lectures, & Seminars
Speaker: Preetham Aghalaya Manjunatha , CE Masters Candidate
Talk Title: Homogenization Procedures for the Constitutive Material Modeling and Analysis of Aperiodic Microstructures
Abstract: Composite materials are the well-known substitutes for traditional metals in various industries because of their micro-structural character. Micro-structures provide a high strength-to-weight ratio, which makes them suitable for manufacturing large variety of applications ranging from simple toys to complicated space/aircraft structures. Since, these materials are widely used in high performance structures, their stress/thermal analysis issues are of major concern. Due to the high degree of material heterogeneity, it is extremely difficult to analyze such structures.
Homogenization (rigorous averaging) is a process that overcomes the difficulty of modeling each micro-structure. It replaces an individual micro-structure by an equivalent material model representation (unit cell). Periodic micro-structures appear in regular intervals throughout the domain, in contrast aperiodic micro-structures follows an irregular pattern. Further, this method bridges the analysis gap between micro and macro domain of the structures. In this thesis, Homogenization procedure based on anti-periodic displacement fields for aperiodic micro-structures and aperiodic boundary conditions are considered to model the constitutive material matrix. This work could be easily implemented with the traditional finite element packages. In addition, it eventually increases the convergence accuracy and reduces the high computational expenses. Different problems are analyzed by the implementation of digital image processing schemes for the extraction of a unit cell around the Gauss quadrature points and the mesh-generation. In the future, this research defines a new path for the analysis of any random heterogeneous materials by its ease of implementation and the state-of-the-art micro-structure material modeling capabilities and digital image based micro-meshing.
Location: Kaprielian Hall (KAP) - 209 Conference Room
Audiences: Everyone Is Invited
Contact: Evangeline Reyes