Bulletin of the American Physical Society
76th Annual Meeting of the Southeastern Section of APS
Volume 54, Number 16
Wednesday–Saturday, November 11–14, 2009; Atlanta, Georgia
Session DD: Gases, Fluids and Metals Under Stress |
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Chair: Elisa Riedo, Georgia Institute of Technology Room: Brussels |
Thursday, November 12, 2009 1:30PM - 1:42PM |
DD.00001: Stress and strain analysis of metal plates with holes Biyu Hu, Sanichiro Yoshida, John Gaffney With our final goal of understanding how metal connectors used for housing respond to hurricane wind loads, we have conducted Finite Element Analysis (FEA) on metal plates with holes. FEA models have been built to analyze the stress and strain distributions in tensile-loaded, rectangular aluminum and tin plates. The specimen is 20 - 25 mm wide, 0.1 - 10 mm thick, and 100 mm long in the direction of the tensile axis along which two holes are drilled. We have varied the plate thickness and the hole-to-hole distance to study their effects. We have also conducted tensile experiment for specimens of the same materials and dimensions as the FEA using an optical interferometer to analyze the in-plane strain field. Comparison of the FEA and experiments indicates that band-like interferometric fringe patterns representing strain concentration coincides with the region where the von-Mises yield criterion is satisfied and that the specimen fractures at the hole that shows more concentrated plastic strain. These band-like patterns run through holes at about 45 deg to the tensile axis. Sometimes two conjugate bands cross at the hole. FEA with various thicknesses indicate that there is an optimum thickness at which the maximum plastic strain observed at an edge of the hole is minimized. This optimum thickness depends on the in-plane dimension of the specimen, but it always makes the plastic strain most evenly distributed between the two holes. [Preview Abstract] |
Thursday, November 12, 2009 1:42PM - 1:54PM |
DD.00002: Study on stress distribution around holes in metal plates and transition to fracture John Gaffney, Biyu Hu, Sanichiro Yoshida With strength analysis of metal connectors used for buildings in mind, stress distribution around holes in tensile-loaded thin metal plates has been investigated. An optical interferometer sensitive to in-plane displacement of the specimen is set up in front of the tensile machine. Interferometric fringe patterns as a whole image of the specimen are formed on a real-time basis at a preset interval in the order of few seconds. A previously observed bright band-like fringe patterns representing stress concentration in similar specimens without holes are observed around the hole at a late stage of deformation. This band-like pattern is found to run at about 45 deg to the tensile axis through the hole. Sometimes two patterns appear simultaneously around the same hole, forming an ``X'' like shape. The appearance of the pattern greatly depends on the thickness of the specimen, the locations of the holes and the type of the metal. To a certain extent, the transition to fracture can be predicted from the shape of the pattern. Comparison with finite element analysis indicates that this pattern appears in the region where the von-Mises yield criterion is satisfied. [Preview Abstract] |
Thursday, November 12, 2009 1:54PM - 2:06PM |
DD.00003: Pattern Control and State Estimation in Rayleigh-B\'{e}nard Convection Adam Perkins, Michael Schatz We report on a new experimental approach to study instability in Rayleigh- B\'{e}nard convection. The convective fluid absorbs incident infrared laser light, thereby altering the fluid flow. Rapid scanning of the light allows nearly simultaneous actuation at many spatial locations of the pattern. This approach is used to impose reproducibly a given convection pattern. Control is demonstrated by preparing repeatedly a pattern near a straight roll instability. Selected perturbations are applied to this ensemble and decay lifetimes are measured as the system relaxes to the base state. We find that decay lifetimes increase near the instability and give a quantitative measure of distance from instability. We also create patterns that undergo the instability, giving a set of systems evolving from nearby initial conditions on both sides of the instability boundary. This set can be used to test systematically the sensitivity of state estimation, a crucial process in forecasting. Preliminary results of applying one state estimation algorithm to these diverging pattern trajectories will be discussed. [Preview Abstract] |
Thursday, November 12, 2009 2:06PM - 2:18PM |
DD.00004: Identifying Exact Coherent Structures in 2D Turbulence: Experiments and Simulations Michael Schatz, Jon Paprocki, Christopher Lesesne, James Andrews, Reuven Ballaban Recent theoretical advances suggest ways to find unstable exact Navier Stokes solutions that capture many features of coherent structures, which have long been observed in turbulent flow. At present, it remains unknown whether these solutions, termed Exact Coherent States, can describe observations of turbulent flow in laboratory experiments. We describe our experimental and numerical investigations, which search for unstable solutions in quasi-2D flows driven by electromagnetic forces. In the experiments, time series of velocity fields are obtained from images of the visualized flow. In the simulations, long time series of velocity fields are calculated for flows that are forced in a similar manner as the experiments. The velocity field data from both experiments and simulations are used to construct recurrence plots that provide evidence for the existence of unstable periodic orbits. [Preview Abstract] |
Thursday, November 12, 2009 2:18PM - 2:30PM |
DD.00005: Extensive Scaling of Computational Homology and Karhunen-Lo\`{e}ve Decomposition in Rayleigh-B\'{e}nard Convection Experiments H\"{u}seyin Kurtuldu, Michael Schatz We apply two different pattern characterization techniques to large data sets of spatiotemporally chaotic flows in Rayleigh-B\'{e}nard convection (RBC) experiments. Both Computational homology (CH) and a modified Karhunen-Lo\`{e}ve decomposition (KLD) are used to analyze the data. The KLD dimension $D_{KLD}$, the number of eigenmodes required to capture a given fraction of the eigenvalue spectrum, is computed for different subsystem sizes. A similar quantity $D_{CH}$ for the same experimental data is acquired by the probability distribution of topological states constructed from the outputs of CH. We show that both $D_{CH}$ and $D_{KLD}$ scale over a large range of subsystem sizes for the state of SDC; moreover, we find the presence of boundaries leads to deviations from extensive scaling that are similar for both methodologies. [Preview Abstract] |
Thursday, November 12, 2009 2:30PM - 2:42PM |
DD.00006: Transient Turbulence in Taylor-Couette Flow: Co/Counter Rotation and Aspect Ratio Effects Daniel Borrero-Echeverry, Randall Tagg, Michael Schatz Wall-bounded shear flows typically make the transition to turbulence through a subcritical bifurcation that requires a finite amplitude perturbation. At low Reynolds numbers the lifetime of the turbulent state is finite and increases with increasing Reynolds number. Recent studies have challenged the view that there is a critical Reynolds number above which turbulence becomes sustained. The issue has been further complicated by recent numerical studies that suggest that even if turbulence decays locally, it may become sustained globally if the system is sufficiently large. We address this issue and present lifetime measurements in linearly stable Taylor-Couette flow at various aspect ratios. We also discuss the effects of various boundary conditions and weak counter/co-rotation on the observed lifetimes. [Preview Abstract] |
Thursday, November 12, 2009 2:42PM - 2:54PM |
DD.00007: Selected Topics in Nonlinear Wave Phenomena: Diffusive Solitons, Singular Surfaces, and Wave Chaos Pedro Jordan, Ashok Puri We explore some recent topics of interest in the field of nonlinear wave phenomena. We do so in the context of problems arising in acoustic/second-sound (e.g., thermal waves) propagation in certain nonlinear media; reaction-diffusion theory, e.g., population and chemical dynamics; and systems described by equations of the nonlinear Klein--Gordon type (e.g., the sine--Gordon equation). We investigate the corresponding governing equations with an emphasis on shock, solitary/traveling, and chaotic wave phenomena. Employing both analytical and numerical techniques, this study is carried out with the purpose of gaining a better understanding of the physical systems represented in the mathematical models. Finally, other applications of this research are noted and discussed, time permitting. [Preview Abstract] |
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