Bradford S.F.,Research Scientist
Coastal Engineering | Year: 2012
A previously developed model for simulating breaking surf zone waves is improved to yield more accurate and computationally efficient predictions. The model employs a sigma coordinate transformation in the vertical direction and solves the Reynolds averaged Navier-Stokes equations in a fractional step manner with the pressure split into hydrostatic and nonhydrostatic components. The hydrostatic equations are first solved with an approximate Riemann solver and the velocity field is then corrected to be divergence free by including the nonhydrostatic pressure. The previous model required a cumbersome modification to accurately predict surf zone wave heights. In this paper, a simpler alteration is presented that is shown to also yield accurate surf zone wave height predictions. In addition, other opportunities for improving model accuracy, robustness, and computational efficiency are also investigated including the discretization of advection terms and the selective neglect of potentially insignificant lateral viscous terms in the governing equations. © 2012.
Gates D.J.,CSIRO |
Gates D.J.,Research Scientist
Journal of Guidance, Control, and Dynamics | Year: 2010
This paper develops a new guidance logic for a vehicle to converge to a specified, desired path of general shape in three dimensions. Conditions are found that ensure exponential convergence to the path for any initial vehicle position and velocity vector, excluding only the velocity vector normal to the desired path. The guidance logic includes a ghost vehicle that follows the desired path. The command accelerations are derived from fictitious forces that act in a frame of reference that moves with the ghost. The forces comprise a springlike force between vehicle and ghost and a drag force createdbya fictitious medium that moves with the ghost. The motion of the ghost is determined indirectly by a nonlinear constraint that controls the real vehicle speed and leads to a differential-algebraic system of equations. The path equations are formulated in terms of differential geometry, in which time is replaced by distance along the vehicle path. The geometric equations are transformed into standard form differential equations. These are transformed again into kinetic equations for both a fixed medium and a moving medium. The transformed equations provide more explicit formulas for command accelerations and are used to ensure that commands do not exceed the capabilities of the vehicle. Often, the vehicle limitation is more constraining than the convergence conditions. Simulations of a miniature aircraft, using these equations, illustrate the effectiveness of the method. Other methods are compared.
McKellar R.C.,Agriculture and Agri Food Canada |
Delaquis P.,Research Scientist
International Journal of Food Microbiology | Year: 2011
Escherichia coli O157:H7, an occasional contaminant of fresh produce, can present a serious health risk in minimally processed leafy green vegetables. A good predictive model is needed for Quantitative Risk Assessment (QRA) purposes, which adequately describes the growth or die-off of this pathogen under variable temperature conditions experienced during processing, storage and shipping. Literature data on behaviour of this pathogen on fresh-cut lettuce and spinach was taken from published graphs by digitization, published tables or from personal communications. A three-phase growth function was fitted to the data from 13 studies, and a square root model for growth rate (μ) as a function of temperature was derived: μ=(0.023*(Temperature-1.20)) 2. Variability in the published data was incorporated into the growth model by the use of weighted regression and the 95% prediction limits. A log-linear die-off function was fitted to the data from 13 studies, and the resulting rate constants were fitted to a shifted lognormal distribution (Mean: 0.013; Standard Deviation, 0.010; Shift, 0.001). The combined growth-death model successfully predicted pathogen behaviour under both isothermal and non-isothermal conditions when compared to new published data. By incorporating variability, the resulting model is an improvement over existing ones, and is suitable for QRA applications. © 2011.
Engineers at Ohio State University, Columbus, developed a new welding technique that could boost the auto industry's efforts to offer vehicles that weigh less and are more fuel efficient. The Department of Energy (DOE) is investing $57 million in 35 new projects aimed at reducing the cost and improving the efficiency of plug-in electric, alternative fuel and conventional vehicles. Ohio State Professor of Materials Science and Engineering Glenn Daehn and his colleagues will receive $2.7 million to further develop vaporizing foil actuator welding (VFAW) as a viable technology for creating multi‐material, lightweight vehicles. "These investments will accelerate the development of innovative vehicle technologies that will save businesses and consumers money at the pump, cut carbon emissions and strengthen our economy," notes DOE Acting Assistant Secretary David Friedman. Daehn's team has amassed more than half a dozen patents for impulse manufacturing and VFAW, where a high-voltage capacitor bank creates a very short electrical pulse inside a thin piece of aluminum foil. Within microseconds, the foil vaporizes, and a burst of hot gas pushes two pieces of metal together at speeds approaching thousands of miles per hour. The pieces don't melt, so there's no seam of weakened metal between them. Instead, the impact directly bonds the atoms of one metal to atoms of the other. This addresses one of the biggest issues in developing affordable, lightweight vehicles—joining advanced and dissimilar metals without producing joints that are much weaker than the base metals. "VFAW consumes less than one-fifth of the energy than a common welding technique," says Daehn, "yet creates bonds that are 50% stronger." Along with Daehn, Materials Science and Engineering Research Scientist Anupam Vivek is co-principal investigator on the new DOE-funded project, which will span four years. "Widely disparate combinations of metals can be welded with nearly 100% joint efficiency by VFAW," explains Vivek. "This enables introduction of high strength-to-weight ratio materials such as aluminum and magnesium into the body of the car, which has traditionally been made of steel." In 2013, Daehn's team received a $600,000 DOE award under the topic of "Breakthrough Technologies for Dissimilar Material Joining." The results from that project and related works have been published extensively. Additionally, a number of grants from the university, State of Ohio and the National Science Foundation's I-Corps program have funded efforts to advance the technology toward commercial usage. Researchers anticipate automakers will start using VFAW for full-scale production within the next several years. Concurrently they will seek early adopters in other industries that depend on advanced manufacturing and joining solutions. Project partners include Alcoa, Ashland Chemical, Pacific Northwest National Laboratory, Coldwater Machine Company, Ohio State's Center for Design and Manufacturing Excellence, Fontana Corrosion Center and Cosma International, a wholly-owned operating unit of Magna International. Detailed information is available at the Impulse Manufacturing Laboratory website.
According to the findings, the extinction rate of the ancient marine plankton was largely influenced by global changes in climate, and the ocean's dramatic changes in temperature and circulation patterns. Professor James Crampton, from Victoria's School of Geography, Environment and Earth Sciences, along with Dr Roger Cooper, Emeritus Research Scientist at GNS Science, used computer-optimised analysis to examine the exact time of origination and extinction of graptolites—an extinct group of ancient marine animal that lived over 400 million years ago. "We found that extinction happens in short bursts or episodes, separated by longer settled spells, rather than gradually and continuously," says Professor Crampton. "When the world had a warm 'greenhouse' climate, there were low rates of extinction among the plankton. Then there was a sharp change to a cooler, fluctuating 'icehouse' climate like today, and several sharp peaks in the extinction rate, including one very severe peak where graptolites were almost wiped out." Professor Crampton and Dr Cooper worked alongside researchers in the United States to examine each of the 2041 species of the plankton through their 70 million year history. "Our analysis also shows that minor changes in the climate affected the newly evolved species of plankton—these new species were unable to compete and became extinct. It seems that nature was generating lots of new species, many of which could not survive," says Professor Crampton. "In contrast, the most abrupt, severe episodes of environmental change affected the old species more profoundly—in this situation, the old guard was disadvantaged. "So it is the severity of the change in the environment that determines if old or new species are prone to extinction. Overall the extinction changes were very rapid and the ecosystem was relatively unstable." The research group believes that the findings demonstrate the effect the current global climate may have on ocean habitats. "Our research suggests that the modern rate of environmental change could alter the balance of extinction risk, so that the old species will be at greatest risk," says Professor Crampton. The research was recently published in Proceedings of the National Academy of Sciences. Explore further: How the fossilized past can help predict our oceans' future