Science blog

Chemical and biological warfare are not an invention of the 20th century. Solon (638-559 BC) used a strong purgative, the herb hellebore, in the siege of Krissa. During the 6th century BC, the Assyrians poisoned enemy wells with rye ergot. In the Peloponnesian War (431-404 BC), the Spartans flung sulfur and pitch at the Athenians and their allies. In the Middle Ages, besiegers used the bloated and dripping bodies of plague victims as readymade "dirty bombs". In 1346, during its siege of Kaffa (present day Feodosia in Crimea), the Tartar army suffered an outbreak of the Plague. They hurled the corpses of their infected dead over the city walls and into the city's water wells. The resulting epidemic led to the city's surrender. It is widely believed that people afflicted with the horrendous disease fled the place and started the Black Death pandemic which consumed at least one third of Europe's population within a few years. Russian troops adopted the same tactic against Sweden in 1710. Smallpox was another favorite. Francisco Pizarro (1476-1541) gave South American natives clothing items deliberately contaminated with the variola virus. During the French and Indian wars in North America (1689-1763), blankets used by smallpox victims were given to American Indians. General Jeffery Amherst (1717-1797) gifted Indians loyal to the French with smallpox-contaminated bedspreads during the French and Indian War of 1754 to 1767. An epidemic broke among the Native American defenders of Fort Carillon and they lost it to the English.

The following four-article series was published in a newsletter of the American Society of Mechanical Engineers (ASME). It serves as an introduction to the recent analysis discipline known as the finite element method. The author is an engineering consultant and expert witness specializing in finite element analysis. FINITE ELEMENT ANALYSIS: Pre-processing by Steve Roensch, President, Roensch & Associates Second in a four-part series As discussed last month, finite element analysis is comprised of pre-processing, solution and post-processing phases. The goals of pre-processing are to develop an appropriate finite element mesh, assign suitable material properties, and apply boundary conditions in the form of restraints and loads. The finite element mesh subdivides the geometry into elements, upon which are found nodes. The nodes, which are really just point locations in space, are generally located at the element corners and perhaps near each midside. For a two-dimensional (2D) analysis, or a three-dimensional (3D) thin shell analysis, the elements are essentially 2D, but may be "warped" slightly to conform to a 3D surface. An example is the thin shell linear quadrilateral; thin shell implies essentially classical shell theory, linear defines the interpolation of mathematical quantities across the element, and quadrilateral describes the geometry. For a 3D solid analysis, the elements have physical thickness in all three dimensions. Common examples include solid linear brick and solid parabolic tetrahedral elements. In addition, there are many special elements, such as axisymmetric elements for situations in which the geometry, material and boundary conditions are all symmetric about an axis. The model's degrees of freedom (dof) are assigned at the nodes. Solid elements generally have three translational dof per node. Rotations are accomplished through translations of groups of nodes relative to other nodes. Thin shell elements, on the other hand, have six dof per node: three translations and three rotations. The addition of rotational dof allows for evaluation of quantities through the shell, such as bending stresses due to rotation of one node relative to another. Thus, for structures in which classical thin shell theory is a valid approximation, carrying extra dof at each node bypasses the necessity of modeling the physical thickness. The assignment of nodal dof also depends on the class of analysis. For a thermal analysis, for example, only one temperature dof exists at each node. Developing the mesh is usually the most time-consuming task in FEA. In the past, node locations were keyed in manually to approximate the geometry. The more modern approach is to develop the mesh directly on the CAD geometry, which will be (1) wireframe, with points and curves representing edges, (2) surfaced, with surfaces defining boundaries, or (3) solid, defining where the material is. Solid geometry is preferred, but often a surfacing package can create a complex blend that a solids package will not handle. As far as geometric detail, an underlying rule of FEA is to "model what is there", and yet simplifying assumptions simply must be applied to avoid huge models. Analyst experience is of the essence. The geometry is meshed with a mapping algorithm or an automatic free-meshing algorithm. The first maps a rectangular grid onto a geometric region, which must therefore have the correct number of sides. Mapped meshes can use the accurate and cheap solid linear brick 3D element, but can be very time-consuming, if not impossible, to apply to complex geometries. Free-meshing automatically subdivides meshing regions into elements, with the advantages of fast meshing, easy mesh-size transitioning (for a denser mesh in regions of large gradient), and adaptive capabilities. Disadvantages include generation of huge models, generation of distorted elements, and, in 3D, the use of the rather expensive solid parabolic tetrahedral element. It is always important to check elemental distortion prior to solution. A badly distorted element will cause a matrix singularity, killing the solution. A less distorted element may solve, but can deliver very poor answers. Acceptable levels of distortion are dependent upon the solver being used. Material properties required vary with the type of solution. A linear statics analysis, for example, will require an elastic modulus, Poisson's ratio and perhaps a density for each material. Thermal properties are required for a thermal analysis. Examples of restraints are declaring a nodal translation or temperature. Loads include forces, pressures and heat flux. It is preferable to apply boundary conditions to the CAD geometry, with the FEA package transferring them to the underlying model, to allow for simpler application of adaptive and optimization algorithms. It is worth noting that the largest error in the entire process is often in the boundary conditions. Running multiple cases as a sensitivity analysis may be required. Next month's article will discuss the solution phase of the finite element method. Roensch & Associates. All rights reserved.

As modern society searches around for alternative energy sources, wind farms are getting mention. There are, however, limitations regarding wind farms as major energy alternatives. Wind Farms – Limitations as Energy Platforms Wind power is an enticing energy platform compared to fossil fuels. The process works by using the inherent energy in wind as a method for producing electricity. The actual methodology is much like hydropower, but with wind used in place of water. Wind turbines catch the wind, which turns their blades. This turning motion cranks a generator that produces electricity. The electricity is stored in batteries or fed into the electrical grid of a utility. Walla, you have power! Using wind power for localized needs has been around for a long time. The Persians are believed to be the first to use it with the purpose being to turn grain grinding stones. In modern times, the sole purpose is to generate electricity. On a large scale, this means wind farms. Wind farms are simply large collections of wind turbines in a defined area. If you have ever driven east out of San Francisco, you have seen the wind farm along the freeway. While it is both intoxicating and a pollution free source of electricity, a wind farm has definite limitations. The biggest limitation of wind farms is the electricity produced. Simply put, they do not produce massive amounts, certainly not on the scale needed in most cities in industrialized nations. Obviously, each location is different, but wind is simply not a constant occurrence in most places. Even when it is, the number of turbines required to produce enough energy for a city is mind boggling. This, of course, leads to a second limitation. Wind farms need to cover a lot of physical space to produce large amounts of electricity. In many industrialized countries, space is at a premium. As a result, the sheer cost of purchasing land for wind farms is prohibitive. This issue, however, is losing some of its grit as offshore wind farms are becoming more prevalent. To some, one of the limitations of wind farms is they are eyesores. Personally, I think they are mesmerizing and have an artistic appearance. Others, however, definitely do not agree. The Cape Wind offshore wind farm project has met with massive resistance for just this reason. The limitations of wind farms are fairly significant at this point in time. As technology and new approaches, such as offshore wind farms, come to the forefront, these problems may fall the wayside.

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