Eutectic Growth in Two-Phase Multicomponent Alloys

A theory of two-phase eutectic growth for a multicomponent alloy is presented. This theory employs the thermodynamic equilibrium at the solid/liquid interface and thus makes it possible to use standard CALPHAD databases to determine the effects of multicomponent phase equilibrium on eutectic growth. Using the same hypotheses as the Jackson Hunt theory, we find that the growth law determined for binary alloys in the Jackson Hunt theory can be generalized to systems with N elements. In particular, a new model is derived from this theory for ternary two-phase eutectics. The use of this model to predict the eutectic microstructure of systems is discussed

Diets of Desert Cottontail on Prairie Dog Colonies in Western South Dakota

Fecal pellets of desert cottontail (Sylvaligus audubonii) were collected during 1981 in May, June, July, August and September for dietary analysis to determine composition of forage plants. Four plants made up 70 percent of the total diet. Forage plants, in order of significance, were western wheat grass (Pascopyrum smithii), fescue (Festuca spp), squirretail (Sitanion hystrix), and plains muhly (Muhlenbergia cuspidata). The most common forb in diets was scarlet globemallow (Sphaeralcea coccinea) and the shrub, plains pricklypear (Opuntia polyacantha). Grasses in the diet ranged from 65 percent to 88 percent while forbs and shrubs ranged from 11 percent to 31 percent, 1 percent to 6 percent, respectively. Botanical composition in the plant community varied throughout the season

Diets of Cattle in North Central South Dakota

This study was conducted over a two year period during the summer months on the Grand River National Grasslands near Lemmon South Dakota on a 2,510 ha pasture to determine cattle diets. Cattle feces were collected monthly near each of 8 permanent water tanks located throughout the pasture. Microhistological analysis of cattle feces was used to identify and quantify diets by plant species. Eight common plant species comprised the greatest portion of the diet. Grasses and grass-like plants accounted for 84 percent to 99 percent of the diets with sedges common in spring (79%) and early summer (53%). Key forage species were, sedges (Carex spp), blue grama (Bouteloua gracilis), needle and thread (Hesperostipa comata) and green needlegrass (Nassella viridula) that comprised 82 percent of the diet. These plants are key forage species for monitoring seasonal grazing on the grasslands. Forbs ranged from less than 1 percent to 14 percent. Shrubs were a minor component of the diet making up less than 1 percent. Similarity indices changed throughout the season and ranged from 0 to 99 percent, indicating that some plants were highly selected or avoided by cattle (low similarities) and other plant species were consumed in the same proportions as available on the grassland. Rank order correlation indicated seasonal selectivity with an overall correlation of 0.75

Regional blood flow during cardiopulmonary resuscitation in dogs

To determine differences in regional blood flow during cardiopulmonary resuscitation (CPR) versus normal cardiac function, we measured regional blood flow to sev~ral organs in 19 pentobarbital-anesthetized dogs (6-12 kg). Regional blood flow was measured during sinus rhythm in 5 dogs and during electrically induced ventricular fibrillation with CPR in the other 14 dogs. Regional blood flow and cardiac output were measured using radioactively labelled polystyrene microspheres of 15 ±3P diameter, injected into the left ventricle. Adequacy of microsphere mixing at low cardiac outputs was verified by comparing flow rates to paired organs. Cardiac output was 175 ml/kg/min during sinus rhythm versus 47 ml/kg/min during CPR. Flow to all organs sampled was less during CPR, but the relative decrease varied widely. The ratios of regional blood flow during CPR to regional blood flow during sinus rhythm were 90% for brain, 35% for heart, 15% for kidneys, 17% for adrenal glands, 14% for pancreas, 3% for spleen, and 33% for small intestine. These results provide baseline values for regional blood flow during CPR which can be used to evaluate alternative CPR techniques and/or drugs which may improve perfusion of vital organs during CPR

An integrated approach to rotorcraft human factors research

As the potential of civil and military helicopters has increased, more complex and demanding missions in increasingly hostile environments have been required. Users, designers, and manufacturers have an urgent need for information about human behavior and function to create systems that take advantage of human capabilities, without overloading them. Because there is a large gap between what is known about human behavior and the information needed to predict pilot workload and performance in the complex missions projected for pilots of advanced helicopters, Army and NASA scientists are actively engaged in Human Factors Research at Ames. The research ranges from laboratory experiments to computational modeling, simulation evaluation, and inflight testing. Information obtained in highly controlled but simpler environments generates predictions which can be tested in more realistic situations. These results are used, in turn, to refine theoretical models, provide the focus for subsequent research, and ensure operational relevance, while maintaining predictive advantages. The advantages and disadvantages of each type of research are described along with examples of experimental results

A quantitative variational phase field framework

The finite solid-liquid interface width in phase field models results in non-equilibrium effects, including solute trapping. Prior phase field modeling has shown that this extra degree of freedom, when compared to sharp-interface models, results in solute trapping that is well captured when realistic parameters, such as interface width, are employed. However, increasing the interface width, which is desirable for computational reasons, leads to artificially enhanced trapping thus making it difficult to model departure from equilibrium quantitatively. In the present work, we develop a variational phase field model with independent kinetic equations for the solid and liquid phases. Separate kinetic equations for the phase concentrations obviate the assumption of point wise equality of diffusion potentials, as is done in previous works. Non-equilibrium effects such as solute trapping, drag and interface kinetics can be introduced in a controlled manner in the present model. In addition, the model parameters can be tuned to obtain ``experimentally-relevant" trapping while using significantly larger interface widths than prior efforts. A comparison with these other phase field models suggests that interface width of about three to twenty-five times larger than current best-in-class models can be employed depending upon the material system at hand leading to a speed-up by a factor of $W^{(d+2)}$, where $W$ and $d$ denote the interface width and spatial dimension, respectively. Finally the capacity to model non-equilibrium phenomena is demonstrated by simulating oscillatory instability leading to the formation of solute bands.Comment: 51 pages, 9 figures, supplemental material