4,960 research outputs found
Boundary layer thickness effect on boattail drag
A combined experimental and analytical program was conducted to investigate the effects of boundary layer changes on the flow over high angle boattail nozzles. The tests were run on an isolated axisymmetric sting mounted model. Various boattail geometries were investigated at high subsonic speeds over a range of boundary layer thicknesses. In general, boundary layer effects were small at speeds up to Mach 0.8. However, at higher speeds significant regions of separated flow were present on the boattail. When separation was present large reductions in boattail drag resulted with increasing boundary layer thickness. The analysis predicts both of these trends
Does organic farming affect biodiversity?
Farmland biodiversity in Europe has been declining for some time, particularly steeply in the second half of the twentieth century. The consensus is that agricultural intensification is largely responsible. Organic farming is a low intensity system, offering benefits to biodiversity. Using two paired-farm studies, we explored the responses of different taxonomic groups to organic farming. In the first, smaller-scale, study, positive impacts of organic farming on butterflies and spiders were found. In the second, large-scale, study of eighty-nine farm pairs across England, organic farming was mainly associated with positive effects on biodiversity, although there was substantial variation in the size of effects among taxonomic groups surveyed. The largest and most consistent (positive) effects were for plants and the smallest effects were for carabid beetles. Spiders were influenced by farming system, surrounding landscape, and their dispersal ability. Hunting spiders, which tend to have lower dispersal ability, were more abundant and species-rich on organic compared to conventional farms (this effect was more pronounced in landscapes with less arable). There were no farming system or landscape effects found on web-building spiders, which have generally higher dispersal abilities. There was little evidence that non-cropped habitat effects explained the observed differences for either group. The chapter suggests that the farming system differences for spiders in the crop were largely attributable to differences in crop management (such as reduced inputs of pesticides and fertilizers). The results highlight the importance of developing strategies for managing farmland at the landscape scale for most effective conservation of biodiversity
Benthic enrichment in the Georgia Bight related to Gulf Stream intrusions and estuarine outwelling
The distribution patterns of benthic biomass (microbiota, meiofauna, and macrofauna) over the expansive continental shelf of the Georgia Bight suggest nutrient inputs from intrusions of deep Gulf Stream waters at the shelf break…
Changes in union membership over time : a panel analysis for West Germany
Despite the apparent stability of the wage bargaining institutions in West Germany, aggregate union membership has been declining dramatically since the early 90's. However, aggregate gross membership numbers do not distinguish by employment status and it is impossible to disaggregate these sufficiently. This paper uses four waves of the German Socioeconomic Panel in 1985, 1989, 1993, and 1998 to perform a panel analysis of net union membership among employees. We estimate a correlated random effects probit model suggested in Chamberlain (1984) to take proper account of individual specfic effects. Our results suggest that at the individual level the propensity to be a union member has not changed considerably over time. Thus, the aggregate decline in membership is due to composition effects. We also use the estimates to predict net union density at the industry level based on the IAB employment subsample for the time period 1985 to 1997. JEL - Klassifikation: J
Understanding Terrorist Organizations with a Dynamic Model
Terrorist organizations change over time because of processes such as
recruitment and training as well as counter-terrorism (CT) measures, but the
effects of these processes are typically studied qualitatively and in
separation from each other. Seeking a more quantitative and integrated
understanding, we constructed a simple dynamic model where equations describe
how these processes change an organization's membership. Analysis of the model
yields a number of intuitive as well as novel findings. Most importantly it
becomes possible to predict whether counter-terrorism measures would be
sufficient to defeat the organization. Furthermore, we can prove in general
that an organization would collapse if its strength and its pool of foot
soldiers decline simultaneously. In contrast, a simultaneous decline in its
strength and its pool of leaders is often insufficient and short-termed. These
results and other like them demonstrate the great potential of dynamic models
for informing terrorism scholarship and counter-terrorism policy making.Comment: To appear as Springer Lecture Notes in Computer Science v2:
vectorized 4 figures, fixed two typos, more detailed bibliograph
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