6,641 research outputs found
The Changing role of agriculture in Dutch society
Dutch agriculture has undergone significant changes in the past century, similar to many countries in the European Union. Due to economies of scale and in order to remain economically profitable, it became necessary for farmers to increase farm size, efficiency and external inputs, while minimizing labour use per hectare. The latter has resulted in fewer people working in the agricultural sector. Consequently, Dutch society gradually lost its connection to agricultural production. This divergence resulted in a poor image for the agricultural sector, because of environmental pollution, homogenization of the landscape, outbreaks of contagious animal diseases and reduced animal welfare. Although the general attitude towards agriculture seems to have improved slightly in recent years, there is still a long way to go in regaining this trust. In order to keep the Dutch countryside viable, farmers are considered indispensable. However, their methods of production should match the demands of society in terms of sustainability. This applies both to farming systems that are used in a monofunctional way (production only) and to multifunctional farming systems. For researchers involved in development of these farming systems, this requires new capabilities; contrary to the situation in the past, citizens and stakeholder groups now demand involvement in the design of farming systems. In the current paper, it is suggested that, besides traditional mainstream agriculture, other alternative farming systems should be developed and implemented. Hence, Dutch agricultural research should remain focused on the cutting edge of economy and society. Despite all efforts, not all of these newly developed systems will acquire a position within the agricultural spectrum. However, some of the successful ones may prove extremely valuable
Viscous vortical flow calculations over delta wings
Two approaches to calculate turbulent vortical flows over delta wing configurations are illustrated. The first is for a simple delta wing at low speeds using the boundary layer approximation to treat the effects of the secondary separation. The second is for the supersonic case of a generic fighter using the NASA Ames parabolized Navier/Stokes method. Test/theory comparisons are given in both cases
Variations in roughness predictions (flume experiments)
Data of flume experiments with bed forms are used to analyze and compare different roughness predictors. In this study, the hydraulic roughness consists of grain roughness and form roughness. We predict the grain roughness by means of the size of the sediment. The form roughness is predicted by three approaches: Van Rijn (1984), Vanoni & Hwang (1967) and Engelund (1966). The total roughness values (friction factors) are compared with the roughness values according to the DarcyWeisbach equation. Results show that the different methods predict different friction factors. In future research uncertainties in the hydraulic roughness will be taken into account to determine their influence on the computed water levels
A functional central limit theorem for a Markov-modulated infinite-server queue
The production of molecules in a chemical reaction network is modelled as a
Poisson process with a Markov-modulated arrival rate and an exponential decay
rate. We analyze the distributional properties of , the number of molecules,
under specific time-scaling; the background process is sped up by ,
the arrival rates are scaled by , for large. A functional central limit
theorem is derived for , which after centering and scaling, converges to an
Ornstein-Uhlenbeck process. A dichotomy depending on is observed. For
the parameters of the limiting process contain the deviation
matrix associated with the background process.Comment: 4 figure
Exact solution of the Zeeman effect in single-electron systems
Contrary to popular belief, the Zeeman effect can be treated exactly in
single-electron systems, for arbitrary magnetic field strengths, as long as the
term quadratic in the magnetic field can be ignored. These formulas were
actually derived already around 1927 by Darwin, using the classical picture of
angular momentum, and presented in their proper quantum-mechanical form in 1933
by Bethe, although without any proof. The expressions have since been more or
less lost from the literature; instead, the conventional treatment nowadays is
to present only the approximations for weak and strong fields, respectively.
However, in fusion research and other plasma physics applications, the magnetic
fields applied to control the shape and position of the plasma span the entire
region from weak to strong fields, and there is a need for a unified treatment.
In this paper we present the detailed quantum-mechanical derivation of the
exact eigenenergies and eigenstates of hydrogen-like atoms and ions in a static
magnetic field. Notably, these formulas are not much more complicated than the
better-known approximations. Moreover, the derivation allows the value of the
electron spin gyromagnetic ratio to be different from 2. For
completeness, we then review the details of dipole transitions between two
hydrogenic levels, and calculate the corresponding Zeeman spectrum. The various
approximations made in the derivation are also discussed in details.Comment: 18 pages, 4 figures. Submitted to Physica Script
On the occurrence of Schistidium subconfertum and S. succulentum in Europe
We here clarify that currently there are no records of the Asiatic Schistidium subconfertum (Broth.) Deguchi in Europe. The only European specimen referred to this species represents S. succulentum Ignatova & H.H. Blom, a taxon rather recently described from Caucasus. Furthermore, we provide new records of the latter species in the French and Swiss Alps. The two species share a reduced peristome, but can be distinguished by characteristics of the gametophyte such as lamina thickness (unistratous in S. subconfertum versus partially bistratous in S. succulentum) and shallowly versus strongly projecting costa
- …