A Free-Algebraic Solution for the Planar Approximation

An explicit solution for the generating functional of n-point functions in the planar approximation is given in terms of two sets of free-algebraic annihilation and creation operators.Comment: 15 pages, added referenc

Exact Markovian kinetic equation for a quantum Brownian oscillator

We derive an exact Markovian kinetic equation for an oscillator linearly coupled to a heat bath, describing quantum Brownian motion. Our work is based on the subdynamics formulation developed by Prigogine and collaborators. The space of distribution functions is decomposed into independent subspaces that remain invariant under Liouville dynamics. For integrable systems in Poincar\'e's sense the invariant subspaces follow the dynamics of uncoupled, renormalized particles. In contrast for non-integrable systems, the invariant subspaces follow a dynamics with broken-time symmetry, involving generalized functions. This result indicates that irreversibility and stochasticity are exact properties of dynamics in generalized function spaces. We comment on the relation between our Markovian kinetic equation and the Hu-Paz-Zhang equation.Comment: A few typos in the published version are correcte

Robust Covariance Matrix Estimation with Data-Dependent VAR Prewhitening Order

This paper analyzes the performance of heteroskedasticity-and-autocorrelation-consistent (HAC) covariance matrix estimators in which the residuals are prewhitened using a vector autoregressive (VAR) filter. We highlight the pitfalls of using an arbitrarily fixed lag order for the VAR filter, and we demonstrate the benefits of using a model selection criterion (either AIC or BIC) to determine its lag structure. Furthermore, once data-dependent VAR prewhitening has been utilized, we find negligible or even counter-productive effects of applying standard kernel-based methods to the prewhitened residuals; that is, the performance of the prewhitened kernel estimator is virtually indistinguishable from that of the VARHAC estimator.

Optimization of commercial net spacers in spiral wound membrane modules

CFD simulations have been used to determine mass transfer coefficients and power consumption of commercial net spacers. The simulations show transversal and longitudinal vortices, vortex shedding and instationary flow behavior leading to the enhanced mass transfer in spacer filled-channels compared to empty channels. The results of the simulations were validated with experiments and compared with data reported in literature, showing satisfactory agreement. Furthermore, CFD simulations were used to optimize the geometry of commercial net spacers in terms of mass transfer and power consumption. The performance of these optimized spacer geometries will be used as reference for future work on the development of new high-performance spacer shapes

Characterization of Water Movement Into and Through Soils During and Immediately After Rainstorms

The movement of water into and through soils in the unsaturated state is basic to many water resources problems including rainfall-runoff models, ground water recharge, irrigation, drainage, evapotranspiration and the movement of pollutants in soils. This study was conducted in an effort to determine if the flow equation based on Darcy\u27s Law and the continuity equation could be used to describe watershed infiltration and thus be incorporated into hydrologic models. The results of the study indicate that even on apparently uniform soils there is a great deal of variability in soil water properties. Handling this variability plus the difficulty of solving the flow equation led to the conclusion that a simpler approach to modelling watershed infiltration is needed. A simple infiltration model was developed and included in a rainfall-runoff model. Tests with the model indicate that it produces satisfactory estimates of monthly runoff from small, rural watersheds

Spin-mediated dissipation and frequency shifts of a cantilever at milliKelvin temperatures

We measure the dissipation and frequency shift of a magnetically coupled cantilever in the vicinity of a silicon chip, down to $25$ mK. The dissipation and frequency shift originates from the interaction with the unpaired electrons, associated with the dangling bonds in the native oxide layer of the silicon, which form a two dimensional system of electron spins. We approach the sample with a $3.43$ $\mu$m-diameter magnetic particle attached to an ultrasoft cantilever, and measure the frequency shift and quality factor as a function of temperature and the distance. Using a recent theoretical analysis [J. M. de Voogd et al., arXiv:1508.07972 (2015)] of the dynamics of a system consisting of a spin and a magnetic resonator, we are able to fit the data and extract the relaxation time $T_1=0.39\pm0.08$ ms and spin density $\sigma=0.14\pm0.01$ spins per nm$^2$. Our analysis shows that at temperatures $\leq500$ mK magnetic dissipation is an important source of non-contact friction.Comment: 5 pages, 3 figure

Innovatieve systemen halen teelten uit de grond

In de open teelten, met name op zandgronden, is het zeer moeilijk om een rendabele, emissieloze grond - gebonden landbouw te verwezenlijken. Het project “Teelt de grond uit” werkt daarom aan een radicale omslag. Met technologie uit de glastuinbouw worden nieuwe teeltsystemen voor vollegrondsteelten ontwikkeld en geïmplementeerd. Ook wordt er gekeken naar emissieloze teeltsystemen voor de nog grondgebonden tuinbouwgewassen in de glastuinbouw

Extremal t processes: Elliptical domain of attraction and a spectral representation

The extremal t process was proposed in the literature for modeling spatial extremes within a copula framework based on the extreme value limit of elliptical t distributions (Davison, Padoan and Ribatet (2012)). A major drawback of this max-stable model was the lack of a spectral representation such that for instance direct simulation was infeasible. The main contribution of this note is to propose such a spectral construction for the extremal t process. Interestingly, the extremal Gaussian process introduced by Schlather (2002) appears as a special case. We further highlight the role of the extremal t process as the maximum attractor for processes with finite-dimensional elliptical distributions. All results naturally also hold within the multivariate domain

Local land management in Benin with special reference to pastoral groups

A review of local land management experiences in West Africa reveals that the resolution of conflicts over the uses of resources between herders and farmers depends on factors like land and water rights, promotion of the interests of pastoral groups and the Intervention of traditional and modern institutions in conflict resolution. This paper on local land management in Benin with special reference to pastoral groups presents some fmdings in Kemon and Kokey villages. In both villages, land is still under common law to varying degrees despite modern law No 65- 25 of 14* August entitled 'Régime de la propriété foncière'. Crop damage by cattle in areas where agriculture has become more widespread and the blocking of cattle routes are identified as the major causes of conflict between herders and farmers. Resolution of this tension calls for the intervention of local organisations. Conflicts are settled either through amicable settlement or compensation but unfortunately it is the herder in many cases who is still blamed for erop damage