67 research outputs found
An internal model approach to (optimal) frequency regulation in power grids with time-varying voltages
This paper studies the problem of frequency regulation in power grids under
unknown and possible time-varying load changes, while minimizing the generation
costs. We formulate this problem as an output agreement problem for
distribution networks and address it using incremental passivity and
distributed internal-model-based controllers. Incremental passivity enables a
systematic approach to study convergence to the steady state with zero
frequency deviation and to design the controller in the presence of
time-varying voltages, whereas the internal-model principle is applied to
tackle the uncertain nature of the loads.Comment: 16 pages. Abridged version appeared in the Proceedings of the 21st
International Symposium on Mathematical Theory of Networks and Systems, MTNS
2014, Groningen, the Netherlands. Submitted in December 201
Phase retrieval via regularization in self-diffraction based spectral interferometry
A novel variant of spectral phase interferometry for direct electric-field
reconstruction (SPIDER) is introduced and experimentally demonstrated. Other
than most previously demonstrated variants of SPIDER, our method is based on a
third-order nonlinear optical effect, namely self-diffraction, rather than the
second-order effect of sum-frequency generation. On one hand, self-diffraction
(SD) substantially simplifies phase-matching capabilities for multi-octave
spectra that cannot be hosted by second-order processes, given manufacturing
limitations of crystal lengths in the few-micrometer range. On the other hand,
however, SD SPIDER imposes an additional constraint as it effectively measures
the spectral phase of a self-convolved spectrum rather than immediately
measuring the fundamental phase. Reconstruction of the latter from the measured
phase and the spectral amplitude of the fundamental turns out to be an
ill-posed problem, which we address by a regularization approach. We discuss
the numerical implementation in detail and apply it to measured data from a
Ti:sapphire amplifier system. Our experimental demonstration used 40-fs pulses
and a 500 m thick BaF crystal to show that the SD SPIDER signal is
sufficiently strong to be separable from stray light. Extrapolating these
measurements to the thinnest conceivable nonlinear media, we predict that
bandwidths well above two optical octaves can be measured by a suitably adapted
SD SPIDER apparatus, enabling the direct characterization of pulses down to
single-femtosecond pulse durations. Such characteristics appear out of range
for any currently established pulse measurement technique
Towards improved 1-D settler modelling : calibration of the BĂŒrger model and case study
Recently, Burger et al. (2011) developed a new 1-D SST model which allows for more realistic predictions of the sludge settling behaviour than traditional 1-D models used to date. However, the addition of a compression function in this new 1-D model complicates the model calibration. This study aims to report advances in the calibration of this novel 1-D model. Data of the evolution of the sludge blanket height during batch settling experiments were collected at different initial solids concentrations. Based on the linear slopes of the batch settling curves the hindered settling velocity functions by Vesilind (1968) and Takacs et al. (1991) were calibrated. Although both settling velocity functions gave a good fit to the experimental data, very large confidence intervals were found for the parameters of the settling velocity by Takacs. Global sensitivity analysis showed that it is not possible to find a unique set of parameter values for the settling function by Takacs based on experimental data of the hindered settling velocity. Subsequently, the calibrated Vesilind settling velocity was implemented in the 1-D model by Burger et al. (2011) and the parameters of the additional compression function were calibrated by fitting the model by Burger et al. (2011) to the batch settling curves. Simulation results showed that while the 1-D model by Takacs et al. (1991) underpredicted the experimental data of sludge blanket heights, the model by Burger et al. (2011) was able to predict the experimental data far more accurately. However, a global sensitivity analysis showed that no unique optimum for the combined set of hindered and compression parameters could be found
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Towards improved 1-D settler modelling : impact on control strategies using the Benchmark Simulation Model
A Computational Tool for Pre-operative Breast Augmentation Planning in Aesthetic Plastic Surgery
AbstractâBreast augmentation was the most commonly performed cosmetic surgery procedure in 2011 in the United States. Although aesthetically pleasing surgical results can only be achieved if the correct breast implant is selected from a large variety of different prosthesis sizes and shapes available on the market, surgeons still rely on visual assessment and other subjective approaches for operative planning because of lacking objective evaluation tools. In this paper we present the development of a software prototype for augmentation mammaplasty simulation solely based on 3D surface scans, from which patient-specific finite element models are generated in a semi-automatic process. The finite element model is used to pre-operatively simulate the expected breast shapes using physical soft tissue mechanics. Our approach uses a novel mechanism based on so-called displacement templates, which, for a specific implant shape and position, describe the respective internal body forces. Due to a highly efficient numerical solver we can provide immediate visual feedback of the simulation results, and thus the software prototype can be integrated smoothly into the medical workflow. The clinical value of the developed 3D computational tool for aesthetic breast augmentation surgery planning is demonstrated in patientspecific use cases
Catch Me if You Can: Adaptation from Standing Genetic Variation to a Moving Phenotypic Optimum
International audienceAdaptation lies at the heart of Darwinian evolution. Accordingly, numerous studies have tried to provide a formal framework for the description of the adaptive process. Out of these, two complementary modelling approaches have emerged: While so-called adaptive-walk models consider adaptation from the successive fixation of de novo mutations only, quantitative genetic models assume that adaptation proceeds exclusively from pre-existing standing genetic variation. The latter approach, however, has focused on short-term evolution of population means and variances rather than on the statistical properties of adaptive substitutions. Our aim is to combine these two approaches by describing the ecological and genetic factors that determine the genetic basis of adaptation from standing genetic variation in terms of the effect-size distribution of individual alleles. Specifically, we consider the evolution of a quantitative trait to a gradually changing environment. By means of analytical approximations, we derive the distribution of adaptive substitutions from standing genetic variation, that is, the distribution of the phenotypic effects of those alleles from the standing variation that become fixed during adaptation. Our results are checked against individual-based simulations. We find that, compared to adaptation from de novo mutations, (i) adaptation from standing variation proceeds by the fixation of more alleles of small effect; (ii) populations that adapt from standing genetic variation can traverse larger distances in phenotype space and, thus, have a higher potential for adaptation if the rate of environmental change is fast rather than slow
Sub-mm/mm optical properties of real protoplanetary matter derived from Rosetta/MIRO observations of comet 67P
Optical properties are required for the correct understanding and modelling
of protoplanetary and debris discs. By assuming that comets are the most
pristine bodies in the solar system, our goal is to derive optical constants of
real protoplanetary material. We determine the complex index of refraction of
the near-surface material of comet 67P/Churyumov-Gerasimenko by fitting the
sub-millimetre/millimetre observations of the thermal emission of the comet's
sub-surface made by the Microwave Instrument for the Rosetta Orbiter (MIRO)
with synthetic temperatures derived from a thermophysical model and
radiative-transfer models. According to the two major formation scenarios of
comets, we model the sub-surface layers to consist of pebbles as well as of
homogeneously packed dust grains. In the case of a homogeneous dusty surface
material, we find a solution for the length-absorption coefficient of for a wavelength of 1.594 mm and for a wavelength of 0.533 mm and a constant thermal
conductivity of . For the pebble scenario, we
find for the pebbles and a wavelength of 1.594 mm a complex refractive index of
for pebble
radii between 1 mm and 6 mm. Taking into account other constraints, our results
point towards a pebble makeup of the cometary sub-surface with pebble radii
between 3 mm and 6 mm. The derived real part of the refractive index is used to
constrain the composition of the pebbles and their volume filling factor. The
optical and physical properties are discussed in the context of protoplanetary
and debris disc observations.Comment: Accepted for publication in MNRA
Der Freiberger Dom: Architektur als Sprache
Das Lehr-Lern-Projekt beinhaltet die Ableitung und Erprobung von Handlungsempfehlungen und Werkzeugen fĂŒr interdisziplinĂ€re Praxis- und Forschungsprojekte anhand einer Projektarbeit, welche auf die Erstellung einer Besucher-App fĂŒr den Freiberger Dom zielt
Untangling effects of proteins as stabilizers for foam films
Foam filmâs properties have a high impact on the properties of the macroscopic foams. This work focusses on protein stabilized foam films. The direct comparison of three different proteins with a concentration normalized to the protein surface enables to distinguish between electrostatic, steric and network stabilization effects. In order to untangle those effects, we study and compare two globular proteins (ÎČ â lactoglobulin, BLG, and bovine serum albumin, BSA) and a disordered, flexible protein (whole casein, CN) at low ionic strengths with varying solution pH. Image intensity measurement as a recently developed image analysis method in this field allows to record spatially resolved disjoining pressure isotherms in a Thin Film Pressure Balance (TFPB). This reveals insights into the structure formation in inhomogeneous protein films. As a novel method we introduce tracking inhomogeneities (features) which enables the measurement of interfacial mobility and stiffness of foam films. Around the isoelectric point (IEP), Newton Black Films (NBF) form which are stable for the globular proteins while they are unstable for the disordered flexible one. This difference in film stability is explained by different characteristics of the network structures which is supported by findings in the bulk and at the surface of the respective protein solutions
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