130 research outputs found
Allocating service parts in two-echelon networks at a utility company
We study a multi-item, two-echelon, continuous-review inventory problem at a Dutch utility company, Liander. We develop a model that optimizes the quantities of service parts and their allocation in the two-echelon network under an aggregate waiting time restriction. Specific aspects that we address are emergency shipments in case of stockout, and batching for regular replenishment orders at the central warehouse. We use column generation as a basic technique to solve this problem, and use various building blocks for single-item models as columns. Further, we study options to derive simple classification rules from the solution of our multi-item, two-echelon service part optimization problem using statistical techniques. Application of our models at Liander yields a solution that reduces costs by 15% and decreases the impact of waiting time for service parts by 52%
Separation technology–Making a difference in biorefineries
In the quest for a sustainable bio-based economy, biorefineries play a central role as they involve the sustainable processing of biomass into marketable products and energy. This paper aims to provide a perspective on applications of separations that can make a great difference in biorefineries, by significantly reducing the costs and thus making the processes competitive without subsidies. A parallel is drawn between bio-refinery and petro-refinery, to highlight the specific separation challenges encountered in biorefineries and point out the impact of separations on the total costs. Existing and foreseen separations in biorefineries are reviewed, and the upcoming challenges in the bio-domain (additional to current fossil) are identified. Relevant industrial examples are provided to illustrate the tremendous eco-efficiency benefits of well-designed separation processes based on process intensification principles (e.g. reactive separations, dividing-wall column, affinity and trigger-enhanced separations). These examples also illustrate the low sustainability of several bio-separations currently practiced, in terms of high relative energy requirements, large amounts of gypsum co-production and/or excess use of caustic
The AARTFAAC All-Sky Monitor: System Design and Implementation
The Amsterdam-ASTRON Radio Transients Facility And Analysis Center (AARTFAAC)
all sky monitor is a sensitive, real time transient detector based on the Low
Frequency Array (LOFAR). It generates images of the low frequency radio sky
with spatial resolution of 10s of arcmin, MHz bandwidths, and a time cadence of
a few seconds, while simultaneously but independently observing with LOFAR. The
image timeseries is then monitored for short and bright radio transients. On
detection of a transient, a low latency trigger will be generated for LOFAR,
which can interrupt its schedule to carry out follow-up observations of the
trigger location at high sensitivity and resolutions. In this paper, we
describe our heterogeneous, hierarchical design to manage the 240 Gbps raw data
rate, and large scale computing to produce real-time images with minimum
latency. We discuss the implementation of the instrumentation, its performance,
and scalability.Comment: Submitted to Journal of Astronomical Instrumentation, Special issue
on 'Digital Signal Processing (DSP) in Radio Astronomy
Apercal-The Apertif calibration pipeline
Apertif (APERture Tile In Focus) is one of the Square Kilometre Array (SKA) pathfinder facilities. The Apertif project is an upgrade to the 50-year-old Westerbork Synthesis Radio Telescope (WSRT) using phased-array feed technology. The new receivers create 40 individual beams on the sky, achieving an instantaneous sky coverage of 6.5 square degrees. The primary goal of the Apertif Imaging Survey is to perform a wide survey of 3500 square degrees (AWES) and a medium deep survey of 350 square degrees (AMES) of neutral atomic hydrogen (up to a redshift of 0.26), radio continuum emission and polarisation. Each survey pointing yields 4.6 TB of correlated data. The goal of Apercal is to process this data and fully automatically generate science ready data products for the astronomical community while keeping up with the survey observations. We make use of common astronomical software packages in combination with Python based routines and parallelisation. We use an object oriented module-based approach to ensure easy adaptation of the pipeline. A Jupyter notebook based framework allows user interaction and execution of individual modules as well as a full automatic processing of a complete survey observation. If nothing interrupts processing, we are able to reduce a single pointing survey observation on our five node cluster with 24 physical cores and 256 GB of memory each within 24 h keeping up with the speed of the surveys. The quality of the generated images is sufficient for scientific usage for 44% of the recorded data products with single images reaching dynamic ranges of several thousands. Future improvements will increase this percentage to over 80%. Our design allowed development of the pipeline in parallel to the commissioning of the Apertif system
Continuum source catalog for the first APERTIF data release
The first data release of Apertif survey contains 3074 radio continuum images
covering a thousand square degrees of the sky. The observations were performed
during August 2019 to July 2020. The continuum images were produced at a
central frequency 1355 MHz with the bandwidth of 150 MHz and angular
resolution reaching 10". In this work we introduce and apply a new method to
obtain a primary beam model using a machine learning approach, Gaussian process
regression. The primary beam models obtained with this method are published
along with the data products for the first Apertif data release. We apply the
method to the continuum images, mosaic them and extract the source catalog. The
catalog contains 249672 radio sources many of which are detected for the first
time at these frequencies. We cross-match the coordinates with the NVSS,
LOFAR/DR1/value-added and LOFAR/DR2 catalogs resulting in 44523, 22825 and
152824 common sources respectively. The first sample provides a unique
opportunity to detect long term transient sources which have significantly
changed their flux density for the last 25 years. The second and the third ones
combined together provide information about spectral properties of the sources
as well as the redshift estimates.Comment: 12 pages, 9 figures; accepted for publication in A&
Characterising the Apertif primary beam response
Context. Phased Array Feeds (PAFs) are multi element receivers in the focal
plane of a telescope that make it possible to form simultaneously multiple
beams on the sky by combining the complex gains of the individual antenna
elements. Recently the Westerbork Synthesis Radio Telescope (WSRT) was upgraded
with PAF receivers and carried out several observing programs including two
imaging surveys and a time domain survey. The Apertif imaging surveys use a
configuration, where 40 partially overlapping compound beams (CBs) are
simultaneously formed on the sky and arranged in an approximately rectangular
shape. Aims. This manuscript aims to characterise the response of the 40
Apertif CBs to create frequency-resolved, I, XX and YY polarization empirical
beam shapes. The measured CB maps can be used for image deconvolution, primary
beam correction and mosaicing of Apertif imaging data. Methods. We use drift
scan measurements to measure the response of each of the 40 CBs of Apertif. We
derive beam maps for all individual beams in I, XX and YY polarisation in 10 or
18 frequency bins over the same bandwidth as the Apertif imaging surveys. We
sample the main lobe of the beams and the side lobes up to a radius of 0.6
degrees from the beam centres. In addition, we derive beam maps for each
individual WSRT dish as well. Results. We present the frequency and time
dependence of the beam shapes and sizes. We compare the compound beam shapes
derived with the drift scan method to beam shapes derived with an independent
method using a Gaussian Process Regression comparison between the Apertif
continuum images and the NRAO VLA Sky Survey (NVSS) catalogue. We find a good
agreement between the beam shapes derived with the two independent methods.Comment: Accepted for publication by A&A, 14 pages, 15 figure
Characterising the Apertif primary beam response
Context. Phased array feeds (PAFs) are multi-element receivers in the focal plane of a telescope that make it possible to simultaneously form multiple beams on the sky by combining the complex gains of the individual antenna elements. Recently, the Westerbork Synthesis Radio Telescope (WSRT) was upgraded with PAF receivers to carry out several observing programs, including two imaging surveys and a time-domain survey. The Apertif imaging surveys use a configuration of 40 partially overlapping compound beams (CBs) simultaneously formed on the sky and arranged in an approximately rectangular shape. Aims. This work is aimed at characterising the response of the 40 Apertif CBs to create frequency-resolved I, XX, and YY polarization empirical beam shapes. The measured CB maps can be used for the image deconvolution, primary beam correction, and mosaicking processes of Apertif imaging data. Methods. We used drift scan measurements to measure the response of each of the 40 Apertif CBs. We derived beam maps for all individual beams in I, XX, and YY polarisation in 10 or 18 frequency bins over the same bandwidth as the Apertif imaging surveys. We sampled the main lobe of the beams and the side lobes up to a radius of 0.6 degrees from the beam centres. In addition, we derived beam maps for each individual WSRT dish. Results. We present the frequency and time dependence of the beam shapes and sizes. We compared the compound beam shapes derived with the drift scan method to beam shapes derived with an independent method using a Gaussian Process Regression comparison between the Apertif continuum images and the NRAO VLA Sky Survey (NVSS) catalogue. We find a good agreement between the beam shapes derived with the two independent methods
A search for radio emission from double-neutron star merger GW190425 using Apertif
ContextDetection of the electromagnetic emission from coalescing binary neutron stars (BNS) is important for understanding the merger and afterglow. Aims. We present a search for a radio counterpart to the gravitational-wave (GW) source GW190425, a BNS merger, using Apertif on the Westerbork Synthesis Radio Telescope (WSRT). MethodsWe observed a field of high probability in the associated localisation region for three epochs at ΔT\ue2€., =\ue2€., 68, 90, 109 d post merger. We identified all sources that exhibit flux variations consistent with the expected afterglow emission of GW190425. We also looked for possible transients. These are sources that are only present in one epoch. In addition, we quantified our ability to search for radio afterglows in the fourth and future observing runs of the GW detector network using Monte Carlo simulations. ResultsWe found 25 afterglow candidates based on their variability. None of these could be associated with a possible host galaxy at the luminosity distance of GW190425. We also found 55 transient afterglow candidates that were only detected in one epoch. All of these candidates turned out to be image artefacts. In the fourth observing run, we predict that up to three afterglows will be detectable by Apertif. ConclusionsWhile we did not find a source related to the afterglow emission of GW190425, the search validates our methods for future searches of radio afterglows
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