309 research outputs found
An update on the BQCD Hybrid Monte Carlo program
We present an update of BQCD, our Hybrid Monte Carlo program for simulating
lattice QCD. BQCD is one of the main production codes of the QCDSF
collaboration and is used by CSSM and in some Japanese finite temperature and
finite density projects. Since the first publication of the code at Lattice
2010 the program has been extended in various ways. New features of the code
include: dynamical QED, action modification in order to compute matrix elements
by using Feynman-Hellman theory, more trace measurements, a more flexible
integration scheme, polynomial filtering, term-splitting for RHMC, and a
portable implementation of performance critical parts employing SIMD.Comment: Poster presented at the 35th International Symposium on Lattice Field
Theory, Granada, Spain, 18-24 June 201
Single flavour filtering for RHMC in BQCD
Filtering algorithms for two degenerate quark flavours have advanced to the
point that, in 2+1 flavour simulations, the cost of the strange quark is
significant compared with the light quarks. This makes efficient filtering
algorithms for single flavour actions highly desirable, in particular when
considering 1+1+1 flavour simulations for QED+QCD. Here we discuss methods for
filtering the RHMC algorithm that are implemented within BQCD, an open-source
Fortran program for Hybrid Monte Carlo simulations.Comment: 8 pages, 3 figures, proceedings of the 35th International Symposium
on Lattice Field Theory, 18-24 June 2017, Granada, Spai
Optimisations to Hybrid Monte Carlo for Lattice QCD
In Lattice Quantum Chromodynamics, we calculate physical quantities on a discrete 4D Euclidean lattice via expectation values, which take the form of path integrals. Due to the high dimensionality of these integrals, the standard technique for evaluating lattice expectation values is Monte Carlo; we generate configurations of gauge fields U and fermion fields distributed according to the lattice action S, then take a weighted average of the observable across the configurations. The most common method used to generate configurations is a Markov Chain technique called Hybrid Monte Carlo. While this technique is functional, it takes a lot of computational resources to generate configurations which are desirably close to the continuum theory. The object of this work is to investigate a variety of improvements over the basic Hybrid Monte Carlo method, and determine which combinations produce independent configurations at the lowest cost. We start by performing a systematic study of filtering for double-flavour simulations, comparing polynomial filtering to the common technique of mass filtering. We show that combining these two methods produces optimal speedup with minimal tuning of parameters, which can be a serious concern when multiple filters are involved. During this investigation, we used the novel technique of overlaid integrators for implementing multiple integration time scales, which expands the possible step-size choices. Next, we investigate improvements to single-flavour simulations, comparing polynomial filtering with a different method that we denote truncated ordered product RHMC. We obtain the best speedup when using truncation filters, but it is highly dependent on the truncation order chosen. To alleviate this problem, we apply a novel integration step-size tuning method called characteristic scale tuning which allows for step-sizes to be better tuned to the energy modes of the system. This improves the performance of our algorithms for a wide range of filter parameters, thus reducing the need to tune filter parameters. Finally, we extend our single-flavour techniques to Lattice QCD+QED simulations, which include electromagnetic effects via a photon field.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 201
Protoneutron stars within the Brueckner-Bethe-Goldstone theory
We study the structure of newly born neutron stars (protoneutron stars)
within the finite temperature Brueckner-Bethe-Goldstone theoretical approach
including also hyperons. We find that for purely nucleonic stars both finite
temperature and neutrino trapping reduce the value of the maximum mass. For
hyperonic stars the effect is reversed, because neutrino trapping shifts the
appearance of hyperons to larger baryon density and stiffens considerably the
equation of state.Comment: 11 pages, 7 figures, submitted to Astronomy & Astrophysic
MR guided high intensity focused ultrasound (MRgHIFU) for treating recurrent gynaecological tumours: a pilot feasibility study.
Objective To assess the feasibility of targeting recurrent gynaecological tumours with MR guided high intensity focused ultrasound (MRgHIFU).Methods 20 patients with recurrent gynaecological tumours were prospectively scanned on a Philips/Profound 3 T Achieva MR/ Sonalleve HIFU system. Gross tumour volume (GTV) and planning target volume (PTV) were delineated on T 2W and diffusion-weighted imaging (DWI). Achievable treatment volumes that (i) assumed bowel and/or urogenital tract preparation could be used to reduce risk of damage to organs-at-risk (TVoptimal), or (ii) assumed no preparations were possible (TVno-prep) were compared with PTV on virtual treatment plans. Patients were considered treatable if TVoptimal ≥ 50 % PTV.Results 11/20 patients (55%) were treatable if preparation strategies were used: nine had central pelvic recurrences, two had tumours in metastatic locations. Treatable volume ranged from 3.4 to 90.3 ml, representing 70 ± 17 % of PTVs. Without preparation, 6/20 (30%) patients were treatable (four central recurrences, two metastatic lesions). Limiting factors were disease beyond reach of the HIFU transducer, and bone obstructing tumour access. DWI assisted tumour outlining, but differences from T 2W imaging in GTV size (16.9 ± 23.0%) and PTV location (3.8 ± 2.8 mm in phase-encode direction) limited its use for treatment planning.Conclusions Despite variation in size and location within the pelvis, ≥ 50 % of tumour volumes were considered targetable in 55 % patients while avoiding adjacent critical structures. A prospective treatment study will assess safety and symptom relief in a second patient cohort.Advances in knowledge Target size, location and access make MRgHIFU a viable treatment modality for treating symptomatic recurrent gynaecological tumours within the pelvis
On Virtual Displacement and Virtual Work in Lagrangian Dynamics
The confusion and ambiguity encountered by students, in understanding virtual
displacement and virtual work, is discussed in this article. A definition of
virtual displacement is presented that allows one to express them explicitly
for holonomic (velocity independent), non-holonomic (velocity dependent),
scleronomous (time independent) and rheonomous (time dependent) constraints. It
is observed that for holonomic, scleronomous constraints, the virtual
displacements are the displacements allowed by the constraints. However, this
is not so for a general class of constraints. For simple physical systems, it
is shown that, the work done by the constraint forces on virtual displacements
is zero. This motivates Lagrange's extension of d'Alembert's principle to
system of particles in constrained motion. However a similar zero work
principle does not hold for the allowed displacements. It is also demonstrated
that d'Alembert's principle of zero virtual work is necessary for the
solvability of a constrained mechanical problem. We identify this special class
of constraints, physically realized and solvable, as {\it the ideal
constraints}. The concept of virtual displacement and the principle of zero
virtual work by constraint forces are central to both Lagrange's method of
undetermined multipliers, and Lagrange's equations in generalized coordinates.Comment: 12 pages, 10 figures. This article is based on an earlier article
physics/0410123. It includes new figures, equations and logical conten
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Determination of global Earth outgoing radiation at high temporal resolution using a theoretical constellation of satellites
New, viable, and sustainable observation strategies from a constellation of satellites have attracted great attention across many scientific communities. Yet the potential for monitoring global Earth outgoing radiation using such a strategy has not been explored. To evaluate the potential of such a constellation concept and to investigate the configuration requirement for measuring radiation at a time resolution sufficient to resolve the diurnal cycle for weather and climate studies, we have developed a new recovery method and conducted a series of simulation experiments. Using idealized wide field-of-view broadband radiometers as an example, we find that a baseline constellation of 36 satellites can monitor global Earth outgoing radiation reliably to a spatial resolution of 1000 km at an hourly time scale. The error in recovered daily global mean irradiance is 0.16 W m-2 and -0.13 W m-2, and the estimated uncertainty in recovered hourly global mean irradiance from this day is 0.45 W m-2 and 0.15 W m-2, in the shortwave and longwave spectral regions, respectively. Sensitivity tests show that addressing instrument-related issues that lead to systematic measurement error remains of central importance to achieving similar accuracies in reality. The presented error statistics therefore likely represent the lower bounds of what could currently be achieved with the constellation approach, but this study demonstrates the promise of an unprecedented sampling capability for better observing the Earth’s radiation budget
Nonlinear regularization techniques for seismic tomography
The effects of several nonlinear regularization techniques are discussed in
the framework of 3D seismic tomography. Traditional, linear, penalties
are compared to so-called sparsity promoting and penalties,
and a total variation penalty. Which of these algorithms is judged optimal
depends on the specific requirements of the scientific experiment. If the
correct reproduction of model amplitudes is important, classical damping
towards a smooth model using an norm works almost as well as
minimizing the total variation but is much more efficient. If gradients (edges
of anomalies) should be resolved with a minimum of distortion, we prefer
damping of Daubechies-4 wavelet coefficients. It has the additional
advantage of yielding a noiseless reconstruction, contrary to simple
minimization (`Tikhonov regularization') which should be avoided. In some of
our examples, the method produced notable artifacts. In addition we
show how nonlinear methods for finding sparse models can be
competitive in speed with the widely used methods, certainly under
noisy conditions, so that there is no need to shun penalizations.Comment: 23 pages, 7 figures. Typographical error corrected in accelerated
algorithms (14) and (20
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