1,000 research outputs found
Building Merger Trees from Cosmological N-body Simulations
Although a fair amount of work has been devoted to growing Monte-Carlo merger
trees which resemble those built from an N-body simulation, comparatively
little effort has been invested in quantifying the caveats one necessarily
encounters when one extracts trees directly from such a simulation. To somewhat
revert the tide, this paper seeks to provide its reader with a comprehensive
study of the problems one faces when following this route. The first step to
building merger histories of dark matter haloes and their subhaloes is to
identify these structures in each of the time outputs (snapshots) produced by
the simulation. Even though we discuss a particular implementation of such an
algorithm (called AdaptaHOP) in this paper, we believe that our results do not
depend on the exact details of the implementation but extend to most if not all
(sub)structure finders. We then highlight different ways to build merger
histories from AdaptaHOP haloes and subhaloes, contrasting their various
advantages and drawbacks. We find that the best approach to (sub)halo merging
histories is through an analysis that goes back and forth between
identification and tree building rather than one which conducts a
straightforward sequential treatment of these two steps. This is rooted in the
complexity of the merging trees which have to depict an inherently dynamical
process from the partial temporal information contained in the collection of
instantaneous snapshots available from the N-body simulation.Comment: 19 pages, 28 figure
Preliminary investigations into the active control of large space structures: Solution of the Timoshenko beam equations by the method of characteristics
An algorithm was developed and incorporated into a computer program for solving the damped, Timoshenko beam equations with free-free boundary conditions and prescribed initial data. A number of special cases are considered and, where appropriate, comparisons are made with known results
Cracks of unequal length at the edge of an elliptic hole in out of plane shear
AbstractIntegral transforms are used to find mode III stress intensity factors for two unequal length cracks at the edge of an elliptic hole in an infinite elastic solid
The Reduction of Ducted Fan Engine Noise Via A Boundary Integral Equation Method
The development of a Boundary Integral Equation Method (BIEM) for the prediction of ducted fan engine noise is discussed. The method is motivated by the need for an efficient and versatile computational tool to assist in parametric noise reduction studies. In this research, the work in reference 1 was extended to include passive noise control treatment on the duct interior. The BEM considers the scattering of incident sound generated by spinning point thrust dipoles in a uniform flow field by a thin cylindrical duct. The acoustic field is written as a superposition of spinning modes. Modal coefficients of acoustic pressure are calculated term by term. The BEM theoretical framework is based on Helmholtz potential theory. A boundary value problem is converted to a boundary integral equation formulation with unknown single and double layer densities on the duct wall. After solving for the unknown densities, the acoustic field is easily calculated. The main feature of the BIEM is the ability to compute any portion of the sound field without the need to compute the entire field. Other noise prediction methods such as CFD and Finite Element methods lack this property. Additional BIEM attributes include versatility, ease of use, rapid noise predictions, coupling of propagation and radiation both forward and aft, implementable on midrange personal computers, and valid over a wide range of frequencies
Toy Models for Galaxy Formation versus Simulations
We describe simple useful toy models for key processes of galaxy formation in
its most active phase, at z > 1, and test the approximate expressions against
the typical behaviour in a suite of high-resolution hydro-cosmological
simulations of massive galaxies at z = 4-1. We address in particular the
evolution of (a) the total mass inflow rate from the cosmic web into galactic
haloes based on the EPS approximation, (b) the penetration of baryonic streams
into the inner galaxy, (c) the disc size, (d) the implied steady-state gas
content and star-formation rate (SFR) in the galaxy subject to mass
conservation and a universal star-formation law, (e) the inflow rate within the
disc to a central bulge and black hole as derived using energy conservation and
self-regulated Q ~ 1 violent disc instability (VDI), and (f) the implied steady
state in the disc and bulge. The toy models provide useful approximations for
the behaviour of the simulated galaxies. We find that (a) the inflow rate is
proportional to mass and to (1+z)^5/2, (b) the penetration to the inner halo is
~50% at z = 4-2, (c) the disc radius is ~5% of the virial radius, (d) the
galaxies reach a steady state with the SFR following the accretion rate into
the galaxy, (e) there is an intense gas inflow through the disc, comparable to
the SFR, following the predictions of VDI, and (f) the galaxies approach a
steady state with the bulge mass comparable to the disc mass, where the
draining of gas by SFR, outflows and disc inflows is replenished by fresh
accretion. Given the agreement with simulations, these toy models are useful
for understanding the complex phenomena in simple terms and for
back-of-the-envelope predictions.Comment: Resubmitted to MNRAS after responding to referee's comments; Revised
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Five-minute Apgar score and educational outcomes: retrospective cohort study of 751 369 children
Background: The Apgar score is used worldwide for assessing the clinical condition and short-term prognosis of newborn infants. Evidence for a relationship with long-term educational outcomes is conflicting. We investigated whether Apgar score at 5 min after birth was associated with additional support needs (ASN) and educational attainment.
Methods: Data on pregnancy, delivery and later educational outcomes for children attending Scottish schools between 2006 and 2011 were collated by linking individual-level data from national educational and maternity databases. The relationship between Apgar score and overall ASN, type-specific ASN and educational attainment was assessed using binary, multinomial and generalised ordinal logistic regression models, respectively. Missing covariate data were imputed.
Results: Of the 751 369 children eligible, 9741 (1.3%) had a low or intermediate Apgar score and 49 962 (6.6%) had ASN. Low Apgar score was independently associated with overall ASN status (adjusted OR for Apgar ≤3, OR 1.52 95% CI 1.35 to 1.70), as well as ASN due to cognitive (OR 1.26, 95% CI 1.09 to 1.47), sensory (OR 2.49 95% CI 1.66 to 3.73) and motor (OR 3.57, 95% CI 2.86 to 4.47) impairments. There was a dose-response relationship between Apgar score and overall ASN status: of those scoring 0–3, 10.1% had ASN, compared with 9.1% of those scoring 4–7 and 6.6% of those scoring 7–10. A low Apgar score was associated with lower educational attainment, but this was not robust to adjustment for confounders.
Conclusions: Apgar scores are associated with long-term as well as short-term prognoses, and with educational as well as clinical outcomes at the population level
What are effective medication combinations for dyslipidemia?
Many combination drug therapies are effective in treating dyslipidemia. Compared with statin monotherapy, combinations that include ezetimibe (Zetia), a bile acid sequestrant, or niacin further lower low- density lipoprotein (LDL) cholesterol (strength of recommendation [SOR]: A), and increase the likelihood of attaining National Cholesterol Education Program (NCEP) LDL cholesterol goals (SOR: B). Adding ezetimibe to a bile acid sequestrant reduces LDL cholesterol (SOR: B). Fibrate or niacin added to statin monotherapy provide mixed lipid-modifying effects for combined dyslipidemia (SOR: A)
Some Triple Sine Series
Two types of triple sine series are investigated. They are reduced to singular integral equations with kernels involving elliptic functions. Closed form solutions are obtained
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