30,129 research outputs found
The economics of garbage collection
This paper argues that economic theory can improve our understanding of memory management. We introduce the allocation curve, as an analogue of the demand curve from microeconomics. An allocation curve for a program characterises how the amount of garbage collection activity required during its execution varies in relation to the heap size associated with that program. The standard treatment of microeconomic demand curves (shifts and elasticity) can be applied directly and intuitively to our new allocation curves. As an application of this new theory, we show how allocation elasticity can be used to control the heap growth rate for variable sized heaps in Jikes RVM
Investigation of Liner Axial Displacement in a Complex Acoustic Environment
This paper investigates the effects of axial liner displacement on modal attenuation and scattering. Tests are conducted with a uniform liner mounted in the NASA Langley Curved Duct Test Rig, with a multimode source at flow conditions of Mach 0.0 and 0.3. The impedance of the liner is educed from data acquired in the NASA Langley Grazing Flow Impedance Tube. This impedance is then used as input to the CDUCT-LaRC propagation code to enable comparisons of predicted and measured modal content upstream and downstream of the liner as the axial displacement is varied. Results show that the liner position has a clear effect on the sound field as the number of modes is increased from two to six. The comparison of measured and predicted modal content is very good when only a few modes are present. For frequencies in which more modes are present, this comparison is less favorable very near resonance, but improves for frequencies away from resonance
Survey of fracture toughness test methods
Comprehensive survey presents current methods of fracture toughness testing that are based on linear elastic fracture mechanics. General principles of the basic two dimensional crack stress field model are discussed in relation to real three dimensional specimens. Methods of test instrumentation and procedure are described
Over-the-Rotor Liner Investigation via the NASA Langley Normal Incidence Tube
NASA Langley and Glenn Research Centers have collaborated on the usage of acoustic liners mounted very near or directly over the rotor of turbofan aircraft engines. This collaboration began over a decade ago with the investigation of a metallic foam liner. Similar to conventional acoustic liner applications, this liner was designed to absorb sound generated by the rotor-alone and rotor-stator interaction sources within the fan duct. Given its proximity to the rotor tips, the expectation was that the liner would also serve as a pressure release and thereby inhibit the amount of noise generated. Initial acoustic results were promising, but there was concern regarding potential aerodynamic penalties. Nevertheless, there were sufficient positive results to warrant further investigation. To that end, the current report presents results obtained in the NASA Langley Normal Incidence Tube for 20 acoustic liner candidates for the OTR application. The majority contain grooves at their surface, designed to minimize aerodynamic penalties caused by placing the liner in close proximity to the fan rotor tips. The intent is to assess the acoustic properties of each liner configuration, and in particular to assess the effects of including the grooves on the overall acoustic performance. An additional intent of this paper is to provide documentation regarding recent enhancements to the NASA Langley Normal Incidence Tube
Fundamental Limitations of Cavity-assisted Atom Interferometry
Atom interferometers employing optical cavities to enhance the beam splitter
pulses promise significant advances in science and technology, notably for
future gravitational wave detectors. Long cavities, on the scale of hundreds of
meters, have been proposed in experiments aiming to observe gravitational waves
with frequencies below 1 Hz, where laser interferometers, such as LIGO, have
poor sensitivity. Alternatively, short cavities have also been proposed for
enhancing the sensitivity of more portable atom interferometers. We explore the
fundamental limitations of two-mirror cavities for atomic beam splitting, and
establish upper bounds on the temperature of the atomic ensemble as a function
of cavity length and three design parameters: the cavity g-factor, the
bandwidth, and the optical suppression factor of the first and second order
spatial modes. A lower bound to the cavity bandwidth is found which avoids
elongation of the interaction time and maximizes power enhancement. An upper
limit to cavity length is found for symmetric two-mirror cavities, restricting
the practicality of long baseline detectors. For shorter cavities, an upper
limit on the beam size was derived from the geometrical stability of the
cavity. These findings aim to aid the design of current and future
cavity-assisted atom interferometers.Comment: 11 pages, 12 figure
Whole family-based physical activity promotion intervention: the Families Reporting Every Step to Health pilot randomised controlled trial protocol
Introduction : Family-based physical activity (PA) interventions present a promising avenue to promote childrenās activity, however, high-quality experimental research is lacking. This paper describes the protocol for the FRESH (Families Reporting Every Step to Health) pilot trial, a child-led family-based PA intervention delivered online.Ā Methods and analysis : FRESH is a three-armed, parallel-group, randomised controlled pilot trial using a 1:1:1 allocation ratio with follow-up assessments at 8- and 52-weeks post-baseline. Families will be eligible if a minimum of one child in school Years 3-6 (aged 7-11 years) and at least one adult responsible for that child are willing to participate. Family members can take part in the intervention irrespective of their participation in the accompanying evaluation and vice versa. Following baseline assessment, families will be randomly allocated to one of three arms: (1) FRESH, (2) pedometer-only, or (3) no-intervention control. All family members in the pedometer-only and FRESH arms receive pedometers and generic PA promotion information. FRESH families additionally receive access to the intervention website; allowing participants to select step challenges to ātravelā to target cities around the world, log steps, and track progress as they virtually globetrot. Control families will receive no treatment. All family members will be eligible to participate in the evaluation with two follow-ups (8 and 52 weeks). Physical (e.g., fitness, blood pressure), psychosocial (e.g., social support), and behavioural (e.g., objectively-measured family PA) measures will be collected each time point. At 8-week follow-up, a mixed-methods process evaluation will be conducted (questionnaires and family focus groups) assessing acceptability of the intervention and evaluation. FRESH familiesā website engagement will also be explored.Ā Ethics and dissemination : This study received ethical approval from the Ethics Committee for the School of the Humanities and Social Sciences at the University of Cambridge. Findings will be disseminated via peer-reviewed publications, conferences, and to participating families
Agricultural interpretation technique development
There are no author-identified significant results in this report
Interfacial motion in flexo- and order-electric switching between nematic filled states
We consider a nematic liquid crystal, in coexistence with its isotropic
phase, in contact with a substrate patterned with rectangular grooves. In such
a system, the nematic phase may fill the grooves without the occurrence of
complete wetting. There may exist multiple (meta)stable filled states, each
characterised by the type of distortion (bend or splay) in each corner of the
groove and by the shape of the nematic-isotropic interface, and additionally
the plateaux that separate the grooves may be either dry or wet with a thin
layer of nematic. Using numerical simulations, we analyse the dynamical
response of the system to an externally- applied electric field, with the aim
of identifying switching transitions between these filled states. We find that
order-electric coupling between the fluid and the field provides a means of
switching between states where the plateaux between grooves are dry and states
where they are wet by a nematic layer, without affecting the configuration of
the nematic within the groove. We find that flexoelectric coupling may change
the nematic texture in the groove, provided that the flexoelectric coupling
differentiates between the types of distortion at the corners of the substrate.
We identify intermediate stages of the transitions, and the role played by the
motion of the nematic-isotropic interface. We determine quantitatively the
field magnitudes and orientations required to effect each type of transition.Comment: 14 pages, 12 fig
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