18,725 research outputs found
Tripropellant combustion process
The addition of small amounts of hydrogen to the combustion of LOX/hydrocarbon propellants in large rocket booster engines has the potential to enhance the system stability. Programs being conducted to evaluate the effects of hydrogen on the combustion of LOX/hydrocarbon propellants at supercritical pressures are described. Combustion instability has been a problem during the development of large hydrocarbon fueled rocket engines. At the higher combustion chamber pressures expected for the next generation of booster engines, the effect of unstable combustion could be even more destructive. The tripropellant engine cycle takes advantage of the superior cooling characteristics of hydrogen to cool the combustion chamber and a small amount of the hydrogen coolant can be used in the combustion process to enhance the system stability. Three aspects of work that will be accomplished to evaluate tripropellant combustion are described. The first is laboratory demonstration of the benefits through the evaluation of drop size, ignition delay and burning rate. The second is analytical modeling of the combustion process using the empirical relationship determined in the laboratory. The third is a subscale demonstration in which the system stability will be evaluated. The approach for each aspect is described and the analytical models that will be used are presented
Why do we observe a small but non zero cosmological constant ?
The current observations seem to suggest that the universe has a positive
cosmological constant of the order of while the most natural value for
the cosmological constant will be where
is the Planck length. This reduction of the cosmological constant from
to may be interpreted as due to the ability of
quantum micro structure of spacetime to readjust itself and absorb bulk vacuum
energy densities. Being a quantum mechanical process, such a cancellation
cannot be exact and the residual quantum fluctuations appear as the ``small''
cosmological constant. I describe the features of a toy model for the spacetime
micro structure which could allow for the bulk vacuum energy densities to be
canceled leaving behind a small residual value of the the correct magnitude.
Some other models (like the ones based on canonical ensemble for the four
volume or quantum fluctuations of the horizon size) lead to an insignificantly
small value of with showing that obtaining the
correct order of magnitude for the residual fluctuations in the cosmological
constant is a nontrivial task, becaue of the existence of the small
dimensionless number .Comment: couple of references added; matches with published versio
Rippled Cosmological Dark Matter from Damped Oscillating Newton Constant
Let the reciprocal Newton 'constant' be an apparently non-dynamical
Brans-Dicke scalar field damped oscillating towards its General Relativistic
VEV. We show, without introducing additional matter fields or dust, that the
corresponding cosmological evolution averagely resembles, in the Jordan frame,
the familiar dark radiation -> dark matter -> dark energy domination sequence.
The fingerprints of our theory are fine ripples, hopefully testable, in the FRW
scale factor; they die away at the General Relativity limit. The possibility
that the Brans-Dicke scalar also serves as the inflaton is favorably examined.Comment: RevTex4, 12 pages, 5 figures; Minor revision, References adde
A symmetry for vanishing cosmological constant
Two different realizations of a symmetry principle that impose a zero
cosmological constant in an extra-dimensional set-up are studied. The symmetry
is identified by multiplication of the metric by minus one. In the first
realization of the symmetry this is provided by a symmetry transformation that
multiplies the coordinates by the imaginary number i. In the second realization
this is accomplished by a symmetry transformation that multiplies the metric
tensor by minus one. In both realizations of the symmetry the requirement of
the invariance of the gravitational action under the symmetry selects out the
dimensions given by D = 2(2n+1), n=0,1,2,... and forbids a bulk cosmological
constant. Another attractive aspect of the symmetry is that it seems to be more
promising for quantization when compared to the usual scale symmetry. The
second realization of the symmetry is more attractive in that it is posible to
make a possible brane cosmological constant zero in a simple way by using the
same symmetry, and the symmetry may be identified by reflection symmetry in
extra dimensions.Comment: Talk in the conference IRGAC 2006, 2nd International Conference on
Quantum Theories and Renormalization Group in Gravity and Cosmology,
Barcelon
A comparison of the quality of life of vulnerable young males with severe emotional and behaviour difficulties in a residential setting and young males in mainstream schooling: QoL of vulnerable young males
Accessible summary: The findings from this study reveal that the notion of anticipated and deleterious differences in quality of life (QoL) between children with severe emotional and behavioural problems and those without such difficulties is not supported. Indeed, results reveal counter-intuitive findings with children with emotional and behavioural problems reporting better QoL than those without such presenting problems on a number of QoL subscales. The type of QoL measure and related subscales appears to be sensitive to differing aspects of self-report QoL, with in some instances, some QoL subscales being more discriminatory between groups compared with other QoL subscales. Consequently, the choice of QoL measure is critically important in accurately and reliably determining QoL in children with significant emotional and behavioural problems. One hundred and seventy-four males completed a quality of life (QoL) assessment utilizing, a generic paediatric quality of life inventory (PedsQL) and the short form (36) health survey (SF36). The adolescents aged 13-16 years were in a Scottish Centre for young males with social, emotional, behavioural and educational problems. To identify similarities and differences, a comparison group (n = 110) of males in the third and fourth year in a mainstream secondary school were also administered the PedsQL and the SF36 self-rating scales. The effectiveness of the PedsQL and the SF36 for assessing QoL for adolescent males was investigated. There were significant differences between the groups in the Centre and between the Centre groups and the comparison group in terms of their QoL. The results between the groups were found in the PedsQL subscales 'physical functioning' where secure > comparison (P = 0.04); secure > residential (P = 0.008); and PedsQL subscale 'social functioning' day > comparison (P = 0.026); secure > comparison (P = 0.037). SF36 subscales 'role physical functioning' secure > residential (P residential (P residential (P = 0.001). This study provides a unique insight into the complex dimensions influencing the QoL of this specific group of young people. © 2013 John Wiley & Sons Ltd
Domain Bubbles of Extra Dimensions
``Dimension bubbles'' of the type previously studied by Blau and Guendelman
[S.K. Blau and E.I. Guendelman, Phys. Rev. D40, 1909 (1989)], which effectively
enclose a region of 5d spacetime and are surrounded by a region of 4d
spacetime, can arise in a 5d theory with a compact extra dimension that is
dimensionally reduced to give an effective 4d theory. These bubbles with thin
domain walls can be stabilized against total collapse in a rather natural way
by a scalar field which, as in the case with ``ordinary'' nontopological
solitons, traps light scalar particles inside the bubble.Comment: 13 pages, no figures; to appear in Phys.Rev.
Out of equilibrium: understanding cosmological evolution to lower-entropy states
Despite the importance of the Second Law of Thermodynamics, it is not
absolute. Statistical mechanics implies that, given sufficient time, systems
near equilibrium will spontaneously fluctuate into lower-entropy states,
locally reversing the thermodynamic arrow of time. We study the time
development of such fluctuations, especially the very large fluctuations
relevant to cosmology. Under fairly general assumptions, the most likely
history of a fluctuation out of equilibrium is simply the CPT conjugate of the
most likely way a system relaxes back to equilibrium. We use this idea to
elucidate the spacetime structure of various fluctuations in (stable and
metastable) de Sitter space and thermal anti-de Sitter space.Comment: 27 pages, 11 figure
Scalable syntheses of well-defined pentadecablock bipolymer and quintopolymer
The one-pot syntheses of two pentadeca-(15)-block methacrylate-based amphiphilic copolymers, specifically a bipolymer (AB)7A and a quintopolymer (ABCDE)3, are being reported using a fast and easy to scale up procedure that does not require any intermediate purification steps. Both syntheses were carried out using sequential group transfer polymerisation (GTP) and took under 3.5 h. Amino-containing (DMAEMA, DEAEMA), ether (THFMA, MEGMA) and alkyl (EtMA) methacrylates were used to produce the multiblock copolymers with a final Ä < 1.3
Astrophysical Probes of Fundamental Physics
I review the theoretical motivation for varying fundamental couplings and
discuss how these measurements can be used to constrain a number of fundamental
physics scenarios that would otherwise be inacessible to experiment. As a case
study I will focus on the relation between varying couplings and dark energy,
and explain how varying coupling measurements can be used to probe the nature
of dark energy, with important advantages over the standard methods. Assuming
that the current observational evidence for varying and is
correct, a several-sigma detection of dynamical dark energy is feasible within
a few years, using currently operational ground-based facilities. With
forthcoming instruments like CODEX, a high-accuracy reconstruction of the
equation of state may be possible all the way up to redshift .Comment: Invited Review talk at the ESO Precision Spectroscopy in Astrophysics
conference, to appear in the proceeding
Spherical Panoramic Image Payload Design for Stratospheric Ballooning
The 2017 total solar eclipse inspired innovation in design and implementation in stratospheric ballooning techniques and payloads to fully document the unique near-space experience. The Arkansas BalloonSAT team, as part of the Eclipse Ballooning Project, designed and fabricated a spherical panoramic image payload using computer aided design (CAD) software to capture images of the eclipse from the high altitude balloon. The light, durable, and water resistant mount consisted of a polyurethane-coated UV-resin based enclosure that housed six Hero 4 Session GoPro cameras, and a relative inertial measurement unit (IMU) PCB equipped with GPS. The camerasâ lenses faced radially outward on the six faces of a cube. The IMU/GPS PCB system computed the absolute orientation of the cameras, which allows the time-lapse pictures to be stitched together into a relatively stable spherical video. The batteries for this system experienced a voltage drop during flight as the payload temperature dropped below -60 degrees Celsius resulting in an auto-shutdown procedure disabling the system before eclipse totality. Future designs for this model will include a temperature regulation or insulation system to prevent this voltage drop
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