14,856 research outputs found
Heat transfer to two-phase air/water mixtures flowing in small tubes with inlet disequilibrium
The cooling of gas turbine components was the subject of considerable research. The problem is difficult because the available coolant, compressor bleed air, is itself quite hot and has relatively poor thermophysical properties for a coolant. Injecting liquid water to evaporatively cool the air prior to its contact with the hot components was proposed and studied, particularly as a method of cooling for contingency power applications. Injection of a small quantity of cold liquid water into a relatively hot coolant air stream such that evaporation of the liquid is still in process when the coolant contacts the hot component was studied. No approach was found whereby heat transfer characteristics could be confidently predicted for such a case based solely on prior studies. It was not clear whether disequilibrium between phases at the inlet to the hot component section would improve cooling relative to that obtained where equilibrium was established prior to contact with the hot surface
Monolithic microwave integrated circuit water vapor radiometer
A proof of concept Monolithic Microwave Integrated Circuit (MMIC) Water Vapor Radiometer (WVR) is under development at the Jet Propulsion Laboratory (JPL). WVR's are used to remotely sense water vapor and cloud liquid water in the atmosphere and are valuable for meteorological applications as well as for determination of signal path delays due to water vapor in the atmosphere. The high cost and large size of existing WVR instruments motivate the development of miniature MMIC WVR's, which have great potential for low cost mass production. The miniaturization of WVR components allows large scale deployment of WVR's for Earth environment and meteorological applications. Small WVR's can also result in improved thermal stability, resulting in improved calibration stability. Described here is the design and fabrication of a 31.4 GHz MMIC radiometer as one channel of a thermally stable WVR as a means of assessing MMIC technology feasibility
Disordered Electrons in a Strong Magnetic Field: Transfer Matrix Approaches to the Statistics of the Local Density of States
We present two novel approaches to establish the local density of states as
an order parameter field for the Anderson transition problem. We first
demonstrate for 2D quantum Hall systems the validity of conformal scaling
relations which are characteristic of order parameter fields. Second we show
the equivalence between the critical statistics of eigenvectors of the
Hamiltonian and of the transfer matrix, respectively. Based on this equivalence
we obtain the order parameter exponent for 3D quantum
Hall systems.Comment: 4 pages, 3 Postscript figures, corrected scale in Fig.
Microarcsecond VLBI pulsar astrometry with PSRPI I. Two binary millisecond pulsars with white dwarf companions
Model-independent distance constraints to binary millisecond pulsars (MSPs)
are of great value to both the timing observations of the radio pulsars, and
multiwavelength observations of their companion stars. Very Long Baseline
Interferometry (VLBI) astrometry can be employed to provide these
model-independent distances with very high precision via the detection of
annual geometric parallax. Using the Very Long Baseline Array, we have observed
two binary millisecond pulsars, PSR J1022+1001 and J2145-0750, over a two-year
period and measured their distances to be 700 +14 -10 pc and 613 +16 -14 pc
respectively. We use the well-calibrated distance in conjunction with revised
analysis of optical photometry to tightly constrain the nature of their massive
(M ~ 0.85 Msun) white dwarf companions. Finally, we show that several
measurements of their parallax and proper motion of PSR J1022+1001 and PSR
J2145-0750 obtained by pulsar timing array projects are incorrect, differing
from the more precise VLBI values by up to 5 sigma. We investigate possible
causes for the discrepancy, and find that imperfect modeling of the solar wind
is a likely candidate for the timing model errors given the low ecliptic
latitude of these two pulsars.Comment: 14 pages, 9 figures, 6 tables; minor revisions in response to referee
comments to match version accepted by Ap
Instabilities in the nonsymmetric theory of gravitation
We consider the linearized nonsymmetric theory of gravitation (NGT) within
the background of an expanding universe and near a Schwarzschild metric. We
show that the theory always develops instabilities unless the linearized
nonsymmetric lagrangian reduces to a particular simple form. This theory
contains a gauge invariant kinetic term, a mass term for the antisymmetric
metric-field and a coupling with the Ricci curvature scalar. This form cannot
be obtained within NGT. Next we discuss NGT beyond linearized level and
conjecture that the instabilities are not a relic of the linearization, but are
a general feature of the full theory. Finally we show that one cannot add
ad-hoc constraints to remove the instabilities as is possible with the
instabilities found in NGT by Clayton.Comment: 29 page
Microarcsecond VLBI pulsar astrometry with PSR II. parallax distances for 57 pulsars
We present the results of PSR, a large astrometric project targeting
radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric
database of 60 pulsars, we have obtained parallax-based distance measurements
for all but 3, with a parallax precision of typically 40 as and
approaching 10 as in the best cases. Our full sample doubles the number of
radio pulsars with a reliable (5) model-independent distance
constraint. Importantly, many of the newly measured pulsars are well outside
the solar neighbourhood, and so PSR brings a near-tenfold increase in the
number of pulsars with a reliable model-independent distance at kpc.
Using our sample along with previously published results, we show that even the
most recent models of the Galactic electron density distribution model contain
significant shortcomings, particularly at high Galactic latitudes. When
comparing our results to pulsar timing, two of the four millisecond pulsars in
our sample exhibit significant discrepancies in the estimates of proper motion
obtained by at least one pulsar timing array. With additional VLBI observations
to improve the absolute positional accuracy of our reference sources and an
expansion of the number of millisecond pulsars, we will be able to extend the
comparison of proper motion discrepancies to a larger sample of pulsar
reference positions, which will provide a much more sensitive test of the
applicability of the solar system ephemerides used for pulsar timing. Finally,
we use our large sample to estimate the typical accuracy attainable for
differential astrometry with the VLBA when observing pulsars, showing that for
sufficiently bright targets observed 8 times over 18 months, a parallax
uncertainty of 4 as per arcminute of separation between the pulsar and
calibrator can be expected.Comment: updated to version accepted by ApJ: 30 pages, 20 figures, 9 table
On Critical Exponents and the Renormalization of the Coupling Constant in Growth Models with Surface Diffusion
It is shown by the method of renormalized field theory that in contrast to a
statement based on a mathematically ill-defined invariance transformation and
found in most of the recent publications on growth models with surface
diffusion, the coupling constant of these models renormalizes nontrivially.
This implies that the widely accepted supposedly exact scaling exponents are to
be corrected. A two-loop calculation shows that the corrections are small and
these exponents seem to be very good approximations.Comment: 4 pages, revtex, 2 postscript figures, to appear in Phys.Rev.Let
Quantum simulation of a Fermi-Hubbard model using a semiconductor quantum dot array
Interacting fermions on a lattice can develop strong quantum correlations,
which lie at the heart of the classical intractability of many exotic phases of
matter. Seminal efforts are underway in the control of artificial quantum
systems, that can be made to emulate the underlying Fermi-Hubbard models.
Electrostatically confined conduction band electrons define interacting quantum
coherent spin and charge degrees of freedom that allow all-electrical
pure-state initialisation and readily adhere to an engineerable Fermi-Hubbard
Hamiltonian. Until now, however, the substantial electrostatic disorder
inherent to solid state has made attempts at emulating Fermi-Hubbard physics on
solid-state platforms few and far between. Here, we show that for gate-defined
quantum dots, this disorder can be suppressed in a controlled manner. Novel
insights and a newly developed semi-automated and scalable toolbox allow us to
homogeneously and independently dial in the electron filling and
nearest-neighbour tunnel coupling. Bringing these ideas and tools to fruition,
we realize the first detailed characterization of the collective Coulomb
blockade transition, which is the finite-size analogue of the
interaction-driven Mott metal-to-insulator transition. As automation and device
fabrication of semiconductor quantum dots continue to improve, the ideas
presented here show how quantum dots can be used to investigate the physics of
ever more complex many-body states
- …