3,366 research outputs found
Constraints on Supersymmetric Models of Hybrid Inflation
We point out that the inclusion of a string component contributing around 5%
to the CMB power spectrum amplitude on large scales can increase the preferred
value of the spectral index n_s of density fluctuations measured by CMB
experiments. While this finding applies to any cosmological scenario involving
strings, we consider in particular models of supersymmetric hybrid inflation,
which predict n_s >= 0.98, in tension with the CMB data when strings are not
included. Using MCMC analysis we constrain the parameter space allowed for F-
and D-term inflation. For the F-term model, using minimal supergravity
corrections, we find that \log\kappa= -2.34\pm 0.38 and M= (0.518\pm 0.059) *
10^16 GeV. The inclusion of non-minimal supergravity corrections can modify
these values somewhat. In the corresponding analysis for D-term inflation, we
find \log\kappa= -4.24\pm 0.19 and m_FI= (0.245\pm 0.031) * 10^16 GeV. Under
the assumption that these models are correct, these results represent precision
measurements of important parameters of a Grand Unified Theory. We consider the
possible uncertainties in our measurements and additional constraints on the
scenario from the stochastic background of gravitational waves produced by the
strings. The best-fitting model predicts a B-mode polarization signal \approx
0.3 \mu K rms peaking at l \approx 1000. This is of comparable amplitude to the
expected signal due to gravitational lensing of the adiabatic E-mode signal on
these scales.Comment: 21 pages, 16 figure
A RANS modelling approach for predicting powering performance of ships in waves
In this paper, a modelling technique for simulating self-propelled ships in waves is presented. The flow is modelled using a RANS solver coupled with an actuator disk model for the propeller. The motion of the ship is taken into consideration in the definition of the actuator disk region as well as the advance ratio of the propeller. The RPM of the propeller is controlled using a PID-controller with constraints added on the maximum permissible RPM increase rate. Results are presented for a freely surging model in regular waves with different constraints put on the PID-controller. The described method shows promising results and allows for the studying of several factors relating to selfpropulsion. However, more validation data is needed to judge the accuracy of the mode
Szego coordinates, quadrature domains, and double quadrature domains
We define Szego coordinates on a finitely connected smoothly bounded planar
domain which effect a holomorphic change of coordinates on the domain that can
be as close to the identity as desired and which convert the domain to a
quadrature domain with respect to boundary arc length. When these Szego
coordinates coincide with Bergman coordinates, the result is a double
quadrature domain with respect to both area and arc length. We enumerate a host
of interesting and useful properties that such double quadrature domains
possess, and we show that such domains are in fact dense in the realm of
bounded finitely connected domains with smooth boundaries.Comment: 19 page
Maximizing Energy Efficiency in Multiple Access Channels by Exploiting Packet Dropping and Transmitter Buffering
Quality of service (QoS) for a network is characterized in terms of various
parameters specifying packet delay and loss tolerance requirements for the
application. The unpredictable nature of the wireless channel demands for
application of certain mechanisms to meet the QoS requirements. Traditionally,
medium access control (MAC) and network layers perform these tasks. However,
these mechanisms do not take (fading) channel conditions into account. In this
paper, we investigate the problem using cross layer techniques where
information flow and joint optimization of higher and physical layer is
permitted. We propose a scheduling scheme to optimize the energy consumption of
a multiuser multi-access system such that QoS constraints in terms of packet
loss are fulfilled while the system is able to maximize the advantages emerging
from multiuser diversity. Specifically, this work focuses on modeling and
analyzing the effects of packet buffering capabilities of the transmitter on
the system energy for a packet loss tolerant application. We discuss low
complexity schemes which show comparable performance to the proposed scheme.
The numerical evaluation reveals useful insights about the coupling effects of
different QoS parameters on the system energy consumption and validates our
analytical results.Comment: in IEEE trans. Wireless communications, 201
Phase field modelling of surfactants in multi-phase flow
A diffuse interface model for surfactants in multi-phase flow with three or
more fluids is derived. A system of Cahn-Hilliard equations is coupled with a
Navier-Stokes system and an advection-diffusion equation for the surfactant
ensuring thermodynamic consistency. By an asymptotic analysis the model can be
related to a moving boundary problem in the sharp interface limit, which is
derived from first principles. Results from numerical simulations support the
theoretical findings. The main novelties are centred around the conditions in
the triple junctions where three fluids meet. Specifically the case of local
chemical equilibrium with respect to the surfactant is considered, which allows
for interfacial surfactant flow through the triple junctions
Ring completion of rig categories
We offer a solution to the long-standing problem of group completing within
the context of rig categories (also known as bimonoidal categories). Given a
rig category R we construct a natural additive group completion R' that retains
the multiplicative structure, hence has become a ring category. If we start
with a commutative rig category R (also known as a symmetric bimonoidal
category), the additive group completion R' will be a commutative ring
category. In an accompanying paper we show how this can be used to prove the
conjecture from [BDR] that the algebraic K-theory of the connective topological
K-theory spectrum ku is equivalent to the algebraic K-theory of the rig
category V of complex vector spaces.Comment: There was a mathematical error in arXiv:0706.0531v2: the map T in the
purported proof of Lemma 3.7(2) is not well defined. Version 4 has been
edited for notational consistenc
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