87,372 research outputs found
The Impacts of Three Flamelet Burning Regimes in Nonlinear Combustion Dynamics
Axisymmetric simulations of a liquid rocket engine are performed using a
delayed detached-eddy-simulation (DDES) turbulence model with the Compressible
Flamelet Progress Variable (CFPV) combustion model. Three different pressure
instability domains are simulated: completely unstable, semi-stable, and fully
stable. The different instability domains are found by varying the combustion
chamber and oxidizer post length. Laminar flamelet solutions with a detailed
chemical mechanism are examined. The Probability Density Function (PDF)
for the mixture fraction and Dirac PDF for both the pressure and the
progress variable are used. A coupling mechanism between the Heat Release Rate
(HRR) and the pressure in an unstable cycle is demonstrated. Local extinction
and reignition is investigated for all the instability domains using the full
S-curve approach. A monotonic decrease in the amount of local extinctions and
reignitions occurs when pressure oscillation amplitude becomes smaller. The
flame index is used to distinguish between the premixed and non-premixed
burning mode in different stability domains. An additional simulation of the
unstable pressure oscillation case using only the stable flamelet burning
branch of the S-curve is performed. Better agreement with experiments in terms
of pressure oscillation amplitude is found when the full S-curve is used.Comment: 25 pages, 12 figures. Submitted to Combustion and Flame for a Special
Issu
On the Design of Secure Full-Duplex Multiuser Systems under User Grouping Method
Consider a full-duplex (FD) multiuser system where an FD base station (BS) is
designed to simultaneously serve both downlink users and uplink users in the
presence of half-duplex eavesdroppers (Eves). Our problem is to maximize the
minimum secrecy rate (SR) among all legitimate users by proposing a novel user
grouping method, where information signals at the FD-BS are accompanied with
artificial noise to degrade the Eves' channel. The SR problem has a highly
nonconcave and nonsmooth objective, subject to nonconvex constraints due to
coupling between the optimization variables. Nevertheless, we develop a
path-following low-complexity algorithm, which invokes only a simple convex
program of moderate dimensions at each iteration. We show that our
path-following algorithm guarantees convergence at least to a local optima. The
numerical results demonstrate the merit of our proposed approach compared to
existing well-known ones, i.e., conventional FD and nonorthogonal multiple
access.Comment: 6 pages, 3 figure
Non-gaussian Noise in Quantum Spin Glasses and Interacting Two-level Systems
We study a general model for non-gaussian noise based on an infinite
range quantum Ising spin system in the paramagnetic state, or equivalently,
interacting two-level classical fluctuators. We identify a dilatation
interaction term in the dynamics which survives the thermodynamic limit and
circumvents the central limit theorem to produce non-gaussian noise even when
the equilibrium distribution is that of {\em non-interacting} spins. The
resulting second spectrum (`noise of the noise') itself has a universal
form which we analyze within a dynamical mean field approximation.Comment: 4 pages, 3 figure
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