25,507 research outputs found
Start-up vortex flow past an accelerated flat plate
Viscous flow past a finite flat plate moving in direction normal to itself is
studied numerically.The plate moves with velocity , where .
We present the evolution of vorticity profiles, streaklines and streamlines,
and study the dependence on the acceleration parameter . Four stages in the
vortex evolution, as proposed by Luchini & Tognaccini (2002), are clearly
identified. The initial stage, in which the vorticity consists solely of a
Rayleigh boundary layer, is shown to last for a time-interval whose length
shrinks to zero like , as . In the second stage, a center of
rotation develops near the tip of the plate, well before a vorticity maximum
within the vortex core develops. Once the vorticity maximum develops, its
position oscillates and differs from the center of rotation. The difference
between the two increases with increasing , and decreases in time. In the
third stage, the center of rotation and the shed circulation closely satisfy
self-similar scaling laws for inviscid flow. Finally, in the fourth stage, the
finite plate length becomes relevant and the flow begins to depart from the
self-similar behaviour. While the core trajectory and circulation closely
satisfy inviscid scaling laws, the vorticity maximum and the boundary layer
thickness follow viscous scaling laws. The results are compared with
experimental results of Pullin & Perry (1980), and Taneda & Honji (1971), where
available
Semilocal Convergence Behavior of Halley's Method Using Kantorovich's Majorants Principle
The present paper is concerned with the semilocal convergence problems of
Halley's method for solving nonlinear operator equation in Banach space. Under
some so-called majorant conditions, a new semilocal convergence analysis for
Halley's method is presented. This analysis enables us to drop out the
assumption of existence of a second root for the majorizing function, but still
guarantee Q-cubic convergence rate. Moreover, a new error estimate based on a
directional derivative of the twice derivative of the majorizing function is
also obtained. This analysis also allows us to obtain two important special
cases about the convergence results based on the premises of Kantorovich and
Smale types.Comment: 17 page
Numerical study of viscous starting flow past a flat plate
Viscous flow past a finite plate which is impulsively started in direction
normal to itself is studied numerically using a high order mixed finite
difference and semi-Lagrangian scheme. The goal is to resolve details of the
vorticity generation at early times, and to determine the effect of viscosity
on flow quantities such as the core trajectory and vorticity, and the shed
circulation. Vorticity contours, streaklines and streamlines are presented for
a range of Reynolds numbers and a range of times . At early times, most of the vorticity is attached to the plate. The
paper proposes a definition for the shed circulation at early as well as late
times, and shows that it indeed represents vorticity that separates from the
plate without reattaching. The contribution of viscous diffusion to the
circulation shedding rate is found to be significant, but, interestingly, to
depend only slightly on the value of the Reynolds number. The shed circulation
and the vortex core trajectories follow scaling laws for inviscid self-similar
flow over several decades in time. Scaling laws describing the core vorticity,
core dissipation, boundary layer thickness, drag and lift forces in time and
Reynolds number are also presented. The simulations provide benchmark results
to evaluate, for example, simpler separation models such as point vortex and
vortex sheet models
Energy Efficiency Optimization for MIMO Broadcast Channels
Characterizing the fundamental energy efficiency (EE) limits of MIMO
broadcast channels (BC) is significant for the development of green wireless
communications. We address the EE optimization problem for MIMO-BC in this
paper and consider a practical power model, i.e., taking into account a
transmit independent power which is related to the number of active transmit
antennas. Under this setup, we propose a new optimization approach, in which
the transmit covariance is optimized under fixed active transmit antenna sets,
and then active transmit antenna selection (ATAS) is utilized. During the
transmit covariance optimization, we propose a globally optimal energy
efficient iterative water-filling scheme through solving a series of concave
fractional programs based on the block-coordinate ascent algorithm. After that,
ATAS is employed to determine the active transmit antenna set. Since activating
more transmit antennas can achieve higher sum-rate but at the cost of larger
transmit independent power consumption, there exists a tradeoff between the
sum-rate gain and the power consumption. Here ATAS can exploit the best
tradeoff and thus further improve the EE. Optimal exhaustive search and
low-complexity norm based ATAS schemes are developed. Through simulations, we
discuss the effect of different parameters on the EE of the MIMO-BC.Comment: submitted for possible publication, 26 pages, 10 figure
Covert Communication with A Full-Duplex Receiver Based on Channel Distribution Information
In this work, we consider a system of covert communication with the aid of a
full-duplex (FD) receiver to enhance the performance in a more realistic
scenario, i.e., only the channel distribution information (CDI) rather than
channel state information (CSI) is known to a warden. Our work shows that
transmitting random AN can improve the covert communication with the infinite
blocklength. Specifically, we jointly design the optimal transmit power and AN
power by minimizing the outage probability at Bob, and we find that the outage
probability decreases and then increases as the maximum allowable AN power
increases. Intuitively, once AN exceeds an optimal value, the performance will
become worse because of the self-interference. The simulation results also show
that the performance behaviors of CDI and CSI are different. When Willie only
knows CDI, there is an optimal AN power that minimizes Bob's outage
probability. However, when Willie knows CSI, the outage probability
monotonically decreases with AN power
On Galerkin Approximations for the Zakai Equation with Diffusive and Point Process Observations
This paper studies Galerkin approximations applied to the Zakai equation of
stochastic filtering. The basic idea of this approach is to project the
infinite-dimensional Zakai equation onto some finite-dimensional subspace
generated by smooth basis functions; this leads to a finite-dimensional system
of stochastic differential equations that can be solved numerically. The
contribution of the paper is twofold. On the theoretical side, existing
convergence results are extended to filtering models with observations of
point-process or mixed type. On the applied side, various issues related to the
numerical implementation of the method are considered; in particular, we
propose to work with a subspace that is constructed from a basis of Hermite
polynomials. The paper closes with a numerical case study.Comment: 28 pages, 7 figure
On realizing Lov\'asz-optimum orthogonal representation in the real Hilbert space
Quantum contextuality is usually revealed by the non-contextual inequality,
which can always be associated with an exclusivity graph. The quantum upper
bound of the inequality is nothing but the Lov\'asz number of the graph. In
this work, we show that if there is a Lov\'asz-optimum orthogonal
representation realized in the -dimensional complex Hilbert space, then
there always exists a corresponding Lov\'asz-optimum orthogonal representation
in the -dimensional real Hilbert space. This in turn completes the
proof that the Lov\'asz-optimum orthogonal representation for any exclusivity
graph can always be realized in the real Hilbert space of suitable dimension
SCMA based resource management of D2D communications for maximum sum-revenue
The device-to-device (D2D) communication is one of the promising technologies
of the future Internet of Things (IoT), but its security-related issues remain
challenging. The block-chain is considered to be a secure and reliable
distributed ledger, so we can treat the device user equipment (D-UE) request
for the reusing resources of cellular user equipment (C-UE) as a transaction
and put it into a transaction pool, then package the record into the
block-chain. In this paper, we study the D2D communication resource allocation
scheme based on sparse code multiple access (SCMA). Firstly, the system's
interference model and block-chain-based transaction flow are analyzed. Then we
propose the optimization problem so that C-UE can get the maximum revenue by
sharing its resources to D-UE. This problem is NP-hard, so we propose a
heuristic algorithm based on semi-definite relaxation (SDR) programming to
solve it. Finally, the performance of the proposed algorithm is verified by
simulation of different system parameters.Comment: This paper was accepted for the WOCC 2019 in Beijin
UAV-Enabled Wireless Power Transfer with Directional Antenna: A Two-User Case
This paper considers an unmanned aerial vehicle (UAV)-enabled wireless power
transfer (WPT) system, in which a UAV equipped with a directional antenna is
dispatched to deliver wireless energy to charge two energy receivers (ERs) on
the ground. Under this setup, we maximize the common (or minimum) energy
received by the two ERs over a particular finite charging period, by jointly
optimizing the altitude, trajectory, and transmit beamwidth of the UAV, subject
to the UAV's maximum speed constraints, as well as the maximum/minimum altitude
and beamwidth constraints. However, the common energy maximization is a
non-convex optimization problem that is generally difficult to be solved
optimally. To tackle this problem, we first ignore the maximum UAV speed
constraints and solve the relaxed problem optimally. The optimal solution to
the relaxed problem reveals that the UAV should hover above two symmetric
locations during the whole charging period, with the corresponding altitude and
beamwidth optimized. Next, we study the original problem with the maximum UAV
speed constraints considered, for which a heuristic hover-fly-hover trajectory
design is proposed based on the optimal symmetric-location-hovering solution to
the relaxed problem. Numerical results validate that thanks to the employment
of directional antenna with adaptive beamwidth and altitude control, our
proposed design significantly improves the common energy received by the two
ERs, as compared to other benchmark schemes.Comment: 6 pages, 3 figures, conferenc
First-principles calculations of current-induced spin-transfer torques in magnetic domain walls
Current-induced spin-transfer torques (STTs) have been studied in Fe, Co and
Ni domain walls (DWs) by the method based on the first-principles noncollinear
calculations of scattering wave functions expanded in the tight-binding
linearized muffin-tin orbital (TB-LMTO) basis. The results show that the
out-of-plane component of nonadiabatic STT in Fe DW has localized form, which
is in contrast to the typical nonlocal oscillating nonadiabatic torques
obtained in Co and Ni DWs. Meanwhile, the degree of nonadiabaticity in STT is
also much greater for Fe DW. Further, our results demonstrate that compared to
the well-known first-order nonadiabatic STT, the torque in the third-order
spatial derivative of local spin can better describe the distribution of
localized nonadiabatic STT in Fe DW. The dynamics of local spin driven by this
third-order torques in Fe DW have been investigated by the
Landau-Lifshitz-Gilbert (LLG) equation. The calculated results show that with
the same amplitude of STTs the DW velocity induced by this third-order term is
about half of the wall speed for the case of the first-order nonadiabatic STT.Comment: 8 pages, 8 figure
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