358,263 research outputs found
Turbulence and surface heat transfer near the stagnation point of a circular cylinder in turbulent flow
A turbulent boundary layer flow analysis of the momentum and thermal flow fields near the forward stagnation point due to a circular cylinder in turbulent cross flow is presented. Turbulence modeling length scale, anisotropic turbulence initial profiles and boundary conditions were identified as functions of the cross flow turbulence intensity and the boundary layer flow far field velocity. These parameters were used in a numerical computational procedure to calculate the mean velocity, mean temperature, and turbulence double correlation profiles within the flow field. The effects of the cross flow turbulence on the stagnation region momentum and thermal flow fields were investigated. This analysis predicted the existing measurements of the stagnation region mean velocity and surface heat transfer rate with cross flow Reynolds number and turbulence intensity less than 250,000 and 0.05, respectively
Density Evolution for Asymmetric Memoryless Channels
Density evolution is one of the most powerful analytical tools for
low-density parity-check (LDPC) codes and graph codes with message passing
decoding algorithms. With channel symmetry as one of its fundamental
assumptions, density evolution (DE) has been widely and successfully applied to
different channels, including binary erasure channels, binary symmetric
channels, binary additive white Gaussian noise channels, etc. This paper
generalizes density evolution for non-symmetric memoryless channels, which in
turn broadens the applications to general memoryless channels, e.g. z-channels,
composite white Gaussian noise channels, etc. The central theorem underpinning
this generalization is the convergence to perfect projection for any fixed size
supporting tree. A new iterative formula of the same complexity is then
presented and the necessary theorems for the performance concentration theorems
are developed. Several properties of the new density evolution method are
explored, including stability results for general asymmetric memoryless
channels. Simulations, code optimizations, and possible new applications
suggested by this new density evolution method are also provided. This result
is also used to prove the typicality of linear LDPC codes among the coset code
ensemble when the minimum check node degree is sufficiently large. It is shown
that the convergence to perfect projection is essential to the belief
propagation algorithm even when only symmetric channels are considered. Hence
the proof of the convergence to perfect projection serves also as a completion
of the theory of classical density evolution for symmetric memoryless channels.Comment: To appear in the IEEE Transactions on Information Theor
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Valuation of cash flows under random rates of interest: A linear algebraic approach
This paper reformulates the classical problem of cash flow valuation under stochastic discount factors into a system of linear equations with random perturbations. Using convergence results, a sequence of uniform approximations is developed. The new formulation leads to a general framework for deriving approximate statistics of cash flows for a broad class of models of stochastic interest rate process. We show applications of the proposed method by pricing default-free and defaultable cash flows. The methodology developed in this paper is applicable to a variety of uncertain cash flow analysis problems
Low-Complexity Reduced-Rank Beamforming Algorithms
A reduced-rank framework with set-membership filtering (SMF) techniques is
presented for adaptive beamforming problems encountered in radar systems. We
develop and analyze stochastic gradient (SG) and recursive least squares
(RLS)-type adaptive algorithms, which achieve an enhanced convergence and
tracking performance with low computational cost as compared to existing
techniques. Simulations show that the proposed algorithms have a superior
performance to prior methods, while the complexity is lower.Comment: 7 figure
Quantum vortex dynamics in two-dimensional neutral superfluids
We derive an effective action for the vortex position degree-of-freedom in a
superfluid by integrating out condensate phase and density fluctuation
environmental modes. When the quantum dynamics of environmental fluctuations is
neglected, we confirm the occurrence of the vortex Magnus force and obtain an
expression for the vortex mass. We find that this adiabatic approximation is
valid only when the superfluid droplet radius , or the typical distance
between vortices, is very much larger than the coherence length . We go
beyond the adiabatic approximation numerically, accounting for the quantum
dynamics of environmental modes and capturing their dissipative coupling to
condensate dynamics. For the case of an optical-lattice superfluid we
demonstrate that vortex motion damping can be adjusted by tuning the ratio
between the tunneling energy and the on-site interaction energy . We
comment on the possibility of realizing vortex Landau level physics.Comment: 14 pages, 10 figures, accepted by PRA with corrected references and
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Skin friction reduction by slot injection at Mach 0.8
Surface skin friction, boundary layer profiles and turbulent intensity due to axially symmetric tangential slot injection into a transonic boundary layer were measured. Effects of slot height, multiple slot injection, and injection mass flow rate on the surface skin friction downstream of the the slot have been investigated. Surface skin friction was a function of the injection mass flow rate for x/s 40. Large normal pressure gradient and relatively large turbulent intensity were found near the slot with small injection mass flow rate; the region the high turbulent intensity moved downstream with increasing injection mass flow rate. The results with two slot injections indicated that the distance between slots should be less than 30 slot heights in order to achieve some benefits from the first slot. Of significant importance in the present investigation is that the skin friction reduction obtained at transonic speed is of the same order as obtained in the hypersonic regime. Additional work is required in order to formulate a correlation between the turbulent intensity and the injection mass flow rate that may be used in future analysis
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