9,553 research outputs found
Dynamical systems analysis of fluid transport in time-periodic vortex ring flows
It is known that the stable and unstable manifolds of dynamical systems theory provide a powerful tool for understanding Lagrangian aspects of time-periodic flows. In this work we consider two time-periodic vortex ring flows. The first is a vortex ring with an elliptical core. The manifolds provide information about entrainment and detrainment of irrotational fluid into and out of the volume transported with the ring. The likeness of the manifolds with features observed in flow visualization experiments of turbulent vortex rings suggests that a similar process might be at play. However, what precise modes of unsteadiness are responsible for stirring in a turbulent vortex ring is left as an open question. The second situation is that of two leapfrogging rings. The unstable manifold shows striking agreement with even the fine features of smoke visualization photographs, suggesting that fluid elements in the vicinity of the manifold are drawn out along it and begin to reveal its structure. We suggest that interpretations of these photographs that argue for complex vorticity dynamics ought to be reconsidered. Recently, theoretical and computational tools have been developed to locate structures analogous to stable and unstable manifolds in aperiodic, or finite-time systems. The usefulness of these analogs is demonstrated, using vortex ring flows as an example, in the paper by Shadden, Dabiri, and Marsden [Phys. Fluids 18, 047105 (2006)]
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Dispersion-engineered silicon nitride waveguides for mid-infrared supercontinuum generation covering the wavelength range 0.8-6.5 mu m
We numerically demonstrate the generation of a mid-infrared supercontinuum (SC) through the design of an on-chip complementary metal oxide semiconductor (CMOS) compatible 10-mm-long air-clad rectangular waveguide made using stoichiometric silicon nitride (Si 3 N 4 ) as the core and MgF 2 glass as its lower cladding. The proposed waveguide is optimized for pumping in both the anomalous and all-normal dispersion regimes. A number of waveguide geometries are optimized for pumping at 1.55 μ m with ultrashort pulses of 50-fs duration and a peak power of 5 kW. By initially keeping the thickness constant at 0.8 μ m, four different structures are engineered with varying widths between 3 μ m and 6 μ m. The largest SC spectral evolution covering a region of 0.8 μ m to beyond 6.5 μ m could be realized by a waveguide geometry with a width of 3 μ m. Numerical analysis shows that increasing width beyond 3 μ m by fixing thickness at 0.8 μ m results in a reduction of the SC extension in the long wavelength side. However, the SC spectrum can be enhanced beyond 6.5 μ m by increasing the waveguide thickness beyond 0.9 μ m with the same peak power and pump source. To the best of our knowledge, this is first time report of a broad SC spectral evolution through numerical demonstration in the mid-infrared region by the silicon nitride waveguide. In the case of all-normal dispersion pumping, a flatter SC spectra can be predicted with the same power and pump pulse but with a reduced bandwidth spanning 950–2100 nm
Clash of Ignorance
The clash of ignorance thesis presents a critique of the clash of civilizations theory. It challenges the assumptions that civilizations are monolithic entities that do not interact and that the Self and the Other are always opposed to each other. Despite some significantly different values and clashes between Western and Muslim civilizations, they overlap with each other in many ways and have historically demonstrated the capacity for fruitful engagement. The clash of ignorance thesis makes a significant contribution to the understanding of intercultural and international communication as well as to the study of inter-group relations in various other areas of scholarship. It does this by bringing forward for examination the key impediments to mutually beneficial interaction between groups. The thesis directly addresses the particular problem of ignorance that other epistemological approaches have not raised in a substantial manner. Whereas the critique of Orientalism deals with the hegemonic construction of knowledge, the clash of ignorance paradigm broadens the inquiry to include various actors whose respective distortions of knowledge symbiotically promote conflict with each other. It also augments the power-knowledge model to provide conceptual and analytical tools for understanding the exploitation of ignorance for the purposes of enhancing particular groups’ or individuals’ power. Whereas academics, policymakers, think tanks, and religious leaders have referred to the clash of ignorance concept, this essay contributes to its development as a theory that is able to provide a valid basis to explain the empirical evidence drawn from relevant cases
Sparse Reconstruction-based Detection of Spatial Dimension Holes in Cognitive Radio Networks
In this paper, we investigate a spectrum sensing algorithm for detecting
spatial dimension holes in Multiple Inputs Multiple Outputs (MIMO)
transmissions for OFDM systems using Compressive Sensing (CS) tools. This
extends the energy detector to allow for detecting transmission opportunities
even if the band is already energy filled. We show that the task described
above is not performed efficiently by regular MIMO decoders (such as MMSE
decoder) due to possible sparsity in the transmit signal. Since CS
reconstruction tools take into account the sparsity order of the signal, they
are more efficient in detecting the activity of the users. Building on
successful activity detection by the CS detector, we show that the use of a
CS-aided MMSE decoders yields better performance rather than using either
CS-based or MMSE decoders separately. Simulations are conducted to verify the
gains from using CS detector for Primary user activity detection and the
performance gain in using CS-aided MMSE decoders for decoding the PU
information for future relaying.Comment: accepted for PIMRC 201
Multiple description video coding for stereoscopic 3D
In this paper, we propose an MDC schemes for stereoscopic 3D video. In the literature, MDC has previously been applied in 2D video but not so much in 3D video. The proposed algorithm enhances the error resilience of the 3D video using the combination of even and odd frame based MDC while retaining good temporal prediction efficiency for video over error-prone networks. Improvements are made to the original even and odd frame MDC scheme by adding a controllable amount of side information to improve frame interpolation at the decoder. The side information is also sent according to the video sequence motion for further improvement. The performance of the proposed algorithms is evaluated in error free and error prone environments especially for wireless channels. Simulation results show improved performance using the proposed MDC at high error rates compared to the single description coding (SDC) and the original even and odd frame MDC
Paper Money Value Change: Comparative Banking Fiqhiyyah Study
This article considered one of the significant issues in Islamicfinancial fiqh because it bears a close relation to financial commitment. Many fiqh treatments from a Sharia perspective were found in Islamic ancient fiqh. This issue was apparent in Islamic banks investment especially in murâbahah. The study concludes that it is a must who owes amount of money and its value changes to high or low to pay the value of money then,and not the actual amount. Furthermore, it shows that the treatment of fluctuations in money value comes by linking its value with some indicators, including stable currency and the linkage to gold or silverDOI: 10.15408/aiq.v5i1.256
Sample, Quantize and Encode: Timely Estimation Over Noisy Channels
The effects of quantization and coding on the estimation quality of
Gauss-Markov processes are considered, with a special attention to the
Ornstein-Uhlenbeck process. Samples are acquired from the process, quantized,
and then encoded for transmission using either infinite incremental redundancy
(IIR) or fixed redundancy (FR) coding schemes. A fixed processing time is
consumed at the receiver for decoding and sending feedback to the transmitter.
Decoded messages are used to construct a minimum mean square error (MMSE)
estimate of the process as a function of time. This is shown to be an
increasing functional of the age-of-information (AoI), defined as the time
elapsed since the sampling time pertaining to the latest successfully decoded
message. Such functional depends on the quantization bits, codewords lengths
and receiver processing time. The goal, for each coding scheme, is to optimize
sampling times such that the long-term average MMSE is minimized. This is then
characterized in the setting of general increasing functionals of AoI, not
necessarily corresponding to MMSE, which may be of independent interest in
other contexts.
We first show that the optimal sampling policy for IIR is such that a new
sample is generated only if the AoI exceeds a certain threshold, while for FR
it is such that a new sample is delivered just-in-time as the receiver finishes
processing the previous one. Enhanced transmissions schemes are then developed
in order to exploit the processing times to make new data available at the
receiver sooner. For both IIR and FR, it is shown that there exists an optimal
number of quantization bits that balances AoI and quantization errors, and
hence minimizes the MMSE. It is also shown that for longer receiver processing
times, the relatively simpler FR scheme outperforms IIR.Comment: Accepted for publication in the IEEE Transactions on Communications.
arXiv admin note: substantial text overlap with arXiv:2004.1298
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