Combustion noise Dowling, Ann P.; Mahmoudi, Yasser
Proceedings of the Combustion Institute,
2015, 2015-00-00, 20150101, Letnik:
35, Številka:
1
Journal Article
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Combustion noise is becoming increasingly important as a major noise source in aeroengines and ground based gas turbines. This is partially because advances in design have reduced the other noise ...sources, and partially because next generation combustion modes burn more unsteadily, resulting in increased external noise from the combustion. This review reports recent progress made in understanding combustion noise by theoretical, numerical and experimental investigations. We first discuss the fundamentals of the sound emission from a combustion region. Then the noise of open turbulent flames is summarized. We subsequently address the effects of confinement on combustion noise. In this case not only is the sound generated by the combustion influenced by its transmission through the boundaries of the combustion chamber, there is also the possibility of a significant additional source, the so-called ‘indirect’ combustion noise. This involves hot spots (entropy fluctuations) or vorticity perturbations produced by temporal variations in combustion, which generate pressure waves (sound) as they accelerate through any restriction at the exit of the combustor. We describe the general characteristics of direct and indirect noise. To gain further insight into the physical phenomena of direct and indirect sound, we investigate a simple configuration consisting of a cylindrical or annular combustor with a low Mach number flow in which a flame zone burns unsteadily. Using a low Mach number approximation, algebraic exact solutions are developed so that the parameters controlling the generation of acoustic, entropic and vortical waves can be investigated. The validity of the low Mach number approximation is then verified by solving the linearized Euler equations numerically for a wide range of inlet Mach numbers, stagnation temperature ratios, frequency and mode number of heat release fluctuations. The effects of these parameters on the magnitude of the waves produced by the unsteady combustion are investigated. In particular the magnitude of the indirect and direct noise generated in a model combustor with a choked outlet is analyzed for a wide range of frequencies, inlet Mach numbers and stagnation temperature ratios. Finally, we summarize some of the unsolved questions that need to be the focus of future research.
In this corrigendum we correct an error in our paper T. Caraballo, R. Colucci, J. López-de-la-Cruz and A. Rapaport. A way to model stochastic perturbations in population dynamics models with bounded ...realizations, Commun Nonlinear Sci Numer Simulat, 77(2019) 239–257. We present a correct way to model real noisy perturbations by considering a slightly different stochastic process based, as in the original paper, on the Ornstein-Uhlenbeck process. Namely, we correct the formulae that generates the noisy realizations to ensure the boundedness property to be satisfied with probability one (which turns out not to be true in our original paper even though it was observed in all the simulations).
The methods for remote sensing image (RSI) scene classification based on deep convolutional neural networks (DCNNs) have achieved prominent success. However, confronted with adversarial examples ...obtained by adding imperceptible perturbations to clean images, the great vulnerability of DCNNs makes it worth exploring effective defense methods. To date, numerous countermeasures for adversarial examples have been proposed, but how to improve the defensive ability for unknown attacks still to be answered. To address this issue, in this article, we propose an effective defense framework specified for RSI scene classification, named perturbation-seeking generative adversarial networks (PSGANs). In brief, a new training framework is designed to train the classifier by introducing the examples generated during the image reconstruction process, in addition to clean examples and adversarial ones. These generated examples can be random kinds of unknown attacks during training and thus are utilized to eliminate the blind spots of a classifier. To assist the proposed training framework, a reconstruction method is developed. First, instead of modeling the distribution of clean examples, we model the distributions of the perturbations added in adversarial examples. Second, to make a tradeoff between the diversity of the reconstructed examples and the optimization of PSGAN, a scale factor named seeking radius is introduced to scale the generated perturbations before they are subtracted by the given adversarial examples. Comprehensive and extensive experimental results on three widely used benchmarks for RSI scene classification demonstrate the great effectiveness of PSGAN when faced with both known and unknown attacks. Our source code is available at https://github.com/xuxiangsun/PSGAN .
Multiresonance (MR)‐induced thermally activated delayed fluorescence (TADF) emitters based on B‐ and N‐embedded polycyclic aromatics are desirable for ultrahigh‐definition organic light‐emitting ...diodes (OLEDs) due to their high photoluminescence quantum yield (PLQY) and narrow bandwidth. But the reverse intersystem crossing (RISC) rates of MR‐TADF emitters are usually small, resulting in severe device efficiency roll‐off at high brightness. To solve this issue, a sensitizer for the MR‐TADF emitter has been required. Herein, a new MR‐TADF emitter is developed through coordination of Au with B/N‐embedded polycyclic ligand. Benefitting from the Au perturbation, the RISC rate is dramatically accelerated to 2.3 × 107 s−1, leading to delayed fluorescence lifetime as short as 4.3 µs. Meanwhile, the PLQY of 95% and full width at half maximum of 39 nm (0.18 eV) are essentially unchanged after metal coordination. Therefore, a high PLQY, short delayed fluorescence lifetime, and high color purity are concurrently realized in a single TADF emitter. Accordingly, vacuum‐deposited OLEDs exhibit high‐performance electroluminescence with a maximum external quantum efficiency (EQE) of 35.8% without sensitization. The EQE is maintained as high as 32.3% at 10 000 cd m−2. Furthermore, solution‐processed OLED based on the emitter also achieves excellent performance with a maximum EQE of 25.7% and a small efficiency roll‐off.
A metal‐perturbation approach is developed for boosting the reverse intersystem crossing rate of multiresonance type B–N molecule by utilizing the spin–orbit coupling effect of a gold atom. The resultant emitter exhibits a short‐lived thermally activated delayed fluorescence, which enables the fabrication of high‐performance nonsensitized organic light‐emitting diode with a high external quantum efficiency, little efficiency roll‐off, and good color purity.
This paper formulates a maximum power point tracking (MPPT) technique that accurately tracks the maximum power point of photovoltaic (PV), which undergoes simultaneous or independent sudden changes ...in load resistance and irradiance. The proposed algorithm is fundamentally divided into three parts; current and voltage perturbation algorithm (IPA/VPA), perturbation step-size reduction algorithm (PSSRA) and a deviation avoidance loop. The use of dual perturbation parameters using IPA and VPA ensures high tracking speed. PSSRA iteratively reduces the perturbation step-size of IPA and VPA and helps in reducing the power oscillations around MPP. Finally, a deviation avoidance loop is developed to detect a change in irradiance by examining the sign of the slope of the two power curves, namely P-V and P-I. The algorithm compares the per unit change in voltage and current of PV, to determine a simultaneous change in both irradiance and load resistance. The proposed algorithm is compared with two recently developed MPPT algorithms. The results show that the proposed technique can track the MPP with high speed and low steady-state oscillations and does not deviate from the MPP tracking path regardless of fast changes in irradiance and load resistance.
Transparency and guaranteed safety are important requirements in the design of wearable exoskeleton actuators for individuals who have lower limb deficits but still maintain a certain level of ...voluntary motor control. Specifically, precision in torque delivery timing and magnitude, robustness, disturbance rejection, and repeatability are desired in the actuator design and control. Motivated by these needs, this study aims to develop a series of elastic actuators with clutch (SEAC) that can precisely generate the desired assistance in terms of both timing and torque magnitude for a wearable hip exoskeleton and guarantee the wearer's safety at the same time. The proposed mechanical design improves actuator transparency and safety by a mechanical clutch that automatically disengages the transmission when needed. A new torque control for the SEAC, based on singular perturbation theory with flexible compensation techniques, is proposed to precisely control the assistive torque by rejecting the undesired human motion disturbance. The mechanical design of the proposed device and the design of a singular perturbation control algorithm are discussed, and the SEAC performance is verified by experiments. Experimental results, derived from a test with a human subject, are presented to demonstrate the precision of the assistive torque and timing control of the SEAC while interacting with a human wearer.
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