119 research outputs found
Skin friction and heat transfer in hypersonic transitional and turbulent boundary layers
The decompositions of the skin-friction and heat transfer coefficients based
on the two-fold repeated integration in hypersonic transitional and turbulent
boundary layers are analyzed to explain the generations of the wall skin
friction and heat transfer. The Reynolds analogy factor slightly increases as
the wall temperature decreases, especially for the extremely cooled wall. The
integral analysis is applied to explain the overshoot behaviours of the
skin-friction and heat transfer coefficients in hypersonic transitional
boundary layers. The overshoot of the skin-friction coefficient is mainly
caused by the drastic change of the mean velocity profiles, and the overshoot
of the heat transfer coefficient is primarily due to the viscous dissipation.
In the hypersonic turbulent boundary layers, the skin-friction and heat
transfer coefficients increase significantly as the wall temperature decreases.
The effects of the mean velocity gradients and the Reynolds shear stress
contribute dominantly to the wall skin friction, and have weak correlations
with the wall temperature, except for the strongly cooled wall condition. The
strongly cooled wall condition and high Mach number can enhance the effect of
the Reynolds shear stress, and weaken the impact of the mean velocity
gradients. Furthermore, the magnitudes of the dominant relative contributions
of the mean temperature gradients, pressure dilatation, viscous dissipation and
the Reynolds heat flux to the heat transfer coefficient increase as the wall
temperature increases in the hypersonic turbulent boundary layers
Deep learning based autoencoder for m-user wireless interference channel physical layer design
Deep learning (DL) based autoencoder (AE) has been proposed recently as a promising, and potentially disruptive approach to design the physical layer of beyond-5G communication systems. Compared to a traditional communication system with a multiple-block structure, the DL based AE approach provides a new paradigm to physical layer design with a pure data-driven and end-to-end learning based solution. In this paper, we address the dynamic interference in a multi-user Gaussian interference channel. We show that standard constellation are not optimal for this context, in particular, for a high interference condition. We propose a novel adaptive DL based AE to overcome this problem. With our approach, dynamic interference can be learned and predicted, which updates the learning processing for the decoder. Compared to other machine learning approaches, our method does not rely on a fixed training function, but is adaptive and applicable to practical systems. In comparison with the conventional system using n-psk or n-QAM modulation schemes with zero force (ZF) and minimum mean square error (MMSE) equalizer, the proposed adaptive deep learning (ADL) based AE demonstrates a significant achievable BER in the presence of interference, especially in strong and very strong interference scenarios. The proposed approach has laid the foundation of enabling adaptable constellation for 5G and beyond communication systems, where dynamic and heterogeneous network conditions are envisaged
Line Inductance Stability Operation Domain Assessment for Weak Grids With Multiple Constant Power Loads
Weak grids are gaining considerable attention since power generation resources are remote from constant power loads (CPLs), which results in low-frequency/harmonic oscillation. Meanwhile, due to the play, and plug demand of modern power system, the line inductance of weak grids often changes, which is also regarded as the variation regarding short circuit ratio (SCR). Based on this, the conventional impedance-based stability operation point assessment approaches should be expanded into stability domain assessment approach considering the line inductance variation. Therefore, the stability-oriented line inductance stability domain assessment approach for weak grids with CPLs is proposed in this paper. Firstly, the source impedance matrix of weak grid, and load admittance matrix of CPLs are separately built. Secondly, an improved stability forbidden domain criterion is proposed through related mapping transformation process, which has lower conservatism than two previous improved stability criteria. Thirdly, the improved stability forbidden domain criterion is switched into the condition that the intermediate matrices are Hurwitz. Meanwhile, the line inductance stability domain is directly obtained through these intermediate matrices, and guardian map theory. Finally, the simulation, and experiment results illustrate that the proposed approach has less conservatism, and high efficiency.This work was supported by National Key Research, and Development Program of China under Grant 2018YFA0702200
Electron Bunch Train Excited Higher-Order Modes in a Superconducting RF Cavity
Higher-order mode (HOM) based intra-cavity beam diagnostics has been proved
effectively and conveniently in superconducting radio-frequency (SRF)
accelerators. Our recent research shows that the beam harmonics in the bunch
train excited HOM spectrum, which have much higher signal-to-noise ratio than
the intrinsic HOM peaks, may also be useful for beam diagnostics. In this
paper, we will present our study on bunch train excited HOMs, including the
theoretic model and recent experiments carried out based on the DC-SRF
photoinjector and SRF linac at Peking University.Comment: Supported by National Natural Science Foundation of China (11275014
Demonstration of chronometric leveling using transportable optical clocks beyond laser coherence limit
Optical clock network requires the establishment of optical frequency
transmission link between multiple optical clocks, utilizing narrow linewidth
lasers. Despite achieving link noise levels of 10, the final accuracy
is limited by the phase noise of the clock laser. Correlation spectroscopy is
developed to transmit frequency information between two optical clocks
directly, enabling optical clock comparison beyond the phase noise limit of
clock lasers, and significantly enhancing the measurement accuracy or shorten
the measurement time. In this letter, two compact transportable
Ca clocks are employed to accomplish the correlation
spectroscopy comparison, demonstrating an 10 cm level measurement accuracy of
chronometric leveling using a mediocre clock laser with linewidth of 200 Hz.
The relative frequency instability reaches ,
which is about 20 times better than the result with Rabi spectroscopy using the
same clock laser. This research greatly reduces the harsh requirements on the
performance of the clock laser, so that an ordinary stable-laser can also be
employed in the construction of optical clock network, which is essential for
the field applications, especially for the chronometric leveling
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