Experimental Investigation of Longitudinal Space-Time Correlations of the Velocity Field in Turbulent Rayleigh-B\'{e}nard Convection

We report an experimental investigation of the longitudinal space-time cross-correlation function of the velocity field, $C(r,\tau)$, in a cylindrical turbulent Rayleigh-B\'{e}nard convection cell using the particle image velocimetry (PIV) technique. We show that while the Taylor's frozen-flow hypothesis does not hold in turbulent thermal convection, the recent elliptic model advanced for turbulent shear flows [He & Zhang, \emph{Phys. Rev. E} \textbf{73}, 055303(R) (2006)] is valid for the present velocity field for all over the cell, i.e., the isocorrelation contours of the measured $C(r,\tau)$ have a shape of elliptical curves and hence $C(r,\tau)$ can be related to $C(r_E,0)$ via $r_E^2=(r-\beta\tau)^2+\gamma^2\tau^2$ with $\beta$ and $\gamma$ being two characteristic velocities. We further show that the fitted $\beta$ is proportional to the mean velocity of the flow, but the values of $\gamma$ are larger than the theoretical predictions. Specifically, we focus on two representative regions in the cell: the region near the cell sidewall and the cell's central region. It is found that $\beta$ and $\gamma$ are approximately the same near the sidewall, while $\beta\simeq0$ at cell center.Comment: 16 pages, 15 figures, submitted to J. Fluid Mec

Modified Generalized-Brillouin-Zone Theory with On-site Disorders

We study the characterization of the non-Hermitian skin effect (NHSE) in non-Hermitian systems with on-site disorder. We extend the application of generalized-Brillouin-zone (GBZ) theory to these systems. By proposing a modified GBZ theory, we give a faithfully description of the NHSE. For applications, we obtain a unified $\beta$ for system with long-range hopping, and explain the conventional-GBZ irrelevance of the magnetic suppression of the NHSE in the previous study.Comment: 7-pages, 3 figure

Toward a direct measurement of the cosmic acceleration: The first observation of HI 21cm absorption line at FAST

In this work, we report the first result from the investgation of Neutral atomic hydrogen(HI) 21cm absorption line in spectrum of PKS1413+135 as a associated type at redshift $z\approx 0.24670041$ observed by FAST using the observing time of 10 minutes for the absorber and the spectral resolution of the raw data was setted to 10 Hz. The full spectral profile is analysed by fitting the absorption line with single Gaussian function as the resolution of 10kHz in 2MHz bandwidth, eventually intending to illustrate the latest cosmic acceleration by the direct measurement of time evolution of the redshift of HI 21cm absorption line with Hubble flow toward a same background Quasar in the time interval of more than a decade or many years as a detectable signal that produced by the accelerated expansion of the Universe in the era of FAST at low redshift space,namely redshift drift $\dot{z}$ or SL effect. The obtained HI gas column density $\rm N_{HI} \approx 2.2867\times 10^{22}/cm^2$ of this DLA system, much equivalent to the originally observed value $\rm N_{HI} \approx 1.3\times 10^{19}\times(T_s/f)/cm^2$ within the uncertainties of the spin temperature of a spiral host galaxy, and the signal to noise ratio SNR highly reaching 57.4357 for the resolution of 10kHz evidently validates the opportunities of the HI 21cm absorption lines of DLA systems to enforce the awareness of the physical motivation of dark energy by the probe of $\rm\dot{z}$ with the enhancement of accuracy in the level of $\sim 10^{-10}$ per decade.Comment: 26 pages,8 figures, 3 tables, submitted to JCA

Pitch motion control of spar-type floating wind turbines

Wave-induced pitch motion has adverse effects on the power generation of Floating Wind Turbines (FWTs). As a main structure to regulate wind energy capture and power generation, the traditional blade-pitch system is not a good choice for controlling such wave-induced pitch motion. There will be trade-offs when fullfilling multiple objectives with a single control system, and frequent active blade pitching is harmful to the wind turbine's bearing and gear system. Hence, many structural control methods have been proposed to mitigate the motion of floating platforms. The present work proposes an active structural control method utilizing a plate hinged at the bottom of a spar-type floating substructure as an external control device. The damping force provided by the hydraulic Power Take Off (PTO) system is the control force during optimization. The plate is relatively small, but can validly increase the moment of inertia of the hinged system in the pitch direction. The controller is developed under an optimal declutching control framework. In declutching control, the damping of the PTO system is set as a binary function. It is found that current optimal declutching control strategy can effectively minimize the pitch motion of the spar-type substructure and maximize the PTO's power capture at the same time. This indicates that the mechanical energy of spar has been converted into electric energy. The proposed controller is able to reduce the wave-induced pitch motion by 21.1% and increase the power capture by 314.3% at the fixed target frequency

Distinguishing the spins of Pc(4440)P_c(4440) and Pc(4457)P_c(4457) with femtoscopic correlation functions

The spins of the pentaquark states $P_c(4440)$ and $P_c(4457)$ play a decisive role in unraveling their nature, but remain undetermined experimentally. Assuming that they are $\Sigma_c\bar{D}^{*}$ bound states, we demonstrate how one can determine their spins by measuring the $\Sigma_c^+\bar{D}^{(*)0}$ correlation functions. We show that one can use the $\Sigma_c^+\bar{D}^0$ correlation function to fix the size of the Gaussian source and then determine the strength of the $\Sigma_c^+\bar{D}^{*0}$ interaction of spin $1/2$ and $3/2$ and therefore the spins of the $P_c(4440)$ and $P_c(4457)$ states. The method proposed can be applied to decipher the nature of other hadronic molecules and thus deepen our understanding of the non-perturbative strong interaction.Comment: 6 pages, 4 figure

Au impact on GaAs epitaxial growth on GaAs (111)B substrates in molecular beam epitaxy

GaAs growth behaviour under the presence of Au nanoparticles on GaAs {111}(B) substrate is investigated using electron microscopy. It has been found that, during annealing, enhanced Ga surface diffusion towards Au nanoparticles leads to the GaAs epitaxial growth into {113}(B) faceted triangular pyramids under Au nanoparticles, governed by the thermodynamic growth, while during conventional GaAs growth, growth kinetics dominates, resulting in the flatted triangular pyramids at high temperature and the epitaxial nanowires growth at relatively low temperature. This study provides an insight of Au nanoparticle impact on GaAs growth, which is critical for understanding the formation mechanisms of semiconductor nanowires. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792053

LSTM RNN-based excitation force prediction for the real-time control of wave energy converters

Wave energy is a type of abundant and dense renewable energy. Wave force prediction is a critical technology that influences power absorption efficiency in the real-time control of wave energy converter (WEC). Could wave elevation be used to predict wave excitation force directly by training artificial neural network? This method results in rapid and suitable prediction for real-time control. A long short-term memory recurrent neural network (LSTM RNN) algorithm is introduced to identify characteristics of wave excitation forces based on wave elevations. In this method, the wave elevations in front of the structure are measured to obtain sufficient time to actuate the control manipulation. A total of 180 regular wave and 12 irregular wave tests are conducted, and the LSTM RNN model is trained based on the experimental results. The performance of the LSTM algorithm is verified. According to the regular cases in the study, the LSTM prediction can identify high-order harmonic loads, and the anti-noise capability of the LSTM algorithm can filter random noises from the measure signals. In the irregular cases, the LSTM RNN algorithm performs effectively to predict the wave force excited on the structure using wave elevations measured by wave probes. The best combinations of the test setting parameters are determined to guide experimental tests and WEC prototypes