bluuebunny/crossref_metadata_2025 huggingface dataset:
Compact all‐metallic cavity‐cascaded antenna
Authors: Chang Le , Li Yue , Zhang Zhijun , Feng Zhenghe
Published: February 2016

A compact all‐metallic cavity‐cascaded antenna is presented. A two‐end‐shorted metallic plate is shorted and divided averagely by five identical shorting walls alternatively, forming five alternatively cascaded open cavities. By selecting proper width of the metallic plate and length of the shorting walls, all the five open cavities operate in the TM1n0 (n = 0.5) mode, generating five fringing fields with the same phase. Thus, effective radiation is achieved. The proposed antenna is operated in the 5‐order mode, e.g. five standing waves are distributed along the proposed antenna. It only occupies a volume of 0.43λ0 × 2.83λ0 (λ0 is the wavelength of the centre frequency). Moreover, the proposed antenna is fabricated by all metal, resulting in low cost, light weight, wide bandwidth, high gain and high efficiency. A prototype that can provide an overlapped impedance bandwidth and 3 dB gain bandwidth of 7.72% from 8.10 to 8.75 GHz and a peak gain of 13.37 dBi at 8.5 GHz is built and measured.
Error performance analysis of STBC‐Alamouti and STBC‐CIOD with phase estimation error over Nakagami‐m fading channels
Authors: Ha Daehan , Lee Hoojin , Kang Joonhyuk
Published: February 2016

The symbol pairwise error probability performance of the Alamouti space‐time block code (STBC‐Alamouti) and the STBC from the coordinate interleaved orthogonal design (STBC‐CIOD) over Nakagami‐m fading channels is investigated, especially with residual phase estimation error (PEE). The derived union upper bounds on the average BER are utilised to evaluate the effect of the residual PEE on error performance. Rigorous numerical results demonstrate that both STBCs can asymptotically achieve full‐diversity even with the adverse effect of PEE in the high SNR regime.
Design considerations on wideband envelope termination for high efficiency RF power amplifiers
Authors: Chung S. , Ma R. , Teo K.H.
Published: February 2016

Design approach of wideband envelope termination (ET) for high efficiency RF power amplifiers (PAs) is discussed, considering power loss subject to design constraints including envelope bandwidth, inductor quality factor, and transistor output impedance. Analysis shows that the optimum ET topology depends on the design constraints. For LTE‐advanced transmission with 80 MHz channel bandwidth, a novel wideband ET is designed using an AC‐terminated third‐order LC filter (LCF‐ET), providing 3 dB envelope impedance bandwidth of 150 MHz. Measured results demonstrate 1.75 dB envelope power loss and −39.2 dB normalised root‐mean‐square error with the LTE envelope signals. For the LTE signal with 10.3 dB peak‐to‐average power ratio, the average power efficiency of an ideal Class‐A RF PA with the LCF‐ET prototype is calculated as 33.4% from the measured 1.75 dB envelope power loss, which exceeds 4.7% and 24.0% average power efficiency of conventional Class‐A and Class‐B PAs without ET.
Modelling by using multiphonon emission theory of data retention characteristics in charge trapping flash memory
Authors: Son Y. , Kang M.
Published: February 2016

Data retention characteristics are investigated in charge trapping flash memory. The physical root cause of the non‐Arrhenius behaviour, which is the general retention characteristic in charge trap flash memories, is numerical modelling that the charge loss mechanism is associated with the trap energy level in the charge storage area. For expression of the charge loss in the relatively shallow traps, multiphonon emission model is adopted. Finally, the ratio of the relatively shallow traps to middle and deep level traps is extracted in a sample data.
Visual surveillance using wide‐angle camera and laser range finder
Authors: Song T.‐H. , Ha J.‐E.
Published: February 2016

In this Letter, a new algorithm to reduce false positive in video surveillance by integrating depth using laser range finder with conventional camera is proposed. Typical video surveillance detects foreground objects by background model using only image. Background model is updated using brightness value on image so that it has difficulty in modelling diverse variations caused by illumination changes or scene structure variations. Inherently, there could be many false positives. Hypothesis generation and verification paradigm is adopted by integrating laser range finder with camera on video surveillance. Conventional background updating algorithm is used for generating foreground objects. They are verified by depth value provided by laser range finder. Rotating 2D laser range finder for targeting and generation of look‐up table is used. Experimental results show that proposed algorithm can reduce false positives in video surveillance.
Feedback stabilisation control design for fractional order non‐linear systems in the lower triangular form
Authors: Zhao Yige , Wang Yuzhen , Zhang Xianfu , Li Haitao
Published: February 2016

Using the Lyapunov function method, this study investigates both state and output feedback stabilisation control design problems for fractional order non‐linear systems in the lower triangular form, and presents a number of new results. First, some new properties for Caputo fractional derivative are presented. Second, by introducing appropriate transformations of coordinates, the feedback stabiliser design problem is converted into the determination of finding some parameters, which can be obtained by solving the Lyapunov equation and relevant matrix inequalities. Finally, based on the Lyapunov function method, both state and output feedback stabilisers are explicitly designed to make the closed‐loop system asymptotically stable. The study of an illustrative example shows that the obtained results are effective in designing feedback stabilisers for fractional order non‐linear systems in the lower triangular form.
l2‐norm‐based prior for haze‐removal from single image
Authors: Jha Deepak Kumar , Gupta Bhupendra , Lamba Subir Singh
Published: February 2016

Climatic and atmospheric phenomena – such as haze, fog and smoke – may lead to deterioration of quality and poor scenic clarity of outdoor images. In computer graphics, the authors can model these images as a linear combination of scene radiance, medium transmission and airlight. Several techniques have been proposed to remove the effects of haze from images using this model. The most effective approach for removing the haze effect from a single image is based on dark channel prior. Dark channel prior is based on statistical observation of outdoor images comprising some regions with dark intensity pixels. Here we propose a new l2‐norm‐based prior to generate a dark channel in order to remove the haze from a single‐input image. The dark channel generated using this new prior is more robust and free from the block‐effect. We also propose a statistical technique for airlight estimation of a given image. The proposed technique for modifying the dark channel prior and the airlight estimation are robust techniques as compared with approaches detailed in currently available literature. By combining this modified dark channel and estimated airlight, the haze can be directly removed and a more accurate haze‐free image can be recovered from single‐input hazy image.
Region matching based on colour invariants in rgb orthogonal space
Authors: Cui Xintong , Kan Jiangming , Li Wenbin
Published: February 2016

Illumination influences the performance of region feature matching based on a grey image. A novel region‐matching algorithm based on the colour invariants and colour‐invariant moments in rgb orthogonal colour space is proposed. First, a colour image is converted in RGB colour space to rgb orthogonal colour space that has colour invariance. Second, the colour invariants H and Cλ are calculated. Then, the maximally stable extremal region is extracted from the colour invariants and the colour‐invariant moments are computed. Finally, the nearest neighbour method is used to find corresponding regions. The proposed method can take advantage of both the colour and geometric properties of the images to solve the problem of illumination influences. Experimental results from the Amsterdam Library of Object Images database and images captured on the Beijing Forestry University campus show that performance of the proposed algorithm is better than that of prior art methods.
Image fusion via feature residual and statistical matching
Authors: Wang Li‐juan , Han Jing , Zhang Yi , Bai Lian‐fa
Published: February 2016

In view of the shortcoming of traditional image fusion based on discrete wavelet transform (DWT) with unclear textural information, an effective visible light and infrared image fusion algorithm via feature residual and statistical matching is proposed in this study. First, the source images are decomposed into low‐frequency coefficients and high‐frequency coefficients by DWT. Second, two different fusion schemes are designed for the low‐frequency coefficients and high frequency ones, respectively. The low‐frequency coefficients are fused by a local feature residual‐based scheme to achieve adaptive fusion; the high‐frequency coefficients are accomplished by a local statistical matching‐based scheme to extract the edge information effectively. Finally, the fused image is obtained by inverse DWT. Experimental results demonstrate that the proposed method can produce a more accurate fused image, leading to an improved performance compared with existing methods.
Alternating direction method for TGV‐TGV* based cartoon‐texture image decomposition
Authors: Lu Chengwu , Wang Minghua
Published: February 2016

This study presents a novel method for separating images into piecewise smooth (cartoon) and texture parts, exploiting both the variational mechanism and Yves Meyer's modelling principle for oscillating patterns. The basic idea presented in this study is the use of total generalised variation (TGV) to model the cartoon components, and its dual TGV* for the oscillation components. Moreover then, the proposed model is numerically implemented by using alternating direction method. Comparative experiments show that the proposed model can better separate cartoon from texture and well preserve small‐scale texture information, at the same time reduce efficiently the staircase effects caused by the classical total variation regularisation in the cartoon components.
Adaptive scene‐text binarisation on images captured by smartphones
Authors: Belhedi Amira , Marcotegui Beatriz
Published: February 2016

The authors address, in this study, a new adaptive binarisation method on images captured by smartphones. This work is part of an application for visually impaired people assistance, which aims at making text information accessible to people who cannot read it. The main advantage of the proposed method is that the windows underlying the local thresholding process are automatically adapted to the image content. This avoids the problematic parameter setting of local thresholding approaches, difficult to adapt to a heterogeneous database. The adaptive windows are extracted based on ultimate opening (a morphological operator) and then used as thresholding windows to perform a local Otsu's algorithm. The authors’ method is evaluated and compared with the Niblack, Sauvola, Wolf, toggle mapping morphological segmentation (TMMS) and maximally stable extremal regions methods on a new challenging database introduced by them. Their database is acquired by visually impaired people in real conditions. It contains 4000 annotated characters (available online for research purposes). Experiments show that the proposed method outperforms classical binarisation methods for degraded images such as low‐contrasted or blurred images, very common in their application.
Infrared dim small target detection with high reliability using saliency map fusion
Authors: Nasiri Mahdi , Mosavi Mohammad Reza , Mirzakuchaki Sattar
Published: February 2016

Detection of dim small targets in infrared (IR) images with high reliability is very important in defence systems. In this study, a new method is introduced based on human visual system and saliency maps fusion to detect the small target in IR images with high reliability. By using the static and motion saliency maps fusion, emphasizing the obtained saliencies from one method to another and applying the information and benefits of all maps in the saliency map fusion, this method suppresses the background clutter and noise with high reliability, makes the target more prominent and finally increases the contrast among them. The experiments are carried out on some real‐life data of IR images containing the moving target. The obtained results show the efficiency and robustness of the proposed method so that this method improves the target signal to the background noise and clutter and increases the contrast between them and also detects the small target in IR images with low false alarm rates and high reliability.
High‐efficiency duty‐cycle controlled full‐bridge converter for ultracapacitor chargers
Authors: Woo‐Young Choi
Published: February 2016

This study proposes a high‐efficiency duty‐cycle controlled full‐bridge dc–dc converter for ultracapacitor chargers. By the asymmetric duty‐cycle control, the proposed converter achieves zero‐voltage switching of power switches without circulating currents. It reduces the output current ripple by the current‐doubler rectifier. A control strategy is suggested for charging ultracapacitors. A constant current and constant voltage charging is performed. The proposed converter achieves a higher efficiency compared with the conventional phase‐shift controlled full‐bridge converter. The performance of the proposed converter is evaluated by experimental results for a 30 F ultracapacitor bank.
Optimisation‐based improved switching technique for static VAr compensator considering minimum source harmonic injection
Authors: Das Sankar , Chatterjee Debashis , Goswami Swapan Kumar
Published: February 2016

A selective harmonic elimination and minimisation‐based improved switching technique is proposed for static VAr compensator (SVC) considering minimum source harmonic injection without external filter. Fixed capacitor–insulated gate bipolar transistor controlled reactor (IGBTCR) is used for the proposed SVC model. The switching function of IGBTCR is optimised for minimum reactor voltage total harmonic distortion (THD) which leads to less harmonic injection to line resulting minimisation of source current THD. The best possible optimised switching angles are computed offline using gravitational search algorithm with varying modulation indices to obtain minimum reactor voltage THD. Piecewise mixed model approximation technique is used to store these switching angles as a function of modulation index in the processor memory for online application. The proposed switching significantly improves source current THD compared to conventional switching. Proper simulations as well as experimental results are presented to confirm the proposed scheme.
Output power stabilisation of wireless power transfer system with multiple transmitters
Authors: Tan Linlin , Guo Jinpeng , Huang Xueliang , Wen Feng
Published: February 2016

Wireless power transfer via strongly coupled magnetic resonance is an effective and convenient technology to transfer energy. The output power splitting in close distances and monotonic decay of the output power in longer distances exist when it comes to single‐transmitter case. For the sake of more stable output power in a broader space, the authors proposed a novel wireless power system with multiple transmitters. Further analysis of features of both single‐transmitter and multiple‐transmitter systems was presented with the adoption of MATLAB and High‐Frequency Structure Simulator. The authors demonstrated the superiority of the multiple‐transmitter system and then investigated the impact of transmitter spacing on output power. To stabilise the output power as much as possible, an optimisation strategy is designed by setting transmitter spacing as decision variables and the minimum power variance as the target. The measurement shows that for a load demand of 5 W, a stable area with a standard deviation of 0.32 can be achieved when transmitters are placed 0.33 m apart. Both theoretical and simulation results make it possible to power the movable devices more stably.
Floating small target detection in sea clutter via normalised Doppler power spectrum
Authors: Li Dong‐Chen , Shui Peng‐Lang
Published: February 2016

This paper, proposes a detector based upon normalised Doppler power spectrum (NDPS) is proposed to find floating small targets in sea clutter. Doppler power spectra (DPS) of sea clutter are modelled by a positive stochastic process in Doppler bins. To measure the power fluctuation of sea clutter at each Doppler bin, the NDPS is constructed which equals the DPS of the received time series subtracting the mean function and divided by the standard deviation function of the stochastic process. In view of the Doppler spread when the target return power is integrated within a long observation time, a double detection scheme is developed. The NDPS at the cell under test (CUT) is first estimated from the DPS at the reference range cells and CUT. Then, the first‐stage detection is conducted by the shape‐parameter‐dependent thresholds to yield the thresholded NDPS at individual Doppler bins. The second‐stage detection is made by selective integration of the thresholded NDPS. The proposed detector is compared with the fractal‐based and tri‐feature‐based detectors by using the real high‐resolution sea clutter datasets. The experimental results show that the proposed detector attains better performance than the fractal‐based detector and has comparable performance with the tri‐feature‐based detector but lower computational complexity.
Classification and separation of partial discharge ultra‐high‐frequency signals in a 252 kV gas insulated substation by using cumulative energy technique
Authors: Zhu Ming‐Xiao , Xue Jian‐Yi , Zhang Jia‐Ning , Li Yuan , Deng Jun‐Bo , Mu Hai‐Bao , Zhang Guan‐Jun , Shao Xian‐Jun , Liu Xiao‐Wei
Published: February 2016

Ultra‐high‐frequency (UHF) method is regarded as an effective approach for partial discharge (PD) detection in gas insulated substation (GIS). The feature parameters representing UHF signal characteristics can be extracted from its waveform, and applied to PD source classification and mixed signals separation. In this study, an UHF signal feature extraction algorithm based on cumulative energy (CE) technique is proposed. The CE functions in time domain (TCE) and frequency domain (FCE) are calculated from UHF waveforms and their fast Fourier transform spectrums, respectively. The mathematical morphology gradient (MMG) operation is applied to TCE and FCE to characterise their rising behavior. The feature parameters including width, area and sharpness are extracted from CEs and MMGs in both time and frequency domain. Moreover, the extracted features are applied to UHF signals classification and mixed signals separation. First, experiments on four kinds of typical defects in a 252 kV GIS are performed, and the classification performance of the extracted features is examined with the acquired signals. Second, by clustering the extracted features with fuzzy maximum‐likelihood algorithm, mixed UHF signals originated from multiple PD sources are successfully separated. The results indicate that the proposed features are effective for representing UHF signal characteristics.
Expandable shipboard decoy including adequate RCS by using trihedral corner reflectors
Authors: Yazdani Mianroodi Reza , Heidar Hamid , Mohseni Armaki Hosein
Published: February 2016

The purpose of this study is to design expandable shipboard decoys by using right angle trihedral corner reflectors (TCRs). It is expected that they show larger radar cross section values than the ships themselves in X‐band. Due to practical impacts, they should be made of lightweight materials in rational dimensions. Furthermore it must be realised in expandable structure. Hence, some kind of conductive textiles are used as the base material. The subject covers two different types of scenarios in specific angular sectors and suitable configuration is proposed for each. In each scenario, various types of TCR configurations are introduced to find the best response. Simulations are performed by CST Microwave Studio software tool based on geometrical theory of diffraction method. The optimum configurations of geometrical models with limited dimensions are evaluated based on actual measured data. The simulated and measured results are in very good agreement.
Adaptive predictive error filter‐based maximum power point tracking algorithm for a photovoltaic system
Authors: Subudhi Bidyadhar , Pradhan Raseswari
Published: February 2016

This study presents a new adaptive predictive error filter‐based maximum power point tracker (MPPT) for a photovoltaic (PV) system. This MPPT is developed using the concept of an adaptive predictive filter (APEF) that consists of a one‐tap finite impulse response. The filter step‐size of the APEF is adapted using a recursive least‐square (RLS) algorithm with an adaptive forgetting factor. The specialty of this MPPT is that it performs MPP tracking operation in a single step. It performs two functions: namely, MPP estimation adjusting operating point of a boost converter at MPP and then filtering of the PV voltage fluctuation after MPPT action. Thus, the proposed MPPT is compact and efficient with less computational complexities than existing MPPTs such as perturb and observe (P&O) and incremental conductance. The filter part of the proposed RLS‐APEF‐MPPT is a discrete PID‐controller, where PD‐part is tuned using pole‐placement strategy and integral‐term is fixed. The performances of the proposed RLS‐APEF‐MPPT were verified using experimentation on a prototype PV system. Its performances were compared with that of P&O, adaptive‐P&O‐MPPTs. From the observed results, it is confirmed that the proposed MPPT exhibits superior performance.
Modelling the flow of a second order fluid through and over a porous medium using the volume averages. II. The stress boundary condition
Authors: Minale Mario
Published: February 2016

In this paper, a stress boundary condition at the interface between a porous medium saturated by a viscoelastic fluid and the free viscoelastic fluid is derived. The volume averages are used to upscale the problem. The boundary condition is obtained on the assumption that the free fluid stress is transferred partially to the fluid within the porous medium and partially to the solid skeleton. To this end the momentum balance on the solid skeleton saturated by the viscoelastic fluid is derived and a generalised Biot’s equation is obtained, which is coupled with the generalised Brinkman’s equation derived in Part I of the paper. They together state that the whole stress carried by the porous medium, sum of that of the fluid and that of the solid skeleton, is not dissipated. The boundary condition here derived does not show any stress jump and as in Part I, to emphasize the effect of elasticity, a second order fluid of Coleman and Noll is considered as viscoelastic fluid. Also the stress boundary condition at the interface between a homogeneous solid and the porous medium saturated by the viscoelastic fluid is obtained.
Dissipation in monotonic and non-monotonic relaxation to equilibrium
Authors: Petersen Charlotte F. , Evans Denis J. , Williams Stephen R.
Published: February 2016

Using molecular dynamics simulations, we study field free relaxation from a non-uniform initial density, monitored using both density distributions and the dissipation function. When this density gradient is applied to colour labelled particles, the density distribution decays to a sine curve of fundamental wavelength, which then decays conformally towards a uniform distribution. For conformal relaxation, the dissipation function is found to decay towards equilibrium monotonically, consistent with the predictions of the relaxation theorem. When the system is initiated with a more dramatic density gradient, applied to all particles, non-conformal relaxation is seen in both the dissipation function and the Fourier components of the density distribution. At times, the system appears to be moving away from a uniform density distribution. In both cases, the dissipation function satisfies the modified second law inequality, and the dissipation theorem is demonstrated.
Spontaneous cavitation in a Lennard-Jones liquid: Molecular dynamics simulation and the van der Waals-Cahn-Hilliard gradient theory
Authors: Baidakov Vladimir G.
Published: February 2016

The process of bubble nucleation in a Lennard-Jones (LJ) liquid is studied by molecular dynamics (MD) simulation. The bubble nucleation rate J is determined by the mean life-time method at temperatures above that of the triple point in the region of negative pressures. The results of simulation are compared with classical nucleation theory (CNT) and modified classical nucleation theory (MCNT), in which the work of formation of a critical bubble is determined in the framework of the van der Waals-Cahn-Hilliard gradient theory (GT). It has been found that the values of J obtained in MD simulation systematically exceed the data of CNT, and this excess in the nucleation rate reaches 8–10 orders of magnitude close to the triple point temperature. The results of MCNT are in satisfactory agreement with the data of MD simulation. To describe the properties of vapor-phase nuclei in the framework of GT, an equation of state has been built up which describes stable, metastable and labile regions of LJ fluids. The surface tension of critical bubbles γ has been found from CNT and data of MD simulation as a function of the radius of curvature of the surface of tension R*. The dependence γ(R*) has also been calculated from GT. The Tolman length has been determined, which is negative and in modulus equal to ≈(0.1 − 0.2) σ. The paper discusses the applicability of the Tolman formula to the description of the properties of critical nuclei in nucleation.
A monolayer of hierarchical silver hemi-mesoparticles with tunable surface topographies for highly sensitive surface-enhanced Raman spectroscopy
Authors: Zhu Shuangmei , Fan Chunzhen , Mao Yanchao , Wang Junqiao , He Jinna , Liang Erjun , Chao Mingju
Published: February 2016

We proposed a facile green synthesis system to synthesize large-scale Ag hemi-mesoparticles monolayer on Cu foil. Ag hemi-mesoparticles have different surface morphologies on their surfaces, including ridge-like, meatball-like, and fluffy-like shapes. In the reaction, silver nitrate was reduced by copper at room temperature in dimethyl sulfoxide via the galvanic displacement reaction. The different surface morphologies of the Ag hemi-mesoparticles were adjusted by changing the reaction time, and the hemi-mesoparticle surface formed fluffy-spherical nanoprotrusions at longer reaction time. At the same time, we explored the growth mechanism of silver hemi-mesoparticles with different surface morphologies. With 4-mercaptobenzoic acid as Raman probe molecules, the fluffy-like silver hemi-mesoparticles monolayer with the best activity of surface enhanced Raman scattering (SERS), the enhancement factor is up to 7.33 × 107 and the detection limit can reach 10−10M. SERS measurements demonstrate that these Ag hemi-mesoparticles can serve as sensitive SERS substrates. At the same time, using finite element method, the distribution of the localized electromagnetic field near the particle surface was simulated to verify the enhanced mechanism. This study helps us to understand the relationship between morphology Ag hemi-mesoparicles and the properties of SERS.
Pressure cell for transport measurements under high pressure and low temperature in pulsed magnetic fields
Authors: Braithwaite Daniel , Knafo William , Settai Rikio , Aoki Dai , Kurahashi Shuhei , Flouquet Jacques
Published: February 2016

We present a new specially designed pressure cell and technique adapted for resistivity measurements in pulsed magnetic fields up to 60 T at pressures up to at least 4 GPa, and temperatures down to 1.5 K. We show that heating effects during the pulse are acceptable (less than 1 K) and can be corrected allowing reliable temperature dependences of the magnetoresistance to be obtained. We illustrate the performance with a study of the phase diagram of the heavy fermion antiferromagnet CeRh2Si2.
Experimental dynamic trapping of electrostatically actuated bistable micro-beams
Authors: Medina Lior , Gilat Rivka , Robert Ilic B. , Krylov Slava
Published: February 2016

We demonstrate a dynamic snap-through from a primary to a secondary statically inaccessible stable configuration in single crystal silicon, curved, doubly clamped micromechanical beam structures. The nanoscale motion of the fabricated bistable micromechanical devices was transduced using a high speed camera. Our experimental and theoretical results collectively show that the transition between the two stable states was solely achieved by a tailored time dependent electrostatic actuation. Fast imaging of the micromechanical motion allowed for direct visualization of dynamic trapping at the statically inaccessible state. These results further suggest that our direct dynamic actuation transcends prevalent limitations in controlling geometrically non-linear microstructures, and may have applications extending to multi-stable, topologically optimized micromechanical logic and non-volatile memory architectures.
On the scaling of the slip velocity in turbulent flows over superhydrophobic surfaces
Authors: Seo Jongmin , Mani Ali
Published: February 2016

Superhydrophobic surfaces can significantly reduce hydrodynamic skin drag by accommodating large slip velocity near the surface due to entrapment of air bubbles within their micro-scale roughness elements. While there are many Stokes flow solutions for flows near superhydrophobic surfaces that describe the relation between effective slip length and surface geometry, such relations are not fully known in the turbulent flow limit. In this work, we present a phenomenological model for the kinematics of flow near a superhydrophobic surface with periodic post-patterns at high Reynolds numbers. The model predicts an inverse square root scaling with solid fraction, and a cube root scaling of the slip length with pattern size, which is different from the reported scaling in the Stokes flow limit. A mixed model is then proposed that recovers both Stokes flow solution and the presented scaling, respectively, in the small and large texture size limits. This model is validated using direct numerical simulations of turbulent flows over superhydrophobic posts over a wide range of texture sizes from L+ ≈ 6 to 310 and solid fractions from ϕs = 1/9 to 1/64. Our report also embarks on the extension of friction laws of turbulent wall-bounded flows to superhydrophobic surfaces. To this end, we present a review of a simplified model for the mean velocity profile, which we call the shifted-turbulent boundary layer model, and address two previous shortcomings regarding the closure and accuracy of this model. Furthermore, we address the process of homogenization of the texture effect to an effective slip length by investigating correlations between slip velocity and shear over pattern-averaged data for streamwise and spanwise directions. For L+ of up to O(10), shear stress and slip velocity are perfectly correlated and well described by a homogenized slip length consistent with Stokes flow solutions. In contrast, in the limit of large L+, the pattern-averaged shear stress and slip velocity become uncorrelated and thus the homogenized boundary condition is unable to capture the bulk behavior of the patterned surface.
Controllable superferromagnetism-induced exchange bias effect in goethite microparticles after zero-field-cooling process
Authors: Ma Ji , Chen Kezheng
Published: February 2016

In this study, goethite microparticles (1.5–5 μm in diameter) were used to investigate the abnormal exchange bias effect after zero-field cooling process. It was found that the superferromagnetic mechanism, which is of great relevance for the superparamagnetic hematite impurities, was the root cause of such exchange-bias effect. Most notably, one can readily control this effect by simply regulating the reaction time during synthesis. It is anticipated that this controllable and intriguing superferromagnetism-induced exchange-bias effect may not only open a new direction to re-examine the conventional exchange-bias mechanism but also has practical applications in magnetic tunnel junctions and high-density memories.
The influence of structural characteristics on the electronic and thermal properties of GaN/AlN core/shell nanowires
Authors: Pavloudis Th. , Termentzidis K. , Komninou Ph. , Latham C. D. , Briddon P. R. , Kioseoglou J.
Published: February 2016

Interatomic potential based molecular dynamics and ab initio calculations are employed to investigate the structural, thermal, and electronic properties of polar GaN/AlN core/shell nanowires. Nanowire models for the molecular dynamics simulations contain hundreds of thousands of atoms with different shell-to-nanowire ratios. The energetic and structural properties are evaluated through a detailed examination of the strain, the stress, and the displacement fields. It is found that the relaxation of the AlN shell is initiated at the edges, with the shell becoming increasingly stress free when the shell-to-nanowire ratio is increased. The basal lattice parameter a of the AlN shell is found to have a smaller value than the value predicted by the elasticity theory. The stresses on the GaN core are strongly influenced by the shell. The core retains the a lattice parameter of bulk GaN only up to a shell-to-nanowire ratio equal to 0.10 and is significantly compressed beyond this point. Concerning the thermal properties, the molecular dynamics simulations conclude that there is a linear relationship between the thermal conductivity and the shell-to-core area ratio of the GaN/AlN core/shell nanowires. The bandgaps of the nanowires are calculated through ab initio calculations of 103 atoms and the influence of the structural characteristics on the electronic properties is investigated. A well-defined relationship that predicts the bandgap of the GaN/AlN nanowires, follows the 2nd order Vegard's law and taking into account the shell-to-nanowire ratio, is established. Finally, the valence band maximum is found to be dominated by the surface N-2p levels, while the conduction band minimum is dominated by the core and interface Ga-3s, and the surface Al-2s levels.
Simulating two-dimensional infrared-Raman and Raman spectroscopies for intermolecular and intramolecular modes of liquid water
Authors: Ito Hironobu , Tanimura Yoshitaka
Published: February 2016

Full classical molecular dynamics (MD) simulations of two-dimensional (2D) infrared-Raman and 2D Raman spectroscopies of liquid water were carried out to elucidate a mode-mode coupling mechanism using a polarizable water model for intermolecular and intramolecular vibrational spectroscopy (POLI2VS). This model is capable of describing both infrared and Raman spectra. Second-order response functions, which consist of one molecular polarizability and two molecular dipole moments for 2D IR-Raman and three molecular polarizabilities for 2D Raman spectroscopies, were calculated using an equilibrium-non-equilibrium hybrid MD approach. The obtained signals were analyzed using a multi-mode Brownian oscillator (BO) model with nonlinear system-bath interactions representing the intramolecular OH stretching, intramolecular HOH bending, hydrogen bonded (HB)-intermolecular librational motion and HB-intermolecular vibrational (translational) motion of liquid water. This model was applied through use of hierarchal Fokker-Planck equations. The qualitative features of the peak profiles in the 2D spectra obtained from the MD simulations are accurately reproduced with the BO model. This indicates that this model captures the essential features of the intermolecular and intramolecular motion. We elucidate the mechanisms governing the 2D signal profiles involving anharmonic mode-mode coupling, the nonlinearities of the polarizability and dipole moment, and the vibrational dephasing processes of liquid water even in the case that the 2D spectral peaks obtained from the MD simulation overlap or are unclear. The mode coupling peaks caused by electrical anharmonic coupling (EAHC) and mechanical anharmonic coupling (MAHC) are observed in all of the 2D spectra. We find that the strength of the MAHC between the OH-stretching and HB-intermolecular vibrational modes is comparable to that between the OH-stretching and HOH bending modes. Moreover, we find that this OH-stretching and HB-intermolecular vibrational coupling should be observed as off-diagonal cross peaks in the 2D spectra.
FALCON: A method for flexible adaptation of local coordinates of nuclei
Authors: König Carolin , Hansen Mads Bøttger , Godtliebsen Ian H. , Christiansen Ove
Published: February 2016

We present a flexible scheme for calculating vibrational rectilinear coordinates with well-defined strict locality on a certain set of atoms. Introducing a method for Flexible Adaption of Local COordinates of Nuclei (FALCON) we show how vibrational subspaces can be “grown” in an adaptive manner. Subspace Hessian matrices are set up and used to calculate and analyze vibrational modes and frequencies. FALCON coordinates can more generally be used to construct vibrational coordinates for describing local and (semi-local) interacting modes with desired features. For instance, spatially local vibrations can be approximately described as internal motion within only a group of atoms and delocalized modes can be approximately expressed as relative motions of rigid groups of atoms. The FALCON method can support efficiency in the calculation and analysis of vibrational coordinates and energies in the context of harmonic and anharmonic calculations. The features of this method are demonstrated on a few small molecules, i.e., formylglycine, coumarin, and dimethylether as well as for the amide-I band and low-frequency modes of alanine oligomers and alpha conotoxin.
Impact of nitrogen doping of niobium superconducting cavities on the sensitivity of surface resistance to trapped magnetic flux
Authors: Gonnella Dan , Kaufman John , Liepe Matthias
Published: February 2016

Future particle accelerators such as the SLAC “Linac Coherent Light Source-II” (LCLS-II) and the proposed Cornell Energy Recovery Linac require hundreds of superconducting radio-frequency (SRF) niobium cavities operating in continuous wave mode. In order to achieve economic feasibility of projects such as these, the cavities must achieve a very high intrinsic quality factor (Q0) to keep cryogenic losses within feasible limits. To reach these high Q0's in the case of LCLS-II, nitrogen-doping of niobium cavities has been selected as the cavity preparation technique. When dealing with Q0's greater than 1 × 1010, the effects of ambient magnetic field on Q0 become significant. Here, we show that the sensitivity to RF losses from trapped magnetic field in a cavity's walls is strongly dependent on the cavity preparation. Specifically, standard electropolished and 120 °C baked cavities show a sensitivity of residual resistance from trapped magnetic flux of ∼0.6 and ∼0.8 nΩ/mG trapped, respectively, while nitrogen-doped cavities show a higher sensitivity of residual resistance from trapped magnetic flux of ∼1 to 5 nΩ/mG trapped. We show that this difference in sensitivities is directly related to the mean free path of the RF surface layer of the niobium: shorter mean free paths lead to less sensitivity of residual resistance to trapped magnetic flux in the dirty limit (ℓ ≪ ξ0), while longer mean free paths lead to lower sensitivity of residual resistance to trapped magnetic flux in the clean limit (ℓ ≫ ξ0). These experimental results are also shown to have good agreement with recent theoretical predictions for pinned vortex lines oscillating in RF fields.
Projection on Proper elements for code control: Verification, numerical convergence, and reduced models. Application to plasma turbulence simulations
Authors: Cartier-Michaud T. , Ghendrih P. , Sarazin Y. , Abiteboul J. , Bufferand H. , Dif-Pradalier G. , Garbet X. , Grandgirard V. , Latu G. , Norscini C. , Passeron C. , Tamain P.
Published: February 2016

The Projection on Proper elements (PoPe) is a novel method of code control dedicated to (1) checking the correct implementation of models, (2) determining the convergence of numerical methods, and (3) characterizing the residual errors of any given solution at very low cost. The basic idea is to establish a bijection between a simulation and a set of equations that generate it. Recovering equations is direct and relies on a statistical measure of the weight of the various operators. This method can be used in any number of dimensions and any regime, including chaotic ones. This method also provides a procedure to design reduced models and quantify its ratio of cost to benefit. PoPe is applied to a kinetic and a fluid code of plasma turbulence.
Fully magnetized plasma flow in a magnetic nozzle
Authors: Merino Mario , Ahedo Eduardo
Published: February 2016

A model of the expansion of a plasma in a magnetic nozzle in the full magnetization limit is presented. The fully magnetized and the unmagnetized-ions limits are compared, recovering the whole range of variability in plasma properties, thrust, and plume efficiency, and revealing the differences in the physics of the two cases. The fully magnetized model is the natural limit of the general, 2D, two-fluid model of Ahedo and Merino [Phys. Plasmas 17, 073501 (2010)], and it is proposed as an analytical, conservative estimator of the propulsive figures of merit of partially magnetized plasma expansions in the near region of the magnetic nozzle.
Manipulation of the spatial distribution of laser-accelerated proton beams by varying the laser intensity distribution
Authors: Aurand B. , Senje L. , Svensson K. , Hansson M. , Higginson A. , Gonoskov A. , Marklund M. , Persson A. , Lundh O. , Neely D. , McKenna P. , Wahlström C.-G.
Published: February 2016

We report on a study of the spatial profile of proton beams produced through target normal sheath acceleration using flat target foils and changing the laser intensity distribution on the target front surface. This is done by either defocusing a single laser pulse or by using a split-pulse setup and irradiating the target with two identical laser pulses with variable spatial separation. The resulting proton beam profile and the energy spectrum are recorded as functions of the focal spot size of the single laser pulse and of the separation between the two pulses. A shaping of the resulting proton beam profile, related to both an increase in flux of low-energy protons in the target normal direction and a decrease in their divergence, in one or two dimensions, is observed. The results are explained by simple modelling of rear surface sheath field expansion, ionization, and projection of the resulting proton beam.
Tunable dielectric properties and excellent microwave absorbing properties of elliptical Fe3O4 nanorings
Authors: Tong Guoxiu , Liu Yun , Cui Tingting , Li Yana , Zhao Yanting , Guan Jianguo
Published: February 2016

Elliptical Fe3O4 nanorings (NRs) with continuously tunable axes that range from 40 nm to 145 nm in length were prepared through a precursor-directed synthetic route to determine the electromagnetic responses generated at 2–18 GHz. The tunability of the dielectric properties of Fe3O4 NRs depends on the long axis rather than on the specific surface area, internal stress, and grain size. Elliptical Fe3O4 NRs exhibit the excellent microwave absorbing properties due to the unique ring-like configuration, which significantly enhances permittivity, multiple scattering, oscillation resonance absorption, microantenna radiation, and interference. These findings indicate that ring-like nanostructures are promising for devising effective microwave absorbers.
Empirical error correction and feature identification for long term wind resource assessment using support vector regression
Authors: Dunstan T. , Lock A. , Skea A.
Published: February 2016

A technique that makes use of supervised machine learning (support vector regression) to correct systematic errors in long-term model wind time series is presented. The technique—Empirical Wind Output Correction (EWOC)—is computationally efficient and is particularly effective over complex terrain. In addition to error correction, EWOC acts as a non-linear feature identification tool by ranking the input site characteristics according to their ability to reduce the overall error. Independent regression functions are constructed for the mean and variance of the wind speed using an observational training data set comprising 109 onshore sites throughout Europe. Corrections are applied to Virtual Met Mast™ (VMM) corrected time series, hindcast Numerical Weather Prediction output (Euro4), and raw Modern-Era Retrospective analysis for Research and Applications and ECMWF Re-Analysis (ERA)-interim reanalysis data. Reductions of approximately 40% in the Mean Absolute Error of the mean and variance of the wind speed compared to the source data are obtained. The quality of fit of the EWOC corrected wind speed distributions is examined through their Kullback-Leibler Divergences (KLD). Reductions of approximately 40% and 60% in the KLD are achieved compared to VMM corrected and raw ERA data, respectively. Possible methods for improving and extending the technique are suggested, including ways of maximising the size and quality of the training data set using a limited set of wind speed observations.
On the long-waves dispersion in Yukawa systems
Authors: Khrapak Sergey A. , Klumov Boris , Couëdel Lénaïc , Thomas Hubertus M.
Published: February 2016

A useful simplification of the quasilocalized charge approximations (QLCA) method to calculate the dispersion relations in strongly coupled Yukawa fluids is discussed. In this simplified version, a simplest possible model radial distribution function, properly related to the thermodynamic properties of the system, is used. The approach demonstrates good agreement with the dispersion relations obtained using the molecular dynamics simulations and the original QLCA in the long-wavelength regime.
Kinetic analysis of the non-isothermal crystallization process, magnetic and mechanical properties of FeCoBSiNb and FeCoBSiNbCu bulk metallic glasses
Authors: Ramasamy Parthiban , Stoica Mihai , Taghvaei A. H. , Prashanth K. G. , Kumar Ravi , Eckert Jürgen
Published: February 2016

The crystallization kinetics of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 and {[(Fe0.5Co0.5)0.75B0.2Si0.05]0.96Nb0.04}99.5Cu0.5 bulk metallic glasses were evaluated using differential scanning calorimetry under non-isothermal condition. The fully glassy rods with diameters up to 2 mm were obtained by copper mold injection casting. Both glasses show good thermal stability, but the addition of only 0.5% Cu completely changes the crystallization behavior. The average activation energy required for crystallization decreases from 645 kJ/mol to 425 kJ/mol after Cu addition. Upon heating, the Cu-free alloy forms only the metastable Fe23B6 phase. In contrast, two well-separated exothermic events are observed for the Cu-added bulk glassy samples. First, the (Fe,Co) phase nucleates and then (Fe,Co)2B and/or (Fe,Co)3B crystallize from the remaining glassy matrix. The Cu-added alloy exhibits a lower coercivity and a higher magnetic saturation than the base alloy, both in as-cast as well as in annealed condition. Besides, the Cu-added glassy sample with 2 mm diameter exhibits a maximum compressive fracture strength of 3913 MPa together with a plastic strain of 0.6%, which is highest plastic strain ever reported for 2 mm diameter ferromagnetic bulk metallic glass sample. Although Cu addition improves the magnetic and mechanical properties of the glass, it affects the glass-forming ability of the base alloy.