bluuebunny/crossref_metadata_2025 huggingface dataset:
Computational analysis of compressibility effect on flow field and aerodynamics at low Reynolds numbers
Authors: Mukohara Hiroki , Anyoji Masayuki
Published: May 2022

The flow around the wing of exploration aircraft flying in the very dilute Martian atmosphere experiences very specific low-Reynolds-number compressible flows. A fundamental understanding of compressibility effects, especially on a separation bubble, is an important issue in the aerodynamic wing design. We investigated the effect of compressibility on the laminar separation bubbles and aerodynamic properties of a blunt flat plate with a thickness of 5% of the chord length in unsteady two-dimensional laminar and large-eddy simulations at chord-based Reynolds numbers (Re) of 6100 and 11 000 in the Mach number (M) range from M = 0.1 to 0.8. In the incompressible flow below M = 0.2 at Re = 11 000, the separated shear layer rolled up due to the Kelvin–Helmholtz (KH) instability reattaches to the surface with turbulent transition and then forms a laminar separation bubble. In contrast, with increasing Mach number, the transition and reattachment are delayed, and the separation bubble is stretched. In particular, at M = 0.8, the rolled-up vortex is not distorted in the span direction and advects downstream with a strong spanwise two-dimensionality. This is due to the compressibility effect that suppresses the KH instability. On the other hand, at Re = 6100, the separated shear layer is stable without spanwise distortion even in the incompressible flow due to the stronger influence of viscous forces. With increasing Mach number, such stable separated shear layer reattaches without rolling up further downstream, and the Strouhal number of the vortex also gradually decreases. This leads that the large lift oscillation owing to periodic and two-dimensional spanwise vortex shedding is significantly suppressed with an increasing Mach number. This is also due to the weakening of the KH instability due to the compressibility effects. However, at Re = 11 000, the reattachment points move downstream more significantly with the increasing Mach number, indicating that the suppression of the KH instability due to the compressibility effect is more pronounced at Re = 11 000 than at Re = 6100. Consequently, the negative skin friction shear stress owing to the reverse flow inside the expanded separation bubble becomes relatively large, decreasing the total drag with the increasing Mach number.
Collective Thomson scattering diagnostic with in situ calibration system for velocity space analysis in large helical device
Authors: Nishiura Masaki , Adachi Shun , Tanaka Kenji , Kubo Shin , Kenmochi Naoki , Shimozuma Takashi , Yanai Ryoma , Saito Teruo , Nuga Hideo , Seki Ryosuke
Published: May 2022

A collective Thomson scattering (CTS) diagnostic with a ±3 GHz band around a 77 GHz gyrotron probe beam was developed to measure the velocity distribution of bulk and fast ions in high-temperature plasmas. We propose a new in situ calibration method for a CTS diagnostic system combined with a raytracing code. The method is applied in two situations for electron cyclotron emission in plasmas and in a CTS diagnostic with a modulated probe beam. Experimental results highlight the importance of refraction correction in probe and receive beams. The CTS spectrum is measured with the in situ calibrated CTS receiver and responds to fast ions originating from a tangential neutral beam with an energy of 170 keV and from a perpendicular beam with an energy of 60 keV, both in the large helical device. From a velocity space analysis model, the results elucidate the measured anisotropic CTS spectrum caused by fast ions. The calibration methods and analyses demonstrated here are essential for CTS, millimeter-wave diagnostics, and electron cyclotron heating required under fusion reactor conditions.
Interface enhanced precessional damping in spintronic multilayers: A perspective
Authors: Swindells C. , Atkinson D.
Published: May 2022

In the past two decades, there have been huge developments in the understanding of damping in multilayered thin films and, more generally, in spin-transport in spintronic systems. In multilayered ferromagnetic (FM)/non-magnetic (NM) thin-film systems, observations of ferromagnetic resonant precession show a strong increase in the fundamental damping when the FM thin films are layered with heavy metals, such as Pt. These observations led to significant theoretical developments, dominated by the “spin-pumping” formalism, which describes the enhancement of damping in terms of the propagation or “pumping” of spin-current across the interface from the precessing magnetization into the heavy metal. This paper presents a perspective that introduces the key early experimental damping results in FM/NM systems and outlines the theoretical models developed to explain the enhanced damping observed in these systems. This is followed by a wider discussion of a range of experimental results in the context of the theoretical models, highlighting agreement between the theory and experiment, and more recent observations that have required further theoretical consideration, in particular, with respect to the role of the interfaces and proximity-induced magnetism in the heavy metal layer. The Perspective concludes with an outline discussion of spin-pumping in the broader context of spin-transport.
Spintronic sources of ultrashort terahertz electromagnetic pulses
Authors: Seifert Tom S. , Cheng Liang , Wei Zhengxing , Kampfrath Tobias , Qi Jingbo
Published: May 2022

Spintronic terahertz emitters are broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current conversion, and current-to-field conversion at terahertz rates. In this Editorial, we review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.
Asymptotic charges for spin-1 and spin-2 fields at the critical sets of null infinity
Authors: Magdy Ali Mohamed Mariem , Valiente Kroon Juan A.
Published: May 2022

The asymptotic charges of spin-1 and spin-2 fields are studied near spatial infinity. We evaluate the charges at the critical sets where spatial infinity meets null infinity with the aim of finding the relation between the charges at future and past null infinities. To this end, we make use of Friedrich’s framework of the cylinder at spatial infinity to obtain asymptotic expansions of the Maxwell and spin-2 fields near spatial infinity, which are fully determined in terms of initial data on a Cauchy hypersurface. With expanding the initial data in terms of spin-weighted spherical harmonics, it is shown that only a subset of the initial data, which satisfy certain regularity conditions, gives rise to well-defined charges at the point where future (past) infinity meets spatial infinity. Given such initial data, the charges are shown to be fully expressed in terms of the freely specifiable part of the data. Moreover, it is shown that there exists a natural correspondence between the charges defined at future and past null infinities.
A stick–slip linear actuator with high speed and nano-resolution by resonance/non-resonance hybrid driving
Authors: Ning Peng , Qiao Guangda , Xia Xiao , Lu Xiaohui , Cheng Tinghai
Published: May 2022

To achieve high speed, nano-resolution, and large stroke, a resonance/non-resonance hybrid piezoelectric stick–slip actuator with a lever-type flexure hinge (LTFH-PSSA) is proposed in this work. The actuator can achieve high speed and large stroke in the resonance mode by the stick–slip working principle and achieve nano-resolution in the non-resonant mode by the direct drive working principle. The excitation electrical signals used in the two working modes are the sine waveform and half-sine waveform, respectively. Compared with the traditional sawtooth waveform, the excitation signal of the sine and half-sine waveforms have no sudden change of voltage, which are more conducive to reduce the impact and vibration of the system. Moreover, a series of static analysis and modal analysis of the stator are carried out by the finite element method. The experimental system is built to test the output characteristics of the LTFH-PSSA. In the resonance state by the stick–slip working principle, the impedance analysis and frequency characteristic test of the LTFH-PSSA are carried out, which states that the tested resonance frequency agrees well with the simulated ones. When the locking force, the voltage, and the frequency are 2 N, 100 Vp–p, and 1850 Hz, the speed of the LTFH-PSSA is up to 52.71 mm/s, and the backward motion is suppressed completely as well. In the non-resonance state, the resolution can reach 2.19 nm and 2.69 nm in the forward and backward motion, respectively. So far, the proposed actuator ranks first in speed and resolution among all reported LTFH-PSSAs.
Strain rate effects on the yielding strength and maximum temperature at shear bands in a Zr-based bulk metallic glass
Authors: Jiao Zhiming , Li Kuo , Wang Zhong , Wang Zhihua , Qiao Junwei , Liaw Peter K.
Published: May 2022

The effects of strain rate on the yielding strength and maximum temperature at shear bands in a typical Zr41.2Ti13.8Ni10Cu12.5Be22.5 (Vit 1) bulk metallic glass are investigated under tension and compression over a wide range of strain rates at ambient temperature. Using the modified cooperative shear model incorporating the notable internal thermal effect at high strain rates, the transition of the strain rate effect of yielding strength from the sudden decrease to the subsequent slow change with increasing the strain rate is quantitatively characterized. The fracture surface temperature evolution under different shear band evolution times is captured by a hierarchical multi-scale model of heat conduction. Dynamic strain rates shorten the shear band evolution time, leading to an increase in the maximum temperature at shear bands compared to quasi-static loadings.
Numerical investigation of laser-driven shock interaction with a deformable particle
Authors: Acharya N. , Aluie H. , Shang J. K.
Published: May 2022

A laser-driven shock propagating through an isolated particle embedded in a plastic (CH) target was studied using the radiation-hydrodynamic code FLASH. Preliminary simulations using IONMIX equations of state (EOS) showed significant differences in the shock Hugoniot of aluminum compared to experimental data in the low-pressure regime [O(10) GPa], resulting in higher streamwise compression and deformation of an aluminum particle. Hence, a simple modification to the ideal gas EOS was developed and employed to describe the target materials and examine the particle dynamics. The evolution of the pressure field demonstrated a complex wave interaction, resulting in a highly unsteady particle drag which featured two drag minima due to shock focusing at the rear end of the particle and rarefaction stretching due to laser shut-off. Although ∼30% lateral expansion and ∼25% streamwise compression were observed, the aluminum particle maintained considerable integrity without significant distortion. Additional simulations examined the particle response for a range of particle densities, sizes, and acoustic impedances. The results revealed that lighter particles such as aluminum gained significant momentum, reaching up to ∼96% of the shocked CH's speed, compared to ∼29% for the heavier tungsten particles. Despite the differences seen in the early stage of shock interaction, particles with varying acoustic impedances ultimately reached the same peak velocity. This identified particle-to-host density ratio is an important factor in determining the inviscid terminal velocity of the particle. In addition, the modified EOS model presented in this study could be used to approximate solid materials in hydrocodes that lack material strength models.
Molecular qubits based on photogenerated spin-correlated radical pairs for quantum sensing
Authors: Mani Tomoyasu
Published: May 2022

Photogenerated spin-correlated radical pairs (SCRPs) in electron donor–bridge–acceptor (D–B–A) molecules can act as molecular qubits and inherently spin qubit pairs. SCRPs can take singlet and triplet spin states, comprising the quantum superposition state. Their synthetic accessibility and well-defined structures, together with their ability to be prepared in an initially pure, entangled spin state and optical addressability, make them one of the promising avenues for advancing quantum information science. Coherence between two spin states and spin selective electron transfer reactions form the foundation of using SCRPs as qubits for sensing. We can exploit the unique sensitivity of the spin dynamics of SCRPs to external magnetic fields for sensing applications including resolution-enhanced imaging, magnetometers, and magnetic switch. Molecular quantum sensors, if realized, can provide new technological developments beyond what is possible with classical counterparts. While the community of spin chemistry has actively investigated magnetic field effects on chemical reactions via SCRPs for several decades, we have not yet fully exploited the synthetic tunability of molecular systems to our advantage. This review offers an introduction to the photogenerated SCRPs-based molecular qubits for quantum sensing, aiming to lay the foundation for researchers new to the field and provide a basic reference for researchers active in the field. We focus on the basic principles necessary to construct molecular qubits based on SCRPs and the examples in quantum sensing explored to date from the perspective of the experimentalist.
Generation of polarized proton beams with gaseous targets from CO2-laser-driven collisionless shock acceleration
Authors: Yan Xue , Wu Yitong , Geng Xuesong , Zhang Hui , Shen Baifei , Ji Liangliang
Published: May 2022

We propose obtaining polarized proton beams based on CO2-laser-driven collisionless shock acceleration (CSA) of the pre-polarized HCl gas. By tailoring the density profile of the pre-polarized HCl gas, the intense CO2 laser pulse heats the plasma target and forms a strong shock that reflects the polarized protons to high energy. According to particle-in-cell simulations implemented with the spin dynamics, directional proton beams of several MeV were generated at a total beam polarization of over 80%. Simulations showed that proton spin precession occurred in the azimuthal magnetic fields generated by the Biermann effect and plasma currents. The latter was the main depolarization mechanism in the early stage of shock wave formation. For CSA at CO2 laser intensities around 1017–1018 W/cm2, the proton depolarization was insignificant and the beam polarization purity was preserved. As pre-polarized hydrogen targets were available at gaseous densities in-state-of-art facilities, CSA driven by relatively long wavelength lasers provided a feasible solution for obtaining ultra-fast polarized proton sources.
Comparison of body definitions for incremental vibrational configuration interaction theory (iVCI)
Authors: Schröder Benjamin , Rauhut Guntram
Published: May 2022

Within incremental vibrational configuration interaction theory (iVCI), the vibrational state energy is determined by means of a many-body expansion, i.e., it is a sum of terms of increasing order, which allow for an embarrassingly parallel evaluation. The convergence of this expansion depends strongly on the definition of the underlying bodies, which essentially decompose the correlation space into fragments. The different definitions considered here comprise mode-based bodies, excitation level-based bodies, and energy-based bodies. An analysis of the convergence behavior revealed that accounting for resonances within these definitions is mandatory and leads to a substantial improvement of the convergence, that is, the expansions can be truncated at lower orders. Benchmark calculations and systematic comparisons of the different body definitions for a small set of molecules, i.e., ketene, ethene, and diborane, have been conducted to study the overall performance of these iVCI implementations with respect to accuracy and central processing unit time.
Experimental and numerical investigation of compressibility effects on velocity derivative flatness in turbulence
Authors: Yamamoto K. , Ishida T. , Watanabe T. , Nagata K.
Published: May 2022

Compressibility effects on the velocity derivative flatness F∂u′/∂x are investigated by experiments with opposing arrays of piston-driven synthetic jet actuators (PSJAs) and direct numerical simulations (DNS) of statistically steady compressible isotropic turbulence and temporally evolving turbulent planar jets with subsonic or supersonic jet velocities. Experiments using particle image velocimetry show that nearly homogeneous isotropic turbulence is generated at the center of a closed box from interactions between supersonic synthetic jets. The dependencies of F∂u′/∂x on the turbulent Reynolds number Reλ and the turbulent Mach number MT are examined both experimentally and using DNS. Previous studies of incompressible turbulence indicate a universal relationship between F∂u′/∂x and Reλ. However, both experiments and DNS confirm that F∂u′/∂x increases relative to the incompressible turbulence via compressibility effects. Although F∂u′/∂x tends to be larger with MT in each flow, the F∂u′/∂x in the turbulent jets and the turbulence generated from PSJAs deviate from those in incompressible turbulence at lower MT compared with isotropic turbulence sustained by a solenoidal forcing. The PSJAs and supersonic planar jets generate strong pressure waves, and the wave propagation can cause an increased F∂u′/∂x, even at low MT. These results suggest that the compressibility effects on F∂u′/∂x are not solely determined from a local value of MT and depend on the turbulence generation process.
Electrostatic tunability of charged, binary nanoparticle assemblies in dielectric colloidal systems
Authors: Sarkar Tamal , Kemp Brandon A. , Sheppard Cheyenne J.
Published: May 2022

Charged nanoparticles exhibit anomalous electrostatic interactions, which can lead to stable, non-touching equilibria in inverted dielectric systems. In this study, we analytically demonstrate minimum-potential energy configurations for binary systems of charged nanoparticles and control of constituent spacing by externally applied electrostatic fields. The field–matter interactions are governed by the electrostatic forces of high order multipoles induced by the charged nanoparticles submerged in dielectric liquids. The particles bind in non-touching configurations due to electrostatic potential wells for each particle induced by other dissimilar particles in their vicinity. Such binary systems are proposed as building blocks with the potential of electromagnetic tunability of novel photonic surfaces.
Two-dimensional parametric design of short pulse laser driven iron opacity experiments
Authors: London Richard A. , Colón Quiñones Roberto A. , Martin Madison E. , Whitley Heather D.
Published: May 2022

High intensity short pulse lasers are commonly used to create high temperature and high density plasmas. These plasmas are used to study fundamental properties such as the equation of state and opacity. The targets often use small-diameter, thin buried layers of the materials of interest, surrounded by a tamper of low atomic number material such as plastic. Computational modeling is used to design and interpret experiments with short pulse lasers. Most of the modeling to date has assumed one-dimensional plane-parallel geometry. In this paper, the effects of radial gradients in the irradiation of thin planar targets are studied with one- and two-dimensional radiation/hydrodynamic simulations. It is found that the main effect of radial gradients is the averaging of plasma conditions and x-ray emission over the pattern of irradiation. Differences between one- and two-dimensional simulations arise because the plasma conditions and x-ray emission are, in general, nonlinear functions of the temperature of the plasma and thereby also nonlinear functions of the irradiation intensity. The differences increase with the ratio of the buried layer radius to the laser spot radius. The root mean square difference in the inferred iron L-shell opacity is less than 30% when the ratio is less than 1.0 and about 70% when the ratio is 1.5.
Reconstruction spectra of multi MeV ions coupled to absolute spectral index
Authors: Roshan M. V. , Razaghi S. , Yap S. L.
Published: May 2022

Shock wave-assisted κ-distribution of the ion spectra enables kinetic particle simulation to reconstruct particular ion spectrum with an index of spectra, n, which is the best-known parameter to adopt standard ion distribution. It is proved that the simulation spectral index of n≅4.51 is consistent with the experimental data, and the spectrum was exceptionally reproduced.
Realization of homojunction PN AlN diodes
Authors: Ahmad Habib , Engel Zachary , Matthews Christopher M. , Lee Sangho , Doolittle W. Alan
Published: May 2022

Aluminum nitride (AlN) is an insulator that has shown little promise to be converted to a semiconductor via impurity doping. Some of the historic challenges for successfully doping AlN include a reconfigurable defect formation known as a DX center and subsequent compensation that causes an increase in dopant activation energy resulting in very few carriers of electricity, electrons, or holes, rendering doping inefficient. Using crystal synthesis methods that generate less compensating impurities and less lattice expansion, thus impeding the reconfiguration of dopants, and using new dopants, we demonstrate: (a) well behaved bulk semiconducting functionality in AlN, the largest direct bandgap semiconductor known with (b) substantial bulk p-type conduction (holes = 3.1 × 1018 cm−3, as recently reported in our prior work), (c) dramatic improvement in n-type bulk conduction (electrons = 6 × 1018 cm−3, nearly 6000 times the prior state-of-the-art), and (d) a PN AlN diode with a nearly ideal turn-on voltage of ∼6 V for a 6.1 eV bandgap semiconductor. A wide variety of AlN-based applications are enabled that will impact deep ultraviolet light-based viral and bacterial sterilization, polymer curing, lithography, laser machining, high-temperature, high-voltage, and high-power electronics.
On self-dual Yang–Mills fields on special complex surfaces
Authors: Araneda Bernardo
Published: May 2022

We derive a generalization of the flat space equations of Yang and Newman for self-dual Yang–Mills fields to (locally) conformally Kähler Riemannian four-manifolds. The results also apply to Einstein metrics (whose full curvature is not necessarily self-dual). We analyze the possibility of hidden symmetries in the form of Bäcklund transformations, and we find a continuous group of hidden symmetries only for the case in which the geometry is conformally half-flat. No isometries are assumed.
Spatiotemporal dynamics of biocrust and vegetation on sand dunes
Authors: Yizhaq H. , Ashkenazy Y.
Published: May 2022

We propose a model to study at the first time the spatiotemporal dynamics of the coupling between biocrust and vegetation cover on sand dunes; previous studies modeled the temporal dynamics of vegetation-biocrust-sand system while other focused only on the spatiotemporal dynamics of vegetation on sand dunes, excluding the effect of biocrust. The model consists of two coupled partial nonlinear differential equations and includes diffusion and advection terms for modeling the dispersal of vegetation and biocrust and the effect of wind on them. In the absence of spatial variability, the model exhibits self-sustained relaxation oscillations and regimes of bistability–the first state is dominated by biocrust and the second by vegetation. We concentrate on the one-dimensional dynamics of the model and show that the front that connects these two states propagates mainly due to the wind advection. In the oscillatory regime the front propagation is complex and very interesting compared to the non-spatial relaxation oscillations. For low wind DP (drift potential) values, a series of spatially oscillatory domains develops as the front advances downwind. These domains form due to the oscillations of the spatially homogeneous states away from the front. However, for higher DP values, the dynamics is much more complex, becoming very sensitive to the initial conditions and exhibiting an irregular spatial pattern as small domains are created and annihilated during the front advance. The irregular spatiotemporal dynamics reported here seems to be unique, at least in the context of vegetation dynamics and possibly also in context of other dynamical systems.
A two-stage reconstruction method for complex networked system with hidden nodes
Authors: Deng Wenfeng , Yang Chunhua , Huang Keke , Wu Wenhan
Published: May 2022

Reconstructing the interacting topology from measurable data is fundamental to understanding, controlling, and predicting the collective dynamics of complex networked systems. Many methods have been proposed to address the basic inverse problem and have achieved satisfactory performance. However, a significant challenge arises when we attempt to decode the underlying structure in the presence of inaccessible nodes due to the partial loss of information. For the purpose of improving the accuracy of network reconstruction with hidden nodes, we developed a robust two-stage network reconstruction method for complex networks with hidden nodes from a small amount of observed time series data. Specifically, the proposed method takes full advantage of the natural sparsity of complex networks and the potential symmetry constraints in dynamic interactions. With robust reconstruction, we can not only locate the position of hidden nodes but also precisely recover the overall network structure on the basis of compensated nodal information. Extensive experiments are conducted to validate the effectiveness of the proposed method and superiority compared with ordinary methods. To some extent, this work sheds light on addressing the inverse problem, of which the system lacks complete exploration in the network science community.
Basis-set correction for coupled-cluster estimation of dipole moments
Authors: Traore Diata , Toulouse Julien , Giner Emmanuel
Published: May 2022

The present work proposes an approach to obtain a basis-set correction based on density-functional theory (DFT) for the computation of molecular properties in wave-function theory (WFT). This approach allows one to accelerate the basis-set convergence of any energy derivative of a non-variational WFT method, generalizing previous works on the DFT-based basis-set correction where either only ground-state energies could be computed with non-variational wave functions [Loos et al., J. Phys. Chem. Lett. 10, 2931 (2019)] or properties could be computed as expectation values over variational wave functions [Giner et al., J. Chem. Phys. 155, 044109 (2021)]. This work focuses on the basis-set correction of dipole moments in coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)], which is numerically tested on a set of 14 molecules with dipole moments covering two orders of magnitude. As the basis-set correction relies only on Hartree–Fock densities, its computational cost is marginal with respect to the one of the CCSD(T) calculations. Statistical analysis of the numerical results shows a clear improvement of the basis convergence of the dipole moment with respect to the usual CCSD(T) calculations.
Optical and strain stabilization of point defects in silicon carbide
Authors: Dietz Jonathan R. , Hu Evelyn L.
Published: May 2022

The photoluminescence and spin properties of ensembles of color centers in silicon carbide are enhanced by fabricating optically isolated slab waveguide structures and carefully controlling annealing and cooling conditions. We find that the photoluminescence signal of an ensemble of implanted defects is enhanced in slab waveguides by an order of magnitude over identically implanted bulk defects. The slab waveguide-enhanced photoluminescence of several defect species is used to study recombination and diffusion in the presence of thermal annealing with both rapid quench cooling and a longer return to ambient conditions. The confined mechanical geometry of a thin film is exploited to measure the spin-strain coupling of the negatively charged silicon monovacancy. The methods in this work can be used to exercise greater control on near-surface emitters in silicon carbide and better understand and control the effects of strain on spin measurements of silicon carbide based color centers.
Effect of vorticity transport on flow structure in the tip region of axial compressors
Authors: Hou Jieuxuan , Liu Yangwei , Zhong Luyang , Zhong Weibo , Tang Yumeng
Published: May 2022

Numerical simulations are carried out to investigate the flow structure in the blade tip region of axial compressors. Various tip clearance heights and end wall motion conditions in a linear compressor cascade are studied to assess the effect of vorticity transport on the tip leakage flow (TLF). Moreover, the effect of vorticity transport on the TLF in a compressor rotor at different operating conditions is studied using delayed detached eddy simulation. The results show that the vorticity transport at both the blade tip and the end wall plays an important role in the roll-up and evolution of the tip leakage vortex (TLV), resulting in great impacts on the loss and stability of the TLV. It is found that the TLV is composed of a two-layer structure. The inner vortex core region formed by the vorticity transport from the blade tip shear layer to the TLV has a great effect on the strength and loss of the vortex, and the structure of the outer shear layer is altered by the secondary vortex formed by the vorticity transport from the end wall shear layer and thus affects the stability of the TLV. By the mechanism of the vorticity transport, the effects of the clearance height, the end wall motion, and the non-uniform clearance as a control method can be explained uniformly. The new understanding of the TLF structure and the vorticity transport mechanism helps to improve the performance of axial compressors by controlling the vorticity transport of the TLF.
Wake modes of a freely vibrating square cylinder
Authors: Sen Subhankar
Published: May 2022

The cross-stream flow-induced vibrations of a square cylinder of mass ratio, m*=10, are studied numerically at a fixed Reynolds number, Re, of 250. The reduced speed, U*, is varied from 1 to 10 independent of Re. The flow-induced vibrations of a square cylinder have been previously investigated either by decoupling U* from Re or by coupling U* with Re. While most of the studies available in the literature follow the former approach, those dealing with m*=10 do not provide a detailed account of the branches of dynamic response, hysteresis, and wake patterns. The current effort aims at contributing to these research gaps. The vibrations are purely vortex-induced, and the dynamic response within synchronization is found to be composed of an initial branch, its extension or extended initial branch, and lower branch. For a square-section oscillator, the extended initial branch is resolved for the first time. The most noteworthy outcome of this work is perhaps the resolution of asymmetric as well as one-sided wake modes at certain reduced speeds. The one-sided shedding occurs either from the top or bottom surface of the oscillator. At U*=4.6, the non-zero mean lift changes sign in successive oscillation cycles, indicating that the solutions are bistable. The resolved asymmetric and one-sided modes are associated with positive and negatives values of mean lift, respectively. A very interesting result of this study is the mismatch of wake modes obtained at non-hysteretic U*=4.6 using forward and backward computations.
A compact portable photoacoustic spectroscopy sensor for multiple trace gas detection
Authors: Zhang Xueshi , Liu Lixian , Zhang Le , Yin Xukun , Huan Huiting , Zhang Lu , Shao Xiaopeng
Published: May 2022

A compact, highly sensitive, and cost-effective photoacoustic spectroscopic sensor for multiple trace gas detection was reported with a self-designed compact distributed feedback laser array and a 2-channel dual phase field programmable gate array based digital lock-in amplifier, only twentieth in size and weight of the commercial instruments. The capability of the portable sensor was verified by measuring CH4 and C2H2 simultaneously. With an integrated nonlocal means algorithm denoising, noise-equivalent concentrations were achieved as 6.89 ppb for CH4 and 2.74 ppb for C2H2, respectively, corresponding to a normalized noise equivalent absorption coefficient of 1.1 × 10−10 cm−1 W/Hz−1/2, a 79 folds higher than the original data. The results demonstrate that the developed minimized sensor has the potential for sensitive and portable measurement of multiple trace gases.
Oxide glasses under pressure: Recent insights from experiments and simulations
Authors: Du Tao , Sørensen Søren S. , To Theany , Smedskjaer Morten M.
Published: May 2022

Deciphering the structure–property relations of densified oxide glasses is a problem of longstanding interest. For example, it is important for understanding the fracture mechanism under sharp contact loading as well as fabricating glasses with tunable physical characteristics. Recent advances in both experimental and simulation techniques have prompted research breakthroughs in understanding the response of glasses to high pressure. In this Perspective, we first briefly discuss the facilities for the high-pressure treatment of glasses, including in situ and ex situ investigations. The recent work on pressure-induced structural changes of archetypical oxide glass families (silicates, germanates, borates, aluminates, phosphates) is discussed and compared to the changes in macroscopic properties induced by densification, as densification treatment can be used to produce oxide glasses with improved hardness, stiffness, and toughness. We also discuss the new insights from atomistic simulations combined with topological analysis tools to unravel the densification mechanism of oxide glasses on the medium-range order length scale. Drawing on these recent studies, we clarify how densification treatment has proved to be an important tool to both understand the disordered nature of glasses and tune their physical properties, although many open questions and challenges remain that require further investigations.
Effects and mitigation of pulsed power radiation on optical fiber velocimetry
Authors: Dolan D. H. , Payne S. , Bell K. , Fox B. , Moore N. W.
Published: May 2022

Optical fiber diagnostics are extensively used in pulsed power experiments, such as the Sandia Z machine. However, radiation produced in a pulsed power environment can significantly affect these measurements. Catastrophic fiber darkening may be mitigated with shielding, but no flexible material can stop all radiation produced by the machine and/or target. Radiation-induced refractive index modulations are particularly challenging for optical interferometry. Several approaches for radiation-tolerant photonic Doppler velocimetry are discussed here.
Elastic–plastic transition of compressional shocks in a perfect 2D Yukawa crystal
Authors: Shen Jue , Lu Shaoyu , Sun Tianyue , Feng Yan
Published: May 2022

Molecular dynamical simulations are performed to systematically investigate the elastic–plastic transition of compressional shocks in a perfect two-dimensional Yukawa crystal. Following the tradition in the theory of elasticity, a stress tensor is used to characterize the state of stress of the simulated systems, and then the variation of the maximum shear stress in the postshock region is precisely obtained. It is found that, as the compression level gradually increases in the 2D Yukawa crystal, the maximum shear stress first increases linearly with the compressional speed until it reaches its extreme value, then decreases drastically to a much lower level. This obtained extreme value of the maximum shear stress is just at the elastic–plastic transition point, corresponding to one-half of the yield stress, which represents the ability to resist the maximum applied shear for the simulated Yukawa crystal. Our calculated Voronoi diagrams and pair correlation functions in the direction perpendicular to the shock compression further confirm this elastic–plastic transition point. It is also found that the critical compressional speed of the elastic–plastic transition point increases with the coupling parameter and decreases with the screening parameter of the 2D Yukawa crystal.
A comparative study of the nonlinear rheological properties of three different cellulose nanofibril suspensions
Authors: Song Hyeong Yong , Park Shin Young , Kim Min Chan , Park Jun Dong , Youn Hye Jung , Hyun Kyu
Published: May 2022

This study focused on the nonlinear rheological characterization of three types of cellulose nanofibril (CNF) suspensions under large amplitude oscillatory shear (LAOS) flow. Three different CNFs were produced, two by mechanical fibrillation alone under different conditions [here named microfibrillated cellulose (MFC) and U-CNF] and the other by mechanical fibrillation after carboxymethylation (CM-CNF). MFC and U-CNF had broad width distributions, whereas CM-CNF had narrower fibril width and width distribution due to the presence of charged carboxymethyl groups. Nonlinear stress responses of the prepared suspensions were analyzed using the sequence of physical processes method. All CNF suspensions exhibited intracycle rheological transitions composed of three physical processes: (1) structure recovery, (2) elastic deformation to early stage yielding, and (3) late-stage yielding. MFC and U-CNF suspensions exhibited similar rheological transitions overall. However, CM-CNF suspension had a higher network recovery rate within a shorter time and showed an additional yielding step due to the complex interplay between recovery and yielding dynamics. This result originated from complete nanofibrillation and charged functional groups on fibril surfaces. Rapid reformation of effective fibril–fibril contacts in CM-CNF suspension was attributed to electrostatic repulsions and complete nanosized lateral dimensions. In addition, excitation frequency was found to influence intracycle rheological transitions. A range of intracycle rheological transitions became narrower on increasing frequency because the time period for each transition was not enough under faster flow conditions. In particular, the characteristic yielding step of CM-CNF suspension disappeared on increasing frequency, which suggested that high-frequency excitation might be unfavorable for the nonlinear viscoelastic characterization of soft materials under LAOS flow.
Propeller–strut interaction tone noise
Authors: Wu Yan , Kingan Michael J. , Go Sung Tyaek
Published: May 2022

This paper presents a detailed investigation of tonal noise produced by an unmanned aerial vehicle propeller operating with a circular strut mounted just downstream. Experimental measurements of the acoustic pressure in near- and far-fields are presented, and it is observed that the measured pressure signals contain a strong impulse caused by the propeller–strut interaction. The magnitude and shape of this impulse vary significantly with an observer location. It was also observed that the magnitude of these impulses was reduced by increasing the distance between the propeller and the strut. In order to investigate the physics of the noise generation mechanism, a number of computational fluid dynamics (CFD) simulations were performed, and a numerical method was developed to predict the radiated acoustic tones using the CFD data. This analysis showed that the unsteady loading sources on the strut and the propeller both made significant contributions to the total impulse, and the directivity of the sound radiated from the strut was different from that radiated from the propeller. Therefore, the addition of the signals from the propeller and the strut produced a pressure impulse, which had a complex directivity. Analytical models were also developed to estimate the unsteady loading on the propeller and strut from which estimates of the radiated noise field could be made. Predictions made using these models are compared with CFD predictions and experimental measurements of the unsteady loading on the strut and also the total radiated noise field and show reasonable agreement.
Modal analysis of non-ducted and ducted propeller wake under axis flow
Authors: Shi Hongda , Wang Tianyuan , Zhao Ming , Zhang Qin
Published: May 2022

Modal decomposition techniques, flow field, and spectral analysis are employed to investigate the wake dynamics and destabilization mechanisms of a four-bladed marine propeller with or without a nozzle. Numerical simulations are conducted using the delayed detached eddy simulation model for the wake and the arbitrary mesh interface method for the blade rotation. The presence of the nozzle significantly reduces the wake's streamwise velocity, delays the wake destabilization, increases the wake length, and changes the morphologies of wake vortices. In particular, the hub vortex in the ducted propeller wake is broken down into chaotic turbulence by the perturbation of the backflow. Two modal decomposition methods, namely, proper orthogonal decomposition and dynamic mode decomposition, are used to decompose the vorticity magnitude in the rotor wake field. From modal analysis, the spatial scale of flow phenomena decreases with the increase in modal frequency. Underlying destabilization mechanisms in the wake correspond to some characteristic frequencies. The interaction of each sheet vortex with the previously shed tip (leakage) vortices occurs at blade passing frequency (BPF). The pairing of adjacent tip (leakage) vortices occurs at half-BPF. The long-wave instability of the hub vortex and the wake meandering are stochastic processes, each of which occurs at a frequency lower or equal to shaft frequency. These four destabilization mechanisms can approximately reconstruct the large-scale flow phenomena in the wake. Moreover, each sheet vortex's alternating connection and disconnection with the previously shed tip (leakage) vortices cause the short-wave instability of the tip (leakage) vortices and generate the secondary vortices. The radial expansion motion of large-scale helical vortices in the outer slipstream dominates the wake meandering phenomenon.
Vortex shedding, flow separation, and drag coefficient in the flow past an ellipsoid of different aspect ratios at moderate Reynolds number
Authors: Li Junwei , Zhou Benmou
Published: May 2022

Incompressible viscous flow past an ellipsoid of different aspect ratios (ARs, the ratio of the vertical to the horizontal axis of the ellipsoid, is ranged from 0.5 to 2) at a Reynolds number of 300 is investigated numerically by a finite volume method with adaptive mesh refinement, and the effects of different aspect ratios on vortex shedding, flow separation, and drag coefficient are analyzed in detail. The accuracy of the present results is ascertained by comparing the present drag coefficient and Strouhal number with other literature studies. The results show that the Strouhal frequency of vortex shedding decreases and the magnitude of vortex shedding becomes weaker with an increase in the aspect ratio. In particular, a secondary frequency will occur within a certain interval of 0.8 ≤ AR ≤ 1.2. The vortex shedding appears as a hairpin vortex at AR ∈ [0.5, 1.6], whereas it becomes a double-line vortex at AR ≥ 1.8. Both the upper flow separation angle and the length of the separation bubble increase with an increase in the aspect ratio. The flow separation is symmetrical about the (x, z)-plane only at 0.5 ≤ AR ≤ 0.7 and AR ≥ 1.8. Furthermore, the total drag coefficient and the pressure drag coefficient both increase gradually with an increase in the aspect ratio. Due to the trend of the contact area between the fluid and the surface of the ellipsoid, the friction drag coefficient decreases first (AR ≤ 1) and then increases (AR ≥1). The pressure drag coefficient reinforces the contribution to the total drag coefficient, and the contribution of the pressure drag coefficient grows with an increase in the aspect ratio.
Broadband spin-unlocked metasurfaces for bifunctional wavefront manipulations
Authors: Dong Shaohua , Li Shiqing , Ling Xiaohui , Hu Guangwei , Li Yi , Zhu Hongyi , Zhou Lei , Sun Shulin
Published: May 2022

Recently, Pancharatnam–Berry (PB) metasurfaces have exhibited powerful capabilities to control spin-polarized light. However, the adopted abrupt PB phase, introduced by simply rotating the basic elements, is spin-locked with opposite signs for different spin excitations, greatly limiting their practical applications. Here, we introduce a high-efficiency and broadband spin-unlocked metasurface with two mechanisms of a resonance phase and a geometric phase perfectly combined together. The design strategy is quite simple just through changing one geometric parameter rather than multi parameter optimization. As a proof of concept, the anomalous photonic spin Hall effect based on the spin-unlocked metasurface is demonstrated first, showing high experimental efficiency (over 80%) in a broad frequency range (11.3–16.6 GHz). Furthermore, another spin-unlocked metasurface is built to demonstrate the completely independent wavefront manipulations, i.e., the focusing effect and anomalous reflection. These findings significantly expand the electromagnetic control ability of a metasurface.
Independent determination of Peltier coefficient in thermoelectric devices
Authors: Dhawan Ruchika , Panthi Hari Prasad , Lazaro Orlando , Blanco Andres , Edwards Hal , Lee Mark
Published: May 2022

Thermoelectric (TE) generators and coolers are one possible solution to energy autonomy for internet-of-things and biomedical electronics and to locally cool high-performance integrated circuits. The development of TE technology requires not only research into TE materials but also advancing TE device physics, which involves determining properties such as the thermopower (α) and Peltier (Π) coefficients at the device rather than material level. Although Π governs TE cooler operation, it is rarely measured because of difficulties isolating Π from larger non-Peltier heat effects such as Joule heating and Fourier thermal conduction. Instead, Π is almost always inferred from α via a theoretical Kelvin relation Π = αT, where T is the absolute temperature. Here, we demonstrate a method for independently measuring Π on any TE device via the difference in heat flows between the thermopile held open-circuit vs short-circuit. This method determines Π solely from conventionally measured device performance parameters, corrects for non-Peltier heat effects, does not require separate knowledge of material property values, and does not assume the Kelvin relation. A measurement of Π is demonstrated on a commercial Bi2Te3 TE generator. By measuring α and Π independently on the same device, the ratio (Π/α) is free of parasitic thermal impedances, allowing the Kelvin relation to be empirically verified to reasonable accuracy.