Inhomogeneous Velocity Research Articles

Ten Thousand Recaptures of a Single DNA Molecule in a Nanopore and Variance Reduction of Translocation Characteristics.

Nanopores have emerged as highly sensitive biosensors operating at the single-molecule level. However, the majority of nanopore experiments still rely on averaging signals from multiple molecules, introducing systematic errors. To overcome this limitation and obtain accurate information from a single molecule, the molecular ping-pong methodology provides a precise approach involving repeated captures of a single molecule. In this study, we have enhanced the molecular ping-pong technique by incorporating a customized electronic system and control algorithm, resulting in a recapture number exceeding 10,000. During the ping-pong process, we observed a significant reduction in the variance of translocation characteristics, providing fresh insights into single-molecule translocation dynamics. An inhomogeneous translocation velocity of folded DNA has been revealed, illustrating a strong interaction between the molecule and the solid-state nanopore. The results not only promise heightened experimental efficiency with reduced sample volume but also increase the precision in statistical analysis of translocation events, marking a significant stride toward authentic single-molecule nanopore sensing.

Crustal structure of the central and southern Kyushu-Palau Ridge: Implications for intra-oceanic arc evolution

As an intra-oceanic arc, the Kyushu–Palau Ridge (KPR) is an exceptional place for studying intra-oceanic arc evolution and related magmatism using its interior velocity structure. Two wide-angle seismic profiles along the KPR provide an opportunity to unveil the crustal structure of its central and southern parts. Strong along-ridge variations are found either in P-wave velocity or crustal thickness of the central and southern KPR. The crustal thickness is between 6 and 12 km, with the velocity increasing from 4.0 km/s to 7.0 km/s from top to bottom. The upper crust has a large velocity gradient of ∼1 s−1 and a thickness between 2 and 4 km, while the lower crust has a lower velocity gradient of <0.2 s−1 and is characterized by inhomogeneous velocity anomalies and a substantial lateral variation in thickness. By comparing with the mature arc in the Izu-Bonin-Mariana arc in the east, we suggest the central and southern KPR is a thickened oceanic crust rather than a mature arc crust. Partly thick crust, velocity anomalies in the lower crust and upper mantle imply the crust is in a transition stage from oceanic crust to mature arc one. Compared with the quantitatively calculated seamount volumes, the lower crustal thickness is generally correlated well with the distribution of the seamount volumes along the central and southern KPR. These observations imply that the central and southern parts of the KPR are at the juvenile stage of arc evolution, with crustal growth primarily attributed to the lower crust thickening.

Wind noise due to flow induced around a windscreen in large-scale turbulence

Turbulent wind noise on microphones is a ubiquitous problem for sound measurement and recording outdoors. Windscreens, such as open-cell foam spheres, are effective at reducing wind noise. The unsteady pressure sources induced on the windscreen surface must contribute less noise on average than the stagnation pressure on a bare microphone, or windscreens would not be effective. To quantify this noise reduction mechanism, a model is presented for the unsteady pressure within an impermeable windscreen of arbitrary shape immersed in an initially isotropic turbulent flow. Only turbulent scales larger than the windscreen are considered, which contribute the most intense low-frequency band of the noise spectrum in most practical situations. The inhomogeneous velocity field around the windscreen is modeled in terms of an induced irrotational blocking field that meets the kinematic boundary condition. The interior pressure is expressed as a convolution of the surface pressure field with a filter function in wavevector space. The wind noise wavevector spectrum is then shown to be only a function of the inflow isotropic velocity spectrum and the windscreen shape. Solutions are obtained for the pressure frequency spectrum within a spherical windscreen for a von Kármán turbulence model and compared with prior results in literature.

Effect of isothermal rough boundaries on the statistics of velocity and temperature fluctuations in turbulent Rayleigh–Bénard convection

Using direct numerical simulations, we investigate the effect of surface roughness on the statistics of fluctuations in a 2D rectangular cell of aspect ratio Γ = 2 with air as the working fluid. We consider roughly two decades of Rayleigh number, 108≤Ra≤4.64×109, with three roughness configurations of R1, R2, and R3 characterized by their maximum heights of 5%, 10%, and 20% of the cell height, respectively. We show that roughened cells trigger stronger fluctuations, which further gets augmented with increasing Ra. Vertical variations of velocity and temperature fluctuations show different trends. While the temperature fluctuation becomes homogeneous in the bulk, it exhibits strong inhomogeneous vertical velocity fluctuations. The comparison of global heat flux with smooth case shows a significant increment beyond Ra=2.15×108. Surface roughness impacts local heat flux through augmented plumes, which is qualitatively ascertained by instantaneous temperature field. Furthermore, probability distribution functions reveal no particular trend for the taller roughness configurations, though the magnitude is amplified. Through identification of plumes and background regions, we show their behavior as a function of Ra for different rough cases. Finally, we decompose the shear production into its three components (based on the nature of mechanical forces) to understand the energy interaction between the mean flow and fluctuating flow field.

Solving the Hydrodynamical System of Equations of Inhomogeneous Fluid Flows with Thermal Diffusion: A Review

The present review analyzes classes of exact solutions for the convection and thermal diffusion equations in the Boussinesq approximation. The exact integration of the Oberbeck–Boussinesq equations for convection and thermal diffusion is more difficult than for the Navier–Stokes equations. It has been shown that the exact integration of the thermal diffusion equations is carried out in the Lin–Sidorov–Aristov class. This class of exact solutions is a generalization of the Ostroumov–Birikh family of exact solutions. The use of the class of exact solutions by Lin–Sidorov–Aristov makes it possible to take into account not only the inhomogeneity of the pressure field, the temperature field and the concentration field, but also the inhomogeneous velocity field. The present review shows that there is a class of exact solutions for describing the flows of incompressible fluids, taking into account the Soret and Dufour cross effects. Accurate solutions are important for modeling and simulating natural, technical and technological processes. They make it possible to find new physical mechanisms of momentum transfer for the design of new types of equipment.

System Matrix Reconstruction Algorithm for Thermoacoustic Imaging With Magnetic Nanoparticles Based on Acoustic Reciprocity Theorem.

According to the acoustic reciprocity theorem (ART), we propose a system matrix reconstruction algorithm of thermoacoustic imaging for magnetic nanoparticles (MNPs) by a single-pulse magnetic field. In both cases of inhomogeneous and homogeneous acoustic velocity, we respectively derive the linear equation between the sound pressure detection value and the distribution of MNPs. The image reconstruction problem is converted to an inverse matrix solution by using the truncated singular value decomposition (TSVD) method. In forward problem, the calculated forward results are consistent with the simulated thermoacoustic signal signals. In inverse problem, we build the two-dimensional breast cancer model. The TSVD method based on the ART faithfully reflects the distribution of abnormal tissue labeled by the MNPs. In the experiment, the biological sample injected with the MNPs is used as the imaging target. The reconstructed image well reflects the cross-sectional images of the MNPs area. The TSVD method based on the ART takes into account energy attenuation and inhomogeneous acoustic velocity, and use a non-focused broadband ultrasonic transducer as the receiver to obtain a larger imaging field-of-view (FOV). By comparing the image metrics, we prove that the algorithm is superior to the traditional time reversal method. The TSVD method based on the ART can better suppress noise, which is expected to reduce the cost by reducing the number of detectors. It is of great significance for future clinical applications.

Method of 2-D Range-Doppler Imaging for Plasma Wake Based on Range Walk Correction

When a hypersonic vehicle flies in the atmosphere, plasma sheath and trails are configured around the vehicle due to aerodynamic heating and ablation, which distort the radar echo signal and then influence the target range information and Doppler frequency extracted from the echoes. In order to examine the impact of plasma laminar wake on radar detection, this article is primarily aimed to establish a laminar wake echo model based on a linear frequency-modulated continuous wave (LFMCW) radar, which takes into account the reaction control system of laminar wake by employing the Born approximation solution and the characteristics of the inhomogeneous and nonunique velocity of the wake medium. An echo signal processing method is also developed, in which the stretch processing technique is combined with the keystone transform of the target range walk correction. Such a treatment not only reduces the sampling rate and bandwidth, but also eliminates the cross-range cells phenomenon caused by hypersonic; finally, the range-Doppler (RD) image of the wake is achieved by 2-D fast Fourier transform (2-D FFT) processing, and radar range and velocity measurement results in the presence of plasma are obtained.

Coupled nonlinear dynamics of acoustic modes in a quasi 1-D duct with inhomogeneous mean properties and mean flow

This study investigates the coupled nonlinear evolution of acoustic modes in a quasi 1-D duct with axially inhomogeneous mean velocity and mean thermodynamic properties. A novel modification of the Krylov–Bogoliubov method of averaging (KBMA) is developed to analytically solve the modal-amplitude equations that are linearly and nonlinearly coupled with both quadratic and cubic nonlinearities. Approximate analytical solutions based on the method of multiple scales (MMS) are also derived for the coupled modal-amplitude equations with cubic nonlinearities. The KBMA and MMS solutions are presented for the cases with and without internal resonance among the acoustic modes. Furthermore, novel initial conditions are derived for the KBMA and MMS modal amplitude and phase equations that are essential to validate the analytical solutions using numerical solutions. While both the analytical solutions are seen to be in good to excellent agreement with the numerical solutions, only KBMA captures the low-frequency oscillations in the peak amplitude of the limit cycle oscillations. The role of the order of nonlinear terms in the evolution to limiting oscillations is elucidated by successively considering coupled modal equations with both quadratic and cubic nonlinearities, only cubic nonlinearities and only quadratic nonlinearities. It is seen that while the cubic nonlinearities determine the asymptotic temporal behavior, the quadratic nonlinearities give rise to complex oscillation patterns characterized by a range of non-integral frequencies. For coupled oscillators with only quadratic nonlinearities, it is observed that for mean temperature gradients below a threshold, the pressure fluctuations are characterized by intermittent bursts of high-amplitude fluctuations, as well as by a frequency spectrum that is essentially continuous. The effects of linear coupling of the modal-amplitude equations on the limit-cycle behavior of the solutions are also illustrated.

Deep Velocity Structure and Seismicity of the Trans-Baikal Region (Along the Reference Geological and Geophysical Profile 1-SB)

The paper presents analysis of the seismicity and deep structure of the Trans-Baikal region in the section of the reference geophysical profile 1-SB. It was determined that the Earth’s crust and upper mantle has a complex heterogeneous structure. The thickness of the Earth’s crust varies from 40 km in the South-Eastern part of the profile and in the areas of intermountain depressions in the North-Western part, and up to 48 km in the areas of mountain ranges. The values of the boundary velocities along the M boundary also vary greatly, from higher values of 8.4‒8.5 km/s for P-waves and 4.9‒4.95 km/s for S-waves (especially in the South-Eastern part of the profile) to reduced values of 7.8‒8.0 km/s for P-waves and 4.6‒4.7 km/s for S-waves in the section of the Baikal rift zone in the North-Western part of the profile. A strong inhomogeneous structure of the medium in terms of elastic wave velocities, Vp/Vs velocity ratios, and the Poisson’s ratio is determined for the upper and the middle crust at depths of 8‒20 km. The authors determined that zones of increased seismicity are referred to blocks of the Earth’s crust with inhomogeneous velocity structure according to data of differently polarized P- and S-waves. The area of the Baikal rift zone, in the immediate vicinity of the largest Muya earthquake of 1957 with M = 7.6, is characterized by elevated inhomogeneity in the upper part of the Earth’s crust according to the elastic wave velocities and secondary parameters of the medium (Vp/Vs ratio, K* = = Vp/(γ – 1), where γ = Vp/Vs, Poisson’s ratio (σ)). A number of other inhomogeneous deep zones have also been identified in the profile based on anomalies of P- and S-waves velocities and secondary parameters of the medium, which correlate to varying degrees with seismically active sites according to long-term instrumental observations. The established unambiguous connection of large inhomogeneous zones of the upper crust of the Trans-Baikal region with the accumulation of stresses and their discharge in the form of strong earthquakes allows us to make a reasonable medium-term forecast of catastrophic events.

Towards shape optimization of flow channels in profile extrusion dies using reinforcement learning

AbstractProfile extrusion is a continuous production process for manufacturing plastic profiles from molten polymer. Especially interesting is the design of the die, through which the melt is pressed to impart the desired shape. However, due to an inhomogeneous velocity distribution at the die exit or residual stresses inside the extrudate, the final shape of the manufactured part often deviates from the desired one [1]. To avoid these deviations, we want to optimize the shape of the flow channel inside the die computationally. This has already been investigated in the literature using conventional optimization approaches [2,3].In this work, we investigate the feasibility of Reinforcement Learning (RL) as a learning‐based approach for shape optimization. RL is based on trial‐and‐error interactions of an agent with an environment. For each action, the agent is rewarded and informed about the subsequent state of the environment, which for this application stems from a high‐fidelity Finite Element Method (FEM) simulation.We will introduce a 2D test case as a proof‐of‐concept, in which an RL agent learns to change the geometry of a flow channel by modifying its representation as computational mesh through a spline‐based deformation method known as Free Form Deformation (FFD) [4]. We will show that an agent can be trained to optimize the geometry for different values of the quantity of interest and that the learning behavior is reproducible, which renders the RL‐based approach promising for further research.

Applications of physics-informed neural networks for property characterization of complex materials

The characterization of in-place material properties is important for quality control and condition assessment of the built infrastructure. Although various methods have been developed to characterize structural materials in situ, many suffer limitations and cannot provide complete or desired characterization, especially for inhomogeneous and complex materials such as concrete and rock. Recent advances in machine learning and artificial neural networks (ANN) can help address these limitations. In particular, physics-informed neural networks (PINN) portend notable advantages over traditional physics-based or purely data-driven approaches. PINN is a particular form of ANN, where physics-based equations are embedded within an ANN structure in order to regularize the outputs during the training process. This paper reviews the fundamentals of PINN, notes its differences from traditional ANN, and reviews applications of PINN for selected material characterization tasks. A specific application example is presented where mechanical wave propagation data are used to characterize in-place material properties. Ultrasonic data are obtained from experiments on long rod-shaped mortar and glass samples; PINN is applied to these data to extract inhomogeneous wave velocity data, which can indicate mechanical material property variations with respect to length.

Investigation of reinforcement learning for shape optimization of 2D profile extrusion die geometries

Profile extrusion is a continuous production process for manufacturing plastic profiles from molten polymer. Especially interesting is the design of the die, through which the melt is pressed to attain the desired shape. However, due to an inhomogeneous velocity distribution at the die exit or residual stresses inside the extrudate, the final shape of the manufactured part often deviates from the desired one. To avoid these deviations, the shape of the die can be computationally optimized, which has already been investigated in the literature using classical optimization approaches [7,32,47].A new approach in the field of shape optimization is the utilization of RL as a learning-based optimization algorithm. RL is based on trial-and-error interactions of an agent with an environment. For each action, the agent is rewarded and informed about the subsequent state of the environment. While not necessarily superior to classical, e.g., gradient-based or evolutionary, optimization algorithms for one single problem, RL techniques are expected to perform especially well when similar optimization tasks are repeated since the agent learns a more general strategy for generating optimal shapes instead of concentrating on just one single problem.In this work, we investigate this approach by applying it to two 2D test cases. The flow-channel geometry can be modified by the RL agent using so-called FFD [34], a method where the computational mesh is embedded into a transformation spline, which is then manipulated based on the control-point positions. In particular, we investigate the impact of utilizing different agents on the training progress and the potential of wall time saving by utilizing multiple environments during training.

SIMULATION OF TWO-COMPONENT POWDER MOLDING MELTING UNDER VACUUM SINTERING CONDITIONS

Vacuum sintering of metal powders under controlled heating conditions belongs to the traditional powder technologies employed to obtain dense composite materials. Despite the long history of studies on sintering processes, modeling in this area remains relevant since it provides deeper understanding of the associated physical phenomena. In the present work, we studied a two-component powder pressing system melting model based on the multiphase filtration theory that takes into account the differences in the melting temperatures of the components. The appearance of the liquid phase was modeled by introducing a melting temperature interval. An algorithm for the numerical realization of the model was developed. Employing a titanium-aluminum as an example, we demonstrate that even a temperature field with weak inhomogeneity can cause an inhomogeneous velocity field and redistribution of the fusible component.

Гидравлическое сопротивление системы управления движением инспекционного снаряда с клапаном обтекаемой формы в газопроводе низкого давления

The main problem while developing inspection shells for low-pressure gas pipelines is mainte-nance of their uniform motion under the action of the pumped gas. In the present work, we propose a solution to this problem based on changing the cross-section of the central flow channel of the smart pig by moving an axially symmetric valve along the axis of the channel. Two types of valves with a drop-shaped and a cone-shaped front part were considered. The choice of the valve shape and size can be made by calculating the forces acting on the smart pig during gas flow. The prob-lem was solved by the methods of computational fluid dynamics under the assumption of axial flow symmetry. The gas was considered to be compressible. The calculations showed that the total force acting on the valve and walls of the flow channel does not depend on its shape and, depending on the valve position, can change by more than two times. The optimum length of a drop-shaped valve is nearly half that of a conical valve. Although the drop-shaped valve produces a more inhomoge-neous velocity field than the conical valve, the former is more acceptable for regulating gas flow through the central channel of the inspection smart pig.

Analysis of the anisotropy aerodynamic characteristics of downstream wind turbine considering the 3D wake expansion based on coupling method

The inhomogeneous wake velocity distribution caused by the variation of relative position has a great impact on aerodynamic anisotropy characteristics of downstream wind turbine (WT). To accurately and efficiently evaluate the aerodynamic anisotropy of the downstream WT under wake inflow, an aero-elastic-servo model combined with the 3D Jensen-Gaussian (3DJG) wake model and OpenFAST was proposed in this paper. The accuracy of the model was verified by comparing with Jonkman's research. The effect of the relative installation position of the two 5 MW turbines on the aerodynamic anisotropy of the downstream WT was analyzed. Results indicate that the power loss of the downstream WT can be up to 17.47%–51.53% in the case investigated in this study. Due to the 3D wake expansion and the momentum mixing effect, relative axial distance and relative radial distance alternately become the dominant factor for rotor power recovery when y/R < 1 and y/R ≥ 1. The phenomenon mentioned in rotor power is also indicated in downstream rotor torque and thrust. Due to the inhomogeneous wind velocity distribution of the wake inflow, the standard deviation and average value of the blade root flapwise moment (BRFM) suffered by downstream WT are 10.84% and 24.30% larger than that suffered by upstream WT respectively. This paper provides better guidance for optimizing the WT layout and quantification of the wake effect on downstream WT.

An efficient model for subgrid-scale velocity enrichment for large-eddy simulations of turbulent flows

In some applications of large-eddy simulation (LES), in addition to providing a closure model for the subgrid-scale stress tensor, it is necessary to also provide means to approximate the subgrid-scale velocity field. In this work, we derive a new model for the subgrid-scale velocity that can be used in such LES applications. The model consists in solving a linearized form of the momentum equation for the subgrid-scale velocity using a truncated Fourier-series approach. Solving within a structured grid of statistically homogeneous sub-domains enables the treatment of inhomogeneous problems. It is shown that the generated subgrid-scale velocity emulates key properties of turbulent flows, such as the right kinetic energy spectrum, realistic strain–rotation relations, and intermittency. The model is also shown to predict the correct inhomogeneous and anisotropic velocity statistics in unbounded flows. The computational costs of the model are still of the same order as the costs of the LES.

Simulation of stationary and nonstationary wind velocity field along a long-span bridge using a numerical truncation method

The spectral representation method (SRM) has been extensively utilized to generate inhomogeneous stochastic wind velocity fields of long-span bridges. However, the simulation process usually needs a large number of computing resources, particularly when plenty of simulation points/samples are required. To address this issue, this study proposes a numerical truncation approach with the aid of a closed-form solution of Cholesky decomposition regarding the coherence matrix. First, the double summation of SRM is optimized as two separate inner and outer summations. Then, the closed-form solution of Cholesky decomposition is cut off to simplify the inner summation by setting an appropriate threshold. Since this inner summation dispenses with the participation of the truncated elements, the computational efforts are reduced significantly. As for the outer summation, it can be fast completed via introducing the fast Fourier transform technique. Finally, numerical examples are used to verify the effectiveness of the proposed method in terms of accuracy and efficiency. The results show that this method possesses a satisfactory accuracy and high efficiency with a given threshold. Therefore, it presents a high potential in simulating large-scale stochastic fluctuating wind velocity fields.

Airy coherent vortices: 3D multilayer self-accelerating structured light

We propose and generate a class of structured light fulfilling the mathematical form of a SU(2) coherent state based on a set of circular Airy vortex modes. Such wave packets possess strong focus with both radial and angular self-accelerations, which exploit more general 3D inhomogeneous velocity control with global spatial symmetry of multilayer rotation akin to galactic kinematics, termed galaxy waves. Galaxy waves are endowed with higher degrees of freedom to control strong focusing and acceleration, which opens a direction of multi-dimensional accelerating of 3D structured light field, promising numerous applications in optical trapping, manufacturing, and nonlinear optics.

Sensitivity analysis to near-surface velocity errors of the Kirchhoff single-stack redatuming of multi-coverage seismic data

We present a study of a Kirchhoff-type redatuming approach for irregular multi-coverage land seismic data to remove the disturbing influence of rugged topography and a considerable near-surface velocity inhomogeneity, like a thick weathering layer, on the subsurface reflections. The redatuming method is physically more justified than the commonly used field-static correction methods that provide - in the indicated situation - non-physical temporal adjustments (field statics) by shifting traces in time, which lead to sub-optimal imaging and inversion of the reflections. The redatuming procedure studied here is different: It transforms the multi-coverage seismic reflection data on the irregular measurement surface to a regular measurement configuration on a pre-specified datum, below the inhomogeneous velocity overburden. This is done by stacking/summing the input traces with certain operators or traveltime curves which are calculated by ray tracing. In this way, incoherent subsurface reflections in the input data become coherent and thus recognizable in the output data. To have this happen, the redatuming operation requires a good approximation of the overburden velocity model and a reasonable one of the sub-burden homogeneous velocity model (below the datum). Then the multi-coverage output data on the new flat datum can be imaged and inverted much better than with standard data processing, i.e., with field-static correction. This work is dedicated to explaining the single-stack Kirchhoff-type redatuming approach kinematically using a synthetic model and data based on a real geological situation. However, our main aim is to study the sensitivity of this redatuming method to errors of the overburden velocity model, which is determined from multi-coverage seismic data using first arrivals traveltime tomography commonly used in the oil industry. We apply the redatuming process to a multi-coverage synthetic seismic dataset generated from the models that represent the real geological situation with irregular measurement surfaces and inhomogeneous overburden or near-surface velocity. To assess accuracy, the redatumed data was migrated in depth-domain and the results showed the correct positioning of the reflectors. The results revealed that this redatuming method produces reliable and accurate redatumed data even using approximate velocity models, which can be useful for prestack time and depth imaging.

Ion-acoustic instability of the cylindrical inhomogeneous helicon discharge plasma with rotating electrons

The kinetic theory for the microinstabilities of a cylindrical plasma, produced by the cylindrical azimuthally symmetric (azimuthal mode number m0=0) helicon wave, based on the Vlasov–Poisson system of equations, is developed. The derived linear integral equation for the Fourier–Bessel transform of the electrostatic potential is the basic equation for the investigations of the parametric and the current-driven instabilities of the radially inhomogeneous cylindrical plasma in the radially inhomogeneous helicon wave. The short-wavelength solution of this equation for the electrostatic potential is derived in the form of the functional equation, which includes an infinite number of its satellites at a frequency separation equal to the helicon wave frequency. The analytical solution is derived for the high-frequency kinetic ion-acoustic instability of the cylindrical helicon plasma, driven by the coupled effect of the electron diamagnetic drift and of the steady azimuthal rotation of electrons relative to the ions with a radially inhomogeneous angular velocity.