Inhomogeneous Flow Research Articles

Influence of the non-monotonic loading and short stops on the regularities of the jerky flow in the Al-Mg alloy

The aim of the paper is to investigate the influence of the non-monotonic loading and short stops on the spatial-time inhomogeneity due to the jerky flow in the Al-Mg alloy. Experimental study on the regularities of inhomogeneous deformation and the processes of macroscopic localization of the plastic flow due to the Portevin-Le Chatelier effect was carried out. Using the digital image correlation technique the data on the initiation and propagation of the strain bands were obtained. Experimental data were obtained that show the effect of short-term stops and non-monotonic loading on the regularities of inelastic deformation of the Al-Mg alloy when the PLC effect manifests itself. Short stops decrease the level of critical strain when the jerky flow starts. Non-monotonic tests led to a change in the type of the PLC effect and a change in the regularities of the spatial-time inhomogeneity of plastic flow.

Influence of the loading system stiffness on the jerky flow in Al-Mg alloy at inelastic and postcritical deformation stages

The work is devoted to studying the issues of experimental studies of the behavior of materials under conditions of inhomogeneous discontinuous flow at the stages of plastic and supercritical deformation (deformation softening). Obtaining reliable experimental data on the indicated stages of inelastic deformation of materials is important for understanding the physical processes occurring in materials during deformation and ultimately leading to fracture. The results of tests are presented, confirming the influence of the rigidity of the loading system in relation to the working part of the sample on the processes of intermittent plastic deformation. The tests were carried out on samples of Al-Mg alloy. The loading rigidity was varied using a rig with variable rigidity. Registration of displacement and deformation fields on the surface of the working part of the samples was carried out by a non-contact video system Vic-3D Correlated Solutions based on the digital image correlation (DIC) technique. The use of this system makes it possible to measure both deformation at the macro level and the distribution of its components in the working part of the specimen under conditions of inhomogeneous discontinuous deformation. As a result of tensile tests, deformation diagrams were obtained for Al-Mg alloy samples tested at different values ​​of the rigidity of the loading system: 120 MN/m (nominal value), 50 MN/m, 18 MN/m, and 5 MN/m. All the diagrams show discontinuous plastic deformation (the Portevin-Le Chatelier effect). A decrease in the rigidity of loading with respect to the working part of the specimen led to a change in the type of discontinuous yield and an increase in the magnitude of the stress drop at each tooth of the diagram. The indicated effects of inhomogeneous plastic deformation are observed both at the stage of strain hardening and at the stage of postcritical deformation.

Effects of rolling-cutting processing parameters on micro-grooved strips forming

High heat flux density in the micro-electronics field urges the demand for functional surface for heat transfer. Micro-grooved surface is the classical heat transfer surface because of the larger specific area and better inducement of turbulence. A novel micro-grooved surface manufacturing process, the rolling-cutting process, is proposed to produce micro-grooved strips. Experiments are conducted to analyze the influence of processing parameters on the strips forming. The optimal combination of processing parameters is given to attain the micro-grooved strips with higher fins height and thinner base thickness. Additionally, the obtained strips are of three-dimensional structures with serrated bulges at the top. The difference between the material flow of serrated bulges and strip base is discussed, and the forming of serrated bulges is mainly due to inhomogeneous sinuous material flow.

Точные решения уравнений Обербека – Буссинеска для сдвиговых течений вязкой бинарной жидкости с учетом эффекта Соре

The paper considers an exact solution to the equations of thermal diffusion of a viscous incompressible fluid in the Boussinesq approximation with neglect of the Dufour effect for a steady shear flow. It is shown that the reduced system of constitutive relations is nonlinear and overdetermined. A nontrivial exact solution of this system is sought in the Lin–Sidorov–Aristov class. The resulting family of exact solutions allows one to describe steady-state inhomogeneous shear flows. This class generalizes the classical Couette, Poiseuille, and Ostroumov–Birikh solutions. It is demonstrated that the system of ordinary differential equations reduced within this class retains the properties of nonlinearity and overdetermination. A theorem on solvability conditions for the overdetermined system is proved; it is reported that, when these conditions are met, the solution is unique. The overdetermined system is solvable owing to the algebraic identity relating the horizontal velocity gradients, which are linear functions of the vertical coordinate. The constructive proof of the computation of hydrodynamic fields consists in the successive integration of the polynomials, the polynomial degree being dependent on the values of the boundary parameters.

Динамика капель, подброшенных над испаряющейся поверхностью воды

The data of a ballistic experiment are analyzed, in which an intense capillary wave injects up microdroplets formed and levitated above a heated region of water due to an ascending convective steam-air flow. The drag force resisting the movement of a droplet is estimated. Using various theoretical approaches, the flow parameters (velocity at different heights, the rate of change of velocity) are estimated. The maximum size of droplets that can levitate freely has been determined. The impossibility of the stationary droplet levitation in a linearly inhomogeneous flow is shown.

Computer simulation of superplastic tensile test

Superplasticity is an ability of polycrystalline materials to archive extremely large deformations, which is utilized in advanced forming technologies demanded mainly in the aerospace industry. Design of such technologies needs an information of the material flow behaviour, which describes the relation of the effective stress on the strain and strain rate taking place during deformation. The most popular experimental method for investigation of the flow behaviour of superplastic materials is tensile testing. The procedure of superplastic tensile testing and interpretation of its results is described by several international standards. At the same time, it is known that the due to flow inhomogeneity in the specimen volume the accuracy of such tests may be violated. Moreover, different standards provide different ratio between the width and the height of the gauge area of a specimen. This work provides the numerical analysis aimed to study how the initial specimen geometry affects the results of tensile tests. A computer program implementing finite element method (FEM) was developed to predict the specimen deformation during the test. A flat specimen is discretized using prismatic elements with specific geometrical constraints reducing the degree of freedom to the order of a plane stressed task. The output stress and strain values were calculated as specified in the ASTM E2448 standard. The effect of the gauge length was studied focusing on the output stress strain curves. The results were compared with the experimental results available in the literature.

X-ray variability of the HMXB Cen X−3: evidence for inhomogeneous accretion flows

ABSTRACT Cen X−3 is a compact high-mass X-ray binary likely powered by Roche lobe overflow. We present a phase-resolved X-ray spectral and timing analysis of two pointed XMM–Newton observations. The first one took place during a normal state of the source, when it has a luminosity LX ∼ 1036 erg s−1. This observation covered orbital phases ϕ = 0.00–0.37, i.e. the egress from the eclipse. The egress light curve is highly structured, showing distinctive intervals. We argue that different intervals correspond to the emergence of different emitting structures. The light-curve analysis enables us to estimate the size of such structures around the compact star, the most conspicuous of which has a size ∼0.3R*, of the order of the Roche lobe radius. During the egress, the equivalent width of Fe emission lines, from highly ionized species, decreases as the X-ray continuum grows. On the other hand, the equivalent width of the Fe K α line, from near-neutral Fe, strengthens. This line is likely formed due to the X-ray illumination of the accretion stream. The second observation was taken when the source was 10 times X-ray brighter and covered the orbital phases ϕ = 0.36–0.80. The X-ray light curve in the high state shows dips. These dips are not caused by absorption but can be due to instabilities in the accretion stream. The typical dip duration, of about 1000 s, is much longer than the time-scale attributed to the accretion of the clumpy stellar wind of the massive donor star, but is similar to the viscous time-scale at the inner radius of the accretion disc.

THz gas discharge in nitrogen as a source of ultraviolet radiation

This work presents the results of investigation of THz gas discharge in inhomogeneous flow of nitrogen. As a source of sub-millimetre radiation we used 40 kW@670 GHz gyrotron created in IAP RAS. It is shown that, despite the tendency of the discharge in electromagnetic wave beams to propagate towards the heating radiation from the breakdown region, in the case of a strongly inhomogeneous gas flow (nitrogen in our case), conditions can be created under which the discharge practically does not propagate and becomes point-like. The features of the glow of a THz discharge in a nitrogen flow in a wide range of background gas pressures are studied. The presence of a powerful afterglow in some bands of the second positive system of nitrogen at high pressures was demonstrated. The prospects of using such a discharge as a source of UV radiation are discussed.

Path Integral Method in the Mean-field Model for the Magnetic Vector Potential

The method of path integrals is used to average the magnetic-induction equation written for the vector potential over the velocity field. This approach avoids the assumption that the random field is two-scale, replacing it with the assumption of short time correlations. As a result of averaging, the classical equation of the mean field is obtained, at least for a homogeneous and isotropic medium. Based on the developed approach, the problem of the average field solenoidality is discussed, as well as a possible generalization of the method to the case of a statistically inhomogeneous and anisotropic flow.

On Energy Cascades in General Flows: A Lagrangian Application

AbstractAn important characteristic of geophysically turbulent flows is the transfer of energy between scales. Balanced flows pass energy from smaller to larger scales as part of the well‐known upscale cascade, while submesoscale and smaller scale flows can transfer energy eventually to smaller, dissipative scales. Much effort has been put into quantifying these transfers, but a complicating factor in realistic settings is that the underlying flows are often strongly spatially heterogeneous and anisotropic. Furthermore, the flows may be embedded in irregularly shaped domains that can be multiply connected. As a result, straightforward approaches like computing Fourier spatial spectra of nonlinear terms suffer from a number of conceptual issues. In this paper, we develop a method to compute cross‐scale energy transfers in general settings, allowing for arbitrary flow structure, anisotropy, and inhomogeneity. We employ Green's function approach to the kinetic energy equation to relate kinetic energy at a point to its Lagrangian history. A spatial filtering of the resulting equation naturally decomposes kinetic energy into length‐scale‐dependent contributions and describes how the transfer of energy between those scalestakes place. The method is applied to a doubly periodic simulation of vortex merger, resulting in the demonstration of the expected upscale energy cascade. Somewhat novel results are that the energy transfers are dominated by pressure work, rather than kinetic energy exchange, and dissipation is a noticeable influence on the larger scale energy budgets. We also describe, but do not employ here, a technique for developing filters to use in complex domains.

Large-eddy simulation of wall-bounded turbulent flow with high-order discrete unified gas-kinetic scheme

In this paper, we introduce the discrete Maxwellian equilibrium distribution function for incompressible flow and force term into the two-stage third-order Discrete Unified Gas-Kinetic Scheme (DUGKS) for simulating low-speed turbulent flows. The Wall-Adapting Local Eddy-viscosity (WALE) and Vreman sub-grid models for Large-Eddy Simulations (LES) of turbulent flows are coupled within the present framework. Meanwhile, the implicit LES are also presented to verify the effect of LES models. A parallel implementation strategy for the present framework is developed, and three canonical wall-bounded turbulent flow cases are investigated, including the fully developed turbulent channel flow at a friction Reynolds number (Re) about 180, the turbulent plane Couette flow at a friction Re number about 93 and lid-driven cubical cavity flow at a Re number of 12000. The turbulence statistics, including mean velocity, the r.m.s. fluctuations velocity, Reynolds stress, etc. are computed by the present approach. Their predictions match precisely with each other, and they are both in reasonable agreement with the benchmark data of DNS. Especially, the predicted flow physics of three-dimensional lid-driven cavity flow are consistent with the description from abundant literature. The present numerical results verify that the present two-stage third-order DUGKS-based LES method is capable for simulating inhomogeneous wall-bounded turbulent flows and getting reliable results with relatively coarse grids.

Signature and energetics of internal gravity waves in stratified turbulence

Internal gravity waves propagating within homogeneous stratified turbulence are the subject of the present study. A spatiotemporal analysis is carried out on the results of direct numerical simulations including a forcing term, with the aim of showing the energy content of the simulations as a function of frequency, ω, and wave-vector inclination to the horizontal, θ. Clear signatures of the dispersion relation of internal gravity waves, ω = ±N cos θ, where N is the Brunt-Vaisala frequency, are observed in all our simulations, which have low Froude number, Frh 1, and increasing buoyancy Reynolds number up to Reb ≈ 10. Interestingly, we observe the presence of high-frequency waves with ω ∼ N and a corresponding low-frequency vortex mode, both containing a non-negligible amount of energy. These waves are large-scale waves, their energy signature being found at scales larger than the forcing scales. We also observe the growth of energy in the shear modes, constituting a horizontal mean flow, and we show that their continuous growth is due to an upscale energy transfer, from the forcing scales to larger horizontal as well as vertical scales. These shear modes are found to be responsible for Doppler shifting the frequency of the large-scale waves. When considering the wave energy across the simulations at varying Reb, such energy is seen to reduce as Reb is increased and the flow enters the strongly stratified turbulence regime. The classical wave-vortex decomposition, based on a purely spatial decomposition of instantaneous snapshots of the flow, is analyzed within the current framework and is seen to correspond relatively well to the “true” wave signal identified by the spatiotemporal analysis, at least for the large-scale waves with ω ∼ N. Distinct energy peaks in θ-ω space highlight that the waves have preferential directions of propagation, specifically θ = 45◦ and θ ≈ 55◦, similar to observations in studies of wave radiation from localized regions of turbulence. This suggests that the same wave-generation mechanisms may be relevant for homogeneous and inhomogeneous stratified turbulent flows.

Sound Production due to Swirl–Nozzle Interaction: Model-Based Analysis of Experiments

Indirect noise due to the interaction of flow inhomogeneities with a choked nozzle is an important cause of combustion instability in solid rocket motors and is believed to be important in aircraft engines. A previously published experiment (Kings, N., and Bake, F., “Indirect Combustion Noise: Noise Generation by Accelerated Vorticity in a Nozzle Flow,” International Journal of Spray and Combustion Dynamics, Vol. 2, No. 3, 2010, pp. 253–266.) demonstrated that interaction of a nozzle with time-dependent axial swirl can also be a source of sound. This axial swirl was generated by intermittent tangential mass injection upstream from a choked nozzle in a so-called vortex wave generator. The present work discusses the impact of swirl–nozzle interaction in this experiment on the acoustic waves detected downstream of the nozzle. The main source of sound appears to be the reduction in mass flux through the choked nozzle, which depends quadratically on the swirl number. This effect is quantitatively predicted by a quasi-steady and quasi-cylindrical analytical model. The model, combined with empirical data for the decay of axial swirl in pipe flows, predicts the observed influence of the distance between the vortex wave generator and the nozzle. The findings presented here contradict the hypothesis found in the literature, which presumes that sound production in the aforementioned experiment is due to the acceleration of vorticity waves through the nozzle.

The Investigation of lengthwise rolling on a stub mandrel by the use of a computer simulation

Lengthwise rolling of pipes on a stub mandrel is one of the most common pipe rolling methods. An urgent problem of the process of lengthwise rolling of pipes on a stub mandrel is the increased wall thickness variation in rough pipes caused by the flow of metal into the groove tapers and the formation of stripes during the rolling process. Therefore, an important direction is the study of the inhomogeneity of the metal flow in the deformation zone during the rolling process. To evaluate the wall thickness variation during the rolling process, a dimensionless parameter was introduced, which is the ratio of the average wall thickness variation in the groove tapers to the average wall thickness at the groove tops. The article presents the results of a study of the influence of mandrel calibration on the wall thickness variation and the tendency to form defects on the inner surface. With the use of computer simulation made for structural alloyed steel AISI-4140, the shape change of the pipe is investigated. Based on the studies, practical recommendations are formulated when choosing the calibration of the mandrel during lengthwise rolling on a stub mandrel, providing increased accuracy of pipes and reducing the level of defects.

Fire-hose instability of inhomogeneous plasma flows with heat fluxes

We study the effects of heat flows and velocity shear on the parallel firehose instability in weakly collisional plasma flow. For this purpose, we apply an anisotropic 16-moment MHD fluid closure model that takes into account the pressure and temperature anisotropy, as well as the effect of anisotropic heat flux. The linear stability analysis of the firehose modes is carried out in the incompressible limit, where the MHD flow is parallel to the background magnetic field, while the velocity is sheared in the direction transverse to the flow direction. It seems that an increase in the velocity shear parameter leads to higher growth rates of the firehose instability. The increase in the instability growth rate is most profound for perturbations with oblique wave-numbers k⊥/k∥<1. Combined action of the velocity shear and heat fluxes introduces an asymmetry of the instability growth in the shear plane: perturbations with wave-vectors with a component in the direction of the velocity shear grow significantly stronger as compared to those with components in the opposite direction. We discuss the implications of the presented study on the observable features of the solar wind and possible measurements of local parameters of the solar wind based on the stability constraints set by the firehose instability.

Front Cover: Electroosmotic Facilitated Protein Capture and Transport through Solid‐State Nanopores with Diameter Larger than Length (Small Methods 11/2020)

In article number 1900893, Yin Zhang, Jingjie Sha, Yunfei Chen, and co-workers study the behaviors of streptavidin electrically facilitated transport through nanopores. The frequency of streptavidin captured in nanopores is non-monotonously dependent on both pore size and bias voltage. Theoretical results attribute this phenomenon to the inhomogeneous electroosmotic flow velocity distribution in the nanopore.

Modeling the Microstructure and Stress in Dense Suspensions under Inhomogeneous Flow.

Under inhomogeneous flow, dense suspensions exhibit behavior that violates the conventional homogeneous rheology. Specifically, one finds flowing regions with a macroscopic friction coefficient below the yielding criterion, and volume fraction above the jamming criterion. We demonstrate the underlying physics by incorporating shear rate fluctuations into a recently proposed tensor model for the microstructure and stress, and applying the model to an inhomogeneous flow problem. The model predictions agree qualitatively with particle-based simulations.

Dissipation element analysis of non-premixed jet flames

Abstract

Differential rotation in cholesteric pillars under a temperature gradient

Steady rotation is induced in cholesteric droplets dispersed in a specific liquid solvent under a temperature gradient. In this phenomenon, two rotational modes have been considered: (1) collective rotation of the local director field and (2) rigid-body rotation of the whole droplet structure. However, here we present another rotational mode induced in a pillar-shaped cholesteric droplet confined between substrates under a temperature gradient, that is, a differential rotation where the angular velocity varies as a function of the radial coordinate in the pillar. A detailed flow field analysis revealed that every pillar under a temperature gradient involves a double convection roll. These results suggested that the differential rotation in the cholesteric pillars was driven by the inhomogeneous material flow induced by a temperature gradient. The present experimental study indicates that the coupling between the flow and the director motion plays a key role in the rotation of the cholesteric droplets under the temperature gradient.

Intrarenal oxygenation determines kidney function during the recovery from an ischemic insult.

Acute kidney injury (AKI) is a significant clinical problem associated with poor outcome. The kidney, due to its inhomogeneous blood flow, is particularly susceptible to changes in oxygen delivery, and intrarenal hypoxia is a hallmark of AKI and progression to chronic kidney disease. However, the role of intrarenal hypoxia per se in the recovery from an ischemic insult is presently unclear. The present study was designed to investigate 1) the role of systemic hypoxia in the acute progression and recovery of AKI and 2) whether increased intrarenal oxygenation improves recovery from an ischemic insult. Anesthetized male Sprague-Dawley rats were subjected to unilateral warm renal ischemia for 45 min followed by 2 h of reperfusion under systemic hypoxia (10% inspired oxygen), normoxia (21% inspired oxygen), or hyperoxia (60% inspired oxygen). Intrarenal oxygen tension was successfully manipulated by altering the inspired oxygen. Glomerular filtration rate (GFR) before the ischemic insult was independent of intrarenal oxygen tension. GFR during the recovery from the ischemic insult was significantly lower compared with baseline in all groups (3 ± 1%, 13 ± 1%, and 30 ± 11% of baseline for hypoxia, normoxia, and hyperoxia, respectively). However, GFR was significantly higher in hyperoxia than hypoxia (P < 0.05, hypoxia vs. hyperoxia). During recovery, renal blood flow was only reduced in hyperoxia, as a consequence of increased renal vascular resistance. In conclusion, the present study demonstrates that renal function during the recovery from an ischemic insult is dependent on intrarenal oxygen availability, and normobaric hyperoxia treatment has the potential to protect kidney function.