Inhomogeneous Flow Research Articles

Single-point structure tensors in turbulent channel flows with smooth and wavy walls

A long-standing problem in turbulence modeling is that the Reynolds stress tensor alone is not necessarily sufficient to characterize the transient and nonequilibrium behaviors of turbulence under arbitrary mean deformation or frame rotation. A more complete single-point characterization of the flow can be obtained using the structure dimensionality, circulicity, and inhomogeneity tensors. These tensors are one-point correlations of local stream vector gradients and carry nonlocal information regarding the structure of the flow field. We explore the potential of these tensors to improve understanding of complex turbulent flows using direct numerical simulation of flows in channels with a smooth wall and a two-dimensional sinusoidal wavy wall. To enforce no-slip and no-penetration conditions at wavy-wall boundaries, an immersed boundary method for the stream vector Poisson equation was adopted within the framework of Stylianou, Pecnik, and Kassinos, “A general framework for computing the turbulence structure tensors,” Comput. Fluids 106, 54–66 (2015). The results show that the effects of wall waviness on the inclination and aspect ratio of the two-point velocity correlation near the wall are reproduced qualitatively by their corresponding single-point tensor representations. In the outer layer, good quantitative agreement is achieved for both parameters. Additional observations on the structural changes of turbulence due to wall waviness and their relevance to turbulence modeling with surface roughness are discussed. The findings of this investigation suggest that single-point structure tensors can be appended to the modeling basis for inhomogeneous flows with geometrically complex boundaries, such as rough-wall flows, to develop improved turbulence models.

Texture evolution and controlling of high-strength titanium alloy tube in cold pilgering for properties tailoring

Abstract As one of the preferred fabrication technologies for high-strength titanium alloy tubes, cold pilgering has unique advantages of large deformation, fine surface, and close dimensional tolerance since materials are incrementally experiencing compressive stress states dominated local loading throughout entire rolling process. However, both complex external loading history and low symmetric hexagonal close-packed (HCP) crystal structure induce heavy crystallographic reorientation of polycrystalline significantly influencing formability and performance of titanium tubular products. How to achieve a bespoken tailoring of texture related properties is still a challenge. Accordingly, taking high-strength Ti-3Al-2.5 V tubes as the case material, this study focuses on the tube texture evolution and the texture controlling guidelines for properties tailoring in cold pilgering. By coupling validated and verified three dimensional finite element (3D-FE) model and viscoplastic self-consistent (VPSC) crystal plasticity model, a macro-meso scaled computation platform is established to predict the inhomogeneous deformation flow and texture evolution during pilgering. Then the tube texture evolution characteristics along rolling process with three typical initial textures are investigated, and the correlation between texture evolution and spatial inhomogeneous deformation allocation is explored, in which the deformation history is fully featured by strain vector, viz., strain ratio α and equivalent plastic strain ee as well as ratios of wall thickness thinning to outer diameter reduction (total Q value and transient Q value). Taking the contractile strain ratio (CSR) as the most important indicator for evaluating the texture and properties, the effect of final texture features of the tube on its tensile mechanical properties is revealed, and then the texture controlling guidelines for properties tailoring of the tube are proposed. To obtain the desired near radial texture and the reasonable CSR, the different pass rolling specifications can be considerably designed to coordinate the spatial strain components based on reasonable range of total Q value and ee, then the rolling tools can be designed for further fine tuning of the transient Q value and strain ratio α.

ПРОФИЛЬ КОНЦЕНТРАЦИИ ВЫСОКОИНЕРЦИОННЫХ ЧАСТИЦ В ПРИСТЕННОЙ ТУРБУЛЕНТНОСТИ ПРИ БОЛЬШИХ ЧИСЛАХ РЕЙНОЛЬДСА

Актуальность работы обусловлена многочисленными применениями турбулентных газодисперсных потоков во многих аппаратах и устройствах, применяемых в добыче полезных ископаемых, при транспортировке природных ресурсов, в энергетике, химической технологии и других отраслях промышленности. В качестве примеров можно привести технологии и установки пневмотранспорта порошкообразных материалов, штреки горных выработок, вентиляционные каналы для помещений различных типов, системы газоочистки и т. д. Течения со взвешенными частицами также широко распространены в природе и являются объектами исследований в метеорологии, геоморфологии, гидравлике русловых процессов и речных наносов и др. Взаимодействие инерционных частиц с неоднородными пристенными турбулентными потоками является весьма сложным явлением, требующим детального моделирования на основе глубокого понимания механизмов взаимодействия частиц с многомасштабными турбулентными вихревыми структурами. Цель: моделирование распределения и статистических параметров движения высокоинерционных частиц в пристенной зоне турбулентного потока при больших числах Рейнольдса на основе стохастической лагранжевой модели турбулентности среды. Методы: статистическое моделирование методом Монте-Карло движения частиц на основе стохастической лагранжевой модели турбулентности среды и теории подобия пристенной турбулентности. Результаты. Стохастическое лагранжево моделирование динамики высокоинерционных частиц в логарифмическом слое пристенной турбулентности при больших числах Рейнольдса показало существенную неравновесность статистики скорости частиц вблизи стенки. Показано, что вблизи стенки имеет место вызванная турбофорезом аккумуляция частиц, вследствие которой концентрация частиц на стенке более чем в 3 раза превышает концентрацию частиц в ядре потока при условии упругого отскока частиц от стенки. В то же время интенсивность пульсаций нормальной к стенке компоненты скорости частиц не равна нулю на стенке и составляет примерно 1/3 от интенсивности пульсаций скорости в ядре потока.

A nonlinear, geometric Hall effect without magnetic field

The classical Hall effect, the traditional means of determining charge-carrier sign and density in a conductor, requires a magnetic field to produce transverse voltages across a current-carrying wire. We demonstrate a use of geometry to create transverse potentials along curved paths without any magnetic field. These potentials also reflect the charge-carrier sign and density. We demonstrate this effect experimentally in curved wires where the transverse potentials are consistent with the doping and change polarity as we switch the carrier sign. In straight wires, we measure transverse potential fluctuations with random polarity demonstrating that the current follows a complex, tortuous path. This geometrically induced potential offers a sensitive characterization of inhomogeneous current flow in thin films.

Comment on “elastic-plastic deformation in ion-exchanged aluminosilicate glass by loading rate dependent nanoindentation”

As part of a recent study Li et al. (2018) [1] proposed as a proof of inhomogeneous shear flow at indentation site in oxide glasses the existence of both a nano pattern on the residual imprint as revealed by atomic force microscopy and numerous discrete events on the load displacement curve, called bursts. Both were reported to be loading rate dependent. Here, using the same experimental set up, we provide experimental evidence that discrete bursts existence and loading rate sensitivity can be attributed to the displacement sensor noise floor and to data sampling rate issues respectively. We also show the nano pattern present on the faces of the indentation imprint to be extremely repeatable, to not depend neither on glass composition nor on loading rate as it is the result of the stamping onto the glass surface of the indenter surface roughness generated by its machining process.

Acoustic Behavior of a Dense Suspension in an Inhomogeneous Flow in a Microchannel

Bulk relocation of coflowing streams in an acoustic field occurs when there is a mismatch in acoustic impedance between the fluids. In separation of red blood cells from whole blood, for example, this relocation is prevented by ensuring that the central buffer stream has the higher impedance. This study shows that for dense suspensions like blood, in addition to typical full-relocation/zero-relocation regimes, an intermediate partial relocation may occur when the buffer's impedance is not high enough. This establishes the influence of particles in acoustic relocation, and provides a way to split any dense suspension into a concentrated central stream and a dilute side stream.

On random walk models for simulation of particle-laden turbulent flows

In this investigation, the accuracy of the discrete and continuous random walk (DRW, CRW) stochastic models for simulation of fluid (material) point particle, as well as inertial and Brownian particles, was studied. The corresponding dispersion, concentration, and deposition of suspended micro- and nano-particles in turbulent flows were analyzed. First, the DRW model used in the ANSYS-Fluent commercial CFD code for generating instantaneous flow fluctuations in inhomogeneous turbulent flows was evaluated. For this purpose, turbulent flows in a channel were simulated using a Reynolds-averaged Navier–Stokes (RANS) approach in conjunction with the Reynolds Stress Transport turbulence model (RSTM). Then spherical particles with diameters in the range of 30 µm to 10 nm were introduced uniformly in the channel. Under the assumption of one-way coupling, ensembles of particle trajectories for different sizes were generated by solving the particle equation of motion, including the drag and Brownian forces. The DRW stochastic turbulence model of the software was used to include the effects of instantaneous velocity fluctuations on particle motion, and the steady state concentration distribution and deposition velocity of particles of various sizes were evaluated. In addition, the improved CRW model based on the normalized Langevin equation was used in an in-house Matlab code. Comparisons of the predicted results of the DRW model of ANSYS-Fluent with the available experimental data and the DNS simulation results and empirical predictions showed that this model is not able to accurately predict the flow fluctuations seen by the particles in that it leads to unreasonable concentration profiles and time-varying deposition velocities. However, the predictions of the improved CRW model were in good agreement with the experimental data and the DNS results. Possible reasons causing the discrepancies between the DRW predictions and the experimental data were discussed. The improved CRW model was also implemented through user-defined functions into the ANSYS-Fluent code, which resulted in accurate concentration distribution and deposition velocity for different size particles.

Experimental investigation of the natural and controlled disturbance development in a supersonic boundary layer on the swept wing

The results of investigation of the transverse flow inhomogeneity effect on the natural and controlled disturbance development in 3D supersonic boundary layer are considered. The transition curves were measured on a model of a smooth swept wing and on a swept wing with roughness elements of two configurations. The influence of periodic surface roughness on the growth of natural pulsations (without using a discharge) in the boundary layer is determined. In experiments with artificial disturbances it is found that the periodic modulation of mean flow can lead to stabilization of controlled disturbances in supersonic boundary layer on a swept wing. It was found out that the weak flow inhomogeneity can influence the effectiveness of the active/passive technology of supersonic boundary layer transition control, including the surface microroughness on a swept wing.

Pore scale numerical simulation of heat transfer in propagating thermal wave during filtration combustion of rich and lean methane-air mixtures

The porous media combustion phenomenon was numerically studied with focus on the heat transfer effect in propagating combustion wave for lean and ultra-rich mixture compositions using the approach when the three-dimensional porous structure and interstitial flow are simulated directly at pore scale with explicit consideration of the thermal interaction between fluid and solid phases including the detailed chemical kinetics model and solid-to-solid radiation. The results demonstrate that irregularity of the porous structure leads to a large spatial variation of the process parameters and the flow inhomogeneity. The most intensive heat sources placed within the cavities where the flow is developed. During the process, the interface heat transfer and radiation contribute to heat recuperation mechanism that leads to thermal non-equilibrium in the combustion wave. The heat is transferred through the bed via radiation layer-by-layer due to restricted visibility of the particles. The numerical data about a local variation of the process parameters were presented. The data of this type can be used for modification of the volume-averaged models within the context of spatial variation consideration. It was shown that there is a correlation between the heat release rate, the interface heat flux, the radiative heat flux, and its root mean square values.

Vortex Intensification of Heat Transfer in Channels and Pipes with Periodic Elements of Discrete Roughness

A complex numerical investigation of vortex intensification of heat transfer in turbulent inhomogeneous medium flow in channels and pipes with periodic elements of discrete roughness has been carried out with the use of multiblock computational technologies, realized in a specialized package VP2/3, and of a modified model of transfer of shear stresses. As roughness elements use was made of asperities and grooves of different geometries, with air and transformer oil used as a working medium. The influence of the geometrical parameters of the indicated channels and pipes as well as of the regime parameters of the flow in them on their thermal and hydraulic efficiency has been analyzed in detail.

Applications of the gas discharge sustained by the powerful radiation of THz gyrotrons

The terahertz frequency range currently remains the least studied from the point of view of gas-discharge physics. Recent progress in its mastering is associated, first of all, with the creation of powerful sources of terahertz radiation - gyrotrons. Nevertheless, the discharge sustained by the powerful radiation of the terahertz frequency range is of interest not only from the fundamental point of view (because of its lack of knowledge), but also from the applied one. This paper presents results of investigations aimed at studying the point-like THz discharge as a source of vacuum ultraviolet radiation and at research of CW THz plasma torch existing at atmospheric pressure and therefore having promising applications in various fields: plasma medicine, plasma chemistry, material processing etc. It is shown that it is possible to create a point-like plasma in an inhomogeneous gas flow with the density above the cut-off one (4·1015 cm−3 for 0.67 THz). It has been demonstrated that such plasma is an effective source of VUV and EUV radiation. Investigations of the atmospheric plasma torch, sustained by CW sub-terahertz radiation (1 kW @ 0.26 THz), demonstrated the possibility of the creation dense non-equilibrium plasma (2·1015 cm−3) with electron temperature in the range from 1 to 2 eV.

Constitutive Behavior and Hot Workability of a Hot Isostatic Pressed Ti-22Al-25Nb Alloy during Hot Compression

A Ti-22Al-25Nb alloy was fabricated from prealloyed powders by hot isostatic pressing for 2 h at a temperature of 1050 °C and pressure of 100 MPa. The hot deformation behavior of the Ti-22Al-25Nb alloy was characterized by isothermal compression testing at deformation temperatures between 900 and 1060 °C and strain rates between 0.001 and 1 s−1. Based on the true stress–strain curves, a constitutive equation was constructed to describe the flow stress as a function of the strain rate and deformation temperature. Three-dimensional (3D) processing maps were developed based on dynamic material model theory using the stress flow data to identify the instability and optimization regions of the hot processing parameters. The results show that the flow stress decreases with increasing deformation temperature and decreasing strain rate, and the softening mechanisms are different under different deformation conditions. The apparent activation energies in the (α2 + β/B2 + O) and (α2 + B2) phase regions are calculated to be 865.177 and 590.661 kJ/mol, respectively, which suggests that the hot isostatic pressed (HIPed) Ti-22Al-25Nb alloy requires a high hot deformation activation energy. Combined with microstructural observations, the optimal processing domains are determined to be a temperature range of 1030-1060 °C and strain rate range of 0.01-0.1 s−1 in the α2 + B2 phase region. Moreover, the results indicate that adiabatic shear bands and severe inhomogeneous deformation cause flow instability at lower temperatures (900-1005 °C) and higher strain rates (> 0.1 s−1).

Stability of space-periodic flow with separation of a laminar boundary layer

An unstable laminar flow over a wavy region of a plate placed parallel to the oncoming air flow is studied. The results have been obtained by the hot-wire method in a low-turbulent wind tunnel at low subsonic velocities. Flow near a wall with transverse undulation, whose amplitude is sufficiently large and can contribute to the formation of local regions of boundary layer separation, is considered. The external harmonic (acoustic) forcing of the periodic flow leads to the suppression of perturbations that dominate in the spectrum of velocity disturbances near the model surface. The results of this work may be useful in developing methods for controlling spatially inhomogeneous flows.

Numerical and experimental investigation on the flow behavior of liquids in narrow gaps

AbstractLapping is one of the manufacturing processes with geometrically undefined cutting edges and is used to produce technical surfaces with high dimensional and shape accuracy. In order to predict the surface generation, a process model is developed that combines models of the material removal mechanism, the kinematics of the lapping particles and the flow of the lapping fluid. Due to many interactions in the process and the inhomogeneous flow of the lapping fluid (loose lapping particles in the fluid) a multiphysical model is needed. In a first step, only the flow behavior of the fluid without and with a simplified lapping particle (cuboid) is numerically and experimentally investigated. Particle Image Velocimetry is used to measure the flow field in the gap. The distance between the two plates, the velocity of the upper plate and the cuboid size can be varied. The experimental setup is simulated using a CFD model. Analogous to the experiment, the velocity of the upper plate and the cuboid size are also varied in the numerical study and the results agree well with the measured flow field. The CFD model helps to analyze further parameters, such as the influence of workpiece velocity on the flow. In the future, the model will be used to analyze the interaction between lapping particles and lapping fluid, which is very important for studying the mechanisms of material removal during lapping.

Experimental study of the liquid velocity and turbulence in a large-scale air-water counter-current bubble column

Measuring the local liquid velocity and turbulence in large-scale bubble columns with optical methods is complex and usually limited to low gas holdups in thin geometries. Comprehensive datasets in large bubble columns are therefore seldom published. Since the importance of Computational Fluid Dynamics (CFD) is increasing for multiphase applications, such data is also important for validating models for dispersed bubbly flows. In the present work the liquid velocity and turbulence in a pilot-scale bubble column is studied and a CFD validation database is generated by completing previous measurements of the gas void fractions and bubble sizes. For this purpose, we used a Particle Shadowgraph Velocimetry (PSV) technique that was intentionally designed to study the fluid dynamics in large-scale facilities. The measurements were realized in the 5.3 m high and 0.24 m diameter counter-current bubble column at Politecnico di Milano. The superficial gas velocity ranged from 0.37 to 1.85 cm/s, the counter current superficial liquid velocity from 0 to 9.2 cm/s. All operation points are in the so-called pseudo-homogeneous flow regime, in which the integral gas holdup (ranging from 1.02 to 7.55%) increases linear with the superficial gas velocity but an inhomogeneous flow is present. The dominant frequencies of the bubbly flow, shear rates, and turbulence levels are increasing with increasing superficial gas velocity. With increasing superficial liquid velocities, the dominant frequencies are decreasing, the averaged liquid velocities are shifted downwards, but the overall turbulence levels remain constant. In order to investigate the smaller scales at which the bubble-induced turbulence is expected, a filtering process is proposed. As a result, the filtered turbulence levels of all operation points fall on a linear trend line when plotted over the local void fraction, which is the same result obtained in other studies in small, homogenous tabletop columns. The now available database for CFD validation contains the averaged liquid velocities, basic turbulence information, local void fractions, and the bubble sizes at two different heights. The data will be in particular useful to validate the capabilities of models to upscale bubbly flows from tabletop to the pilot-scale bubble columns.

Narrow escape problem for Brownian particles in a microsphere with internal circulation.

This paper considers the narrow escape problem of a Brownian particle within a two-dimensional domain with two escape windows and an internal circulation modeled by the flow within a Hill's vortex. To account for the spatially inhomogeneous flow within the domain, a Lagrangian study is undertaken using the complete equations of motion for a dense particle which is necessary to distinguish between the various regimes as the strength of the internal circulation is varied. For very low internal circulation the particle undergoes the conventional narrow escape problem, and agreement is good with the asymptotic expression. As the internal circulation is increased, regimes are identified with different scaling for the mean escape time. The potential application of this for drug delivery (were nanoparticles are encased in a microsphere) is discussed.

Finite element analysis of plasticity behaviour of aluminium alloys in high-pressure torsion compressive loading stage

Purpose The purpose of this paper is to examine the plasticity behaviour of aluminium alloys in high-pressure torsion (HPT) compressive loading stage. It is a part of the strengthen lightweight material development through severe plastic deformation. Design/methodology/approach A finite element simulation of HPT compression stage by displacement control incremental loading was proposed by taking into account an unconstraint HPT configuration. The quasi-static condition was utilised, by embedding strain hardening plasticity constitutive model and considering frictional effects, to assess the plasticity behaviour of aluminium alloys, particularly AA2024 and AA6082. Findings The present investigation clearly indicates that the deviation of material flow as a result of sticking condition of µ⩾0.5, was found to be negligible. An inhomogeneous material flow along the sample radial and thickness direction was evident, producing a stress concentration at the edge of the loaded surface, indicating the anticipated region of failure. The effective plastic strain in the compression stage was also found to be significant. Based on the effective strain response, plasticity behaviour of the compressed sample was predicted. Originality/value This paper demonstrates the plasticity behaviour of the analysed aluminium alloys. Since the mechanical properties produced by the deformed material are closely related to the exerted plastic deformation, understanding the phenomenon associated with the plastic strain development is essential. The outcome of this research will assist in seizing the opportunities of improving both material properties and the HPT procedures.

On the Velocity Profile of Couette Flow of Lubricant Within a Micro/Submicro Gap

The hydrodynamic lubrication properties of sliding bearings are influenced by the velocity profile under shear. However, at present, there are very few reports on the velocity profile distribution of the fluid film lubrication in conformal contacts. In this article, an apparatus was built for in situ measurement of the velocity profile of thin oil film under a micro/submicro gap at ambient pressure using a photobleached imaging technique and imaging the shape evolution of the bleached area under shear. The film thickness is calibrated by interferometry. The results show that the velocity profile of oligomer PB450 doped with a fluorescent agent is a typical linear distribution under higher film thickness, which is in accordance with the classical lubrication theory. When the gap of the disk and slider is less than 2 μm, there are obvious partial inhomogeneous shear flows at ambient pressure, and the slip length increases with the decreasing film thickness. Lubricants of different viscosities and molecular structures show an inhomogeneous flow transition at different confinements. Correspondingly, abnormal velocity profile results combined with generalized Reynolds equation could explain the phenomenon that the convergence ratio of the maximum load-carrying capacity is larger than 1.2 experiencing hydrodynamic lubrication. Hence, this work contributes to an improved understanding of rheology as well as more accurate predictions of tribological properties.

Cavity resonator sensor and temporal signals analysis for object detection in granular flows

We present a microwave measurement system for detection of objects in granular media flowing through pipes. The system comprises a resonant cavity sensor operated in several high-order modes that yield spatially periodic sensitivities, and a microwave transceiver for fast S-parameter measurements. The temporal signatures from the passage of an object and the temporal correlations of the noise caused by flow inhomogeneities are incorporated in a matched filter detector, which is shown to perform uniformly better than two reference power detectors. The detection capabilities of the system are evaluated based on many object passage events in granulates flowing down a vertical pipe. We study the detection of dielectric and metal objects in microcrystalline cellulose pellets and glass beads, where the minimum detectable object size is 2–4 mm for metal objects and 4–6 mm for dielectric objects at a mass flow rate of 100 kg/h.

Pair dispersion in inhomogeneous turbulent thermal convection

Due to large scale flow inhomogeneities and the effects of temperature, turbulence small-scale structure in thermal convection is still an active field of investigation, especially considering sophis- ticated Lagrangian statistics. Here we experimentally study Lagrangian pair dispersion (one of the canonical problems of Lagrangian turbulence) in a Rayleigh-B enard convection cell. A sufficiently high temperature difference is imposed on a horizontal layer of fluid to observe a turbulent flow. We perform Lagrangian tracking of sub-millimetric particles on a large measurement volume in- cluding part of the Large Scale Circulation (LSC) revealing some large inhomogeneities. Our study brings to light several new insights regarding our understanding of turbulent thermal convection: (i) by decomposing particle Lagrangian dynamics into the LSC contribution and the turbulent fluc- tuations, we highlight the relative impact of both contributions on pair dispersion; (ii) using the same decomposition, we estimate the Eulerian second-order velocity structure functions from pair statistics and show that after removing the LSC contribution, the remaining statistics recover usual homogeneous and isotropic behaviours which are governed by a local energy dissipation rate to be distinguished from the global dissipation rate classically used to characterise turbulence in thermal convection; and (iii) we revisit the super-diffusive Richardson-Obukhov regime of particle dispersion and propose a refined estimate of the Richardson constant.