Inhomogeneous Flow Research Articles

Adaptive metal flow control in stamping through ferrofluidic actuators

Inhomogeneous metal flow in stamping non-axisymmetric parts makes material deformation in the flange critical, resulting in uneven sheet thickness distribution, high blank holder forces or even the need to use draw beads to compensate for large material draw-in. The paper presents an innovative mechatronic system for the adaptive control of the blank holder force, which integrates ferrofluidic actuators and non-contact sensors for the in-line material flow measurement, with the aim of exploring, through laboratory-scale experiments, the feasibility of its implementation in stamping processes and understanding the potential benefits to enable more reliable process design.

Inhomogeneous flow stresses in FSW jointed aluminum alloy sheets inversely identified by FE-VFM

Inhomogeneous plastic flow stresses are identified for the friction stir welded aluminum alloy 6061-T6 sheets using a new finite element-based virtual fields method (FE-VFM). The FE-VFM is an enhanced virtual fields method developed based on the formulations in a common FE scheme. In the present study, the following novel numerical schemes are newly adopted in the FE-VFM: (1) a two-step identification process, (2) partitioning of the whole domain of interest with an interpolation of the constitutive parameter within each subdomain, and (3) the normal distribution type virtual fields. The non-uniform stress method (nUSM) is also considered for comparison. Before applying the developed method to a real experiment, the FE-VFM is verified via numerical feasibility tests using ideal deformation data of the FE simulations conducted with the artificially generated target welding materials. The feasibility tests show that the proposed FE-VFM can calculate the inhomogeneous flow stresses of the welded aluminum sheets with reliable accuracy. In the experimental applications, plastic properties measured by FE-VFM and nUSM are compared in terms of the distributions of yield stress, uniform elongation, and ultimate tensile tests. Also, identified inhomogeneous flow stresses are validated by simulating the two tensile tests loaded parallel and perpendicular to the weld line. The validation reveals that the proposed FE-VFM can give more accurate measurements over the nUSM owing to its ability to capture the mechanical properties in the narrow thermo-mechanically affected zone (TMAZ).

Sound Generation in Multicomponent Nozzle Flows With Dissipation

Abstract Low emission aircraft engines burn in a lean regime, which makes the combustor susceptible to unsteady combustion. Along with improper mixing and air cooling, the unsteady combustion process gives rise to flow inhomogeneities. The acceleration of these inhomogeneities in the nozzle downstream of the combustor generates indirect combustion noise. If the acoustic waves that are reflected off the nozzle are sufficiently in phase with the heat released by the flame, thermoacoustic instabilities can occur. The generation and transmission of sound through the nozzle guide vane are typically modeled with a compact and isentropic nozzle model. Because the flow is non-isentropic due to losses from wall friction and recirculation zones, in the literature, a mismatch is observed between experimental and theoretical predictions in subsonic-choked regimes. In this work, we propose a low-order physical model to predict indirect noise in a multicomponent nozzle flow with dissipation using conservation laws whilst modeling non-isentropicity using a friction factor. The model is generalized for finite-length (non-compact) arbitrary geometry nozzles. We show that the friction factor can account for wall friction and two (or three) dimensional effects, such as flow recirculation in a cross-averaged sense. We analyze the model numerically for both subsonic and supersonic nozzles, emphasizing the importance of non-isentropic and non-compact assumptions with compositional inhomogeneities. Further, we show the effect of the nozzle geometry. The results are validated with existing experimental data from the literature.

Study of deformation structures maps in metals under tension

The paper considers the regularities of macroscopic inhomogeneity of plastic flow during uniaxial tension of flat samples of Fe-Cr-Ni 2 mm thick. Their tension axis was oriented along the rolling direction. The average grain size was 12.5 ± 3 mm. The plastic flow curves of the alloy had long stages of linear strain hardening over the entire test temperature range 180 K < T < 297 K. For the experimental study of plastic deformation, we used the method of accurately reconstructing the fields of displacement vectors and calculating the components of the plastic distortion tensor using speckle photography with increments of the total strain between exposures 0.001. The field of displacement vectors as a whole over the sample during loading is inhomogeneous both in the directions of the displacement vectors and in values; in some areas, the displacement vectors nonmonotonically change directions relative to the tension axis. It has been established that in the test temperature range 180 K < T < 297 K, plastic flow is localized at all stages of the process. The appearance of the a′-martensite phase during the deformation of the alloy under study leads to a change in the mechanical characteristics, the work hardening coefficient, and the deformation localization parameters. The maps of deformation structures are analyzed in the form of spatial distributions of the components of the plastic distortion tensor: local elongations, narrowings, shifts and rotations. The non-linear nature of the change in the coefficient of transverse deformation from the level of acting stresses is established. The general form and quantitative parameters of the evolution of the components of the plastic distortion tensor indicate the connection of this process with the self-organization of a defective subsystem in a deformable medium.

Coherent propagation of vortex rings at extremely high Reynolds numbers

We take advantage of the extremely small kinematic viscosity of superfluid $^4$ He to investigate the propagation of macroscopic vortex rings at Reynolds numbers between $2 \times 10^4$ and $4 \times 10^6$ . These inhomogeneous flow structures are thermally generated by releasing short power pulses into a small volume of liquid, open to the surrounding bath through a vertical tube $2$ mm in diameter. We study specifically the ring behaviour between $1.30$ and $1.80$ K using the flow visualization and second sound attenuation techniques. From the obtained data sets, containing more than $2600$ realizations, we find that the rings remain well-defined in space and time for distances up to at least $40$ tube diameters, and that their circulation depends significantly on the travelled distance, in a way similar to that observed for turbulent vortex rings propagating in Newtonian fluids. Additionally, the ring velocity and circulation appear to be influenced solely by a single, experimentally accessible parameter, combining the liquid temperature with the magnitude and duration of the power pulse. Overall, our results support the view that macroscopic vortex rings moving in superfluid $^4$ He closely resemble their Newtonian analogues, at least in the absence of significant thermal effects and at sufficiently large flow scales.

Wave disturbances of the atmosphere in a spatially inhomogeneous flow

Analysis of measurements by the Dynamics Explorer 2 satellite indicates a close connection between atmospheric wave disturbances and wind circulation in the polar thermosphere. According to satellite observations, large-amplitude acoustic-gravity waves are systematically observed in the regions of formation of powerful wind systems. At the same time, the azimuths of AGW propagation are spatially consistent with the directions of wind circulation. Waves propagate mainly against the wind, and their amplitude is approximately proportional to the wind speed. In order to explain the experimental data, the paper theoretically investigated the changes in the amplitudes of AGW in a horizontally inhomogeneous wind flow. The dispersion equation of the AGW in the reference frame of the medium, which moves with a non-uniform speed, was obtained. When obtaining it, inertial forces were taken into account, as well as the change in the background density of the atmosphere in an inhomogeneous flow. It is shown that under the slow change in wind speed, the real part of this equation coincides with the dispersion equation of AGW for a stationary medium. An expression for the change in the amplitude of waves in a moving medium was obtained, according to which, in an oncoming inhomogeneous wind, the amplitude of the wave increases approximately according to a linear law, which is consistent with the data of satellite observations.

Effects of Background Periodic Flow on MHD Fast-wave Propagation to a Coronal Loop

This paper investigates the propagation of MHD fast waves into a cylindrical coronal loop through an inhomogeneous stationary flow region. The background flow is assumed to have a small, spatially periodic structure in addition to a constant speed. This study focuses on the absorption of the wave energy in Alfvén resonance, comparing it with the constant flow case. A new flow (absorption) regime is induced by the periodic flow structure, which enhances the absorption for the antiparallel flow and inverse absorption (overreflection) for the parallel flow with respect to the axial wavevector, depending on the transitional layer and flow profiles. A giant overreflection and anomalous absorption behavior arises for some flow configurations. In the other flow regimes, its effect on the absorption is shown to be weak.

Influence of attack angle on ventilated supercavitating flow at high Froude number condition

To study the influence of attack angle on ventilated supercavitating flow, a numerical model is established to simulate the supercavitation flow by using the inhomogeneous multiphase flow model and the SST k-ω turbulence model. And then, the model is verified and validated by comparing with the experimental results. The effect of attack angle on the profile of ventilated supercavity was numerically investigated over high Froude number. Results show that the attack angle causes the linear deviation of the ventilated supercavity axis, the deviation angle is in direct proportion to the attack angle, the deviation direction is opposite to the lift of the cavitator, and the deformation of the supercavity contour increases along the flow direction. The attack angle changes the gas flow structure inside the ventilated supercavity, causing the different deformation of the upper profile and lower profile. Under the condition of positive attack angle, the deformation of the lower profile of the supercavity generated by the disk cavitator is significantly greater than that of the upper profile.

CFD investigation on thermal hydraulics of the double-wall bayonet tube heat exchanger in CLEAR-S facility

The double-wall bayonet tube type heat exchanger design can monitor and mitigate the occurrence of Steam Generator Tube Rupture (SGTR) accidents, which is one of the most important accidents of GEN-Ⅳ lead-cooled Fast Reactor (LFR). The lead-based reactor engineering validation facility CLEAR-S will be used for the integrated testing and verification of the megawatt-level double-wall bayonet tube heat exchanger. In this study, the Shear Stress Transport turbulence model (SSTk-ω) was used to pre-simulate the thermal characteristics of the CLEAR-S double-wall bayonet tube heat exchanger before the experiment. The verification shows that the numerical simulation results of the single bayonet tube structure are in good agreement with the RELAP5 simulation results and experimental data from CLEAR-S. CFD analysis reveals that the non-effective heat exchange area will preheat the liquid water on the side of the tube, so that the overall heat exchange performance decreases by about 4%. The heat conduction of the powder interlayer is the largest heat transfer resistance, which weakened the influence of the lead-bismuth eutectic (LBE) side flow and temperature inhomogeneity on the heat transfer of the tube bundle and significantly reduced the wall radial temperature difference of each bayonet tube. The conclusions of this study can provide a reference for the experiment and design of double-wall bayonet tube heat exchangers.

Detecting the Oscillation and Propagation of the Nascent Dynamic Solar Wind Structure at 2.6 Solar Radii Using Very Long Baseline Interferometry Radio Telescopes

Probing the solar corona is crucial to study the coronal heating and solar wind acceleration. However, the transient and inhomogeneous solar wind flows carry large-amplitude inherent Alfvén waves and turbulence, which make detection more difficult. We report the oscillation and propagation of the solar wind at 2.6 solar radii (Rs) by observation of China’s Tianwen and ESA’s Mars Express with radio telescopes. The observations were carried out on 2021 October 9, when one coronal mass ejection (CME) passed across the ray paths of the telescope beams. We obtain the frequency fluctuations (FFs) of the spacecraft signals from each individual telescope. First, we visually identify the drift of the frequency spikes at a high spatial resolution of thousands of kilometers along the projected baselines. They are used as traces to estimate the solar wind velocity. Then we perform the cross-correlation analysis on the time series of FF from different telescopes. The velocity variations of solar wind structure along radial and tangential directions during the CME passage are obtained. The oscillation of tangential velocity confirms the detection of a streamer wave. Moreover, at the tail of the CME, we detect the propagation of an accelerating fast field-aligned density structure indicating the presence of magnetohydrodynamic waves. This study confirms that the ground-station pairs are able to form particular spatial projection baselines with high resolution and sensitivity to study the detailed propagation of the nascent dynamic solar wind structure.

Developing a modelling approach for sound propagation through turbulent boundary layer and applying to train pantograph

Sound propagation through turbulent boundary layer (TBL) is of great significance in transports where a TBL is attached to their external surfaces, for instance, air planes and high speed trains. Precise predictions of sound propagation through TBLs to their external surfaces can be extremely useful for their corresponding interior noise control. It is well known that a TBL is classified as an inhomogeneous flow medium and existing approaches are not capable to well predict its influence on sound propagation. Insight of sound propagation through TBLs is required but relevant research on this topic is limited in the literature. In this work, a modelling approach is developed to predict the influence of TBLs on sound propagation. In the proposed approach, the ray tracing method is employed to predict the evolution of sound waves when they are passing through a TBL, with which the equivalent phase speed of sound can be obtained and used to calculate the the Doppler factor so that the traditional methods can be extended to predict sound distribution in inhomogeneous flow media. The validation of this modelling approach is performed by using two cases. The first case is to predict the convective effect of sound propagation in a uniform mean flow and the predicted results are compared with analytical solutions. The second case is sound propagation through a TBL and the predictions obtained from this proposed approach are compared with numerical simulations obtained from the finite element (FE) method. In both cases, the convective effect obtained from this proposed approach is close to the analytical solution or the FE prediction. After that, the proposed approach is used to predict the sound propagation from a high speed train pantograph to the external surfaces of the train with considering the influence of the TBL. The corresponding results are verified by using FE simulations. This work shows that the proposed approach is effective and efficient, and also, it enhances the understanding of sound propagation in inhomogeneous flow media.

Macroscopic traffic flow modelling of large-scale freeway networks with field data verification: State-of-the-art review, benchmarking framework, and case studies using METANET

Macroscopic traffic flow models are of paramount importance to traffic surveillance and control. Before their employments in applications, the models need to be calibrated and validated against real traffic data. The model calibration determines an optimal set of model parameters that minimizes the discrepancy between the modeling results and real traffic data. The model validation is furthermore performed to corroborate the accuracy of a calibrated model using data other than used for calibration. The model calibration aims to reflect traffic reality, while model validation focuses on the prediction of future traffic using calibrated models. This paper delivers a comprehensive review of state-of-the-art works on macroscopic model calibration and validation, proposes a benchmarking framework on traffic flow modeling, and has conducted a large number of case studies based on the framework using macroscopic traffic flow model METANET with respect to the urban expressway network in Shanghai. In comparison to previous works, quite more comprehensive results on model calibration have been presented in this paper, in consideration of congestion tracking, traffic flow inhomogeneity, capacity drop, stop-and-go waves, scattering, adverse weather conditions, and accidents. The paper has also reported many results of model validation with respect to the same field examples. The results demonstrate that METANET is able to model complex traffic flow dynamics in large-scale freeway networks with sufficient accuracy. The paper is closed with discussion on limitations and future works.

Local bistability under microwave heating for spatially mapping disordered superconductors

We theoretically study a strongly disordered superconducting layer heated by near-field microwave radiation from a nanometric metallic tip. The microwaves heat up the quasiparticles, which cool by phonon emission and conduction away from the heated area. Due to a bistability with two stable states of the electron temperature under the tip, the heating can be tuned to induce a submicrometer-sized normal region bounded by a sharp domain wall between high- and low-temperature states. We propose this as a local probe to access different physics from existing methods, for example, to map out inhomogeneous superfluid flow in the layer. The bistability-induced domain wall can significantly improve its spatial resolution.

Recent Developments in Theories of Inhomogeneous and Anisotropic Turbulence

Understanding inhomogeneous and anisotropic fluid flows requires mathematical and computational tools that are tailored to such flows and distinct from methods used to understand the canonical problem of homogeneous and isotropic turbulence. We review some recent developments in the theory of inhomogeneous and anisotropic turbulence, placing special emphasis on several kinds of quasi-linear approximations and their corresponding statistical formulations. Aspects of quasi-linear theory that have received insufficient attention in the literature are discussed, and open questions are framed.

Electrodeposited from Ion-Organic Melt Ti-B Protective Coatings on Al-Foil

The great demand for advanced batteries in applications such as mobile devices, the transportation sector, and stationary storage has spurred intensive research and development on rechargeable lithium batteries. The corrosion behavior of Al-foil, as material for theirs current collectors, has long been a concern due to uniform or localized pitting corrosion occurs in contact with an organic electrolyte. Pitting corrosion is attributed to inhomogeneous current flow through the aluminum / electrode interface and results in local breakdown of the passive film on Al-foil. Particular attention among different protective coatings of Al-foil excites Ti-B coatings as conductive and electrochemically inactive protective layers.The main objectives of the present work are investigation of regimes for electrodeposition of Ti-B protective coatings onto Al-foil and examination of the corrosion behavior of Al-foil with such electrodeposited Ti-B protective coatings in contact with an organic electrolyte under various electrodes charging conditions.Low melting point of aluminium (tm = 6600C) made impossible electrodeposition of Ti-B coatings from chloride-fluoride melts (tel = 7000C). But it is possible from ion-organic melts at 70−2000C. In this work was investigated electrodeposition of Ti-B coatings from carbamide melt contains titanium and boron oxides. Cyclic voltammetry has shown the limit current density and potential of electrochemical synthesis. Ultra fine Ti-B coatings were electrodeposited onto Al-foil at 1300C. Figure shows the surface morphologies of an obtained Ti-B coating. Such cost-effective method of obtaining Ti-B protective coatings on Al-foils gives excellent results under theirs using as current collectors in rechargeable lithium batteries. Figure 1

Self-diffusion in inhomogeneous granular shearing flows.

In this Letter, we discuss how flow inhomogeneity affects the self-diffusion behavior in granular flows. Whereas self-diffusion scalings have been well characterized in the past for homogeneous shearing, the effect of shear localization and nonlocality of the flow has not been studied. We, therefore, present measurements of self-diffusion coefficients in discrete numerical simulations of steady, inhomogeneous, and collisional shearing flows of nearly identical, frictional, and inelastic spheres. We focus on a wide range of dense solid volume fractions, that correspond to geophysical and industrial shearing flows that are dominated by collisional interactions. We compare the measured values first with a scaling based on shear rate and, then, on a scaling based on the granular temperature. We find that the latter does much better than the former in collapsing the data. The results lay the foundations of diffusion models for inhomogeneous shearing flows, which should be useful in treating problems of mixing and segregation.

Evaluation of constitutive models for shear-banding wormlike micellar solutions in simple and complex flows

Wormlike micellar solutions possess complex rheology: when exposed to a flow field, the wormlike micelles may orientate, stretch, and break into smaller micelles. Entangled wormlike micellar solutions exhibit shear banding characteristics: macroscopic bands with different local viscosities are organized and stacked along the velocity gradient direction, leading to a non-monotonic flow curve in simple shear. We present a systematic analysis of four commonly used constitutive models that can predict a non-monotonic flow curve and potentially describe the rheology of entangled wormlike micellar solutions with shear-banding characteristics: the Johnson–Segalman, the Giesekus, the thixotropic viscoelastic, and the Vasquez–Cook–McKinley (VCM) models. All four constitutive models contain a stress diffusion term, to account for a smooth transition between the shear bands and ensure a uniqueness of the numerical solution. Initially, the models are fitted to shear and extensional experimental data of a shear-banding wormlike micellar solution. Subsequently, they are employed to solve three non-homogeneous flows: the Poiseuille flow in a planar channel, the flow in a cross-slot geometry, and the flow past a cylinder in a straight channel. Each of these flows exposes the wormlike micellar solution to different flow kinematics (shear, extensional, and mixed), revealing different aspects of its rheological response. The predictive capability of each model is evaluated by directly comparing the numerical results to previously published experimental data obtained from microfluidic devices with corresponding flow configurations. While all the models can describe qualitatively the characteristic features observed experimentally in the benchmark flows, such as plug-like velocity profiles and elastic instabilities, none of them yields a quantitative agreement. Based on the overall performance of the models and also accounting for their differing numerical complexity, we conclude that the Giesekus model is at present the most suitable constitutive equation for simulating shear banding wormlike micellar solutions in flows that exhibit both shear and extensional deformations. However, the quantitative mismatch between model predictions and experiments with wormlike micellar solutions demand that improved constitutive models be developed in future works. • Comparison of popular constitutive models for wormlike micellar solutions. • Evaluation of their performance in rheometric and inhomogeneous shear flows. • Investigation of their predictions in extensional flows and elastic instabilities. • The models can only make qualitative predictions.

Spin imaging of Poiseuille flow of a viscous electronic fluid

Recent progress in fabricating high-quality conductors with small densities of defects has initiated the studies of the viscous electron fluid and has motivated the search for the evidences of the hydrodynamic regime of electron transport. In this work we come up with the spin imaging technique allowing us to attest to the emergence of electron hydrodynamic flows. Based on numerical calculations we demonstrate that the injected electron spin density is inhomogeneous across the channel when the viscous electron fluid forms the Poiseuille flow. We also argue that the Hanle curves at different positions across the channel acquire relative phase shifts resulting from the variation of the electron drift velocity in inhomogeneous hydrodynamic flows. The studied effects can be employed to evidence and study the viscous electron fluid non-invasively.

Preconditioned central moment lattice Boltzmann method on a rectangular lattice grid for accelerated computations of inhomogeneous flows

Convergence acceleration of flow simulations to their steady states at lower Mach numbers can be achieved via preconditioning the lattice Boltzmann (LB) schemes that alleviate the associated numerical stiffness, which have so far been constructed on square lattices. We present a new central moment LB method on rectangular lattice grids for efficient computations of inhomogeneous and anisotropic flows by solving the preconditioned Navier–Stokes (PNS) equations. Moment equilibria corrections are derived via a Chapman–Enskog analysis for eliminating the truncation errors due to grid-anisotropy arising from the use of the rectangular lattice and the non-Galilean invariant cubic velocity errors resulting from an aliasing effect on the standard D2Q9 lattice for consistently recovering the PNS equations. Such corrections depend on the diagonal components of the velocity gradients, which are locally obtained from the second order non-equilibrium moments and parameterized by an associated grid aspect ratio r and a preconditioning parameter γ, and the speed of sound in the collision model is naturally adapted to r via a physically consistent strategy. We develop our approach by using a robust non-orthogonal moment basis and the central moment equilibria are based on a matching principle, leading to simpler expressions for the corrections for using the rectangular grids and for representing the viscosities as functions of the relaxation parameters, r and γ, and its implementation is modular allowing a ready extension of the existing LB schemes based on the square lattice. Numerical simulations of inhomogeneous and anisotropic shear-driven bounded flows using the preconditioned rectangular central moment LB method demonstrate the accuracy and significant reductions in the numbers of steps to reach the steady states for various sets of characteristic parameters.

Rotational viscosity in spin resonance of hydrodynamic electrons

In novel ultrapure materials electrons can form a viscous fluid, which is fundamentally different in its dynamics from the electron gas in ordinary conductors with a significant density of defects. The shape of the nonstationary flow of such an electron fluid is similar to the alternating flow of blood in large-radius arteries [J. R. Womersley, J. Physiol. 127, 553 (1955)]. The rotational viscosity effect is responsible for the interconnection between the dynamics of electron spins and flow inhomogeneities. In particular, it induces the spin polarization of electrons in a curled flow via an internal spin-orbit torque acting on electron spins. Here, we show that this effect in an electron fluid placed in a magnetic field leads to a correction to the ac sample impedance, which has a resonance at the Larmor frequency of electrons. In this way, via the electrically detected spin resonance, the Womersley flow of an electron fluid can be visualized and the rotational viscosity can be measured.