Velocity Gradient Research Articles

Evaluation of mixed convection-radiation flow of a viscous fluid restricted to a vertical porous channel: A comparative study

This article examines the steady mixed convection flow in an upstanding porous channel of a hydrodynamic viscous fluid that is electrically conductive due to the thermal radiation effect. The homotopy perturbation method (HPM) is used to generate analytical solutions to the governing differential equations that characterize the velocity and temperature flow properties. For relevant temperature jump and velocity slip situations, the effects of mixed convection parameters, thermal radiation parameters, and magnetic field effects on the velocity field, skin friction coefficient, temperature distribution, and heat transfer rate have been described. This research also analyses and compares the results obtained by Abbas et al. (2020) with this present work when HPM was used for the limiting cases. It is interesting to report that an excellent agreement was established, thereby authenticating and validating the accuracy of HPM as a strong tool for obtaining approximate solutions. According to the results of this study, it is possible to effectively control the velocity and temperature gradients by varying the main relevant parameters such as: the mixed convection parameter for a constant pressure gradient, the thermal radiation parameter, the magnetic field effect, the rarefaction parameter, and the wall ambient temperature difference ratio parameter

A FEM Flow Impact Acoustic Model Applied to Rapid Computation of Ocean-Acoustic Remote Sensing in Mesoscale Eddy Seas

Mesoscale eddies have an impact on the marine environment mainly in two areas, namely, currents and changes in the sound velocity gradient due to temperature and salt stirring. The traditional underwater-related remote sensing acoustic remote sensing model is capable of analyzing the acoustic field under the change in sound velocity gradient, but it is not capable of analyzing the acoustic field under the influence of ocean currents. In order to more effectively analyze the changes in the acoustic field caused by mesoscale eddies, this paper proposes a FEM flow impact model applied to the rapid computation of acoustic remote sensing of mesoscale eddies in the sea area. The algorithm first performs a grid optimization of the sea area model based on vertical sound velocity variations and completes the classification of sound velocity layer junctions. At the same time, we construct the sound velocity gradient environment affected by the mesoscale eddy and then simplify the fluid flow in the mesoscale eddy into a non-viscous and non-rotating velocity potential, and then participate in the solution of the three-dimensional spatial fluctuation equations in the form of time-harmonic in the frequency domain, from which we can obtain the truncated sound pressure as well as the propagation loss, and quickly and completely characterize the acoustic remote sensing of the sea area of the mesoscale eddy. The paper verifies the effectiveness of the algorithm through SW06-contained flow experiments and further proposes an optimization formula for sound velocity inversion. We analyze this using measured data of mesoscale eddy fields in the Bering Sea waters during 2022 and find that eddies have a greater effect on the propagation of the acoustic field along their flow direction.

Investigation of hydraulic losses in different clocking positions of pump-turbine in pump mode

With the increasing significance of pumped storage in the power system, there is a growing emphasis on improving the hydraulic performance of pump-turbine at pump mode. Consequently, this study aims to investigate the clocking effects of the stay vane and guide vane in order to enhance hydraulic performance. Moreover, the local hydraulic loss rate method is employed to visualize the loss distribution of each component. The results demonstrate that the clocking effect significantly impacts off-design conditions. Specifically, for different clocking positions of the stay vane, the maximum efficiency difference at 0.8Qd and 1.2Qd conditions is 0.14 % and 0.86 % respectively. Similarly, for the guide vane, the values are 0.54 % and 0.34 % respectively. Furthermore, different clocking schemes result in significant changes in hydraulic losses in the volute and stay vane. For the 0.8Qd condition, the hydraulic losses inside the impeller and guide vanes vary significantly with the change in the velocity gradient at the impeller outlet. The optimal clocking effect position for the guide vanes occurs when the trailing edge and the tongue are aligned, and similarly, the position for the stay vanes is when the tongue is located at the 2/3 point of the stay vane flow passage. The study can provide a theoretical guiding significance for structural optimization and installation position of pump-turbine.

Energy equality of weak solutions of the Navier–Stokes–Fourier equations allowing vacuum

In this paper, we are concerned with the conservation of energy criterion of the weak solutions to the Navier–Stokes–Fourier equations on the torus allowing vacuum. We establish the energy conservation criteria via the gradient of velocity for the weak solutions of this system, which generalizes the corresponding recent energy conservation criteria in terms of the velocity obtained by Aoki and Iwabuchi in Aoki and Iwabuchi (2022). Moreover, sufficient conditions for weak solutions keeping the energy balance allowing vacuum are shown for the full Navier–Stokes equations.

Ionized gas extends over 40 kpc in an odd radio circle host galaxy.

A new class of extragalactic astronomical sources discovered in 2021, named odd radio circles (ORCs)1, are large rings of faint, diffuse radio continuum emission spanning approximately 1 arcminute on the sky. Galaxies at the centres of several ORCs have photometric redshifts of z ≃ 0.3-0.6, implying physical scales of several 100 kpc in diameter for the radio emission, the origin of which is unknown. Here we report spectroscopic data on an ORC including strong [O II] emission tracing ionized gas in the central galaxy of ORC4 at z = 0.4512. The physical extent of the [O II] emission is approximately 40 kpc in diameter, larger than expected for a typical early-type galaxy2 but an order of magnitude smaller than the large-scale radio continuum emission. We detect an approximately 200 km s-1 velocity gradient across the [O II] nebula, as well as a high velocity dispersion of approximately 180 km s-1. The [O II] equivalent width (approximately 50 Å) is extremely high for a quiescent galaxy. The morphology, kinematics and strength of the [O II] emission are consistent with the infall of shock ionized gas near the galaxy, following a larger, outward-moving shock. Both the extended optical and radio emission, although observed on very different scales, may therefore result from the same dramatic event.

Does shear viscosity play a key role in the flow across a normal shock wave?

Once there is a velocity gradient in a viscous fluid flow, such as that across a shock wave, a viscous force and viscous energy loss exist inside the flow according to the Navier-Stokes equation (NSE), which may confuse the relative contribution of compressibility and viscosity. In this paper, a viscous shear vector is defined as the component of gradient vector of local velocity magnitude perpendicular to the velocity vector. Then, a local viscous energy flux vector is defined from the viscous shear vector after being multiplied by the viscosity and the velocity magnitude. The divergence of the viscous energy flux vector results in new expressions for viscous force and loss of viscous energy, in which all the square terms of derivative of velocity components correspond to irreversible energy loss. The rest part can be taken as a kind of mechanical energy transfer done by the viscous force, from which the viscous force components can be got based on the assumption that the viscous force vector is parallel to the velocity vector. The new equations are different from and more complex than those in the traditional NSE. By the new theory, it is shown that there is no shear viscous force and shear viscous energy loss in the flow across a normal shock wave without velocity gradient perpendicular to the flow direction.

Experimental Investigation and CFD Analysis of Wind Turbine Blades with Different Attack Angles

The escalating global demand for energy coupled with escalating environmental concerns has underscored the imperative of efficacious energy conversion from renewable reservoirs. Among these, wind energy has ascended as a pragmatic and ecologically conscientious solution. Its ascent, outpacing conventional fuels such as coal, underscores the necessity to comprehend its performance intricately. This study zeroes in on an airfoil model, subjecting it to a dual scrutiny encompassing empirical investigation and computational simulation. Employing Computational Fluid Dynamics (CFD) analyses executed in ANSYS software, the study prognosticates pressure and velocity patterns for the 2D iteration of the Model 1 airfoil by the National Advisory Committee for Aeronautics (NACA). This exhaustive scrutiny spans across velocities of 10 m/s and diverse angles of attack (-5°, 2°, and 8°). Remarkably, a robust 90% correlation manifests between the outcomes of empirical experimentation and computational simulation. Within the aerodynamic schema of the Horizontal Axis Wind Turbine (HAWT), the 8° angle of attack emerges as the vanguard, distinctly illustrating the pinnacle of optimal pressure distribution and velocity gradient. Noteworthy is the consistent augment in airfoil performance as the blade angle escalates, substantiated by elevated apex velocities and pressures in juxtaposition to the -5° and 2° angles. The findings of this inquiry engender a significant stride in airfoil refinement for the optimization of wind turbine blades, thereby conferring invaluable insights in the realms of blade design, aerodynamic contemplations, and the augmentation of wind turbine performance.

Visualizing flow dynamics and restart of Carbopol gel solutions in tube and parallel-plates geometries with wall slip.

The present study examines the impact of slip in Carbopol solutions during the restart flow in pipelines utilizing in situ visualization techniques. Rheological tests were conducted using smooth and hatched parallel plate geometries to obtain the rheological characteristics of the solutions. The behavior of the solutions in the creep tests is compared with those in the experimental unit. Three flow regimes were identified through rheological and experimental setup tests: non-flow, slip, and yielded. The slip regime allowed the establishment of a slip static yield stress value, indicating significant deformation states, and a restart pressure decrease of about 61% when compared to the static yield stress. The flow dynamics under the wall slip effect is captured by velocity profiles, velocity contour maps and velocity gradient. Transient correlations of the scaling law type were determined, with wall slip in proportion to the velocity gradient and wall shear stress. Additionally, the concentration of viscoplastic material in the solution increased the scaling law index. This research seeks to provide valuable findings by quantifying the effects of apparent wall slip through in situ measurements. Such insights are crucial for designing and managing pipeline transport systems that handle yield stress fluids, applicable across various industries including cosmetics, food processing, and the production of waxy oils.

Modeling the trajectories of floating and non-floating microplastic particles in the water column

The distribution and movement of microparticles of plastic (MP) in freshwater and marine environments are determined by the characteristics of both MP and the surrounding flow. The most common plastic polymers in the aquatic environment, such as polyethylene and polypropylene, have a density lower than the density of water. Therefore, based only on buoyancy, it is expected that most MP will be present in the surface layers of the aquatic environment. In both freshwater and marine environments, turbulence-induced mixing depends on factors such as velocity gradient and convective flow. Consequently, the surface wind, the deep temperature gradient and the breaking of waves in the surf zone can lead to the formation of turbulence and deep vertical mixing. In such conditions, in addition to gravity and buoyancy, the motion caused by turbulent mixing can affect the vertical transport and distribution of particles. The paper presents the results of modeling Lagrangian trajectories for floating particles and non-floating particles, under wave conditions they correspond to regular waves, while the particle sizes range from 10 microns to 5 mm, and the density ranges from 0.88 to 2.80 g/cm3, which is within some of the most common densities of microplastic particles.

Polygon model for solution of non-linear velocity gradient of interface and asymmetric break-up of droplet

In this study, a novel model is proposed to accurately depict the changes in the droplet-matrix interface as a function of the velocity gradient at different times. Using the iteration method and drawing the droplet shape, the interface velocity gradient-matrix shear rate non-linear equation is simultaneously solved. The initial droplet shape is assumed to be a polygon, with each side independently undergoing modifications under stress. By sequentially adjusting each side at different time points, the overall shape of the droplet is reconstructed. The experimental section examines droplets consisting of alcohols (polyvinyl alcohol and polyethylene glycol) in water-soluble form with different concentrations and glycerol. The matrix material used is poly dimethyl siloxane. The viscosity ratio between the droplet and matrix falls within the range of 0.18–3.08. The experimental findings show two types of droplet breakup: a single-end bulb and two non-uniform end bulbs of different sizes. The simulation results from the model align well with the experimental observations, accurately capturing the dimensions and volume of the droplets and the aspect ratio of the bulbs. The model also predicts the aspect ratio, lateral area, and rotation of the droplets before breakup with minimal deviation from the experimental data.

РАЗРАБОТКА ПРОГРАММНОГО ОБЕСПЕЧЕНИЯ МОДЕЛИРОВАНИЯ ПРОЦЕССОВ ТЕПЛОМАССОПЕРЕНОСА ПРИ МНОГОФАЗНОМ ПОТОКЕ

Mathematical models simulating the process of heat and mass transfer in a multiphase flow and taking into account the thermal conductivity of the working fluid, taking into account its location in space, physical properties and other characteristics, are difficult to implement in the form of software. Usually such models are represented by nonlinear equations, which makes it difficult to implement them correctly using numerical methods. When developing such mathematical models, hydrodynamics, heat transfer of a moving working fluid and the dependence of viscosity on temperature and velocity gradient are not taken into account. (Research purpose) The research purpose is developing a mathematical model of the functioning of the heat supply system as a set of cylindrical pipelines with a heat source through which the working fluid moves. Additional conditions are introduced to achieve high accuracy of calculations at boundary points. The possibility of using analytical equations to calculate the internal heat dissipation during the transportation of the working fluid inside the pipeline is studied. (Materials and methods) Using the two-phase delay theory (DPL-model) and the finite element method (FEM) as flow balancing methods, we obtain a model of the heat exchange of the working fluid motion in the pipeline with the possibility of taking into account relaxation properties. (Results and discussion) The developed algorithm is implemented using the high-level interpreted programming language MATLAB and its simulation environment for complex technical systems Simulink. (Conclusions) Equations modeling the real motion of a multiphase working fluid in locally nonequilibrium systems, including heat supply systems, are obtained. The results obtained make it possible to compile an algorithm and implement it using the high-level interpreted programming language MATLAB, defining a working version of the software for modeling heat and mass transfer processes in a multiphase flow.

Denoising four-dimensional flow magnetic resonance imaging data using a split-and-overlap approach via singular value decomposition

Four-dimensional flow magnetic resonance imaging (4D flow MRI) offers a powerful tool for visualizing fluid flows, critical for both diagnosing cardiovascular diseases and analyzing engineering fluid dynamics. Despite its potential in medical research, the clinical applicability of 4D flow MRI often faces challenges due to inherent noise. To mitigate this, we introduce the split-and-overlap singular value decomposition (SOSVD) filter, a distinctive noise reduction approach. Unlike traditional singular value decomposition methods, the SOSVD filter partitions the primary data matrix into overlapping subdomains and then applies singular value decomposition to each subdomain, preserving only the dominant mode for noise attenuation. Evaluations on simulated and experimental flow data within a square duct revealed a significant decrease in root mean square noise metrics. Moreover, when applied to in vivo aortic data, the SOSVD filter enhanced various flow determinants, including divergence, velocity gradients, streamlines, and velocity coherence. Thus, the SOSVD method presents a promising avenue for augmenting noise reduction in 4D flow MRI, potentially elevating diagnostic accuracy and enriching cardiovascular disease research.

Role of wall roughness on interaction of leakage flow and main flow in a mixed flow pump with tip clearance

Due to machining precision, spraying coating, cavitation, and erosion, walls of pump flow passage components have different roughness, which will affect the flow state near the wall and even the main flow. In the present work, four types of roughness Ra = 0, 0.1, 1, and 10 μm are set to investigate the influence of wall roughness on the interaction of leakage flow and main flow in a mixed flow pump with tip clearance. Results show that the interaction area of leakage flow and main flow can be divided into three regions: (a) tip clearance zone, (b) shear flow zone, and (c) primary vortex zone. In the tip clearance zone, as roughness increases, the velocity magnitude and its derivative at the same wall distance gradually decrease, and the velocity angle at blade tip continuously increases in the inner circular boundary layer. As the roughness increases, the leakage flow gradually decreases, especially in the center chord region. In the shear flow zone, the velocity gradient rapidly decreases along the direction of pressure side to suction side, and finally, it becomes stable. As roughness increases, the velocity gradient intensity decreases, and the main reason is that the differences of velocity magnitude and velocity direction between the inner circular boundary layer and potential region decrease. In the primary vortex zone, the tip leakage vortex core indicated by the Liutex vortex method coincides with the low-pressure core location. As the roughness increases, the tip leakage vortex core becomes closer to the next blade. The pressure coefficient and vorticity of the tip leakage vortex core decrease with increasing of roughness, and the slopes remain constant on different cross sections with value of 5.9 × 103 and 2.4 × 103 m−1, respectively.

Diagnosis of Circumstellar Matter Structure in Interaction-powered Supernovae with Hydrogen Line Features

Some supernovae (SNe) are powered by the collision of the SN ejecta with dense circumstellar matter (CSM). Their emission spectra show characteristic line shapes of combined broad emission and narrow P Cygni lines, which should closely relate to the CSM structure and the mass-loss mechanism that creates the dense CSM. We quantitatively investigate the relationship between the line shape and the CSM structure by Monte Carlo radiative transfer simulations, considering two representative cases of dense CSM formed by steady and eruptive mass loss. Comparing the Hα emission between the two cases, we find that a narrow P Cygni line appears in the eruptive case but does not appear in the steady case due to the difference in the velocity gradient in the dense CSM. We also reproduce the blueshifted photon excess observed in some Type IIn SNe, which is formed by photon transport across the shock wave, and find the relationship between the velocity of the shocked matter and the amount of blueshift of the photon excess. We conclude that the presence or absence of narrow P Cygni lines can distinguish the mass-loss mechanism and suggest high-resolution spectroscopic observations with λ/Δλ ≳ 104 after the light-curve peak for applying this diagnostic method.

A numerical investigation of momentum flux and kinetic energy transfers between turbulent wind and propagating waves

This paper focuses on simulating turbulent flow over propagating waves by solving the full Navier–Stokes equations in a moving frame. A careful comparison of flow statistics with previous experimental and numerical results demonstrates, to some extent, the rationality of simplifying wind waves as turbulent flow over moving wave boundaries. The phase-averaging method is then applied to investigate the momentum and energy transfers between turbulent wind and waves propagating at slow, intermediate and fast speeds. The results suggest that the dominant mechanism for producing Reynolds shear stress (RSS) and turbulent kinetic energy (TKE) is related to the wave age. Slow waves produce RSS and TKE similar to a two-dimensional shear turbulence. However, a fast wave enhances the streamwise Reynolds normal stress, the windward side's negative RSS and the gradient of both streamwise and vertical velocities, leading to additional RSS and TKE productions that can be ignored under the slow wave regimes. A strengthening wave–turbulence exchange is also found for fast waves. The intermediate wave can be regarded as a transitional condition determining this change.

ELECTROTHERMO-RHEOLOGICAL PERISTALSIS: INVESTIGATING VELOCITY SLIP AND MODIFIED DARCY’S POROSITY EFFECTS

This study examines velocity slip and modified Darcy's porosity in an incompressible Carreau material flowing through an inclined channel under the influence of electroosmotic peristalsis. Modified Darcy's resistance, Gauss's law, and Carreau model equations are utilized in the fundamental equations of motion, Poisson's equation, and heat transfer equation. The objectives and methodology of the study are specified in order to effectively discuss the model results. The governing equations are susceptible to long wave and Debye-Huckle approximations. The non-dimensionalized equations identify controlling variables that facilitate the detection of temperature, velocity, and pressure gradients. Mathematica is used to solve the resultant nonlinear problem in order to analyze the variation in physical quantities of interest and temperature in the Carreau fluid flow pattern. The investigation provides primary findings, including quantitative statistics regarding velocity slip, modified Darcy's porosity, and temperature distribution. The results of the nonlinear system are graphically analyzed and discussed. It is noticed that higher values of electroosmotic parameter cause a decrease in temperature profiles, while a rise in Darcy's number causes a rise in the axial velocity's magnitude. Understanding the behaviors of Carreau fluids under the influence of electroosmotic peristalsis has potential applications in a vast array of biological microfluidic devices.

Effect of triangular ribs on the flow and heat transfer characteristics of heat exchanger tube

To improve the heat exchange tube?s comprehensive performance and achieve enhanced heat transfer with lower flow resistance. The flow and heat transfer characteristics of a newly enhanced tube with triangular ribs were studied by numerical simulation. The results show that the multi-vortex longitudinal swirl developed in the triangular rib enhanced tube can enhance the cold and hot fluid mixing, and make the temperature distribution in the tube more uniform. The field synergy of velocity and temperature gradient was improved and the heat transfer capacity was enhanced. In the triangular rib enhanced tube, reducing the dimensionless pitch ratio of triangular ribs (P* = 0.5, 0.75, 1, and 1.25) and appropriately increasing the area of triangular ribs (A = 8, 12, 16, and 20 mm2) can improve the performance evaluation criteria (PEC). When Re = 8475, P* = 0.5, and A = 20 mm2, the maximum PEC = 1.324 is obtained.

Numerical evaluation of the heat transfer performance of water-cooled system for electric vehicle drive motor based on the field synergy principle

Thermal management of drive motors is a challenging design for transport electrification industries such as electric vehicles. Due to the high heat transfer performance, water-cooled system have become one of the technologies to meet the needs of these industries. Meeting the synergy between the heat transfer performance of the water-cooled system and the heat generated by the motor is key to taking advantage of the performance of drive motors. This study numerically evaluates and experimentally investigates the heat transfer performance of the electric vehicles drive motor water-cooled system based on field synergy principle, discussing the synergy between the temperature gradient field and velocity field of the water-cooled system under different cooling conditions, and the calculated results are consistent with the test results. By observing the interaction between the fluid vortexes and the main flow, the distribution pattern of the synergy angle in the cooling channels was determined. The results show that when the heat transfer capacity of the water-cooled system reaches its peak, the increase in Reynolds number instead leads to the increase in the average synergy angle of the whole field, which ultimately causes a deterioration of synergy between the velocity and temperature gradient fields.

Two fluid dynamics in solar prominences

Aims. Solar prominences contain a significant number of neutral species. The dynamics of the ionised and neutral fluids composing the prominence plasma can be slightly different if the collisional coupling is not strong enough. The differential dynamics can be discerned by tracing line-of-sight velocities using observational techniques. Large-scale velocities can be quantified by measuring the global local and instantaneous displacement of spectral lines by the Doppler effect. Small-scale velocities leave their imprint on the width of spectral lines. In addition, these small-scale velocities can have a thermal (pure stochastic motion) nature or a non-thermal (small-scale unresolved instabilities, high-frequency waves, etc.) origin. For this work, we used one spectral line of ionised and two spectral lines of neutral elements to measure the resolved and unresolved velocities in a prominence with the aim to investigate the possible decoupling of the observed charged and neutral species. Methods. A faint prominence was observed with the German Vacuum Tower Telescope (VTT) on June 17, 2017. Time series consisting of repeated ten-position scans over the prominence were performed while simultaneously recording the intensity spectra of the Ca II IR 854.2 nm, Hα 656.28 nm, and He I D3 587.56 nm lines. The line-of-sight velocities and the Doppler width of the three spectral lines were determined at every spatial position and temporal moment. To make sure all spectral lines were sampling the same plasma volume, we applied selection criteria to identify locations with optically thin plasma. In addition, asymmetric or double-peaked profiles were also excluded for the analysis, since (even in an optically thin regime) they are indicative of the presence of strong velocity gradients or multiple components in the line of sight. Thus, only optically thin, symmetric, single-lobed profiles were retained for this study. As an additional reliability test of the selection criteria, we have also compared our results with optical thickness calculations. Results. After the application of all the selection criteria, only a region close to the prominence border met all requirements. The velocities of the three spectral lines turned out to be very similar over this region, with the ionised Ca II IR showing velocity excursions systematically larger compared to those of the neutral lines of Hα and He I at some moments. The latter was found to be much closer to each other. Most of the velocity differences were below 1 km s−1. The analysis of the Doppler widths indicated that the Ca II IR line shows an excess of unresolved motions. We cannot establish whether these velocities are related to a different temperature of the ions or to unresolved small-scale motions due to any non-thermal mechanism. Conclusions. The dynamics of the ionised and neutral plasma components in the observed prominence were very close to one another. The differences found may indicate that a localised decoupling between ions and neutrals may appear at particular spatial locations or instants of time. Indications of different unresolved motions between those species have also been obtained.

Low-Prandtl-number effects on global and local statistics in two-dimensional Rayleigh–Bénard convection

We present global and local statistical properties of turbulent Rayleigh–Bénard (RB) convection at low Prandtl numbers in this work. A series of high resolution two-dimensional (2D) direct numerical simulations are carried out in a square box for the Prandtl number ranges 0.005≤Pr≤0.07 and 0.01≤Pr≤0.15 at Rayleigh numbers Ra=107 and Ra=108, respectively. The global heat and momentum transport expressed as Nusselt number Nu and Reynolds number Re are found to scale as Nu∼Pr0.14 and Re∼Pr−0.82 for Ra=107, and Nu∼Pr0.11Re∼Pr−0.93 for Ra=108. The local velocity fluctuation at the cell center shows larger amplitudes at lowered Pr, indicating a stronger turbulence in the bulk. The magnitudes of kinetic and thermal energy dissipation rates in the bulk also increase with the decreasing of Pr, due to the intensified velocity gradient and larger thermal diffusivity, respectively. In the cell central region, probability density functions (PDFs) of velocity show a bimodal distribution, and it approaches the Gaussian distribution at higher Pr, while the PDFs of temperature display a stretched exponential shape with intermittent behavior. The kinetic energy spectra further reveal that the velocity cascade follows the Bolgiano–Obukhov scaling in the bulk of the convective flow.