Effective Kinematic Viscosity Research Articles

A Wall Effects and Means of Controlling the Evolution of Swirling Flows with Vortex Breakdown

This paper investigates numerically the bubble-type vortex breakdown apparition in the case of closed rotating flows of a viscous, axisymmetric, and incompressible fluid. First, a truncated conical/cylindrical cavity of spherical end disks is used to simulate and analyze the vortex structure under rigid surface conditions. The geometric effects of the enclosure are also studied. Vortex breakdown is demonstrated beyond the lower disk rotation rate threshold by introducing the no-slip condition imposed on the upper wall. The objective is to explore ways of controlling the evolution of this physical event by modifying the confinement conditions upstream of the vortex rupture. Particular attention is also paid to the effective kinematic viscosity, thermal diffusivity and geometric control of recirculation zones on the axis of rotation (axial bubble type). The second geometry consists of a spherical annulus formed by two concentric hemispheres in differential rotation under plat-free surface conditions. The results show that rotation of the inner hemisphere induces a vortex bubble on the polar axis. In contrast, the outer hemisphere rotation induces a toroidal vortex on the equator.

Effect of Kinematic Viscosity on Ion Acoustic Waves in Superthermal Plasma Comprising Cylindrical and Spherical Geometry

Effect of Kinematic Viscosity on Ion Acoustic Waves in Superthermal Plasma Comprising Cylindrical and Spherical Geometry

Falling Film Flow and Heat Transfer of Cryogenic Liquid Oxygen on Different Structural Surfaces

The accurate prediction of the falling film characteristics of cryogenic liquids is necessary to ensure good evaporation performance, due to their special physical properties. In this study, the film flow and heat transfer characteristics on four different structures were investigated, and the performance of the cryogenic liquid oxygen was compared with other fluids with higher temperatures, which demonstrates the influence of structures and liquid mediums. The VOF model was used to capture the film surface in the simulation model. The results show that for the four structures, liquids with higher kinematic viscosity tend to have greater film thickness, and the sensible heat transfer coefficients are inversely related to the nominal thermal resistance of falling film flow. Both on the smooth plate and the corrugated plate, the film wettability depends on the kinematic viscosity, rather than the dynamic viscosity, and the effect of kinematic viscosity is greater than that of surface tension. Both the local heat transfer coefficient and its fluctuation amplitude decrease gradually along the flow direction on the triangular corrugated plate, and the vortices are easier to produce at the wall troughs when the film viscosity is higher. At the bottom of the horizontal tube, the increases in local film thickness of the liquid oxygen are less than those of the water and the seawater. More liquid tends to accumulate at the bottom of the round tube, while it easily detaches from the film surface of the elliptical tube. For the horizontal tubes, the local heat transfer coefficients decrease rapidly when θ = 0–5°, and increase sharply at θ = 175–180°.

A computational fluid dynamic-based method for analyzing the nonlinear relationship between windage loss and pressure in a geotechnical centrifuge

Temperature control is an important limitation to further increase in geotechnical centrifuge power. Although vacuum pumps can reduce windage loss, they also negatively affect heat transfer performance. Therefore, in this study, we aim to accurately determine the rate at which windage loss decreases with pressure to help assess whether reducing pressure is beneficial to temperature control. A computational fluid dynamic method based on the multi-reference model and k–ω shear-stress transport turbulence model is used to simulate the ZJU400gt geotechnical centrifuge. The windage loss and temperature of ZJU400 at 0–150 gravity acceleration under normal pressure conditions are simulated. Compared with the experimental data, the error is < 20.7%, indicating simulation reliability. Furthermore, the simulation model is used to simulate the windage loss power under low-pressure conditions and predict the relationship between the windage loss power and pressure. Compared with current calculation methods, which yield a linear relationship between windage loss and operating pressure, the simulation results indicate a slightly nonlinear relationship. At 5,000 Pa, the simulated windage loss is 40% larger than the calculated value, severely affecting the temperature control design. Moreover, the velocity exhibits minimal variation with pressure, whereas the effective kinematic viscosity varies substantially. The nonlinear relationship between the windage loss and pressure can be attributed to increased turbulent kinetic energy and the size of the wake region caused by vacuum pumping. A formula for nonlinear windage loss with pressure is proposed, providing a basis for the future design of super-gravity geotechnical centrifuges.

Studies on the effect of kinematic viscosity on electron-acoustic cylindrical and spherical solitary waves in a plasma with trapped electrons

Journal of Applied Mathematics and Computational Mechanics, Prace Naukowe Instytutu Matematyki i Informatyki, Politechnika Częstochowska, Scientific Research of the Institute of Mathematics and Computer Science, Czestochowa University of Technology

Ultrashort Pulse Detection and Response Time Analysis Using Plasma-Wave Terahertz Field-Effect Transistors

We report on the response characteristics of plasmonic terahertz field-effect transistors (TeraFETs) fed with femtosecond and picosecond pulses. Varying the pulse width (tpw) from 10-15 s to 10-10 s under a constant input power condition revealed two distinctive pulse detection modes. In the short pulse mode (tpw << L/s, where L is the gated channel length, s is the plasma velocity), the source-to-drain voltage response is a sharp pulse oscillatory decay preceded by a delay time on the order of L/s. The plasma wave travels along the channel like the shallow water wave with a relatively narrow wave package. In the long pulse mode (tpw > L/s), the response profile has two oscillatory decay processes and the propagation of plasma wave is analogues to oscillating rod with one side fixed. The ultimate response time at the long pulse mode is significantly higher than that under the short pulse conditions. The detection conditions under the long pulse mode are close to the step response condition, and the response time conforms well to the analytical theory for the step function response. The simulated waveform agrees well with the measured pulse response. Our results show that the measurements of the pulse response enable the material parameter extraction from the pulse response data (including the effective mass, kinematic viscosity and momentum relaxation time).

Critical insights into thermohaline stratification for geothermal energy recovery from flooded mines with mine water

Geothermal applications with waste water in abandoned mines are a sustainable way of recycling wastes in abandoned facilities for utilizing clean energy. Thermohaline stratification in mine water is significant to this energy application, because it dominates the heat and mass transport in the mine-water-geologic-formation system and consequently determines the efficiency and sustainability of geothermal energy systems. This study addresses six unresolved issues for understanding the formation and evolution of thermohaline stratification via multiphysics simulations, including effects of key transport parameters on thermohaline stratification; mechanisms underlying layer-merging; effects of the buoyancy ratio on thermohaline stratification, and predictions of the initial distributions of temperature and salinity for thermohaline stratification. Our results showed that the effective kinematic viscosity is the most dominant transport parameter to determine the layer-merging speed and layer number of thermohaline stratification, where seven more thermohaline stratification layers could be observed if two orders of magnitude of this parameter are increased. For layer-merging, relatively “weak” interfaces, which have a small buoyancy ratio across the neighboring layers, disappear and are eroded first. Our results also revealed that the buoyancy ratio determines the layer number, where increasing the buoyancy ratio from 2.16 to 4 can induce twenty more layers. The initially linear temperature and salinity distributions in mine water are needed for predicting the present and future thermohaline stratification, especially the energy recovery. To meet this need, an approach was proposed to accurately predict such initial distributions via back-calculating field measurements. This study provides insights into understanding the key energy transport mechanisms in mine water and recommendations for facilitating future implementations of geothermal energy recovery with mine water dominated by thermohaline stratification.

Experimental investigation on a new hybrid laser process for surface structuring by vapor pressure on Ti6Al4V

Besides conventional structuring processes such as turning, milling or photo-chemical etching, laser processes are increasingly being used for surface structuring of metals. These laser processes differ fundamentally in that structuring is carried out either by material removal or by material redistribution. In this study, a new hybrid process of material ablation by means of pulsed laser radiation and material redistribution based on a remelting process by means of cw laser radiation is experimentally investigated. Besides an introduction to this new hybrid process, we give a detailed description of the equipment and methods used as well as surface structures produced on Ti6Al4V. A melt pool was generated on a prepared Ti6Al4V surface using cw laser radiation with a laser beam diameter of 520 μm, laser power of 220 W, and a scanning velocity of 100 mm/s. In order to create surface structures, simultaneously, superimposed pulsed laser radiation with a laser beam diameter of 65 μm, pulse duration of 60 ns, a maximum pulse energy of 0.35 mJ, and a pulse frequency of 50 kHz was used to evaporate small amounts of molten material from the melt pool. This localized evaporation of molten material is assumed to create vapor pressure that deforms the melt pool surface and therefore leads to surface structures. Our results indicate that by pulsed laser radiation capillary surface waves with a wavelength of the doubled laser beam diameter are excited on the melt pool surface. This forced excitation of capillary surface waves result in surface structures that are analyzed after solidification by means of white light interferometry. Based on this analysis we derived an oscillation frequency of ν = 2.27 (± 0.16) kHz for the excited capillary surface wave as well as an effective kinematic viscosity of μ = 0.1328 cm2 s−1 for the damping of this surface oscillation during solidification. In terms of structural features, we achieved surface structures with heights of up to 100 μm. Furthermore, structure height controllably scales in dependence on pulse energy and number of laser pulses as long as no ejection of molten material takes place. Finally, a comparison of the redistributed material volume per time shows that we achieved a volume redistribution rate of 28.37 mm3/min, which is significantly bigger than has been achieved with other laser texturing techniques so far and demonstrates the high potential of this new hybrid technique not only for surface structuring purposes.

Kinematic viscosity measurement of granular flows via low Reynolds number cylinder drag experiment

Confined, vibration-driven grain piles exhibit fluid-like properties, in particular, predictable, non-random flow patterns, hydrodynamic modal response to vibrational forcing, and a persistent, spatially uniform tendency toward local statistical mechanical equilibrium. This paper presents a technique that combines particle image velocimetry of vibration-driven grain flow over a submerged, instrumented cylinder and measurement of the flow-induced drag force on the cylinder to determine the grain flows effective kinematic viscosity. The fundamental basis of such measurements is provided by recent work showing that high-restitution grain piles subject to low-amplitude vibration are macroscopic dynamical analogs of liquid-state molecular hydrodynamic systems. Practically, the proposed viscometric method provides a key material property, the kinematic, or equivalently, dynamic viscosity, for use in computational fluid dynamic simulations of a variety of materials processing operations that utilize vibration-driven grain flows.

Thermal driven flows inside a square enclosure saturated with nanofluids: Convection heat functions and transfer rate revisions from a homogenous model

In former theoretical researches of nanofluid flows, numerical investigations could not agree with experimental observations, particularly regarding whether the mixing nanoparticles will enhance or deteriorate the heat transfer. In the present work, thermal driven buoyancy flows of nanofluids in a square enclosure were modeled by the use of homogeneous assumptions and the effective kinematic viscosity and thermal conductivity formulas. Thoroughly developed heat transfer coefficient is subsequently proposed, aiming to critically evaluate the performance of nanofluid heat transport. Numerical results are presented over a wide range of thermal Rayleigh number (103 ≤ Ra ≤ 106) and nanoparticles volume fraction (0.001 ≤ φ ≤ 0.04). Present modeling results accurately predict both the enhancement and deterioration of the natural convection heat transfer, fully validated by former experimental observations. Overall, mathematical models and Nusselt number definitions proposed in the present work effectively enhance the reliability of numerical modeling researches on the nanofluid heat transfer. Present clarification research on the Nusselt unifications could benefit future development of thermal carrier fluid enhanced by nano-particles.

Prediction of the density and viscosity of biodiesel and the influence of biodiesel properties on a diesel engine fuel supply system

Biodiesel has been considering as promising fuel in the situation of ever-depleting fossil fuels and serious environment pollution. Beside clearly known advantages, high viscosity and density, and high surface tension are the acute disadvantages that need to be improved to use directly for an unmodified diesel engine. In this work, 24 samples of biodiesel–diesel fuel blends created by mixing respectively 5%, 10%, 20%, 40%, 50%, 60%, 75% and 100% of fractions of Coconut oil biodiesel (COB), Jatropha oil biodiesel (JOB), Waste oil biodiesel (WOB) in diesel fuel were used for experimental purposes. In the first stage, the effects of biodiesel fractions and temperature on kinematic viscosity and density were developed. In the second stage, the effects of kinematic viscosity and density on fuel supply system, fuel pump network, fuel filters, and spray characteristics (including penetration length (Lb ) and Sauter Mean Diameter (SMD)) were investigated. Obtained results of this study based on experimental correlation and mathematical equation showed over 95% of the confident level from a regression correlation equation regarding kinematic viscosity and density.

Effect of kinematic viscosity on the staining performance of caries detector dyes

The relationship between performance of caries detector dyes and solvent molecular weight has been reported, but viscosity has not yet been investigated. Therefore, using extracted human teeth, we compared 6 experimental caries detector dyes with different viscosities to clarify the relationship between dye viscosity and staining performance in sclerotic dentin. Carious dentin started showing dentin staining when stained with caries detector dyes with kinematic viscosity≥263 mm2/s, showed no immunoreactivity for Streptococcus mutans on immunohistochemistry, and contained dentinal tubules free of debris on scanning electron microscopy. Samples stained using a caries detector dye with kinematic viscosity of 332 mm2/s contained calcification product-like debris in the dentinal tubules. These findings suggest that the caries detector dye has a threshold kinematic viscosity between 263 and 332 mm2/s at which sclerotic dentin and tissues beyond are not stained. The caries detector dye appears to control tooth dyeability based on solvent viscosity.

Dissipation in quantum turbulence in superfluid He4 above 1 K

There are two commonly discussed forms of quantum turbulence in superfluid $^4$He above 1K: in one there is a random tangle of quantizes vortex lines, existing in the presence of a non-turbulent normal fluid; in the second there is a coupled turbulent motion of the two fluids, often exhibiting quasi-classical characteristics on scales larger than the separation between the quantized vortex lines in the superfluid component. The decay of vortex line density, $L$, in the former case is often described by the equation $dL/dt=-\chi_2 (\kappa/2\pi)L^2$, where $\kappa$ is the quantum of circulation, and $\chi_2$ is a dimensionless parameter of order unity. The decay of total turbulent energy, $E$, in the second case is often characterized by an effective kinematic viscosity, $\nu'$, such that $dE/dt=-\nu' \kappa^2 L^2$. We present new values of $\chi_2$ derived from numerical simulations and from experiment, which we compare with those derived from a theory developed by Vinen and Niemela. We summarise what is presently known about the values of $\nu'$ from experiment, and we present a brief introductory discussion of the relationship between $\chi_2$ and $\nu'$, leaving a more detailed discussion to a later paper.

Mixed convection heat transfer of double immiscible fluids in functional gradient material preparation

Double immiscible flow has been extensively emerged in science and technology. In this research, mixed convection heat transfer of double immiscible fluids in a vertical parallel channel is investigated. One layer is pure fluid while the other is nanofluid. The effective kinematic viscosity and the effective thermal conductivity are regarded as linear functions of the nanoparticle loading parameter. A method of translation and scaling the computational domain for calculating the coupled equations subject to eight boundary conditions is developed. This method is applicable no matter the widths of the two regions are equal or not. Two interesting phenomena are revealed: the nanofluid with lower particle loading parameter is a better option for higher effectiveness of fluid driving; adding more nanoparticles in one layer can enhance heat transfer of both layers.

Visualization of grid-generated turbulence in He II using PTV

Due to its low viscosity, cryogenic He II has potential use for simulating large-scale, high Reynolds number turbulent flow in a compact and efficient apparatus. To realize this potential, the behavior of the fluid in the simplest cases, such as turbulence generated by flow past a mesh grid, must be well understood. We have designed, constructed, and commissioned an apparatus to visualize the evolution of turbulence in the wake of a mesh grid towed through He II. Visualization is accomplished using the particle tracking velocimetry (PTV) technique, where μm-sized tracer particles are introduced to the flow, illuminated with a planar laser sheet, and recorded by a scientific imaging camera; the particles move with the fluid, and tracking their motion with a computer algorithm results in a complete map of the turbulent velocity field in the imaging region. In our experiment, this region is inside a carefully designed He II filled cast acrylic channel measuring approximately 16 × 16 × 330 mm. One of three different grids, which have mesh numbers M = 3, 3.75, or 5 mm, can be attached to the pulling system which moves it through the channel with constant velocity up to 600 mm/s. The consequent motion of the solidified deuterium tracer particles is used to investigate the energy statistics, effective kinematic viscosity, and quantized vortex dynamics in turbulent He II.

Higher order corrections to dust-acoustic shock waves in a strongly coupled cryogenic dusty plasma

To investigate the contribution of higher-order nonlinearity and dissipation to nonlinear ultra-low-frequency dust-acoustic shock waves (DASWs), a reductive perturbation technique is employed in a strongly coupled cryogenic dusty plasma. The model consists of Boltzmann distributed electrons and ions, as well as equations for strongly coupled charged dust grains. A nonlinear Burger equation and a linear inhomogeneous Burger-type equation are derived. The present model admits both compressive and rarefactive dust-acoustic (DA) shocks. Including these higher-order corrections results in creating new shock wave structures called “humped DASWs.” It is shown that the effects of kinematic viscosity, the number of electrons residing on the dust grain surface, Zd, and the dust number density via β have important roles in the basic features of the produced DA shocks and the associated electric fields. It has been shown that it is more important to be included. These findings are devoted to explaining the observed waves propagating in the laboratory plasma experiments at cryogenic temperature. This kind of plasma is used for etching nano-patterns without defects, used in nonideal systems, and physics of nucleation to deposit nanosize Si compounds.

On mixed convection of two immiscible layers with a layer of non-Newtonian nanofluid in a vertical channel

Mixed convection in a vertical parallel plate channel is analyzed with one region filled with Newtonian fluids and another region with non-Newtonian nanofluids. Pseudo-plastic and dilatant fluids are used as working base flow of nanofluids. Power-law modelling is adopted to predict the effect of non-Newtonian behavior on the fluid flow and thermal performance of nanofluids, and the effective kinematic viscosity and thermal conductivity are determined using linear relations as functions of nanoparticle loading parameter. The velocity and temperature fields are investigated, considering the influence of power-law index n(0.6≤n≤1.4) and nanoparticle volume fraction ϕ(0≤ϕ≤4%). Two cases have been paid special attention in the present work, that is, Case I: in the absence of viscous dissipation (Br=0, GR≠0), and Case II: in the absence of buoyancy forces (GR=0, Br≠0) when purely forced convection happens. It is observed that the power-law index has a strong effect on the shape of velocity profile, and on the scale of temperature curves. The results reveal that the power-law effect on both velocity and temperature is more observable in shear-thinning fluids than that in the dilatant flow. These findings demonstrate that, in some potential applications, the shear-thinning fluid should be chosen for its higher effectiveness on fluid driving where the non-Newtonian fluid is used to drive Newtonian flow via interface shear.

Numerical Modeling of Flow Dynamics in The Aluminum Smelting Process: Comparison Between Air–Water and CO2–Cryolite Systems

Air–water models have been widely applied as substitutes for CO2–cryolite systems in the study of the complex bubble dynamics and bubble-driven flow that occurs in the molten electrolyte phase in the aluminum electrolytic process, but the detailed difference between the two systems has not been studied. This paper makes a numerical comparison between the bubble dynamics for the two systems. Simulations of both single bubble and continuous bubbling were conducted using a three-dimensional computational fluid dynamics (3D CFD) modeling approach with a volume of fluid (VOF) method to capture the phase interfaces. In the single bubble simulations, it was found that bubbles sliding under an anode in a CO2–cryolite system have a smaller bubble thickness and a higher sliding velocity than those in the air–water system for bubbles of the same volume. Dimensionless analysis and numerical simulation show that contact angle is the dominant factor producing these differences; the effects of kinematic viscosity, surface tension, and density are very small. In the continuous bubbling simulations, the continuous stream of air bubbles detaches from the anode sidewall after a period of climbing, just as it does in the single bubble simulation, but bubbles have less tendency to migrate away from the wall. Quasi-stable state flow characteristics, i.e., time-averaged bath flow pattern, turbulence kinetic energy, turbulence dissipation rate, and gas volume fraction, show a remarkable agreement between the two systems in terms of distribution and magnitude. From the current numerical comparisons, it is believed that the air–water model is a close substitutive model for studying bubble-driven bath flow in aluminum smelting processes. However, because of the difference in bubble morphologies between the two systems, and also the reactive generation and growth of bubbles in the real system, there will likely be some differences in bubble coverage of the anode in the anode–cathode gap.

Determination of the effective kinematic viscosity for the decay of quasiclassical turbulence in superfluidHe4

The energy dissipation of quasiclassical homogeneous turbulence in superfluid $^{4}\mathrm{He}$ (He II) is controlled by an effective kinematic viscosity ${\ensuremath{\nu}}^{\ensuremath{'}}$, which relates the energy decay rate $dE/dt$ to the density of quantized vortex lines $L$ as $dE/dt=\ensuremath{-}{\ensuremath{\nu}}^{\ensuremath{'}}{(\ensuremath{\kappa}L)}^{2}$. The precise value of ${\ensuremath{\nu}}^{\ensuremath{'}}$ is of fundamental importance in developing our understanding of the dissipation mechanism in He II, and it is also needed in many high-Reynolds-number turbulence experiments and model testing that use He II as the working fluid. However, a reliable determination of ${\ensuremath{\nu}}^{\ensuremath{'}}$ requires the measurements of both $E(t)$ and $L(t)$, which was never achieved. Here we discuss our study of the quasiclassical turbulence that emerges in the decay of thermal counterflow in He II above 1 K. We were able to measure $E(t)$ by using a recently developed flow-visualization technique and $L(t)$ via second-sound attenuation. We report the ${\ensuremath{\nu}}^{\ensuremath{'}}$ values in a wide temperature range determined from a comparison of the time evolution of $E(t)$ and $L(t)$.

A Comparison of Three Approaches to Compute the Effective Reynolds Number of the Implicit Large-Eddy Simulations

The implicit large-eddy simulation (ILES) has been utilized as an effective approach for calculating many complex flows at high Reynolds number flows. Richtmyer–Meshkov instability (RMI) induced flow can be viewed as a homogeneous decaying turbulence (HDT) after the passage of the shock. In this article, a critical evaluation of three methods for estimating the effective Reynolds number and the effective kinematic viscosity is undertaken utilizing high-resolution ILES data. Effective Reynolds numbers based on the vorticity and dissipation rate, or the integral and inner-viscous length scales, are found to be the most self-consistent when compared to the expected phenomenology and wind tunnel experiments.