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Song, Fuquan, Heying Ding, Lintao Huang, Yong Wang i Yeheng Sun. "Research on non-Newtonian characteristics of crude oil flow at micro-nano scale". Physics of Fluids 35, nr 4 (kwiecień 2023): 042011. http://dx.doi.org/10.1063/5.0145727.
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The characteristic scale of flow in micro–nanochannels is generally in the range of 0.01 μm∼1 μm. When crude oil passes through micro-nano channels and tight reservoirs, it shows obvious nonlinear seepage characteristics, which does not conform to the continuity assumption of fluid. Therefore, a non-Newtonian model of crude oil flowing in micro-nano channels and tight reservoirs under the action of shear stress is established, and the relationship between flow rate and apparent viscosity and shear rate is analyzed. The experiment of crude oil flow in micro-nano channels and tight oil reservoir cores shows that the model can be used to describe the nonlinear seepage law of liquid through the nonlinear fitting. The power law index of the oil-phase power-law non-Newtonian fluid is greater than 1 at the micro-nano scale, which conforms to the flow characteristics of the expansive fluid, thus verifying the effectiveness of the non-Newtonian model. In addition, the study of apparent viscosity and shear rate of non-Newtonian fluid shows that the increasing and decreasing trends of flow rate and shear rate and the changing trends of flow rate and pressure gradient are consistent, and shear rate can be used to describe the characteristics of fluid instead of the pressure gradient.
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Luding, S. "Meso-scale transport in sticky granular fluids". Journal of Fluid Mechanics 864 (7.02.2019): 1–4. http://dx.doi.org/10.1017/jfm.2019.34.
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Fluid mechanics and rheology involve many unsolved challenges related to the transport mechanisms of mass, momentum and energy – especially when it comes to realistic, industrially relevant materials. Very interesting are suspensions or granular fluids with solid, particulate ingredients that feature contact mechanics on the micro-scale, which affect the transport properties on the continuum- or macro-scale. Their unique ability to behave as either fluid, or solid or both, can be quantified by non-Newtonian rheological rules, and results in interesting mechanisms such as super-diffusion, shear thickening, fluid–solid transitions (jamming) or relaxation/creep. Focusing on the steady state flow of a granular fluid, one can attempt to answer a long-standing question: how do realistic material properties such as dissipation, stiffness, friction or cohesion influence the rheology of a granular fluid? In a recent paper Macaulay & Rognon (J. Fluid Mech., vol. 858, 2019, R2) shed new light on the effect cohesion can have on mass transport in sheared, sticky granular fluids. On top of the usual diffusive, stochastic modes of transport, cohesion can create and stabilise clusters of particles into bigger agglomerates that carry particles over large distances – either ballistically in the dilute regime, or by their rotation in the dense regime. Importantly, these clusters must not only be larger than the particles (defining the intermediate, meso-scale), but they must also have a finite lifetime, in order to be able to exchange mass with each other, which can seriously enhance transport in sticky granular fluids by rotection, i.e. a combination of rotation and convection.
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Velho Rodrigues, Marcos F., Maciej Lisicki i Eric Lauga. "The bank of swimming organisms at the micron scale (BOSO-Micro)". PLOS ONE 16, nr 6 (10.06.2021): e0252291. http://dx.doi.org/10.1371/journal.pone.0252291.
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Unicellular microscopic organisms living in aqueous environments outnumber all other creatures on Earth. A large proportion of them are able to self-propel in fluids with a vast diversity of swimming gaits and motility patterns. In this paper we present a biophysical survey of the available experimental data produced to date on the characteristics of motile behaviour in unicellular microswimmers. We assemble from the available literature empirical data on the motility of four broad categories of organisms: bacteria (and archaea), flagellated eukaryotes, spermatozoa and ciliates. Whenever possible, we gather the following biological, morphological, kinematic and dynamical parameters: species, geometry and size of the organisms, swimming speeds, actuation frequencies, actuation amplitudes, number of flagella and properties of the surrounding fluid. We then organise the data using the established fluid mechanics principles for propulsion at low Reynolds number. Specifically, we use theoretical biophysical models for the locomotion of cells within the same taxonomic groups of organisms as a means of rationalising the raw material we have assembled, while demonstrating the variability for organisms of different species within the same group. The material gathered in our work is an attempt to summarise the available experimental data in the field, providing a convenient and practical reference point for future studies.
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dell'Isola, F., L. Rosa i C. Woźniak. "A micro-structured continuum modelling compacting fluid-saturated grounds: the effects of pore-size scale parameter". Acta Mechanica 127, nr 1-4 (marzec 1998): 165–82. http://dx.doi.org/10.1007/bf01170371.
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Pence, Deborah V., Paul A. Boeschoten i James A. Liburdy. "Simulation of Compressible Micro-Scale Jet Impingement Heat Transfer". Journal of Heat Transfer 125, nr 3 (20.05.2003): 447–53. http://dx.doi.org/10.1115/1.1571082.
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A computational study is presented of the heat transfer performance of a micro-scale, axisymmetric, confined jet impinging on a flat surface with an embedded uniform heat flux disk. The jet flow occurs at large, subsonic Mach numbers (0.2 to 0.8) and low Reynolds numbers (419 to 1782) at two impingement distances. The flow is characterized by a Knudsen number of 0.01, based on the viscous boundary layer thickness, which is large enough to warrant consideration of slip-flow boundary conditions along the impingement surface. The effects of Mach number, compressibility, and slip-flow on heat transfer are presented. The local Nusselt number distributions are shown along with the velocity, pressure, density and temperature fields near the impingement surface. Results show that the wall temperature decreases with increasing Mach number, M, exhibiting a minimum local value at r/R=1.6 for the highest M. The slip velocity also increases with M, showing peak values near r/R=1.4 for all M. The resulting Nusselt number increases with increasing M, and local maxima are observed near r/R=1.20, rather than at the centerline. In general, compressibility improves heat transfer due to increased fluid density near the impinging surface. The inclusion of slip-velocity and the accompanying wall temperature jump increases the predicted rate of heat transfer by as much as 8–10% for M between 0.4 and 0.8.
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Squires, Todd M. "Micro-plumes for nano-velocimetry". Journal of Fluid Mechanics 832 (26.10.2017): 1–4. http://dx.doi.org/10.1017/jfm.2017.688.
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Fluid flows through nano-scale channels depend sensitively on the physical and chemical properties of the walls that surround them. The sub-micron dimensions of such channels, however, are impossible to resolve optically, which rules out most methods for flow visualization. Classic calculations by Squire (Q. J. Mech. Appl. Maths, vol. IV, 1951, pp. 321–329) and Landau & Lifshitz (Fluid Mechanics, vol. 6, 1959, Pergamon) showed that the laminar flow driven outside a capillary, by fluid emerging from the end of the capillary, is identical to the flow driven by a point force proportional to the average velocity in the capillary. Secchi et al. (J. Fluid Mech. 826, R3) analyze the dispersion of a solute that is injected along with the fluid, whose concentration decays slowly with distance but with a strong angular dependence that encodes the intra-capillary velocity. Fluorescence micrographs of the concentration profile emerging from the nanocapillary can be related directly to the average fluid velocity within the nanocapillary. Beyond their remarkable capacity for nano-velocimetry, Landau–Squire plumes will likely appear throughout micro- and nano-fluidic systems.
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Blanchard, Danny, i Phillip M. Ligrani. "Micro-scale and millimeter-scale rotating disk couette flows, experiments and analysis". Experiments in Fluids 41, nr 6 (10.10.2006): 893–903. http://dx.doi.org/10.1007/s00348-006-0208-8.
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Youjun, Ji, i K. Vafai. "Analysis of pore scale fluid migration in a porous medium- application to coal rock seam". International Journal of Numerical Methods for Heat & Fluid Flow 27, nr 8 (7.08.2017): 1706–19. http://dx.doi.org/10.1108/hff-05-2016-0198.
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Purpose The purpose of this study is to digitize the porous structure and reconstruct the geometry of the rock by using the image processing software photoshop (PS) and ant colony algorithm coded with compiler Fortran PowerStation (fps) 4.0 based on the microscopic image of a typical rock mass. Design/methodology/approach The digital model of the microstructure of the porous coal rock was obtained, and imported into the numerical simulation software to build the finite element model of microstructure of the porous coal rock. Creeping flow equations were used to describe the fluid flow in the porous rock. Findings The simulation results indicate that the method utilized for reconstructing the microstructure of the porous coal rock proposed in this work is effective. The results demonstrate that the transport of fluid in a porous medium is significantly influenced by the geometric structure of the pore and that the heterogeneous porous structure would result in an irregular flow of the fluid. Research limitations/implications The authors did not experience a limitation. Practical implications The existence of the pores with dead ends would hinder the fluid to flow through the coal rock and reduce the efficiency of extracting fluid from the porous coal rock. It is also shown that the fluid first enters the large pores and subsequently into the small pore spaces. Social implications The paper provides important and useful results for several industries. Originality value Image processing technology has been utilized to incorporate the micro image of the porous coal rock mass, based on the characteristics of pixels of the micro image. The ant colony algorithm was used to map out the boundary of the rock matrix and the pore space. A FORTRAN code was prepared to read the micro image, to transform the bmp image into a binary format, which contains only two values. The digital image was obtained after analyzing the image features. The geometric structure of the coal rock pore was then constructed. The flow process for the micro fluid in the pore structure was illustrated and the physical process of the pore scale fluid migration in the porous coal seam was analyzed.
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Bultreys, T., S. Van Offenwert, W. Goethals, M. N. Boone, J. Aelterman i V. Cnudde. "X-ray tomographic micro-particle velocimetry in porous media". Physics of Fluids 34, nr 4 (kwiecień 2022): 042008. http://dx.doi.org/10.1063/5.0088000.
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Fluid flow through intricate confining geometries often exhibits complex behaviors, certainly in porous materials, e.g., in groundwater flows or the operation of filtration devices and porous catalysts. However, it has remained extremely challenging to measure 3D flow fields in such micrometer-scale geometries. Here, we introduce a new 3D velocimetry approach for optically opaque porous materials, based on time-resolved x-ray micro-computed tomography (CT). We imaged the movement of x-ray tracing micro-particles in creeping flows through the pores of a sandpack and a porous filter, using laboratory-based CT at frame rates of tens of seconds and voxel sizes of 12 μm. For both experiments, fully three-dimensional velocity fields were determined based on thousands of individual particle trajectories, showing a good match to computational fluid dynamics simulations. Error analysis was performed by investigating a realistic simulation of the experiments. The method has the potential to measure complex, unsteady 3D flows in porous media and other intricate microscopic geometries. This could cause a breakthrough in the study of fluid dynamics in a range of scientific and industrial application fields.
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Siginer, Dennis. "Special Section on the Fluid Mechanics and Rheology of Nonlinear Materials at the Macro, Micro, and Nano Scale". Journal of Fluids Engineering 128, nr 1 (1.01.2006): 1–5. http://dx.doi.org/10.1115/1.2163070.
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Zacharoudiou, Ioannis, Emily M. Chapman, Edo S. Boek i John P. Crawshaw. "Pore-filling events in single junction micro-models with corresponding lattice Boltzmann simulations". Journal of Fluid Mechanics 824 (6.07.2017): 550–73. http://dx.doi.org/10.1017/jfm.2017.363.
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The aim of this work is to better understand fluid displacement mechanisms at the pore scale in relation to capillary-filling rules. Using specifically designed micro-models we investigate the role of pore body shape on fluid displacement during drainage and imbibition via quasi-static and spontaneous experiments at ambient conditions. The experimental results are directly compared to lattice Boltzmann (LB) simulations. The critical pore-filling pressures for the quasi-static experiments agree well with those predicted by the Young–Laplace equation and follow the expected filling events. However, the spontaneous imbibition experimental results differ from those predicted by the Young–Laplace equation; instead of entering the narrowest available downstream throat the wetting phase enters an adjacent throat first. Thus, pore geometry plays a vital role as it becomes the main deciding factor in the displacement pathways. Current pore network models used to predict displacement at the field scale may need to be revised as they currently use the filling rules proposed by Lenormandet al.(J. Fluid Mech., vol. 135, 1983, pp. 337–353). Energy balance arguments are particularly insightful in understanding the aspects affecting capillary-filling rules. Moreover, simulation results on spontaneous imbibition, in excellent agreement with theoretical predictions, reveal that the capillary number itself is not sufficient to characterise the two phase flow. The Ohnesorge number, which gives the relative importance of viscous forces over inertial and capillary forces, is required to fully describe the fluid flow, along with the viscosity ratio.
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ASADI, H., K. JAVAHERDEH i S. RAMEZANI. "MICROPOLAR FLUID MODEL FOR BLOOD FLOW THROUGH A STENOSED ARTERY". International Journal of Applied Mechanics 05, nr 04 (grudzień 2013): 1350043. http://dx.doi.org/10.1142/s1758825113500439.
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Various experimental observations have demonstrated that the classical fluid theory is incapable of explaining many phenomena at micro and nano scales. On the other hand, micropolar fluid dynamics can naturally pick up the physical phenomena at these scales owing to its additional degrees of freedom caused by incorporating the effects of fluid molecules on the continuum. Therefore, one of the aims of this paper is to investigate the applicability of the theory of micropolar fluids to modeling and calculating flows in circular microchannels depending on the geometrical dimension of the flow field. Hence, a finite element formulation for the numerical analysis of micropolar laminar fluid flow is developed. In order to validate the results of the FE formulation, the analytical and exact solution of the micropolar Hagen–Poiseuille flow in a circular microchannel is presented, and an excellent agreement between the results of the analytical solution and those of the FE formulation is observed. It is also shown that the micropolar viscosity and the length scale parameter have significant roles on changing the flow characteristics. Then, the behavior of an incompressible viscous fluid flow such as blood flow in a stenosed artery, having multiple kinds of stenoses, is investigated. The obtained results are compared to the results reported in the literature, and an excellent agreement is observed.
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Johnson, Perry L., i Charles Meneveau. "Predicting viscous-range velocity gradient dynamics in large-eddy simulations of turbulence". Journal of Fluid Mechanics 837 (20.12.2017): 80–114. http://dx.doi.org/10.1017/jfm.2017.838.
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The detailed dynamics of small-scale turbulence are not directly accessible in large-eddy simulations (LES), posing a modelling challenge, because many micro-physical processes such as deformation of aggregates, drops, bubbles and polymers dynamics depend strongly on the velocity gradient tensor, which is dominated by the turbulence structure in the viscous range. In this paper, we introduce a method for coupling existing stochastic models for the Lagrangian evolution of the velocity gradient tensor with coarse-grained fluid simulations to recover small-scale physics without resorting to direct numerical simulations (DNS). The proposed approach is implemented in LES of turbulent channel flow and detailed comparisons with DNS are carried out. An application to modelling the fate of deformable, small (sub-Kolmogorov) droplets at negligible Stokes number and low volume fraction with one-way coupling is carried out and results are again compared to DNS results. Results illustrate the ability of the proposed model to predict the influence of small-scale turbulence on droplet micro-physics in the context of LES.
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ASADI, H., K. JAVAHERDEH i S. RAMEZANI. "FINITE ELEMENT SIMULATION OF MICROPOLAR FLUID FLOW IN THE LID-DRIVEN SQUARE CAVITY". International Journal of Applied Mechanics 05, nr 04 (grudzień 2013): 1350045. http://dx.doi.org/10.1142/s1758825113500452.
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The micropolar fluid theory augments the laws of classical continuum mechanics by incorporating the effects of fluid molecules on the continuum. So, the micropolar theory has been able to explain many phenomena at micro and nano scales. In this paper, a finite element formulation for the numerical analysis of micropolar laminar fluid flow is developed. In order to validate the results of the FE formulation, analytical solution of the Poiseuille flow of micropolar fluid in a microchannel is presented, and an excellent agreement between the results of the analytical solution and those of the FE formulation is observed. It is shown that the micropolar viscosity and the length scale parameter have significant roles on changing the flow characteristics. Then, the behavior of an incompressible viscous fluid flow in a lid-driven square cavity is investigated. The obtained results are compared to the results reported in the literature, and an excellent agreement is observed.
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Ostrikov, N. N., i E. M. Zhmulin. "Vortex dynamics of viscous fluid flows. Part 1. Two-dimensional flows". Journal of Fluid Mechanics 276 (10.10.1994): 81–111. http://dx.doi.org/10.1017/s0022112094002478.
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The method of product integration is applied to the vortex dynamics of two-dimensional incompressible viscous media. In the cases of both unbounded and bounded flows under the no-slip boundary condition, the analytic solutions of the Cauchy problem are obtained for the Helmholtz equation in the form of linear and nonlinear product integrals. The application of product integrals allows the generalization in a natural way of the vortex dynamics concept to the case of viscous flows. However, this new approach requires the reconsideration of some traditional notions of vortex dynamics. Two lengthscales are introduced in the form of a micro- and a macro-scale. Elementary ‘vortex objects’ are defined as two types of singular vortex filaments with equal but opposite intensities. The vorticity is considered as the macro-value proportional to the concentration of elementary vortex filaments inhabiting the micro-level. The vortex motion of a viscous medium is represented as the stochastic motion of an infinite set of elementary vortex filaments on the micro-level governed by the stochastic differential equations, where the stochastic velocity component of every filament simulates the viscous diffusion of vorticity, and the regular component is the macro-value induced according to the Biot–Savart law and simulates the convective transfer of vorticity.In flows with boundaries, the production of elementary vortex filaments at the boundary is introduced to satisfy the no-slip condition. This phenomenon is described by the application of the generalized Markov processes theory. The integral equation for the production intensity of elementary vortex filaments is derived and solved using the no-slip condition reformulated in terms of vorticity. Additional conditions on this intensity are determined to avoid the many-valuedness of the pressure in a multi-connected flow domain. This intensity depends on the vorticity in the flow and the boundary velocity at every time instant, together with boundary acceleration.As a result, the successive and accurate application of the product-integral method allows the study of vortex dynamics in a viscous fluid according to the concepts of Helmholtz and Kelvin.
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Mistry, Dhiren, Jimmy Philip i James R. Dawson. "Kinematics of local entrainment and detrainment in a turbulent jet". Journal of Fluid Mechanics 871 (30.05.2019): 896–924. http://dx.doi.org/10.1017/jfm.2019.327.
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In this paper we investigate the continuous, local exchange of fluid elements as they are entrained and detrained across the turbulent/non-turbulent interface (TNTI) in a high Reynolds number axisymmetric jet. To elucidate characteristic kinematic features of local entrainment and detrainment processes, simultaneous high-speed particle image velocimetry and planar laser-induced fluorescence measurements were undertaken. Using an interface-tracking technique, we evaluate and analyse the conditional dependence of local entrainment velocity in a frame of reference moving with the TNTI in terms of the interface geometry and the local flow field. We find that the local entrainment velocity is intermittent with a characteristic length scale of the order of the Taylor micro-scale and that the contribution to the net entrainment rate arises from the imbalance between local entrainment and detrainment rates that occurs with a ratio of two parts of entrainment to one part detrainment. On average, an increase in local entrainment is correlated with excursions of the TNTI towards jet centreline into regions of higher streamwise momentum, convex surface curvature facing the turbulent side of the jet and along the leading edges of the interface. In contrast, detrainment is correlated with excursions of the TNTI away from the jet centreline into regions of lower streamwise momentum, concave surface curvature and along the trailing edge. We find that strong entrainment is characterised by a local counterflow velocity field in the frame of reference moving with the TNTI which enhances the transport of rotational and irrotational fluid elements. On the other hand, detrainment is characterised by locally uniform flow fields with the local fluid velocity on either side of the TNTI advecting in the same direction. These local flow patterns and the strength of entrainment or detrainment rates are also observed to be strongly influenced by the presence and relative strength of vortical structures which are of the order of the Taylor micro-scale that populate the turbulent region along the jet boundary.
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Šarler, Božidar, Tadej Dobravec, Gašper Glavan, Vanja Hatić, Boštjan Mavrič, Robert Vertnik, Peter Cvahte, Filip Gregor, Marina Jelen i Marko Petrovič. "Multi-Physics and Multi-Scale Meshless Simulation System for Direct-Chill Casting of Aluminium Alloys". Strojniški vestnik – Journal of Mechanical Engineering 65, nr 11-12 (18.11.2019): 658–70. http://dx.doi.org/10.5545/sv-jme.2019.6350.
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This paper represents an overview of the elements of the user-friendly simulation system, developed for computational analysis and optimization of the quality and productivity of the electromagnetically direct-chill cast semi-products from aluminium alloys. The system also allows the computational estimation of the design changes of the casting equipment. To achieve this goal, the electromagnetic and the thermofluid process parameters are coupled to the evolution of Lorentz force, temperature, velocity, concentration, strain and stress fields as well as microstructure evolution. This forms a multi-physics and multi-scale problem of great complexity, which has not been demonstrated before. The macroscopic fluid mechanics, solid mechanics, and electromagnetic solution framework is based on local strong-form meshless formulation, involving the radial basis functions and monomials as trial functions, and local collocation or weighted least squares approximation. It is coupled to the micro-scale by incorporating the point automata solution concept. The entire macro-micro solution concept does not require meshing and space integration. The solution procedure can be easily and efficiently automatically adapted in node redistribution and/or refinement sense, which is of utmost importance when coping with fields exhibiting sharp gradients, which occur in the phase-change problems. The simulation system is coded from scratch in modern Fortran. The elements of the experimental validation of the system and the demonstration of its use for round billet casting in IMPOL Aluminium Industry are shown.
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Gireesha, B. J., i S. Sindhu. "Entropy generation analysis of Casson fluid flow through a vertical microchannel under combined effect of viscous dissipation, joule heating, hall effect and thermal radiation". Multidiscipline Modeling in Materials and Structures 16, nr 4 (13.12.2019): 713–30. http://dx.doi.org/10.1108/mmms-07-2019-0139.
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Purpose Fully developed Casson fluid flow through vertical microchannel is deliberated in the presence of thermal radiation. The two predominant features of micro scale phenomenon such as velocity slip and temperature jump are considered. The paper aims to discuss this issue. Design/methodology/approach The governing equations of the physical phenomenon are solved using Runge–Kutta–Fehlberg fourth fifth order method. Findings The outcome of the present work is discussed through graphs. This computation shows that entropy generation rate decreases with enhancing wall ambient temperature difference ratio and fluid wall interaction parameter. Also, it is found that Bejan number is fully retarded with rise in fluid wall interaction parameter. Enhancement in heat transfer or Nusselt number is achieved by increasing the wall ambient temperature ratio and fluid wall interaction parameter. Originality/value Casson liquid flow through microchannel is analyzed by considering temperature jump and velocity slip. This computation shows that entropy generation rate decreases with enhancing wall ambient temperature difference ratio.
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Pawell, Ryan S., Robert A. Taylor, David W. Inglis i Tracie J. Barber. "Jet Formation in Micro Post Arrays". Applied Mechanics and Materials 553 (maj 2014): 367–72. http://dx.doi.org/10.4028/www.scientific.net/amm.553.367.
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Micropost arrays serve as a plaform for the next generation of diagnostic devices. These arrays are found in microfluidic devices for peripheral blood-based diagnostics and metastatic cancer management. The function and performance of these devices is determined by the underlying micro-scale fluid mechanics. Typically, these devices operate in the creeping regime (Re << 1) where the viscous forces of the fluids dominate. Recent advances in manufacturing allow for higher Reynolds number flows (Re >> 1) where the inertial forces dominate. In this work, we use computational simulations to show there is a transitional region (1 < Re < 20) in between the laminar and creeping regimes for two different micropost array geometries. Numerical analysis is employed to investigate jet formation both within the array and at the array exit. The peak-to-peak amplitude of the streamwise normalized velocity profile is used to quantify jet formation within the array; the streamwise velocity profile at the end of the array exit is used to determine jet length at the exit of the array. Above the transitional region (Re > 20) significant jets form downstream of the posts, amplitude scales exponentially and jet length scales with Re according to power law.
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Chang, Fun Liang, i Yew Mun Hung. "Gravitational effects on electroosmotic flow in micro heat pipes". International Journal of Numerical Methods for Heat & Fluid Flow 30, nr 2 (17.07.2019): 535–56. http://dx.doi.org/10.1108/hff-01-2019-0008.
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Purpose This paper aims to investigate the coupled effects of electrohydrodynamic and gravity forces on the circulation effectiveness of working fluid in an inclined micro heat pipe driven by electroosmotic flow. The effects of the three competing forces, namely, the capillary, the gravitational and the electrohydrodyanamic forces, on the circulation effectiveness of a micro heat pipe are compared and delineated. Design/methodology/approach The numerical model is developed based on the conservations of mass, momentum and energy with the incorporation of the Young–Laplace equation for electroosmotic flow in an inclined micro heat pipe incorporating the gravity effects. Findings By inducing electroosmotic flow in a micro heat pipe, a significant increase in heat transport capacity can be attained at a reasonably low applied voltage, leading to a small temperature drop and a high thermal conductance. However, the favorably applied gravity forces pull the liquid toward the evaporator section where the onset of flooding occurs within the condenser section, generating a throat that shrinks the vapor flow passage and may lead to a complete failure on the operation of micro heat pipe. Therefore, the balance between the electrohydrodyanamic and the gravitational forces is of vital importance. Originality/value This study provides a detailed insight into the gravitational and electroosmotic effects on the thermal performance of an inclined micro heat pipe driven by electroosmotic flow and paves the way for the feasible practical application of electrohydrodynamic forces in a micro-scale two-phase cooling device.
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Dehghani, Hamidreza, i Andreas Zilian. "ANN-aided incremental multiscale-remodelling-based finite strain poroelasticity". Computational Mechanics 68, nr 1 (5.05.2021): 131–54. http://dx.doi.org/10.1007/s00466-021-02023-3.
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AbstractMechanical modelling of poroelastic media under finite strain is usually carried out via phenomenological models neglecting complex micro-macro scales interdependency. One reason is that the mathematical two-scale analysis is only straightforward assuming infinitesimal strain theory. Exploiting the potential of ANNs for fast and reliable upscaling and localisation procedures, we propose an incremental numerical approach that considers rearrangement of the cell properties based on its current deformation, which leads to the remodelling of the macroscopic model after each time increment. This computational framework is valid for finite strain and large deformation problems while it ensures infinitesimal strain increments within time steps. The full effects of the interdependency between the properties and response of macro and micro scales are considered for the first time providing more accurate predictive analysis of fluid-saturated porous media which is studied via a numerical consolidation example. Furthermore, the (nonlinear) deviation from Darcy’s law is captured in fluid filtration numerical analyses. Finally, the brain tissue mechanical response under uniaxial cyclic test is simulated and studied.
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Liu, L. X., C. J. Teo, A. H. Epstein i Z. S. Spakovszky. "Hydrostatic Gas Journal Bearings for Micro-Turbomachinery". Journal of Vibration and Acoustics 127, nr 2 (5.08.2004): 157–64. http://dx.doi.org/10.1115/1.1897738.
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Several years ago an effort was undertaken at MIT to develop high-speed rotating MEMS (Micro Electro-Mechanical Systems) using computer chip fabrication technology. To enable high-power density the micro-turbomachinery must be run at tip speeds of order 500m∕s, comparable to conventional scale turbomachinery. The high rotating speeds (of order 2 million rpm), the relatively low bearing aspect ratios (L∕D<0.1) due to fabrication constraints, and the laminar flow regime in the bearing gap place the micro-bearing designs to an exotic spot in the design space for hydrostatic gas bearings. This paper presents a new analytical model for axially fed gas journal bearings and reports the experimental testing of micro gas bearings to characterize and to investigate their rotordynamic behavior. The analytical model is capable of dealing with all the elements of, (1) micro-devices, (2) dynamic response characteristics of hydrostatic gas bearings, (3) evaluation of stiffness, natural frequency and damping, (4) evaluation of instability boundaries, and (5) evaluation of effects of imbalance and bearing anisotropy. First, a newly developed analytical model for hydrostatic gas journal bearings is introduced. The model consists of two parts, a fluid dynamic model for axially fed gas journal bearings and a rotordynamic model for micro-devices. Next, the model is used to predict the natural frequency, damping ratio and the instability boundary for the test devices. Experiments are conducted using a high-resolution fiber optic sensor to measure rotor speed, and a data reduction scheme is implemented to obtain imbalance-driven whirl response curves. The model predictions are validated against experimental data and show good agreement with the measured natural frequencies and damping ratios. Last, the new model is successfully used to establish bearing operating protocols and guidelines for high-speed operation.
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Zhang, Zhentong, Dominique Legendre i Rémi Zamansky. "Model for the dynamics of micro-bubbles in high-Reynolds-number flows". Journal of Fluid Mechanics 879 (1.10.2019): 554–78. http://dx.doi.org/10.1017/jfm.2019.662.
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We propose a model for the acceleration of micro-bubbles (smaller than the dissipative scale of the flow) subjected to the drag and fluid inertia forces in a homogeneous and isotropic turbulent flow. This model, that depends on the Stokes number, Reynolds number and the density ratio, reproduces the evolution of the acceleration variance as well as the relative importance and alignment of the two forces as observed from direct numerical simulations (DNS). We also report that the bubble acceleration statistics conditioned on the local kinetic energy dissipation rate are invariant with the Stokes number and the dissipation rate. Based on this observation, we propose a stochastic model for the instantaneous bubble acceleration vector accounting for the small-scale intermittency of the turbulent flows. The norm of the bubble acceleration is obtained by modelling the dissipation rate along the bubble trajectory from a log-normal stochastic process, whereas its orientation is given by two coupled random walks on a unit sphere in order to model the evolution of the joint orientation of the drag and inertia forces acting on the bubble. Furthermore, the proposed stochastic model for the bubble acceleration is used in the context of large eddy simulations (LES) of turbulent flows laden with small bubbles. To account for the turbulent motion at scales smaller than the mesh resolution, we decompose the instantaneous bubble acceleration in its resolved and residual parts. The first part is given by the drag and fluid inertia forces computed from the resolved velocity field, and the second term refers to the random contribution of small unresolved turbulent scales and is estimated with the stochastic model proposed in the paper. Comparisons with DNS and standard LES, show that the proposed model improves significantly the statistics of the bubbly phase.
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Cotta, Renato M., Péricles C. Pontes, Adam H. R. Sousa, Carolina P. Naveira-Cotta i Kleber M. Lisboa. "Computational-analytical simulation of microsystems in process intensification". High Temperatures-High Pressures 50, nr 6 (2021): 469–95. http://dx.doi.org/10.32908/hthp.v50.1189.
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Heat and mass transfer enhancement techniques, either passive or active, have an important role in the more general goal of process intensification in modern engineering developments. In this context, the study of transport phenomena at the nano- and micro-scales aims far beyond the plain miniaturization of devices, being mainly directed towards process efficiency improvement and lower energy and raw materials consumption. The analysis of heat and mass transfer at such scales has required the development or extension of both theoretical and experimental methodologies. In light of the inherent multiscale nature of microfluidic devices, classical fully numerical methodologies often require large refined meshes with associated costly computations. A hybrid numerical-analytical approach for the analysis of microfluidic and thermal micro-systems is here reviewed, which includes a computational-analytical integral transform method for partial differential direct problems, that, together with mixed symbolic-numerical computations, lead to robust cost-effective algorithms for micro-scale transport phenomena analysis. Examples of this hybrid approach in selected applications are then examined more closely, including micro-reactors for continuous biodiesel synthesis with multiple reactive interfaces and three-dimensional thermal micro-devices with solid-fluid thermal conjugation.
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Vu, Van Huyen, Benoît Trouette, Quy Dong TO i Eric Chénier. "Hybrid atomistic-continuum multiscale method for fluid flow with density variation in microchannels". International Journal of Numerical Methods for Heat & Fluid Flow 28, nr 1 (2.01.2018): 3–30. http://dx.doi.org/10.1108/hff-11-2016-0473.
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Purpose This paper aims to extend the hybrid atomistic-continuum multiscale method developed by Vu et al. (2016) to study the gas flow problems in long microchannels involving density variations. Design/methodology/approach The simulation domain is decomposed into three regions: the bulk where the continuous Navier–Stokes and energy equations are solved, the neighbourhood of the wall simulated by molecular dynamics and the overlap region which connects the macroscopic variables (density, velocity and temperature) between the two former regions. For the simulation of long micro/nanochannels, a strategy with multiple molecular blocks all along the fluid/solid interface is adopted to capture accurately the macroscopic velocity and temperature variations. Findings The validity of the hybrid method is shown by comparisons with a simplified analytical model in the molecular region. Applications to compressible and condensation problems are also presented, and the results are discussed. Originality/value The hybrid method proposed in this paper allows cost-effective computer simulations of large-scale problems with an accurate modelling of the transfers at small scales (velocity slip, temperature jump, thin condensation films, etc.).
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Xu, Qianghui, Xiongyu Chen, Junyu Yang, Zhiying Liu i Lin Shi. "Pore-scale study of coke combustion in a matrix-fracture system based on the micro-continuum approach". Physics of Fluids 34, nr 3 (marzec 2022): 036603. http://dx.doi.org/10.1063/5.0082518.
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In situ combustion is an advanced recovery technique used to exploit heavy oil in the fractured reservoirs that make up approximately one-third of global heavy-oil resources. However, the mesoscopic mechanisms of coke combustion in the multiscale matrix-fracture system are not well understood because of the difficulty of performing pore-resolved simulations. In the present study, a pore-resolved micro-continuum approach was used to investigate fully coupled thermal and reactive flows through fractured media that contain nanometer-range coke pores, micrometer-range matrix pores, and sub-millimeter range natural fractures. Image-based simulations were implemented using synthetic geological models to mimic coke deposition patterns based on tomography images. The combustion regime diagram for the fractured media was mapped based on the ignition temperature and the air flux to exhibit three combustion regimes. The regime diagram was compared with that for unfractured media to address the impact of natural fractures on oxygen transport and the burning temperature. The oxygen diffusion mechanism dominated oxygen transport from the fracture into the matrix and led to a desirable smoldering combustion temperature regardless of the air injection rate. Effects of fracture geometries were quantified to demonstrate tortuous and discrete fractures, and matching air injection rates with fracture apertures can suppress air-channeling risk effectively. Possible discrepancies between lab measurements and field operations were demonstrated, and their potential to drive misinterpretation of experimental results was considered. The present pathway from tomography images to synthetic images and numerical simulations extends the “image and compute” technique to resolution of multiscale and nonlinear reactive transport.
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Clausse, Alejandro, Nicolás Silin i Gustavo Boroni. "A multiscale method for producing homogenized drag laws of a permeable medium by conflating experimental data with Lattice-Boltzmann simulations". International Journal of Numerical Methods for Heat & Fluid Flow 29, nr 11 (4.11.2019): 4394–407. http://dx.doi.org/10.1108/hff-01-2019-0058.
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Purpose The purpose of this paper is to obtain a permeability law of a gas flow through a permeable medium using particle image velocimetry experimental data as primal information, which is conflated with numerical calculations by means of a multi-scale method. Design/methodology/approach The D2Q9 single-relaxation-time Lattice Boltzmann model (LBM) implemented in GPU is used for the numerical calculations. In a first homogenized micro-scale, the drag forces are emulated by means of an effective Darcy law acting only in the close neighborhood of the solid structures. A second mesoscopic level of homogenization makes use of the effective drag forces resulting from the first-scale model. Findings The procedure is applied to an experiment consisting of a regular array of wires. For the first level of homogenization, an effective drag law of the individual elemental obstacles is produced by conflating particle image velocimetry measurements of the flow field around the wires and numerical calculations performed with a GPU implementation of the LBM. In the second homogenization, a Darcy–Forchheimer correlation is produced, which is used in a final homogenized LBM model. Research limitations/implications The numerical simulations at the first level of homogenization require a substantial amount of calculations, which in the present case were performed by means of the computational power of a GPU. Originality/value The homogenization procedure can be extended to other permeable structures. The micro-scale-level model retrieves the fluid-structure forces between the flow and the obstacles, which are difficult to obtain experimentally either from direct measurement or by indirect assessment from velocity measurements.
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Karimi, A., i A. M. Ardekani. "Gyrotactic bioconvection at pycnoclines". Journal of Fluid Mechanics 733 (26.09.2013): 245–67. http://dx.doi.org/10.1017/jfm.2013.415.
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AbstractBioconvection is an important phenomenon in aquatic environments, affecting the spatial distribution of motile micro-organisms and enhancing mixing within the fluid. However, stratification arising from thermal or solutal gradients can play a pivotal role in suppressing the bioconvective flows, leading to the aggregation of micro-organisms and growth of their patchiness. We investigate the combined effects by considering gyrotactic motility where the up-swimming cells are directed by the balance of the viscous and gravitational torques. To study this system, we employ a continuum model consisting of Navier–Stokes equations with the Boussinesq approximation coupled with two conservation equations for the concentration of cells and stratification agent. We present a linear stability analysis to determine the onset of bioconvection for different flow parameters. Also, using large-scale numerical simulations, we explore different regimes of the flow by varying the corresponding boundary conditions and dimensionless variables such as Rayleigh number and Lewis number ($\mathit{Le}$) and we show that the cell distribution can be characterized using the ratio of the buoyancy forces as the determinant parameter when $\mathit{Le}\lt 1$ and the boundaries are insulated. But, in thermally stratified fluids corresponding to $\mathit{Le}\gt 1$, temperature gradients are demonstrated to have little impact on the bioconvective plumes provided that the walls are thermally insulated. In addition, we analyse the dynamical behaviour of the system in the case of persistent pycnoclines corresponding to constant salinity boundary conditions and we discuss the associated inhibition threshold of bioconvection in the light of the stability of linearized solutions.
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Li, Kai, Yihui Zhao, Maiqi Liu, Xiaoying Wang, Fangyuan Zhang i Dazhi Wang. "A multi-scale E-jet 3D printing regulated by structured multi-physics field". Journal of Micromechanics and Microengineering 32, nr 2 (31.12.2021): 025005. http://dx.doi.org/10.1088/1361-6439/ac43d1.
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Abstract Micro/nano scale structure as important functional part have been widely used in wearable flexible sensors, gas sensors, biological tissue engineering, microfluidic chips super capacitors and so on. Here a multi-scale electrohydrodynamic jet (E-jet) 3D printing approach regulated by structured multi-physics fields was demonstrated to generate 800 nm scale 2D geometries and high aspect ratio 3D structures. The simulation model of jetting process under resultant effect of top fluid field, middle electric field and bottom thermal field was established. And the physical mechanism and scale law of jet formation were studied. The effects of thermal field temperature, applied voltage and flow rate on the jet behaviors were studied; and the range of process parameters of stable jet was obtained. The regulation of printing parameters was used to manufacture the high resolution gradient graphics and the high aspect ratio structure with tight interlayer bonding. The structural features could be flexibly adjusted by reasonably matching the process parameters. Finally, polycaprolactone/polyvinylpyrrolidone (PCL/PVP) composite scaffolds with cell-scale fiber and ordered fiber spacing were printed. The proposed E-jet printing method provides an alternative approach for the application of biopolymer materials in tissue engineering.
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Khatib, Fadi Al, Afif Gouissem, Armin Eilaghi i Malek Adouni. "The Effect of Enzymatic Crosslink Degradation on the Mechanics of the Anterior Cruciate Ligament: A Hybrid Multi-Domain Model". Applied Sciences 11, nr 18 (15.09.2021): 8580. http://dx.doi.org/10.3390/app11188580.
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The anterior cruciate ligament’s (ACL) mechanics is an important factor governing the ligament’s integrity and, hence, the knee joint’s response. Despite many investigations in this area, the cause and effect of injuries remain unclear or unknown. This may be due to the complexity of the direct link between macro- and micro-scale damage mechanisms. In the first part of this investigation, a three-dimensional coarse-grained model of collagen fibril (type I) was developed using a bottom-up approach to investigate deformation mechanisms under tensile testing. The output of this molecular level was used later to calibrate the parameters of a hierarchical multi-scale fibril-reinforced hyper-elastoplastic model of the ACL. Our model enabled us to determine the mechanical behavior of the ACL as a function of the basic response of the collagen molecules. Modeled elastic response and damage distribution were in good agreement with the reported measurements and computational investigations. Our results suggest that degradation of crosslink content dictates the loss of the stiffness of the fibrils and, hence, damage to the ACL. Therefore, the proposed computational frame is a promising tool that will allow new insights into the biomechanics of the ACL.
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Borg, Matthew K., Duncan A. Lockerby i Jason M. Reese. "A hybrid molecular–continuum method for unsteady compressible multiscale flows". Journal of Fluid Mechanics 768 (10.03.2015): 388–414. http://dx.doi.org/10.1017/jfm.2015.83.
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We present an internal-flow multiscale method (‘unsteady-IMM’) for compressible, time-varying/unsteady flow problems in nano-confined high-aspect-ratio geometries. The IMM is a hybrid molecular–continuum method that provides accurate flow predictions at macroscopic scales because local microscopic corrections to the continuum-fluid formulation are generated by spatially and temporally distributed molecular simulations. Exploiting separation in both time and length scales enables orders of magnitude computational savings, far greater than seen in other hybrid methods. We apply the unsteady-IMM to a converging–diverging channel flow problem with various time- and length-scale separations. Comparisons are made with a full molecular simulation wherever possible; the level of accuracy of the hybrid solution is excellent in most cases. We demonstrate that the sensitivity of the accuracy of a solution to the macro–micro time-stepping, as well as the computational speed-up over a full molecular simulation, is dependent on the degree of scale separation that exists in a problem. For the largest channel lengths considered in this paper, a speed-up of six orders of magnitude has been obtained, compared with a notional full molecular simulation.
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Xia, Qing, Gangming Sun, Junseok Kim i Yibao Li. "Multi-scale modeling and simulation of additive manufacturing based on fused deposition technique". Physics of Fluids 35, nr 3 (marzec 2023): 034116. http://dx.doi.org/10.1063/5.0141316.
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The issue of multi-scale modeling of the filament-based material extrusion has received considerable critical attention for three-dimensional (3D) printing, which involves complex physicochemical phase transitions and thermodynamic behavior. The lack of a multi-scale theoretical model poses significant challenges for prediction in 3D printing processes driven by the rapidly evolving temperature field, including the nonuniformity of tracks, the spheroidization effect of materials, and inter-track voids. Few studies have systematically investigated the mapping relationship and established the numerical modeling between the physical environment and the virtual environment. In this paper, we develop a multi-scale system to describe the fused deposition process in the 3D printing process, which is coupled with the conductive heat transfer model and the dendritic solidification model. The simulation requires a computational framework with high performance because of the cumulative effect of heat transfer between different filament layers. The proposed system is capable of simulating the material state with the proper parameter at the macro- and micro-scale and is directly used to capture multiple physical phenomena. The main contribution of this paper is that we have established a totally integrated simulation system by considering multi-scale and multi-physical properties. We carry out several numerical tests to verify the robustness and efficiency of the proposed model.
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Xing, Z. B., Xingchao Han, Hanbing Ke, Q. G. Zhang, Zhiping Zhang, Huijin Xu i Fuqiang Wang. "Multi-phase lattice Boltzmann (LB) simulation for convective transport of nanofluids in porous structures with phase interactions". International Journal of Numerical Methods for Heat & Fluid Flow 31, nr 8 (22.03.2021): 2754–88. http://dx.doi.org/10.1108/hff-07-2020-0481.
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Purpose A combination of highly conductive porous media and nanofluids is an efficient way for improving thermal performance of relevant applications. For precisely predicting the flow and thermal transport of nanofluids in porous media, the purpose of this paper is to explore the inter-phase coupling numerical methods. Design/methodology/approach Based on the lattice Boltzmann (LB) method, this study combines the convective flow, non-equilibrium thermal transport and phase interactions of nanofluids in porous matrix and proposes a new multi-phase LB model. The micro-scale momentum and heat interactions are especially analyzed for nanoparticles, base fluid and solid matrix. A set of three-phase LB equations for the flow/thermal coupling of base fluid, nanoparticles and solid matrix is established. Findings Distributions of nanoparticles, velocities for nanoparticles and the base fluid, temperatures for three phases and interaction forces are analyzed in detail. Influences of parameters on the nanofluid convection in the porous matrix are examined. Thermal resistance of nanofluid convective transport in porous structures are comprehensively discussed with the models of multi-phases. Results show that the Rayleigh number and the Darcy number have significant influences on the convective characteristics. The result with the three-phase model is mildly larger than that with the local thermal non-equilibrium model. Originality/value This paper first creates the multi-phase theoretical model for the complex coupling process of nanofluids in porous structures, which is useful for researchers and technicians in fields of thermal science and computational fluid dynamics.
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Guo, Xiaoyu, i Chiang C. Mei. "Liquid film on a hydrophobic radome or roof top in rain". Journal of Fluid Mechanics 870 (16.05.2019): 1158–74. http://dx.doi.org/10.1017/jfm.2019.319.
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The water film due to rain falling on a radome surface causes severe losses in radio wave transmission. Hydrophobic coatings have been applied as a remedy to reduce the film thickness and to minimize the losses. However, quantitative accounts of the wave scattering are mostly based on empirical estimates of the film thickness. We describe a fluid-mechanical theory for the film under steady rain falling on a textured surface formed by a square array of pillars. Assuming the water surface on top of the pillars to be in the Cassie–Baxter state, the analysis is carried out by making use of the sharp contrast of length scales between the film thickness and the radome radius. The textured surface is viewed as a periodic array of cells around pillars. The macro-scale flow is simple and linear but the micro-scale flow in a typical lattice period is fully nonlinear. These two problems are coupled and are solved iteratively to obtain the slip length and the spatial variation of the film thickness. Numerical results are presented to show the effect of solid fraction on local flow field, the slip length and the non-uniform reduction of the film thickness. To examine the influence of the macro-scale geometry on film formation, the theory is also modified for a hydrophobic roof top formed by two inclined planes.
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ISHIKAWA, TAKUJI, i T. J. PEDLEY. "Diffusion of swimming model micro-organisms in a semi-dilute suspension". Journal of Fluid Mechanics 588 (24.09.2007): 437–62. http://dx.doi.org/10.1017/s0022112007007847.
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The diffusive behaviour of swimming micro-organisms should be clarified in order to obtain a better continuum model for cell suspensions. In this paper, a swimming micro-organism is modelled as a squirming sphere with prescribed tangential surface velocity, in which the centre of mass of the sphere may be displaced from the geometric centre (bottom-heaviness). Effects of inertia and Brownian motion are neglected, because real micro-organisms swim at very low Reynolds numbers but are too large for Brownian effects to be important. The three-dimensional movement of 64 or 27 identical squirmers in a fluid otherwise at rest, contained in a cube with periodic boundary conditions, is dynamically computed, for random initial positions and orientations. The computation utilizes a database of pairwise interactions that has been constructed by the boundary element method. In the case of (non-bottom-heavy) squirmers, both the translational and the orientational spreading of squirmers is correctly described as a diffusive process over a sufficiently long time scale, even though all the movements of the squirmers were deterministically calculated. Scaling of the results on the assumption that the squirmer trajectories are unbiased random walks is shown to capture some but not all of the main features of the results. In the case of (bottom-heavy) squirmers, the diffusive behaviour in squirmers' orientations can be described by a biased random walk model, but only when the effect of hydrodynamic interaction dominates that of the bottom-heaviness. The spreading of bottom-heavy squirmers in the horizontal directions show diffusive behaviour, and that in the vertical direction also does when the average upward velocity is subtracted. The rotational diffusivity in this case, at a volume fractionc=0.1, is shown to be at least as large as that previously measured in very dilute populations of swimming algal cells (Chlamydomonas nivalis).
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Luo, Li, Jingxuan Wang, Yundong Sha, Yanping Hao i Fengtong Zhao. "Experimental and Numerical Analysis of the Progressive Damage and Failure of SiCf/TC4 Composite Shafts". Applied Sciences 13, nr 10 (19.05.2023): 6232. http://dx.doi.org/10.3390/app13106232.
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Long fibre-reinforced metal matrix composite materials, which are widely used in industry, have complex and diverse damage modes due to their structural characteristics. In this study, the progressive damage process and failure mode analysis of the SiCf/TC4 composite shafts were thoroughly investigated under single torsional loads. A bearing performance test was carried out, the damage process was monitored using acoustic emissions, and the fracture specimens were analysed using a scanning electron microscope (SME). More specifically, under reverse torque loading, the damage process was slow-varying, the interface was subjected to tensile force, and fracture occurred mostly in the form of interface cracking; further, the breaking load of the specimen was 11,812 Nm. Under forward loading, the damage process was fast-varying. The fibres were subjected to tensile forces, and the fracture form was mostly fibre fracture; the breaking load of the specimen was 10,418 Nm. Under torque loading, the first damage to the specimens appeared in the outermost layer of the composite material’s reinforced section, and the initial cracking position was at the interface, expanding from the outside to the inside. Based on the principles of macro-mechanics and micro-mechanics theory, the cross-scale models were proposed, which contain the shaft with the same dimensions as the specimen and a micro-mechanics representative volume element (RVE) model. The initial interface damage load was 6552 Nm under reverse torque loading. Under forward loading, the initial interface damage load was 9108 Nm. In comparison to the acoustic emission test results, the main goal was to calculate the progressive damage process under the same conditions as the experiment, verifying the effectiveness of the cross-scale models.
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Namazian, Zafar, i S. A. M. Mehryan. "The impacts of non-uniform magnetic field on free convection heat transfer of a magnetizable micropolar nanofluid". International Journal of Numerical Methods for Heat & Fluid Flow 29, nr 10 (7.10.2019): 3685–706. http://dx.doi.org/10.1108/hff-10-2018-0551.
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Purpose The purpose of this study is to numerically study the heat transfer of free convection of a magnetizable micropolar nanofluid inside a semicircular enclosure. Design/methodology/approach The flow domain is under simultaneous influences of two non-uniform magnetic fields generated by current carrying wires. The directions of the currents are the same. Although the geometry is symmetric, it is physically asymmetric. The impacts of key parameters, including Rayleigh number Ra = 103-106, Hartman number Ha = 0-50, vortex viscosity parameter Δ = 0-4, nanoparticles volume fraction φ = 0-0.04 and magnetic number Mnf = 0-1000, on the macro- and micro-scales flows, temperature and heat transfer rate are studied. Finding The outcomes show that dispersing of the nanoparticles in the host fluid increases the strength of macro- and micro-scale flows. When Mnf = 0, the increment of the vortex viscosity parameter increases the strength of the particles micro-rotations, while this characteristic is decreased by growing Δ for Mnf ≠ 0. The increment of Δ and Ha decreases the rate of heat transfer. The increment of Ha decreases the enhancement percentage of heat transfer rate because of dispersing nanoparticles, known as En parameter. In addition, the value of Δ has no effect on En. Moreover, the average Nusselt number Nuavg and En remain constant by increasing the magnetic number Mnf for different volume fraction values. Originality/value The authors believe that all of the results, both numerical and asymptotic, are original and have not been published elsewhere yet.
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Bourhis, M., M. Pereira i F. Ravelet. "Performance and flow characteristics of the optimum rotors of Betz, Joukowsky, and Glauert at low tip-speed ratio". Physics of Fluids 34, nr 10 (październik 2022): 105105. http://dx.doi.org/10.1063/5.0107962.
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The advent of the Internet of Things technology has led to a renewed interest in the use of low tip-speed ratio micro-scale wind turbines to supply power to battery-less microsystems. At low tip-speed ratio ( λ), the blade geometry varies significantly depending on the optimal flow conditions used in the classical design method and the blade element/momentum theory (BEMT), and very few papers have examined this controversy. This experimental study aims to investigate the airflow and power characteristics of three 200-cm wind turbines designed according to the BEMT with three different optimum flow conditions at λ = 1: the Betz model, the Glauert model, and the Joukowsky model. Glauert optimum rotor achieves higher maximum power coefficient ([Formula: see text]) than the optimum rotors of Betz ([Formula: see text]) and Joukowsky ([Formula: see text]). The two latter turbines have lower cut-in wind speed and their torque coefficient decreases linearly with the tip-speed ratio. Betz optimum rotor has a highly stable and persistent wake, whereas large recirculation bubbles and vortex breakdown are observed downstream the runners of Glauert and Joukowsky. The airflow velocity fields and induction factor distributions computed from stereoscopic particle image velocimetry acquisitions show significant differences between each rotor and also between the theoretical developments and the experimental results, especially for the Joukowsky rotor. In addition, even though the optimum flow conditions of Glauert or Betz appear to be the most appropriate models, a method based on flow deflection rather than on airfoil polar plots may be more pertinent for the design of low tip-speed ratio micro-scale wind turbines.
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Giorgini, Antonio, Saverio Avino, Pietro Malara, Paolo De Natale i Gianluca Gagliardi. "Liquid Droplet Microresonators". Sensors 19, nr 3 (24.01.2019): 473. http://dx.doi.org/10.3390/s19030473.
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We provide here an overview of passive optical micro-cavities made of droplets in the liquid phase. We focus on resonators that are naturally created and suspended under gravity thanks to interfacial forces, illustrating simple ways to excite whispering-gallery modes in various slow-evaporation liquids using free-space optics. Similar to solid resonators, frequency locking of near-infrared and visible lasers to resonant modes is performed exploiting either phase-sensitive detection of the leakage cavity field or multiple interference between whispering-gallery modes in the scattered light. As opposed to conventional micro-cavity sensors, each droplet acts simultaneously as the sensor and the sample, whereby the internal light can detect dissolved compounds and particles. Optical quality factors up to 107–108 are observed in liquid-polymer droplets through photon lifetime measurements. First attempts in using single water droplets are also reported. These achievements point out their huge potential for direct spectroscopy and bio-chemical sensing in liquid environments. Finally, the first experiments of cavity optomechanics with surface acoustic waves in nanolitre droplets are presented. The possibility to perform studies of viscous-elastic properties points to a new paradigm: a droplet device as an opto-fluid-mechanics laboratory on table-top scale under controlled environmental conditions.
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Saisorn, Sira, Adirek Suriyawong, Pochai Srithumkhant, Pakorn Wongpromma i Somchai Wongwises. "An investigation of horizontal and vertical flow boiling in a single channel with a confinement number beyond the threshold of micro-scale flow". Physics of Fluids 33, nr 11 (listopad 2021): 113302. http://dx.doi.org/10.1063/5.0062287.
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Wasekar, Vivek M., i Raj M. Manglik. "Short-Time-Transient Surfactant Dynamics and Marangoni Convection Around Boiling Nuclei". Journal of Heat Transfer 125, nr 5 (23.09.2003): 858–66. http://dx.doi.org/10.1115/1.1599367.
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The effects of surfactant concentration on the initial short-time-scale Marangoni convection around boiling nuclei in aqueous solutions have been computationally investigated. The model consists of a hemispherical bubble (1–100 μm radius) on a downward-facing constant-temperature heated wall in a fluid pool with an initial uniform temperature gradient. Time-dependent transport of liquid mass, momentum, energy, and surfactant bulk and surface convection along with the adsorption kinetics are considered. Conditions for bubble sizes, surfactant bulk concentrations, and wall heat flux levels are represented by a range of thermocapillary and diffusocapillary Marangoni numbers (6⩽MaT⩽103,0⩽MaS⩽8.6×105) over a micro-scale time period (1 μs–1 ms). With a surfactant in solution, a surface concentration gradient develops at the bubble interface that tends to oppose the temperature gradient and reduce the overall Marangoni convection. The maximum circulation strength, which is dependent on the bubble size, corresponds to a characteristic surfactant adsorption time. This, when scaled by a ratio of bubble radius, is found to depend solely on the surfactant bulk concentration. Moreover, the interfacial surfactant adsorption does not display a stagnant cap behavior for the range of parameters and time scales covered in this study.
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Okulov, V. L., B. R. Sharifullin, N. Okulova, J. Kafka, R. Taboryski, J. N. Sørensen i I. V. Naumov. "Influence of nano- and micro-roughness on vortex generations of mixing flows in a cavity". Physics of Fluids 34, nr 3 (marzec 2022): 032005. http://dx.doi.org/10.1063/5.0083503.
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Experiments were carried out in a water-filled elongated cup of a “kitchen scale,” where motion was created by a rotating disk with various micro- and nano-roughness in the top of the cup. The obtained results have shown that for some patterns of nanostructures, there is a noticeable growth of a vortex, generated by the disk, while other roughnesses do not make visible changes in the flow structure. The results are of interest in assessing the efficiency of surfaces with nanoscale roughnesses. Indeed, the first type of nano-roughness may become useful for enhancing soft mixing in chemical and bio-reactors, including in the preparation of special food delicacies. On the other hand, the use of nanostructured surfaces that do not affect the main flow can help to solve some industrial problems of water and ice erosion, for example, in wind turbines or any other objects where disturbances of the main flow are undesirable.
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Pujara, N., M. A. R. Koehl i E. A. Variano. "Rotations and accumulation of ellipsoidal microswimmers in isotropic turbulence". Journal of Fluid Mechanics 838 (12.01.2018): 356–68. http://dx.doi.org/10.1017/jfm.2017.912.
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Aquatic micro-organisms and artificial microswimmers locomoting in turbulent flow encounter velocity gradients that rotate them, thereby changing their swimming direction and possibly providing cues about the local flow environment. Using numerical simulations of ellipsoidal particles in isotropic turbulence, we investigate the effects of body shape and swimming velocity on particle motion. Four particle shapes (sphere, rod, disc and triaxial ellipsoid) are investigated at five different swimming velocities in the range $0\leqslant V_{s}\leqslant 5u_{\unicode[STIX]{x1D702}}$, where $V_{s}$ is the swimming velocity and $u_{\unicode[STIX]{x1D702}}$ is the Kolmogorov velocity scale. We find that anisotropic, swimming particles preferentially sample regions of lower fluid vorticity than passive particles do, and hence they accumulate in these regions. While this effect is monotonic with swimming velocity, the particle enstrophy (variance of particle angular velocity) varies non-monotonically with swimming velocity. In contrast to passive particles, the particle enstrophy is a function of shape for swimming particles. The particle enstrophy is largest for triaxial ellipsoids swimming at a velocity smaller than $u_{\unicode[STIX]{x1D702}}$. We also observe that the average alignment of particles with the directions of the velocity gradient tensor are altered by swimming leading to a more equal distribution of rotation about different particle axes.
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Azarmanesh, Milad, Mousa Farhadi i Pooya Azizian. "Simulation of the double emulsion formation through a hierarchical T-junction microchannel". International Journal of Numerical Methods for Heat & Fluid Flow 25, nr 7 (7.09.2015): 1705–17. http://dx.doi.org/10.1108/hff-09-2014-0294.
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Purpose – The purpose of this paper is to present a practical way to create three kinds of double emulsions such as double emulsion, double-component double emulsion and viscoelastic double emulsion. Design/methodology/approach – A hierarchical T-junction microfluidic device is selected to simulate this phenomenon. A system of the three-phase flows consists of the inner, middle and outer phases were simulated by the direct numerical simulation (DNS) method. The dripping regime is considered for the droplet formation in both T-junctions. The adaptive mesh refinement technique is used to simulate the droplet formation and determine the interface rupture. Findings – The one-step and two-step encapsulation are used to create the double emulsion and the viscoelastic double emulsion, respectively. In both T-junctions, droplets are created by the balance of three parameters which are instability, viscous drag and pressure buildup. The one-step formation of double emulsion is presented for encapsulates the viscoelastic fluid. Originality/value – The simulated hierarchical microchannel shows some desirable features for creating the complex compounds. The encapsulation process is simulated in micro-scale that is useful for drug delivery applications.
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Papon, Easir Arafat, i Anwarul Haque. "Review on process model, structure-property relationship of composites and future needs in fused filament fabrication". Journal of Reinforced Plastics and Composites 39, nr 19-20 (19.06.2020): 758–89. http://dx.doi.org/10.1177/0731684420929757.
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This paper presents the state-of-the-art of additive manufacturing of composites for processing functional, load-bearing components. A general overview of different additive manufacturing methods is provided, and specific attention is focused on fused filament fabrication-based composites processing. Different process modeling strategies are summarized, and key aspects of these models are discussed. Significant results such as thermal and fluid flow characteristics, effects of nozzle geometry on melt flow, fiber orientation, bead spreading, and solidification, the formation of residual stresses, and deformation behavior are discussed from computational modeling perspective. The scientific advancement, model limitations, and future modeling needs are prescribed reviewing the current works. A general overview of material development in nano-micro-macro-scale reinforcement is also presented. Different length-scales of reinforcement has its own challenges and promises. The continuous fiber reinforcement has a great potential for being the next-generation composites manufacturing technology. However, the challenges in reducing the void content, better bonding between the fiber–matrix, and layer-layer adhesion, and process uncertainty are some of the key areas yet to advance. Based on the current limitations on computational modeling, materials development, and process modeling studies, future research needs and recommendations are provided.
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GULITSKI, G., M. KHOLMYANSKY, W. KINZELBACH, B. LÜTHI, A. TSINOBER i S. YORISH. "Velocity and temperature derivatives in high-Reynolds-number turbulent flows in the atmospheric surface layer. Part 1. Facilities, methods and some general results". Journal of Fluid Mechanics 589 (8.10.2007): 57–81. http://dx.doi.org/10.1017/s0022112007007495.
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This is a report on a field experiment in an atmospheric surface layer at heights between 0.8 and 10m with the Taylor micro-scale Reynolds number in the range Reλ = 1.6−6.6 ×103. Explicit information is obtained on the full set of velocity and temperature derivatives both spatial and temporal, i.e. no use of Taylor hypothesis is made. The report consists of three parts. Part 1 is devoted to the description of facilities, methods and some general results. Certain results are similar to those reported before and give us confidence in both old and new data, since this is the first repetition of this kind of experiment at better data quality. Other results were not obtained before, the typical example being the so-called tear-drop R-Q plot and several others. Part 2 concerns accelerations and related matters. Part 3 is devoted to issues concerning temperature, with the emphasis on joint statistics of temperature and velocity derivatives. The results obtained in this work are similar to those obtained in experiments in laboratory turbulent grid flow and in direct numerical simulations of Navier–Stokes equations at much smaller Reynolds numbers Reλ ~ 102, and this similarity is not only qualitative, but to a large extent quantitative. This is true of such basic processes as enstrophy and strain production, geometrical statistics, the role of concentrated vorticity and strain, reduction of nonlinearity and non-local effects. The present experiments went far beyond the previous ones in two main respects. (i) All the data were obtained without invoking the Taylor hypothesis, and therefore a variety of results on fluid particle accelerations became possible. (ii) Simultaneous measurements of temperature and its gradients with the emphasis on joint statistics of temperature and velocity derivatives. These are reported in Parts 2 and 3.
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Collado, Francisco J., Carlos Monné, Antonio Pascau, Daniel Fuster i Andrés Medrano. "Thermodynamics of Void Fraction in Saturated Flow Boiling". Journal of Heat Transfer 128, nr 6 (18.10.2005): 611–15. http://dx.doi.org/10.1115/1.2190696.
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Recently, Collado (Proc, IMECE 2001, Symposium on Fluid Physics and Heat Transfer for Macro- and Micro-Scale Gas-Liquid and Phase Change Flows) suggested calculating void fraction, an essential element in thermal-hydraulics, working with the “thermodynamic” quality instead of the usual “flow” quality. The “thermodynamic” quality is a state variable, which has a direct relation with the actual vapor volumetric fraction, or void fraction, through phase densities. This approach provides a procedure for predicting void fraction, if values of “thermodynamic” quality are available. However, the standard heat balance is usually stated as a function of the “flow” quality. Therefore, we should search for a new heat balance between the mixture enthalpy, based on “thermodynamic” quality, and the absorbed heat. This paper presents the results of such analysis based on the accurate measurements of the outlet void fraction measured during the Cambridge project by Knights (1960, “A Study of Two-Phase Pressure Drop and Density Determination in a High-Pressure Steam-Water Circuit,” Ph.D. thesis, Cambridge University Engineering Lab, Cambridge, UK) in the 1960s for saturated flow boiling. In the 286 tests analyzed, the pressure and mass fluxes range from 1.72 MPa to 14.48 MPa and from 561.4 to 1833.33 kgm−2s−1, respectively. As the main result, we find that the slip ratio would close this new thermodynamic heat balance. This has allowed the accurate calculation of void fraction from this balance, provided we can predict the slip ratio. Finally, the strong connection of this new thermodynamic heat balance with the standard one through the slip ratio is highlighted.
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Rani, Sarma L., Rohit Dhariwal i Donald L. Koch. "Clustering of rapidly settling, low-inertia particle pairs in isotropic turbulence. Part 2. Comparison of theory and DNS". Journal of Fluid Mechanics 871 (22.05.2019): 477–88. http://dx.doi.org/10.1017/jfm.2019.294.
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Part 1 (Rani et al. J. Fluid Mech., vol. 871, 2019, pp. 450–476) of this study presented a stochastic theory for the clustering of monodisperse, rapidly settling, low-Stokes-number particle pairs in homogeneous isotropic turbulence. The theory involved the development of closure approximations for the drift and diffusion fluxes in the probability density function (p.d.f.) equation for the pair relative positions $\boldsymbol{r}$. In this part 2 paper, the theory is quantitatively analysed by comparing its predictions of particle clustering with data from direct numerical simulations (DNS) of isotropic turbulence containing particles settling under gravity. The simulations were performed at a Taylor micro-scale Reynolds number $Re_{\unicode[STIX]{x1D706}}=77.76$ for three Froude numbers $Fr=\infty ,0.052,0.006$, where $Fr$ is the ratio of the Kolmogorov scale of acceleration and the magnitude of gravitational acceleration. Thus, $Fr=\infty$ corresponds to zero gravity, and $Fr=0.006$ to the highest magnitude of gravity among the three DNS cases. For each $Fr$, particles of Stokes numbers in the range $0.01\leqslant St_{\unicode[STIX]{x1D702}}\leqslant 0.2$ were tracked in the DNS, and particle clustering quantified both as a function of separation and the spherical polar angle. We compared the DNS and theory values for the exponent $\unicode[STIX]{x1D6FD}$ characterizing the power-law dependence of clustering on separation. The $\unicode[STIX]{x1D6FD}$ from the $Fr=0.006$ DNS case are in reasonable agreement with the theoretical predictions obtained using the second drift closure (referred to as DF2). To quantify the anisotropy in clustering, we calculated the leading–order coefficient in the spherical harmonics expansion of the p.d.f. of pair relative positions. The coefficients predicted by the theory (DF2) again show reasonable agreement with those calculated from the DNS clustering data for $Fr=0.006$. However, we note that in spite of the high magnitude of gravity, the clustering is only marginally anisotropic both in DNS and theory. The theory predicts that the spherical harmonic coefficient scales with $\unicode[STIX]{x1D6FD}(=\unicode[STIX]{x1D6FD}_{2}St_{\unicode[STIX]{x1D702}}^{2})$, where $\unicode[STIX]{x1D6FD}_{2}$ is the ratio of the drift and diffusion flux coefficients. Since the drift flux, and thereby $\unicode[STIX]{x1D6FD}_{2}$, is seen to decrease with gravity for $St_{\unicode[STIX]{x1D702}}<1$, the anisotropy is also correspondingly diminished.
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Chaudhary, Indresh, Piyush Garg, V. Shankar i Ganesh Subramanian. "Elasto-inertial wall mode instabilities in viscoelastic plane Poiseuille flow". Journal of Fluid Mechanics 881 (24.10.2019): 119–63. http://dx.doi.org/10.1017/jfm.2019.759.
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A linear stability analysis of plane Poiseuille flow of an upper-convected Maxwell (UCM) fluid, bounded between rigid plates separated by a distance $2L$, has been carried out to investigate the interplay of elasticity and inertia on flow stability. The stability is governed by the following dimensionless groups: the Reynolds number $Re=\unicode[STIX]{x1D70C}U_{max}L/\unicode[STIX]{x1D702}$ and the elasticity number $E\equiv W/Re=\unicode[STIX]{x1D706}\unicode[STIX]{x1D702}/(\unicode[STIX]{x1D70C}L^{2})$, where $W=\unicode[STIX]{x1D706}U_{max}/L$ is the Weissenberg number. Here, $\unicode[STIX]{x1D70C}$ is the fluid density, $\unicode[STIX]{x1D702}$ is the fluid viscosity, $\unicode[STIX]{x1D706}$ is the micro-structural relaxation time and $U_{max}$ is the maximum base-flow velocity. The stability is analysed for two-dimensional perturbations using both pseudo-spectral and shooting methods. We also analyse the linear stability of plane Couette flow which, along with the results for plane Poiseuille flow, yields insight into the structure of the complete elasto-inertial eigenspectrum. While the general features of the spectrum for both flows remain similar, plane Couette flow is found to be stable over the range of parameters examined ($Re\leqslant 10^{4},E\leqslant 0.01$). On the other hand, plane Poiseuille flow appears to be susceptible to an infinite hierarchy of elasto-inertial instabilities. Over the range of parameters examined, there are up to seven distinct neutral stability curves in the $Re$–$k$ plane (here $k$ is the perturbation wavenumber in the flow direction). Based on the symmetry of the eigenfunctions for the streamwise velocity about the centreline, four of these instabilities are antisymmetric, while the other three are symmetric. The neutral stability curve corresponding to the first antisymmetric mode is shown to be a continuation (to finite $E$) of the Tollmien–Schlichting (TS) instability already present for Newtonian channel flow. As $E$ is increased beyond $0.0016$, a new elastic mode appears at $Re\sim 10^{4}$, which coalesces with the continuation of the TS mode for a range of $Re$, thereby yielding a single unstable mode in this range. This trend persists until $E\sim 0.0021$, beyond which this neutral curve splits into two separate ones in the $Re$–$k$ plane. The new elastic mode which arises out of this splitting has been found to be the most unstable, with the lowest critical Reynolds number $Re_{c}\approx 1210.9$ for $E=0.0066$. The neutral curves for both the continuation of the original TS mode, and the new elastic antisymmetric mode, form closed loops upon further increase in $E$, which eventually vanish at sufficiently high $E$. For $E\ll 1$, the critical Reynolds number and wavenumber scale as $Re_{c}\sim E^{-1}$ and $k_{c}\sim E^{-1/2}$ for the first two of the symmetric modal families, and as $Re_{c}\sim E^{-5/4}$ for first two of the antisymmetric modal families; $k_{c}\sim E^{-1/4}$ for the third antisymmetric family. The critical wave speed for all of these unstable eigenmodes scales as $c_{r,c}\sim E^{1/2}$ for $E\ll 1$, implying that the modes belong to a class of ‘wall modes’ in viscoelastic flows with disturbances being confined in a thin region near the wall. The present study shows that, surprisingly, even in plane shear flows, elasticity acting along with inertia can drive novel instabilities absent in the Newtonian limit.
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Nan, Keyi, Zhongyan Hu, Wei Zhao, Kaige Wang, Jintao Bai i Guiren Wang. "Large-Scale Flow in Micro Electrokinetic Turbulent Mixer". Micromachines 11, nr 9 (28.08.2020): 813. http://dx.doi.org/10.3390/mi11090813.
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In the present work, we studied the three-dimensional (3D) mean flow field in a micro electrokinetic (μEK) turbulence based micromixer by micro particle imaging velocimetry (μPIV) with stereoscopic method. A large-scale solenoid-type 3D mean flow field has been observed. The extraordinarily fast mixing process of the μEK turbulent mixer can be primarily attributed to two steps. First, under the strong velocity fluctuations generated by μEK mechanism, the two fluids with different conductivity are highly mixed near the entrance, primarily at the low electric conductivity sides and bias to the bottom wall. Then, the well-mixed fluid in the local region convects to the rest regions of the micromixer by the large-scale solenoid-type 3D mean flow. The mechanism of the large-scale 3D mean flow could be attributed to the unbalanced electroosmotic flows (EOFs) due to the high and low electric conductivity on both the bottom and top surface.