Academic literature on the topic 'Micro-scale Fluid Mechanics'

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.

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.

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.

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.

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.

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.

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.

Thesis (M.S. in chemical engineering)--Washington State University, May 2009.
Title from PDF title page (viewed on June 8, 2009). "Department of Chemical Engineering." Includes bibliographical references (p. 70-71).

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, February 2007.
Includes bibliographical references (p. 193-208).
Magnetorheological fluids belong to the class of field-responsive fluids that undergo large, reversible and fast changes in their rheological properties when acted upon by an external magnetic field. 'Smart' or controllable composite materials have been obtained by doping polymers, foams, fabrics etc. with these field-responsive fluids. The resulting composite materials have potential applications in numerous fields ranging from adaptive energy absorption, automotive crash protection to microfluidic valves, mixers and separation devices. A series of stable magnetorheological (MR) fluids have been systematically characterized under steady shearing, creep and large amplitude oscillatory shear (LAOS) flow conditions. A rheometer fixture for applying nearly uniform magnetic fields up to 0.4 T has been fabricated to measure both steady-state and transient changes in the fluid properties under applied fields. Stable MR fluids with a markedly improved dynamic response (yield stress as a function of magnetic field) compared to commercial fluids have been formulated by increasing the constituent particle size and by stabilizing the system against sedimentation. A new "soft-glassy rheology" model has been used to model the fluid response time and visco-elasto-plastic response under creep conditions and oscillatory loadings.
(cont.) The experiments and model show that the evolution of chain structure and plastic collapse in these suspensions exhibits a universal scaling with the dimensionless stress s = [sigma]/[sigma]y. Structure evolution, pattern formation and dynamics of MR fluid flow in microchannel geometries has been analyzed using high-speed digital video microscopy. In order to elucidate the mechanisms that control MR structure formation, experiments have been performed while varying the magnetic field, particle size, channel geometry, concentration and fluid composition. Excellent qualitative agreement has been obtained with Brownian Dynamics simulations and useful scalings based on interplay of magnetostatic & viscous forces have been extracted to understand the field-dependent fluid response on the macro & micro scale. Novel MR elastomeric materials and microparticles have been synthesized by doping photo-curable or thermo-curable polymers with field-responsive fluids. A high-throughput micromolding technique for synthesis of controllable particles of anisotropic shapes and sizes has been developed. Flexible and permanent chain-like structures have also been synthesized using amidation chemistry. Potential microfluidic applications such as field-responsive valves, mixers and separation devices using these 'smart' materials have also been investigated.
by Suraj Sharadchandra Deshmukh.
Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.
Includes bibliographical references (leaves 187-189).
The aim of this thesis was to further characterize the effectiveness of field responsive fluids (FRFs) in geometries pertinent to the soldier and to examine the effects of specific geometric and kinematic parameters, including patterned surface geometry, electrode gap distance, and normal force on the performance of homogeneous ERF composites. Field responsive fluid composites designed for variable impedance energy absorption incorporated electrorheological fluid (ERF) and shear-thickening fluid (STF) in novel geometries to absorb compressive and tensile/shear forces. ER and ST fluids change their apparent viscosity in the presence of elevated electric and shear fields, respectively, and the magnitude of this effect can be adjusted using the magnitude of the input field, allowing variable impedance operation. Several test fixtures were developed to test these novel FRF composites. A compression apparatus was designed and constructed to test STF-filled foam over a range of strain rates not previously examined in the literature. Silicon-based microchannel devices with etched features on the order of 100 pm and etch depths of 7-90 pm were fabricated to test homogeneous ER fluids in small electrode gaps.
(cont.) Tests using these silicon devices allowed creation of 5 kV/mm (5 V/pm) electric fields across electrode gaps as small as 20 pm, with increases of measured shear force as high as 350% from no electric field to full 5 kV/mm operation. Production of these devices in bulk using established silicon processing techniques was demonstrated, and factors affecting the manufacture of these devices were investigated.
by Ryan A. Griffin.
S.M.

Thesis (Mechanical Engineer and M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2005.
Thesis Advisor(s): Gopinath, Ashok ; Sinibaldi, Jose O. "September 2005." Description based on title screen as viewed on March 12, 2008. Includes bibliographical references (p. 85-87). Also available in print.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.
Includes bibliographical references (leaf 37).
Biological cells and organisms employ a different method of propulsion when in viscous, viscolelastic fluids rather than Newtonian fluids. By studying the dynamics of a flag under a flow of a viscoelastic fluid, we hope to better understand the swimming dynamics in these biological fluids. A slender polysiloxane rod was placed in a rotating annulus filled with a cetyl pyridnium chloride micellar solution and also with a xanthan gum solution. Flapping of the rod was observed with the micellar solution for Weissenberg numbers greater than 1, where elastic forces in the fluid dominated the elastic force in the flag. Flapping was not observed in the xanthan gum for Weissenberg numbers up to 250, where the elastic force in the flag dominated the elastic force in the fluid. The observation of a flapping flag in a viscoelastic fluid indicates that, unlike in a Newtonian fluid, the polymers in the fluid can interact with an elastic body to cause a flapping motion which may indicate why the swimming dynamics of sperm change with their fluid environment.
by Batya A. Fellman.
S.B.

Controlling internal flow in evaporating sessile droplets is desirable across applications ranging from lab-on-chip medical diagnostics, DNA profiling to surface patterning. Diffusion-limited evaporation in droplets exhibits very low internal flow velocities [∼O(10^−6) m/s]. Enhancement of internal flow is helpful for applications that demand in situ mixing at small-scale fluidic systems but are limited by the low Reynolds number. To overcome this limitation, we present a non-intrusive methodology to enhance flow inside the droplets without affecting their global evaporation pattern. A highly volatile ethanol droplet is positioned asymmetrically in the vicinity of a water droplet. The ethanol molecules are consequently adsorbed asymmetrically on the air-water interface creating a gradient in surface tension. This causes an internal Marangoni convection with flow rates ∼O(10^3) times higher than a naturally evaporating water droplet. The inter-droplet distance between ethanol-water is used as a control parameter to vary the strength of Marangoni convection. The flow pattern transitions through several regimes from asymmetric to symmetric double toroid once the ethanol droplet completely evaporates. Experimental flow visualization and quantification by micro-particle image velocimetry have been used alongside simple scaling arguments to quantify the physical mechanism at play. We can also switch between different flow patterns by strategic dispensing of ethanol droplets. Mixing at small fluidic length scales is especially challenging in viscous and non-volatile droplets frequently encountered in biochemical assays. In situ mixed methods, which depend on diffusion or evaporation-driven capillary flow, are typically slow and inefficient, while thermal or electro-capillary methods are either complicated to implement or may cause sample denaturing. As a consequence of increased velocity by vapor-mediated interactions, we can use it to enhance mixing in droplets. We demonstrate a decrease in mixing timescale in a sessile droplet of glycerol by simply introducing a droplet of ethanol in its near vicinity. The fast evaporation of ethanol introduces molecules in the proximity of the glycerol droplet, which is preferentially adsorbed (more on the side closer to ethanol), creating a gradient of surface tension driving the Marangoni convection in the droplet. We conclusively show that the mixing time reduces by ∼10 hours due to the vapor-mediated Marangoni convection for the given volume of the droplet. Simple scaling arguments are used to predict the enhancement of the mixing timescale. Experimental evidence obtained from fluorescence imaging is used to quantify mixing and validate the analytical results. This is the first proof of enhanced mixing in a viscous, sessile droplet using the vapor mediation technique. Further, sessile droplets of contrasting volatilities that can communicate via long-range (∼O(1) mm) vapor-mediated interactions are used to remote control the flow-driven self-assembly of nanoparticles in the drop of lower volatility. This allows morphological control of the buckling instability observed in evaporating nanofluid droplets. A nanofluid droplet is dispensed adjacent to an ethanol droplet. Asymmetrical adsorption-induced Marangoni flow (∼O(1) mm/s) internally segregates the particle population. Particle aggregation occurs preferentially on one side of the droplet, leaving the other side to develop a relatively weaker shell that buckles under the effect of evaporation-driven capillary pressure. The inter-droplet distance is varied to demonstrate the effect on the precipitate shape (flatter to dome-shaped) and the location of the buckling (top to side). In addition to being a simple template for hierarchical self-assembly, the presented exposition also promises to enhance mixing rates in droplet-based bioassays with minimal contamination. Vapor-mediated interaction in droplets can have implications in controlling agglomeration in functional droplets. A functional sessile droplet containing buoyant colloids (ubiquitous in applications like chemical sensors, drug delivery systems, and nanoreactors) forms self-assembled aggregates. The particles initially dispersed over the entire drop-flocculates at the center. We attribute the formation of such aggregates to the finite radius of curvature of the drop and the buoyant nature of particles. Initially, larger particles rise to the top of the droplet (due to higher buoyancy force), and later the smaller particles join the league, leading to the graded size distribution of the central aggregate. This can be used to segregate polydisperse hollow spheres based on size. The proposed scaling analysis unveils insights into the distinctive particle transport during evaporation. However, the formation of prominent aggregates can be detrimental in applications like spray painting, pesticide industries, washing, coating, lubrication, etc. One way to avoid the central aggregate is to spread the droplets completely (contact angle ~ 00), thus theoretically creating an infinite radius of curvature leading to uniform deposition of buoyant particles. Practically, this requires a highly hydrophilic surface, and even a tiny inhomogeneity on the surface would pin the droplet giving it a finite radius of curvature. We demonstrate that using non-intrusive vapor mediated Marangoni convection higher than the evaporation-driven convection) can be vital to an efficient and on-demand manipulation of the suspended micro-objects. The interplay of surface tension and buoyancy force results in the transformation of flow inside the droplet leads to spatiotemporal disbanding of agglomeration at the center of the droplet. We also showcase a mechanism of asymmetric solvent depletion using vapor-mediated interaction that can non-intrusively regulate the site of crystal precipitation. In general, the flow pattern inside a drying sessile saline droplet leads to the circumferential deposition of salt crystals at the end of evaporation. Instead, we show that our proposed approach can manipulate the spatial location of crystal precipitation. The introduction of a pendant ethanol droplet near the sessile saline droplet’s vicinity creates an asymmetric ethanol vapor gradient around the sessile drop. The vigorous and non-uniform Marangoni flow promotes targeted contact line depinning, ensuring preferential segregation of the salt crystals. Using this methodology, we can inhibit crystal formation at selected locations and favorably control its deposition in definite regions. The interplay of flow hydrodynamics and the associated contact line motion governs this phenomenon marked by the inception and growth of crystals at a preferential site. The universal character of such a phenomenon is verified for a variety of salt solutions on the glass substrate. Deposits of biofluid droplets on surfaces (such as respiratory droplets formed during an expiratory) are composed of water-based salt-protein solution that may also contain an infection (bacterial/viral). The final patterns of the deposit formed are dictated by the composition of the fluid and flow dynamics within the droplet. This work reports the spatio-temporal, topological regulation of deposits of respiratory fluid droplets and control of motility of bacteria by tweaking flow inside droplets using non-contact vapor-mediated interactions. Respiratory droplets form multiscale dendritic, cruciform-shaped precipitates when evaporated on a glass substrate. However, we showcase that using non-intrusive vapor mediation as a tool can control these deposits at nano-micro-millimeter scales. We morphologically control dendrite orientation, size and subsequently suppress cruciform-shaped crystals. The nucleation sites are controlled via preferential transfer of solutes in the droplets; thus, achieving control over crystal occurrence and growth dynamics. As a result, active living matter in respiratory fluids like bacteria is preferentially segregated and agglomerated with controlled motility without attenuation of its viability and pathogenesis. For the first time, we have experimentally presented a proof-of-concept to control the motion of live active matter like bacteria in a near non-intrusive manner. The methodology can have ramifications in biomedical applications like disease detection and bacterial segregation.

Supercritical CO2 fluid has been widely used in chemical extraction, chemical synthesis, micro-manufacturing and heat transfer apparatus, and so forth. The current chapter deals with near-critical CO2 micro-scale thermal convective flow and the effects of thermal-mechanical process. When the scale becomes smaller, new and detailed figures of near-critical thermal effects emerges. To explore this new area, theoretical developments and numerical investigations discussed and explained in this chapter. From a theoretical point of view, the thermal-mechanical nature of near-critical fluid would play a leading role in small time and spatial scales. This effect is found to dominants the thermal dynamic responses and convective structures of micro-scale fluid behaviors. The scaling effects, boundary thermal-mechanical process, instability evolutions, mixing flows and characteristics, possible extensions and applications are also discussed in this chapter.

Supercritical CO2 fluid has been widely used in chemical extraction, chemical synthesis, micro-manufacturing, and heat transfer apparatus, and so forth. The current chapter deals with near-critical CO2 micro-scale thermal convective flow and the effects of thermal-mechanical process. When the scale becomes smaller, new, and detailed figures of near-critical thermal effects emerges. To explore this new area, theoretical developments and numerical investigations are discussed and explained in this chapter. From a theoretical point of view, the thermal-mechanical nature of near-critical fluid would play a leading role in small time and spatial scales. This effect is found dominant to the thermal dynamic responses and convective structures of micro-scale fluid behaviors. The scaling effects, boundary thermal-mechanical process, instability evolutions, mixing flows and characteristics, possible extensions, and applications are also discussed in this chapter.

Microelectromechanical systems (MEMS) refer to systems with characteristic length ranging between 1 µm and 1 mm, fabricated by integrated circuits batch processing technologies and unite mechanical and electrical components. MEMS devices and systems have wide applications in multifarious medical and industrial applications with worldwide market of billions of dollars. Examples of MEMS devices are accelerometers for automobile airbags; micropumps for inkjet printing, electronic cooling, and environmental testing; infrared detectors, digital light processing chip for projection display, etc. Microfluidics refers to fluid flow at a small size scale that causes change in fluid behavior. Microfluidic devices/systems handle a small quantity (micro- or nanoliter) of fluids (liquid or gas). The major application for handling fluids in microfluidics relates to chemical and biomedical analyses. The benefit of application of microfluidics in chemical and biomedical analysis is that they provide a total solution from sample utilization to display of analytical results.

Computational fluid dynamics (CFD) study of the behavior of red blood cells (RBCs) in flow provides us informative insight into the mechanics of blood flow in microvessels. However, the size of computational domain is limited due to computational expense. Recently, we proposed a graphics processing unit (GPU) computing method for patient-specific pulmonary airflow simulations (Miki et al., in press). In this study, we extend this method to micro-scale blood flow simulations, where a lattice Boltzmann method (LBM) of fluid mechanics is coupled with a finite element method (FEM) of membrane mechanics by an immersed boundary method (IBM). We also present validation and performance of our method for micro-scale blood flow simulations.

Engineering surfaces are never perfectly flat. They contain micro and nano-scale features on multiple length scales. Predicting the amount of fluid trapped in these minute surface crevices and its controlled release could benefit a variety of practical applications. In a sliding contact, the released fluid could create an ultra-thin film, reducing the direct contact and consequently the boundary friction. Transdermal patches are the least invasive of available subcutaneous drug delivery techniques. The drug is stored in a micro-reservoir and it is released to the skin either through a permeable membrane or through a series of micro needles. The aim of the current paper represents the first attempt to investigate whether a modeling approach encompassing two complementary simulation techniques in an integrated framework can be used to predict the volume of fluid stored in a nano-scale surface feature. Molecular dynamics (MD) simulation could provide accurate modeling of fluid behavior at nano-scale, and statistical mechanics (SM) could provide a fast prediction.

The concept of slip length, related to surface velocity and shear rate, is often used to analyze the slip surface property for flow in micro or nanochannels. In this study, a hybrid scheme that couples Molecular dynamics simulation (used near the solid boundary to include the surface effect) and a continuum solution (to study the fluid mechanics) is validated and used for the study of slip length behavior in the Couette flow problem. By varying the height of the channel across multiple length scales, we investigate the effect of channel scale on surface slip length. In addition, by changing the velocity of the moving-solid wall, the influence of shear rate on the slip length in a certain range of the channel height is studied. The results show that within a certain range of the channel heights, the slip length is size-dependant. This upper bound of the channel height can vary with the shear rate.

The question of “where and how the turbulent drag arises” is one of the most fundamental problems unsolved in fluid mechanics. However, the physical mechanism responsible for the friction drag reduction is still not well understood. Over decades, it is found that the turbulence production and self-containment in a boundary layer are organized phenomena and not random processes as the turbulence looks like. The further study in the boundary layer should be able to help us know more about the mechanisms of drag reduction. The wavelet-based vector multi-resolution technique was proposed and applied to the two dimensional PIV velocities for identifying the multi-scale turbulent structures. The intermediate and small scale vortices embedded within the large-scale vortices were separated and visualized. By analyzing the fluctuating velocities at different scales, coherent eddy structures were obtained and this help us obtain the important information on the multi-scale flow structures in the turbulent flow. By comparing the eddy structures in different operating conditions, the mechanism to explain the drag reduction caused by micro bubbles in turbulent flow was proposed.

Conventional fluid mechanics (Navier–Stokes equations with linear constitutive relations) is, on the whole, applicable for simulating very small scale liquid and gas systems. This changes (for simple fluids) only in the vicinity of solid surfaces (approximately 5 molecular diameters for liquids, or one mean free path for gases) or under very high temperature or velocity gradients. It is shown that typical experimental conditions in practical systems do not give rise to gradients of this magnitude. Therefore, only surface effects cause significant deviation from results expected by conventional fluid mechanics. In micro and nano systems, however, large surface area to volume ratio means that the detail of boundary conditions and near surface dynamics can dominate the flow characteristics. In this paper, the use of non–equilibrium molecular dynamics (NEMD) to study these fluid mechanics problems in an engineering simulation context is discussed. The extent of systems that can be studied by NEMD, given current computational capabilities, is demonstrated. Methods for reducing computational cost, such as hybridisation with continuum based fluid mechanics and extracting information from a small representative systems are also discussed. Non–equilibrium surface effects in gas micro systems may also been studied using NEMD. These occur at boundaries in the form of discontinuities (velocity slip and temperature jump) and within approximately one mean free path of a surface, in the form of a Knudsen layer. The distributions of molecular velocities, free path between collisions and time spent in collision have been calculated for an unbounded equilibrium fluid. The influence of a solid surface on the state of a fluid or flow can be investigated by measuring how these fundamental properties are affected.

Flow maldistribution is a fluid-management problem of interest in engineering. It consists in the non-uniform distribution of various flows in industrial applications like multiple accesses, manifolds, bifurcations, spreaders, etc., where the fluid currents separate, detach and reattach, break from the main body of the stream and splash within the conducts, etc. Several effects in the operation of most systems and apparatus where it occurs are common, including the interruption of fluid currents, malfunctioning, and high energy consumptions. The problem at industrial scale has been identified and treated mainly through empirical considerations, but no totally solved. It has been scarcely investigated however, in micro-technology applications with unforeseeable situations in theses very small area/volume scales, with appealing engineering tasks to solve. In any case, detailed analyses must be conducted using fluid mechanics models with experimental validation. In this paper the most relevant aspects of flow maldistribution in micro-systems where intense heat and mass transfer occur, are described. A review of their consequences and the current trends for their remediation, are presented. Examples in MEMS, fuel cells, micro-heat exchangers and micro-structured chemical reactors, are considered.

The non-mechanical valvular conduit, which uses no moving parts but instead relies on a complex geometry to regulate flow, is studied through a combination of numerical, computational and experimental methods. This study is based on using water as the fluid at standard state properties. A numerical model is developed to evaluate the effectiveness of the non-mechanical valve’s intricate geometry. Then computational simulations of the oscillating/pumping sequence of the valvular conduit are conducted to examine the effectiveness of the valve when placed in use for a diaphragm pump. Results demonstrate that the non-mechanical valvular conduit can be an effective application for a diaphragm pump at the micro or macro-scale without requiring valvular mechanics. In computational simulations, when non-mechanical valves are positioned at both the inlet and exit of a diaphragm, the positive circulation of fluid is enhanced by 38% which is sufficient to meet the thermal dissipation requirements of an Intel Pentium D processor (i.e. 130 W). In addition, the experimental results in steady state condition demonstrated that the valvular design regulates the flow direction by producing diodicity (a measure of favorable flow direction) of 2.44.

This paper presents experimental results concerning water flow in smooth and rough rectangular micro-channels. It is part of a work intended to test the classical fluid mechanics laws when the characteristic length scale of inner liquid flows falls below 500μm. The method consists in determining experimental friction coefficients as a function of the Reynolds number. This implies simultaneous measurements of pressure drop and flow rates in microstructures. The two experimental apparatus used in this study enabled us to explore a wide range of length scales (7μm to 300μm) and of Reynolds number (0.01 to 8,000). Classical machining technologies were used to make micro-channels of various heights down to a scale of 100μm. Smaller silicon-Pyrex micro-channels were also made by means of silicon-based micro technologies. In these structures, friction coefficients have been measured locally with Cu-Ni strain gauges. For every height tested, both smooth and rough walls were successively used. When compared to macro-scale correlation the results demonstrate that i) In the smooth case, friction is correctly predicted by the Navier-Stokes equations with the classical kinematic boundary conditions, ii) For 200μm high channels, visualizations show transition to turbulence at Reynolds number of about 3,000. The presence of roughness elements did not significantly influence this result and iii) Roughness considerably increases the friction coefficient in the laminar regime. However, the Poiseuille number remains independent of the Reynolds number.

Unless protected by an inert gas atmosphere, micro-scale conjunctions are often separated by molecularly-thin adhered films. Therefore, predicting contact load, friction or adhesion, must consider the contribution of this layer to the overall contact problem. The contribution of an adhered layer can be accounted for using a simplified solution (e.g. an adjustment to the energy of adhesion to account for the liquid film). However, these methods cannot account for layers consisting of multiple species of molecules. The most common approach, which accounts for inter-molecular forces between molecules of various species, is a molecular dynamics simulation. However, this is time consuming, and therefore, often limited for small volumes of fluid and small scale contacts. The current paper proposes an alternative approach, where the pressure and shear between two smooth surfaces separated by an ultra-thin film is predicted using a statistical mechanics based model. This method accounts for the chemical structure of each species of molecules comprising the ultra-thin film, their concentration, intermolecular forces and adsorption to the wall. This approach is very fast, therefore, it can be easily included in a larger scale code predicting the behavior of the entire micro-scale mechanism. It was found that for a specified material of the solid boundary the model can predict the optimal concentration of each species of molecule in the intervening ultra-thin film, to minimize friction or adhesion.

The study of liquid dynamics in LNG tanks is getting more and more important with the actual trend of LNG tankers sailing with partially filled tanks. The effect of sloshing liquid in the tanks on pressure levels at the tank walls and on the overall ship motion indicates the relevance of an accurate simulation of the fluid behaviour. This paper presents the simulation of sloshing LNG by a compressible two-phase model and the validation of the numerical model on model-scale sloshing experiments. The details of the numerical model, an improved Volume Of Fluid (iVOF) method, are presented in the paper. The program has been developed initially to study the sloshing of liquid fuel in spacecraft. The micro-gravity environment requires a very accurate and robust description of the free surface. Later, the numerical model has been used for calculations for different offshore applications, including green water loading. The model has been extended to take two-phase flow effects into account. These effects are particularly important for sloshing in tanks. The complex mixture of the liquid and gas phase around the free surface imposes a challenge to numerical simulation. The two-phase flow effects (air entrapment and entrainment) are strongly affected by both the filling ratio of the tank and the irregular motion of the tank in typical offshore conditions. The velocity field and pressure distribution around the interface of air and LNG, being continuous across the free surface, requires special attention. By using a newly-developed gravity-consistent discretisation, spurious velocities at the free surface are prevented. The equation of state applied in the compressible cells in the flow domain induces the need to keep track on the pressure distribution in both phases, as the gas density is directly coupled to the gas pressure. The numerical model is validated on a 1:10 model-scale sloshing model experiment. The paper shows the results of this validation for different filling ratios and for different types of motion of the sloshing tank.