Academic literature on the topic 'Hydrodynamic slip length'
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Journal articles on the topic "Hydrodynamic slip length"
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Tsekov, Roumen. "Nonuniform Slip Effect in Wetting Films." Coatings 10, no. 6 (June 25, 2020): 597. http://dx.doi.org/10.3390/coatings10060597.
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The slip effect in wetting films is theoretically studied, and a nonlinear dependence of the hydrodynamic velocity on the slip length is discovered. It is demonstrated that the hydrodynamic flow is essentially affected by the presence of a nonuniform slip length distribution, leading also to enhancement of the energy dissipation in the films. This effect could dramatically slow the usually quick hydrodynamic flows over superhydrophobic surfaces, for instance.
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DAVIS, ANTHONY M. J., and ERIC LAUGA. "Hydrodynamic friction of fakir-like superhydrophobic surfaces." Journal of Fluid Mechanics 661 (August 23, 2010): 402–11. http://dx.doi.org/10.1017/s0022112010003460.
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A fluid droplet located on a superhydrophobic surface makes contact with the surface only at small isolated regions, and is mostly in contact with the surrounding air. As a result, a fluid in motion near such a surface experiences very low friction, and superhydrophobic surfaces display strong drag reduction in the laminar regime. Here we consider theoretically a superhydrophobic surface composed of circular posts (so-called fakir geometry) located on a planar rectangular lattice. Using a superposition of point forces with suitably spatially dependent strength, we derive the effective surface-slip length for a planar shear flow on such a fakir-like surface as the solution to an infinite series of linear equations. In the asymptotic limit of small surface coverage by the posts, the series can be interpreted as Riemann sums, and the slip length can be obtained analytically. For posts on a square lattice, our analytical prediction of the dimensionless slip length, in the low surface coverage limit, is in excellent quantitative agreement with previous numerical computations.
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Michelin, Sébastien, Giacomo Gallino, François Gallaire, and Eric Lauga. "Viscous growth and rebound of a bubble near a rigid surface." Journal of Fluid Mechanics 860 (December 3, 2018): 172–99. http://dx.doi.org/10.1017/jfm.2018.876.
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Motivated by the dynamics of microbubbles near catalytic surfaces in bubble-powered microrockets, we consider theoretically the growth of a free spherical bubble near a flat no-slip surface in a Stokes flow. The flow at the bubble surface is characterised by a constant slip length allowing us to tune the hydrodynamic mobility of its surface and tackle in one formulation both clean and contaminated bubbles as well as rigid shells. Starting with a bubble of infinitesimal size, the fluid flow and hydrodynamic forces on the growing bubble are obtained analytically. We demonstrate that, depending on the value of the bubble slip length relative to the initial distance to the wall, the bubble will either monotonically drain the fluid separating it from the wall, which will exponentially thin, or it will bounce off the surface once before eventually draining the thin film. Clean bubbles are shown to be a singular limit which always monotonically get repelled from the surface. The bouncing events for bubbles with finite slip lengths are further analysed in detail in the lubrication limit. In particular, we identify the origin of the reversal of the hydrodynamic force direction as due to the change in the flow pattern in the film between the bubble and the surface and to the associated lubrication pressure. Last, the final drainage dynamics of the film is observed to follow a universal algebraic scaling for all finite slip lengths.
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Li, Da Yong, Da Lei Jing, Yun Lu Pan, Khurshid Ahmad, and Xue Zeng Zhao. "Slip Length Measurement of Water Flow on Graphite Surface Using Atomic Force Microscope." Advanced Materials Research 941-944 (June 2014): 1581–84. http://dx.doi.org/10.4028/www.scientific.net/amr.941-944.1581.
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In this paper, we present experimental measurements of slip length of deionized (DI) water flow on a silicon surface and a graphite surface by using atomic force microscope. The results show that the measured hydrodynamic drag force is higher on silicon surface than that on graphite surface, and a measured slip length about 10 nm is obtained on the later surface.
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PANZER, PETER, MARIO LIU, and DIETRICH EINZEL. "THE EFFECTS OF BOUNDARY CURVATURE ON HYDRODYNAMIC FLUID FLOW: CALCULATION OF SLIP LENGTHS." International Journal of Modern Physics B 06, no. 20 (October 20, 1992): 3251–78. http://dx.doi.org/10.1142/s0217979292001420.
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The slip description of fluid flow past solid boundaries is reconsidered. We find that the traditional picture of fluid slip as a mean free path correction to hydrodynamics has to be revised whenever the particle scattering becomes close to specular. Then the microscopic slip length may diverge and it is the boundary’s curvature which is decisive for the momentum transfer between fluid and wall. By explicitly considering surface roughness we can explain discrepancies between experimentally observed data and traditional slip theory.
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Bharti, Partha P. Gopmandal, R. K. Sinha, and H. Ohshima. "Effect of core hydrophobicity on the electrophoresis of pH-regulated soft particles." Soft Matter 17, no. 11 (2021): 3074–84. http://dx.doi.org/10.1039/d0sm02278k.
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Wang, Yun-Lei, Jiu-Hui Wu, Mu-Ming Hao, and Lu-Shuai Xu. "Improved hydrodynamic performance of liquid film seal by considering boundary slip and cavitation." Industrial Lubrication and Tribology 71, no. 9 (November 4, 2019): 1108–15. http://dx.doi.org/10.1108/ilt-03-2019-0088.
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Purpose The purpose of this paper is to investigate the effect of boundary slip on hydrodynamic performance of liquid film seal considering cavitation. Design/methodology/approach A mathematical model of liquid film seal with slip surface was established based on the Navier slip model and Jakobsson–Floberg–Olsson (JFO) boundary condition. Liquid film governing equation was discretized by the finite difference method and solved by the SOR relaxation iterative algorithm and the hydrodynamic performance parameters of liquid film seal were obtained considering boundary slip and cavitation. Findings The results indicate that the values of performance parameters are affected significantly by the slip length under the condition of high speed and low differential pressure. Originality/value The performances of liquid film seal are investigated considering slip surface and cavitation. The results presented in the study are expected to provide a theoretical basis to improve the design method of liquid film seal.
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Vidal, A., and L. Botto. "Slip flow past a gas–liquid interface with embedded solid particles." Journal of Fluid Mechanics 813 (January 17, 2017): 152–74. http://dx.doi.org/10.1017/jfm.2016.842.
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We simulate shear flow past a stationary monolayer of spherical particles embedded in a flat gas–liquid interface. This problem is relevant to the understanding of the microhydrodynamics of particle-laden interfacial structures, including particle-laden drops, bubbles and foams. The combination of the free-shear condition at the gas–liquid interface and the no-slip condition at the particle surfaces gives rise to a velocity slip at the particle-laden interface. We study the characteristics of the flow near the monolayer, focusing on slip velocity, slip length and interfacial shear stress. Two microstructures are compared: a square array, and a reticulated array mimicking a percolating network of aggregated particles. We demonstrate that the scaling laws for the dependence of the slip length on solid area fraction developed for flow past superhydrophobic microstructured surfaces apply to the case of interfacial particles. The calculated slip lengths are in general smaller that those reported for microstructured superhydrophobic surfaces. This difference, which is due to the significant protrusion of the spherical particles in the liquid, can be accounted for in the case of the square array by an approximate argument. For a given area fraction, the reticulated array yields a larger slip length than the square array. We analyse the hydrodynamic forces acting on the particles, and the corresponding tangential stress exerted by the bulk ‘subphase’.
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Hodes, Marc, Toby L. Kirk, Georgios Karamanis, and Scott MacLachlan. "Effect of thermocapillary stress on slip length for a channel textured with parallel ridges." Journal of Fluid Mechanics 814 (February 6, 2017): 301–24. http://dx.doi.org/10.1017/jfm.2017.8.
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We compute the apparent hydrodynamic slip length for (laminar and fully developed) Poiseuille flow of liquid through a heated parallel-plate channel. One side of the channel is textured with parallel (streamwise) ridges and the opposite one is smooth. On the textured side of the channel, the liquid is in the Cassie state. No-slip and constant heat flux boundary conditions are imposed at the solid–liquid interfaces along the tips of the ridges, and the menisci between ridges are considered to be flat and adiabatic. The smooth side of the channel is subjected to no-slip and adiabatic boundary conditions. We account for the streamwise and transverse thermocapillary stresses along menisci. When the latter is sufficiently small, Stokes flow may be assumed. Then, our solution is based upon a conformal map. When, additionally, the ratio of channel height to half of the ridge pitch is of order 1 or larger, an accurate but less cumbersome solution follows from a matched asymptotic expansion. When inertial effects are relevant, the slip length is numerically computed. Setting the thermocapillary stress equal to zero yields the slip length for an adiabatic flow.
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BAHGA, SUPREET S., OLGA I. VINOGRADOVA, and MARTIN Z. BAZANT. "Anisotropic electro-osmotic flow over super-hydrophobic surfaces." Journal of Fluid Mechanics 644 (February 10, 2010): 245–55. http://dx.doi.org/10.1017/s0022112009992771.
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Patterned surfaces with large effective slip lengths, such as super-hydrophobic surfaces containing trapped gas bubbles, have the potential to greatly enhance electrokinetic phenomena. Existing theories assume either homogeneous flat surfaces or patterned surfaces with thin double layers (compared with the texture correlation length) and thus predict simple surface-averaged, isotropic flows (independent of orientation). By analysing electro-osmotic flows over striped slip-stick surfaces with arbitrary double-layer thickness, we show that surface anisotropy generally leads to a tensorial electro-osmotic mobility and subtle, nonlinear averaging of surface properties. Interestingly, the electro-osmotic mobility tensor is not simply related to the hydrodynamic slip tensor, except in special cases. Our results imply that significantly enhanced electro-osmotic flows over super-hydrophobic surfaces are possible, but only with charged liquid–gas interfaces.
Dissertations / Theses on the topic "Hydrodynamic slip length"
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Cowley, Adam M. "Hydrodynamic and Thermal Effects of Sub-critical Heating on Superhydrophobic Surfaces and Microchannels." BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6572.
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This dissertation focuses on the effects of heating on superhydrophobic (SHPo) surfaces. The work is divided into two main categories: heat transfer without mass transfer and heat transfer in conjunction with mass transfer. Numerical methods are used to explore the prior while experimental methods are utilized for the latter. The numerical work explores convective heat transfer in SHPo parallel plate microchannels and is separated into two stand-alone chapters that have been published archivally. The first considers surfaces with a rib/cavity structure and the second considers surfaces patterned with a square lattice of square posts. Laminar, fully developed, steady flow with constant fluid properties is considered where the tops of the ribs and posts are maintained at a constant heat flux boundary condition and the gas/liquid interfaces are assumed to be adiabatic. For both surface configurations the overall convective heat transfer is reduced. Results are presented in the form of average Nusselt number as well as apparent temperature jump length (thermal slip length). The heat transfer reduction is magnified by increasing cavity fraction, decreasing Peclet number, and decreasing channel size relative to the micro-structure spacing. Axial fluid conduction is found to be substantial at high Peclet numbers where it is classically neglected. The parameter regimes where prior analytical works found in the literature are valid are delineated. The experimental work is divided into two stand-alone chapters with one considering channel flow and the other a pool scenario. The channel work considers high aspect ratio microchannels with one heated SHPo wall. If water saturated with dissolved air is used, the air-filled cavities of SHPo surfaces act as nucleation sites for mass transfer. As the water heats it becomes supersaturated and air can effervesce onto the SHPo surface forming bubbles that align to the underlying micro-structure if the cavities are comprised of closed cells. The large bubbles increase drag in the channel and reduce heat transfer. Once the bubbles grow large enough, they are expelled from the channel and the nucleation and growth cycle begins again. The pool work considers submerged, heated SHPo surfaces such that the nucleation behavior can be explored in the absence of forced fluid flow. The surface is maintained at a constant temperature and a range of temperatures (40 - 90 °C) are explored. Similar nucleation behavior to that of the microchannels is observed, however, the bubbles are not expelled. Natural convection coefficients are computed. The surfaces with the greatest amount of nucleation show a significant reduction in convection coefficient, relative to a smooth hydrophilic surface, due to the insulating bubble layer.
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Nakano, Hiroyoshi. "Singular behavior near surfaces: boundary conditions on fluids and surface critical phenomena." Kyoto University, 2019. http://hdl.handle.net/2433/242589.
Full textBooks on the topic "Hydrodynamic slip length"
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Charlaix, E., and L. Bocquet. Hydrodynamic slippage of water at surfaces. Oxford University Press, 2017. http://dx.doi.org/10.1093/oso/9780198789352.003.0004.
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The boundary condition (B.C.) for hydrodynamic flows at solid surfaces is usually assumed to be that of no slip. However a number of molecular simulations and experimental investigations over the last two decades have demonstrated violations of the no-slip B.C., leading to hydrodynamic slippage at solid surfaces. In this short review, we explore the molecular mechanisms leading to hydrodynamic slippage of water at various surfaces and discuss experimental investigations allowing us to measure the so-called slip length
Conference papers on the topic "Hydrodynamic slip length"
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Parthasarathy, Ramkumar N., Kapil Ranganathan, and Vu Lam. "Laminar Channel Flow With Hydrodynamic Slip." In ASME 2008 International Mechanical Engineering Congress and Exposition. ASMEDC, 2008. http://dx.doi.org/10.1115/imece2008-67655.
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A theoretical and experimental investigation of slip flow over an ultrahydrophobic surface in a rectangular cross-sectional channel was conducted. The experiments consisted of the flow of water over a LOTUS-paint coated surface in a rectangular channel. The tests included flows at three different Reynolds numbers, Re = 129.6, Re = 103.6, Re = 69.1. Velocity measurements were made using a Laser Doppler Velocimeter (LDV). Velocity measurements for the no-slip conditions were also made for a flow rate of Re = 129.6 and compared with the slip profile. An uncoated surface was used for these measurements. The velocity profile and the flow rate for fully-developed channel flow were derived as a function of the slip length and slip velocity. The experimental data was used to determine the slip velocities and slip lengths for different Reynolds numbers from the theoretical equation. These quantities, used in the theoretical equation generated the necessary theoretical profiles for comparison with experiments. The slip velocities and slip lengths were evaluated by considering two locations, y = h and y = h/3. The profiles drawn from the theoretical equation evaluated from the slip velocities and lengths using the locations y = h and y = h/3 compared reasonably well with the experimental measurements. The volumetric flow rates computed theoretically using the slip lengths and slip velocities also compared well with the experimental values. The measurements highlight the effects of slip on one surface of the channel.
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Galatro, Daniela, Daniel Fürtbauer, Xiangyu Hu, David-Emilio Nerucci, and Flavio Marín. "Considerations for Hydrodynamic Slug Analysis in Pipelines." In 2014 10th International Pipeline Conference. American Society of Mechanical Engineers, 2014. http://dx.doi.org/10.1115/ipc2014-33098.
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Hydrodynamic slugs in pipelines are usually analyzed by using a steady-state flow assurance simulator as a first approximation. The pipelines are then modeled in transient simulation software to get more accurate values. Comparisons between an empirical and a mechanistic method are made in this work by running simulations in steady-state simulators in order to explain the differences in the calculated slug properties. It has been demonstrated that both methods cannot accurately estimate the maximum slug length in pipelines since the relative errors are significant; nevertheless the mechanistic model is more accurate than the empirical one with lower relative errors. Additionally, slug sizes for operational slugging have been analyzed by using a new alternative pseudo transient approach to the Lagrangian slug tracking scheme. The model expresses an unsteady state mass balance in a pipeline, formulated utilizing the slip velocity written in terms of the void fraction and superficial gas velocity. Our model includes a constitutive equation for slip velocity, elevation changes to represent the hydraulic profile of the pipeline, a method for the calculation of the maximum slug length, a modified correlation for the slug length calculation and the variation of the fluid density along the pipeline profile. The results yielded by this model have been compared with field data and results performed by using a transient simulation software, showing fairly accurate values.
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Zhou, Qi, and Chiu-On Ng. "Dispersion due to Electroosmotic Flow Through a Circular Tube With Axial Step Changes of Zeta Potential and Hydrodynamic Slippage." In ASME 2013 Fluids Engineering Division Summer Meeting. American Society of Mechanical Engineers, 2013. http://dx.doi.org/10.1115/fedsm2013-16468.
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The hydrodynamic dispersion of a neutral non-reacting solute due to steady electro-osmotic flow in a circular channel with longitudinal step changes of zeta potential and hydrodynamic slippage is analyzed in this study. The channel wall is periodically micro-patterned along the axial position with alternating slip-stick stripes of distinct zeta potentials. Existing studies on electrically driven hydrodynamic dispersion are based on flow subject to either the no-slip boundary condition on the capillary surface or the simplification of lubrication approximation. Taking wall slippage into account, a homogenization analysis is performed in this study to derive the hydrodynamic dispersion coefficient without subject to the long-wave constraint of the lubrication approximation, but for a general case where the length of one periodic unit of wall pattern is comparable with the channel radius. The flow and the hydrodynamic dispersion coefficient are calculated numerically, using the packages MATLAB and COMSOL, as functions of controlling parameters including the period length of the wall pattern, the area fraction of the slipping region (EOF-suppressing) in a periodic unit, the ratio of the two zeta potentials, the intrinsic hydrodynamic slip length, the Debye parameter, and the Péclet number. The dispersion coefficient is found to show notable, non-monotonic in certain situations, dependence on these controlling parameters. It is noteworthy that the introduction of hydrodynamic slippage will generate much richer behaviors of the hydrodynamic dispersion than the situation with no-slip boundary condition, as slippage interacts with zeta potentials in the EOF-suppressing and EOF-supporting regions (either likewise or oppositely charged).
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Yang, JinFu, Ce Chen, ShengBo Yang, DaRen Yu, Ying Cui, Kun Yang, and ZhongGuang Fu. "An Analytic Model of Oil-Film Force on Hydrodynamic Journal Bearing of Finite Length." In ASME Turbo Expo 2008: Power for Land, Sea, and Air. ASMEDC, 2008. http://dx.doi.org/10.1115/gt2008-50251.
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In order to more clearly express the interrelation between the oil-film force on hydrodynamic journal bearing of finite length and the wedging, whiling, and squeezing motions of journals, an analytic model with well-defined physical meaning is proposed in this paper by introducing the non-slip boundary condition for the oil-film velocity gradient without modifying the basic assumption for Reynolds equation to formulate the expression of oil-film pressure distribution, obtaining the analytic solution of oil-film force through integration of circumferential pressure, and defining the effect coefficients for wedging, whirling and squeezing motions, which are related to the clearance ratio and eccentricity ratio of bearings. The proposed model is compared with an existing model to show off its advantage. The proposed model was also applied to a 200MW steam turbine low-pressure rotor-bearing system to simulate the dynamic response of the rotor during the speed-up process. The analytic results of this application proved the validity of the proposed model.
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Raju, Reni, and Subrata Roy. "Hydrodynamic Model for Microscale Flows in a Channel With Two 90° Bends." In ASME/JSME 2003 4th Joint Fluids Summer Engineering Conference. ASMEDC, 2003. http://dx.doi.org/10.1115/fedsm2003-45535.
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Many microfluidic devices require serpentine channels to allow longer contact length within a compact area. The necessity of understanding the physical laws governing these complicated small geometries is crucial for better design of practical microfluidic systems. At micro-scales the continuum assumption of standard Navier-Stokes equation is no longer valid as the mean free path of the fluid becomes comparable to the dimension of the system. A finite element based hydrodynamic algorithm has been developed recently for analyzing slip flows through two-dimensional micro geometries. This paper documents numerical results for gas flow through a micro-column with two sharp 90° bends. The results obtained show increase in slip effect for higher pressure ratios. To the best of our knowledge, this is the first such published report addressing microflow in this particular geometry.
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Sadeghi, Arman, Abolhassan Asgarshamsi, and Mohammad Hassan Saidi. "Analysis of Laminar Flow in the Entrance Region of Parallel Plate Microchannels for Slip Flow." In ASME 2009 7th International Conference on Nanochannels, Microchannels, and Minichannels. ASMEDC, 2009. http://dx.doi.org/10.1115/icnmm2009-82012.
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Microscale fluid dynamics has received intensive interest due to the emergence of microelectromechanical systems (MEMS) technology. Fluid flow in microdevices has some characteristics which one of them is rarefaction effect related with gas flow. In this work, the steady state laminar rarefied gas flow in the entrance region of parallel plate microchannels is investigated by the integral method with slip flow conditions at solid surface. The effects of Knudsen number on friction factor and Nusselt number are presented in graphical form as well as analytical form. Also the effect of Knudsen number on hydrodynamic entry length is presented. The results show that as Knudsen number increases the local friction factor and Nusselt number decrease. Also an increment of Knudsen number leads to a larger amount of hydrodynamic entry length.
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Kim, Daejong, and Michael D. Bryant. "Hydrodynamic Performance of Meso Scale Gas Journal Bearings." In ASME 2004 International Mechanical Engineering Congress and Exposition. ASMEDC, 2004. http://dx.doi.org/10.1115/imece2004-62026.
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Recent advancement of micro machining technology enabled a fabrication of micro scale gas bearings for high performance miniature scale rotating machinery. This paper presents recent development of micro gas bearings fabricated through X-ray lithography, and suggests fabrication method of various meso scale (~mm) gas journal bearings, which include Rayleigh step gas journal bearings, pressure dam bearings, grooved pressure dam bearings. Also presented are simulations performed using orbit method for the suggested meso scale gas bearings with diameter of 2mm, length of 2mm, and clearance of 1~2 micron. Pan’s first order slip model was adopted to simulate molecular rarefaction effects. The rotor mass was 0.3g, simulating very small Laser scanner unit. From the simulations, Rayleigh step journal bearing had highest threshold speed reaching 300,000 rpm. Effects of molecular rarefaction in sub micron bearing clearance on the dynamic performance at very high bearing numbers, were also investigated.
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Rojas, Guillermo, Oscar E. Bautista, and Federico Mendez. "Effect of Hydrodynamic Slippage on Oscillating Electroosmotic Flows in Infinitely Extended Microcapillary." In ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels collocated with the ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems. American Society of Mechanical Engineers, 2015. http://dx.doi.org/10.1115/icnmm2015-48776.
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In this work we conduct a numerical analysis of the time periodic electroosmotic flow in a cylindrical microcapillary, whose wall is considered hydrophobic. The fluid motion is driven by the sudden imposition of a time-dependent electric field. The electrical potential is obtained by solving the nonlinear Poisson-Boltzmann equation for high zeta potential, under the assumption that the electrokinetic potential is not affected by the oscillatory external field. In addition, we neglect the channel entry and exit effects, in such manner that the flow is fully developed. The governing equations are nondimensionalized, and the solution is obtained as a function of three dimensionless parameters: the ratio of the Navier slip length to the radius of the microcapillary, δ; Rω, which is the dimensionless frequency for the flow or Strouhal number and measures the competition between the diffusion time to the time scale associated to the frequency of the oscillatory electric field; and κ, which represents the ratio of the radius of the microcapillary to the Debye length. The principal results show that using slippage, the bulk velocity increases for increasing values of δ. For the values of the dimensionless parameters used in this analysis, by using hydrophobic walls, the bulk velocity can be increased in about 20% in comparison with the case of no-slip boundary condition. On the other hand, the dimensionless frequency for the flow or Strouhal number plays a fundamental role in determining the motion of the fluid. For Rω ≪ 1, the dissipation is found in resonance with the frequency of the oscillatory electric field. For Rω ≫ 1, the dissipation is not in phase with the frequency and, therefore, the velocity in the center of the microcapillary, in some cases, is almost null, and the maximum value of the velocity is near to the microcapillary wall.
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Chen, Min, Bing-Yang Cao, and Zeng-Yuan Guo. "Micro/Nano-Scale Fluid Flows on Structured Surfaces." In ASME 2008 6th International Conference on Nanochannels, Microchannels, and Minichannels. ASMEDC, 2008. http://dx.doi.org/10.1115/icnmm2008-62023.
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Understanding the effects of surface nanostructures on fluid flow in micro- and nano-channels is highly desirable for micro/nano-electro-mechanical systems. By way of equilibrium and non-equilibrium molecular dynamics simulations, wetting on nano-structured surfaces and liquid flow in nano-channels with structured surfaces are simulated. The surfaces show dual effects on the boundary slip and friction of the liquid flow in nano-channels. Generally, the nanostructures enhance the surface hydrophilicity for a hydrophilic liquid-solid interaction, and increase the hydrophobicity for a hydrophobic interaction. Simultaneously, the nanostructures distort the nanoscale streamlines of the liquid flow near the channel surface and block the flow, which decreases the apparent slip length. The twofold effects of the nanostructures on the surface wettability and the hydrodynamic disturbance result in a non-monotonic dependence of the slip length on the structure’s size. However, the surface structure may lead to a very high contact angle of about 170° in some cases, which cause the surface show super-hydrophobicity and lead to a remarkable velocity slip. The surface nanostructures can thus be applied to control the friction of micro- and nano-flows. In addition, the gaseous flows in micro- and nano-channels with structured surfaces are simulated. The geometry of the surface is modeled by triangular, rectangular, sinusoidal and randomly triangular nanostructures respectively. The results show that the velocity slips, including negative slip, depend not only on the Knudsen number but also the surface structure. The impacts of the surface nanostructure and the gas rarefaction are strongly coupled. In general, the slip length of a gaseous flow over a structured surface is less than what predicted by the Maxwell model, and depends not only on the Knudsen number but also the size of the surface nanostructures.
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Cowley, A., D. Maynes, J. Crockett, and B. W. Webb. "Effective Temperature Jump Length and Influence of Axial Conduction for Thermal Transport Through Channels With Superhydrophobic Walls." In ASME 2013 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2013. http://dx.doi.org/10.1115/imece2013-63858.
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This paper presents a numerical investigation of thermal transport in a parallel-plate channel comprised of superhydrophobic walls. The scenario analyzed in this paper is laminar, fully developed, steady flow with constant properties. The superhydrophobic walls considered here have alternating micro-ribs and cavities aligned perpendicular to the flow direction. The cavities are assumed to be non-wetting and contain air. The thermal transport through the ribs is considered to have a constant heat flux while the thermal transport through the air/fluid interface over the cavity is considered to be negligible. Numerical results have been obtained over a range a Peclet numbers, cavity fractions, and relative rib/cavity widths. Results were also obtained where axial conduction was neglected and these results are compared to previous analytical work with excellent agreement. When the influence of axial conduction is not neglected, however, the results for local wall temperatures and Nusselt numbers show departure from the previous analytical results. The departure is more pronounced at low Peclet numbers and at large relative channel diameters. This paper provides a comparison over a wide range of parameters that characterize the overall influence of axial conduction. In general, the results show that the relative size of the cavity compared to the total rib/cavity module width (cavity fraction) and the flow Peclet number have a significant impact on the total thermal transport properties. Also, the rib/cavity module width compared to the hydraulic diameter affects the overall thermal transport behavior. Lastly, this paper explores the concept of a temperature jump length which is analogous to the hydrodynamic slip length. The ratio of temperature jump length to hydrodynamic slip length is presented in terms of cavity fraction, Peclet number, and relative size of the rib cavity module.