Effective Slip Research Articles

Characterization and Simulation of the Bond Response of NSM FRP Reinforcement in Concrete.

The near-surface mounted (NSM) technique with fiber reinforced polymer (FRP) reinforcement as strengthening system for concrete structures has been broadly studied during the last years. The efficiency of the NSM FRP-to-concrete joint highly depends on the bond between both materials, which is characterized by a local bond–slip law. This paper studies the effect of the shape of the local bond–slip law and its parameters on the global response of the NSM FRP joint in terms of load capacity, effective bond length, slip, shear stress, and strain distribution along the bonded length, which are essential parameters on the strengthening design. A numerical procedure based on the finite difference method to solve the governing equations of the FRP-to-concrete joint is developed. Pull-out single shear specimens are tested in order to experimentally validate the numerical results. Finally, a parametric study is performed. The effect of the bond–shear strength slip at the bond strength, maximum slip, and friction branch on the parameters previously described is presented and discussed.

Effective permeability tensor of confined flows with wall grooves of arbitrary shape

Pressure and shear-driven flows of a confined film of fluid overlying a periodic one-dimensional topography of arbitrary shape are considered for prediction of the effective hydraulic permeability in the Stokes flow regime. The other surface confining the fluid may be a planar no-slip wall, an identically patterned wall, a free surface or a surface with prescribed shear. Analytical predictions are obtained using spectral analysis and the domain perturbation method under the assumption of small pattern size to pitch ratio. Using a novel decomposition of the channel height effects into exponentially and algebraically decaying components, a simple surface-metrology-dependent relationship which connects the eigenvalues of the effective permeability tensor is obtained. Two representative topographies are assessed numerically: the infinitely differentiable topography of a phase-modulated sinusoid which has multiple local extrema and zero crossings and the non-differentiable triangular-wave topography. Non-differentiability in the form of corners of triangular patterns and the cusps of scalloped patterns are not found to be an impediment to meaningful and numerically accurate asymptotic predictions of effective permeability and effective slip, contradicting an earlier suggestion from the literature. Several distinct applications of the theory to the friction-reduction and shear-stability performance of the recently developed lubricant impregnated patterned surfaces as well as to scalloped and trapezoidal drag-reduction riblets are discussed, with comparison to experimental data from the literature for the last application. Analytical approximations which have an extended domain of numerical accuracy are also proposed.

Capillary imbibition of confined water in nanopores

The spontaneous capillary imbibition of confined nanopores is investigated using an analytical model that includes the slip effect, wettability and effective viscosity at the water surface interface. The results show that the effective viscosity of confined water is larger than that of bulk water and decreases with diameter and wettability. The effective slip length is negative for a contact angle of 0◦, and the effective slip length is positive and increases with diameter. The results of the presented model show that the capillary imbibition length for nanoconfined water can vary up to 0.389-1.033 times that determined by the Lucas-Washburn equation with no-slip boundary conditions for nanopores due to the effective viscosity and slippage with various dimensions and contact angles. The enhancement increases with diameter and contact angle. These results elucidate the confined movement through nanopores, which can be used to understand fracturing-fluid flow in the nanopores of shale reservoir formations. Cited as : Zeng, F., Zhang, Q., Guo, J., Meng, Y., Shao, X., Zheng, Y. Capillary imbibition of confined water in nanopores. Capillarity, 2020, 3(1): 8-15, doi: 10.26804/capi.2020.01.02

The local slip length and flow fields over nanostructured superhydrophobic surfaces

The local slip behavior and flow fields near the gas-liquid interface (GLI) of a Newtonian liquid flowing past a superhydrophobic surface with periodic rectangular grooves are investigated using molecular dynamics (MD) simulations. The saturated vapor of the liquid fills the groove to form the GLI. A flat GLI is introduced by carefully adjusting the channel height to make the liquid bulk pressure equal to the coexistence pressure. The setup with the flat GLI allows for an accurate determination of the local slip velocity, shear rate and slip length. We find that the local slip velocity and shear rate at the GLI are well described by the elliptical and exponential functions, respectively. By directly computing the local slip length from the local flow fields, we propose a novel distribution function for the slip length along the GLI for both transverse and longitudinal flows. Moreover, we demonstrate that the relationship between the local and the effective slip lengths in the transverse and longitudinal cases deviates from the continuum assumptions as the groove width is reduced to the nanoscale dimensions. The functional form for the local slip length can be potentially used as a boundary condition in the continuum analysis without considering the explicitly gas flow in the grooves of superhydrophobic surfaces.

Bayesian Self‐Adapting Fault Slip Inversion With Green's Functions Uncertainty and Application on the 2016Mw7.1 Kumamoto Earthquake

AbstractKinematic finite‐extent models of earthquake sources can be determined by inverse modeling of observed waveforms and/or geodetic data. Such models are subject to significant uncertainty as a result of inaccurate observations and imperfect physical description of the complex properties of the Earth's crust. For slip inversions of large earthquakes, the major source of uncertainty is related to the uncertainty of Green's functions due to the imperfect description of the crustal model and selected parameterization of the source model. To account for both, we introduce an effective nonlinear Bayesian slip inversion with transdimensional source parameterization, including analytical representation of uncertainties of Green's functions. Our nonlinear slip inversion method relies on a self‐adapting spatial parametrization of the slip distribution by means of a varying number of spline control points on the assumed fault. For the temporal parameterization, it utilizes the regularized Yoffe function with spatially varying rise time and rupture velocity. Rake angle is also treated as an unknown spatially dependent parameter. The Green's function uncertainties are included using full covariance matrices. The posterior probability density function is sampled by the transdimensional Markov chain Monte Carlo algorithm with parallel tempering. The performance of our slip inversion method is demonstrated on a synthetic test from the Source Inversion Validation project and real‐data inversion of the 2016Mw7.1 Kumamoto earthquake. In the latter test, we infer an ensemble of ~7,300,000 possible rupture models, representing samples of the posterior probability density, and inspect which features of these models are reliable and which are rather artifacts.

Pipe flow of sphere suspensions having a power-law-dependent fluid matrix

Measurement and prediction of suspension flow properties in cylindrical geometries represent a significant challenge for both the experimentalist and modeler. In this paper, results of a computational study of the suspension flow in a pipe geometry using a computational method based on the smoothed particle hydrodynamics are presented. Flow fields and the spatial distribution of solid spherical inclusions for the case of pipe flow will be shown as a function of the matrix fluid properties (including Newtonian, shear thinning, and shear thickening), driving force, and volume fraction of particles. Simulation results of suspension flow are consistent with a previous work of suspensions with a Newtonian fluid matrix. A strong effective slip phenomenon is shown for the case of a suspension with a shear thinning fluid matrix despite adherence to no-slip boundary conditions. A simple scaling ansatz is given to describe the change of flow rate with driving force. At a volume fraction of approximately 40 % and higher, there is strong evidence of ordering. This feature is illuminated using a small-angle scattering algorithm and compared to experimental studies using neutron scattering. Issues related to proper experimental characterization of rheological properties for pumping and printing of cement-based materials are discussed.Measurement and prediction of suspension flow properties in cylindrical geometries represent a significant challenge for both the experimentalist and modeler. In this paper, results of a computational study of the suspension flow in a pipe geometry using a computational method based on the smoothed particle hydrodynamics are presented. Flow fields and the spatial distribution of solid spherical inclusions for the case of pipe flow will be shown as a function of the matrix fluid properties (including Newtonian, shear thinning, and shear thickening), driving force, and volume fraction of particles. Simulation results of suspension flow are consistent with a previous work of suspensions with a Newtonian fluid matrix. A strong effective slip phenomenon is shown for the case of a suspension with a shear thinning fluid matrix despite adherence to no-slip boundary conditions. A simple scaling ansatz is given to describe the change of flow rate with driving force. At a volume fraction of approximately 40 % and ...

Liquid film entrainment during dip coating on a saturated porous substrate

The present work examines the effect of substrate surface porosity on the coating thickness and meniscus profile during dip coating under saturated porous media conditions. The classical Landau-Levich formulation is modified by encoding the influence of porosity in an effective Navier slip boundary condition at the porous substrate surface. It is shown that simplified Navier slip-based model works well for creeping flow through the porous medium. The film height profile equation is derived as a function of a rescaled capillary number (Ca‾) and a substrate permeability factor, with inertial effects neglected. Numerical solutions show that the classical 2/3rd power dependence of film thickness on capillary number is recovered only at sufficiently high Ca‾ values. As Ca‾ is decreased, a marked deviation is seen. The shrinking of the entrainment meniscus and the change in meniscus curvature are analyzed in detail. The theoretical results are also validated with a suitable experimental system.

Testing for gravitational preferred directions with galaxy and lensing surveys

We analyze the sensitivity of galaxy and weak-lensing surveys to detect preferred directions in the gravitational interaction. We consider general theories of gravity involving additional vector degrees of freedom with non-vanishing spatial components in the background. We use a model-independent parametrization of the perturbations equations in terms of four effective parameters, namely, the standard effective Newton constant Geff and slip parameter γ for scalar modes and two new parameters μQ and μh for vector and tensor modes respectively, which are required when preferred directions are present. We obtain the expressions for the multipole galaxy power spectrum in redshift space and for the weak-lensing shear, convergence and rotation spectra in the presence of preferred directions. By performing a Fisher matrix forecast analysis, we estimate the sensitivity of a future Euclid-like survey to detect this kind of modification of gravity. We finally compare with the effects induced by violations of statistical isotropy in the primordial power spectrum and identify the observables which could discriminate between them.

Inertial migration of neutrally buoyant particles in superhydrophobic channels

The effective anisotropic hydrodynamic slip of a channel wall decorated by superhydrophobic grooves is shown to alter the equilibrium positions of neutrally buoyant particles and to generate their motion transverse to the pressure gradient.

The effects of surface topography and non-uniform wettability on fluid flow and interface slip in rough nanochannel

The flow of argon liquid in an asymmetric nanochannel with smooth and rough wall is investigated by molecular dynamics method. Considering the relationships of related factors which include characteristic dimensions, the number and the dense arrangement pattern of nanostructure, the effects of surface topography on fluid flow and interface slip are probed. The results show that the variation of the surface topography height has a significant influence on the fluid flow and interface slip. However, the effect of the variation of topography width is greatly related to the number and dense arrangement pattern of nanostructure. Surface topography causes an increase in interfacial resistance and more reduction in velocity slip at the rough wall, which even results in the forming of sticky boundary condition. The effect of non-uniform interface wettability of rough wall on the flow behavior and slip at the solid–liquid interface is investigated as well. The results show that the influence of non-uniform interface wettability on the flow in the hydrophobic channel is more obvious than that in the hydrophilic channel. Hydrophobic substrate with hydrophobic nanostructures has a smaller interfacial hydrodynamic resistance than that with hydrophilic nanostructures, which makes the average velocity and velocity slip become greater. Obviously, non-uniform interfacial wettability strengthens the effect of surface topography on the fluid flow and interface slip. It is possible to achieve effective velocity slip and obtain drag reduction by applying the non-uniform wettability of nanostructures under the same surface topography.

On a volume averaged measure of macroscopic reinforcement slip in two‐scale modeling of reinforced concrete

SUMMARYA two‐scale model for reinforced concrete, in which the large‐scale problem formulation is enriched by an effective reinforcement slip variable, is derived from the single‐scale model describing the response of plain concrete, reinforcement steel, and the bond between them. The subscale problem on the representative volume element (RVE) is correspondingly defined as finding the response of the RVE subjected to effective variables (strain, slip, and slip gradient) imposed from the large scale. A novel volumetric definition of effective reinforcement slip and its gradient is devised, and the corresponding subscale problem is formulated. The newly defined effective variables are imposed on the RVE in a weak sense using Lagrange multipliers. The response of the RVEs of different sizes was investigated by means of pull‐through tests, and the novel boundary condition type was used in FE2 analyses of a deep beam. Locally, prescribing the macroscopic reinforcement slip and its gradient in the proposed manner resulted in reduced RVE‐size dependency of effective work conjugates, which allows for more objective description of reinforcement slip in two‐scale modeling of reinforced concrete. Globally, this formulation produced more consistent amplitudes of effective slip fluctuations and more consistent maximum crack width predictions.

Analysis of microstructural facet fatigue failure in ultra-fine grained powder metallurgy Ti-6Al-4V produced through hydrogen sintering

The hydrogen sintering and phase transformation (HSPT) process is a powder metallurgy method that produces Ti-6Al-4V with fatigue properties rivaling wrought processed Ti-6Al-4V. Previous fatigue work on HSPT Ti-6Al-4V has shown failure to be frequently caused by microstructural facets, which were believed to be grain boundary (GB) α phase. This belief conflicts with other reports as fatigue failure typically occurs across α colonies or primary α grains, not GB α. In this work, electron backscattered diffraction is used to identify these facets as GB α. Furthermore, a simple geometric model predicting the effective slip length of GB α is presented.

Influence of texture shape and arrangement on thermo-hydraulic performance of the textured microchannels

Textured superhydrophobic surfaces (TSS) are purported to reduce flow friction in microchannels due to velocity slip at liquid-gas interface. At the same time, the liquid-gas interface inhibits heat transfer in textured microchannels due to the low thermal conductivity of entrapped gas phase. Despite significant understanding on fluid flow and thermal transport on the TSS, the interplay of texture shape and arrangement on thermo-hydraulic performance has not been investigated in detail hitherto. To this end, we have numerically investigated the pressure-driven flow through textured microchannels with an aim to enhance the thermo-hydraulic performance. The effective slip length and temperature jump length were estimated as a function of flow and geometry parameters for three types of micropillar shapes viz., square, triangular and herringbone, decorated in microchannels in regular and staggered manner. Scaling relations for the effective slip length and temperature jump length have been shown to be valid for triangular and herringbone shaped micropillars at different flow and geometry related parameters. Herringbone shaped micropillars exhibit more flow friction and allow a significant heat transfer in microchannels within the parameter range investigated, followed by triangular and square shaped micropillars. Although the arrangement of textures in microchannels was found to affect the flow friction substantially, its effect on heat transfer was found to be marginal. Subsequently, the overall thermo-hydraulic performance was observed to be superior in regularly arranged herringbone shaped micropillars, at moderate to high constriction ratios (a ratio of texture pitch to half channel height) and high Peclet numbers over the other texture shapes. The results presented in this work would serve as a useful guide to attain maximum thermo-hydraulic performance in textured microchannels.

Boundary-condition and geometry engineering in electronic hydrodynamics

We analyze the role of boundary geometry in viscous electronic hydrodynamics. We address the twin questions of how boundary geometry impacts flow profiles, and how one can engineer boundary conditions -- in particular the effective slip parameter -- to manipulate the flow in a controlled way. We first propose a micropatterned geometry involving finned barriers, for which we show by an explicit solution that one can obtain effectively no-slip boundary conditions regardless of the detailed microscopic nature of the channel surface. Next we analyse the role of mesoscopic boundary curvature on the effective slip length, in particular its impact on the Gurzhi effect. Finally we investigate a hydrodynamic flow through a circular junction, providing a solution, which suggests an experimental set-up for determining the slip parameter. We find that its transport properties differ qualitatively from the case of ballistic conduction, and thus presents a promising setting for distinguishing the two.

Turbulent drag reduction in Taylor-Couette flows using different super-hydrophobic surface configurations

Turbulent drag reduction (DR) in an incompressible Taylor-Couette flow configuration using different patterns of “idealized” superhydrophobic surfaces (SHS) on rotating inner-wall is investigated using direct numerical simulations (DNS). Three dimensional DNS studies based on the finite difference method in cylindrical annuli of aspect ratio (Γ) = 6.0 and radius ratios (η) = 0.5 and 0.67 have been performed at Reynolds numbers (Re) 4000 and 5000. The SHS comprised of streamwise or azimuthal microgrooves (MG), spanwise or longitudinal MG, grooves inclined to the streamwise direction (spiral), and microposts. The SHS have been modeled as shearfree areas. We were able to achieve a maximum DR up to 34% for the streamwise aligned SHS, while we got drag enhancement of 4% for the spiral SHS at η = 0.67. The SHS cause slip at the wall as well as near-wall turbulence modification, both governing the DR. We have tried to understand the role of the effective slip and modified turbulence dynamics responsible for DR by analyzing the statistics of mean flow, velocity fluctuations, Reynolds stresses, turbulence kinetic energy (TKE), and near-wall streaks. Most of the results show enhanced production of near-wall streamwise velocity fluctuations and TKE resulting in near-wall turbulence enhancement, yet we observed DR for most of the cases, thereby implying slip to be the dominant contributor to DR in comparison to modified near-wall turbulence.

Predictions of the effective slip length and drag reduction with a lubricated micro-groove surface in a turbulent channel flow

We perform direct numerical simulations of a turbulent channel flow with a lubricated micro-grooved surface to investigate the effects of this surface on the slip characteristics at the interface and the friction drag. The interface between water and lubricant is assumed to be flat, i.e. the surface-tension effect is neglected. The solid substrate, where a lubricant is infused, is composed of straight longitudinal grooves. The flow rate of water inside the channel is maintained constant, and a lubricant layer under the interface is shear driven by the turbulent water flow above. A turbulent channel flow with a superhydrophobic (i.e. air-lubricated) surface having the same solid substrate configuration is also simulated for comparison. The results show that the drag reduction with the liquid-infused surface highly depends on the lubricant viscosity as well as the groove width and aspect ratio. The amounts of drag reduction with the liquid-infused surfaces are not as good as those with superhydrophobic surfaces, but are still meaningfully large. For instance, the maximum drag reduction by the heptane-infused surface is approximately 13 % for a rectangular groove whose spanwise width and depth in wall units are 12 and 14.4, respectively, whereas a superhydrophobic surface with the same geometry results in a drag reduction of 21 %. The mean slip length normalized by the viscosity ratio and groove depth depends on the groove aspect ratio. The ratio of fluctuating spanwise slip length to the streamwise one is between 0.25 (ideal surface without groove structures) and 1 (i.e. isotropic slip), indicating that the slip is anisotropic. Using the Stokes flow assumption, the effective streamwise and spanwise slip lengths are expressed as a function of groove geometric parameters and lubricant viscosity. We also suggest a predictive model for drag reduction with the heptane-lubricated surface by combining the predicted effective slip lengths with the drag reduction formula used for riblets (Luchini et al., J. Fluid Mech., vol. 228, 1991, pp. 87–109). The predicted drag reductions are in good agreements with those from the present and previous direct numerical simulations.

The optimal Stokes-Brinkman coupling for two-dimensional transverse flows in dual-scale fibrous porous media using the effective Navier slip approach

Optimal values of the effective viscosity and the stress jump coefficient in the Stokes-Brinkman model with continuous and jump stress conditions, respectively, have been accurately characterized and then applied to solve various two-dimensional transverse dual-scale flows in fibrous porous media. In this work, the effective viscosity and the stress jump coefficient are determined such that the interfacial slip velocity can be identified to that in the effective Navier-slip description and therefore it naturally facilitates the accurate prediction of the slip velocity and its gradient (stress) at the fluid/porous interface along with the velocity fields in both the porous media and the pure fluid domain. With these optimal values of the effective viscosity and the stress jump coefficient, the Stokes-Brinkman coupling can be employed to accurately describe the dual-scale porous flow at low computational cost, which may provide an effective computational framework in investigating particle deposition/filtration and void transports within composites.

Inertial Sensor-Based Slip Detection in Human Walking

Slip is one of the leading causes of fall-related injuries among occupational and elderly population. Existing research supports proactive slip and fall prevention approaches, while active strategies remain underdeveloped. Development of active slip-induced fall prevention systems requires fast, effective slip detection. This paper aims to develop a novel, real-time slip detection and estimation algorithm during human walking. The slip estimation is built on a slip dynamic model for biped walkers with the integration of the human locomotion constraints. The slip detection uses a set of wearable inertial sensors attached on the lower limbs. A slip indicator is introduced to detect the slip shortly after the heel-strike event. We present an extended Kalman filter-based slip estimation to characterize the slipping distance. One attractive property of the algorithm is its fast, accurate slip onset detection and slipping distance estimation with low-cost, nonintrusive sensing features. Experiments are conducted to validate and demonstrate the performance of the proposed slip detection and estimation scheme. Note to Practitioners —Slip-induced falls often lead to injuries among elderly and professional workers. Active slip and fall prevention could potentially mitigate the negative societal and economic impacts due to slip-induced injuries. However, no assistive device is yet commercially available or reported for slip and fall prevention. This paper presents a first step toward this goal by development and evaluation of a fast slip detection scheme. The wearable slip detection system is small size and lightweight, consisting of five small inertial measurements units placed on the leg. The system can detect the initiation of foot slip several times faster compared to the human sensorimotor mechanisms. The simplicity of the system design, easy implementation, and the fast detection results demonstrate superior reliability and robustness compared to the other slip detection approaches. The proposed slip detection algorithm can find potential applications, such as humanoid robots and control, and healthcare, or home automation.

Wall-bounded flow over a realistically rough superhydrophobic surface

Direct numerical simulation (DNS) is performed for two wall-bounded flow configurations: laminar Couette flow at $Re=740$ and turbulent channel flow at $Re_{\unicode[STIX]{x1D70F}}=180$, where $\unicode[STIX]{x1D70F}$ is the shear stress at the wall. The top wall is smooth and the bottom wall is a realistically rough superhydrophobic surface (SHS), generated from a three-dimensional surface profile measurement. The air–water interface, which is assumed to be flat, is simulated using the volume-of-fluid (VOF) approach. The two flow cases are studied with varying interface heights $h$ to understand its effect on slip and drag reduction ($DR$). For the laminar Couette flow case, the presence of the surface roughness is felt up to $40\,\%$ of the channel height in the wall-normal direction. Nonlinear dependence of $DR$ on $h$ is observed with three distinct regions. A nonlinear curve fit is obtained for gas fraction $\unicode[STIX]{x1D719}_{g}$ as a function of $h$, where $\unicode[STIX]{x1D719}_{g}$ determines the amount of slip area exposed to the flow. A power law fit is obtained from the data for the effective slip length as a function of $\unicode[STIX]{x1D719}_{g}$ and is compared to those derived for structured geometry. For the turbulent channel flow, statistics of the flow field are compared to that of a smooth wall to understand the effects of roughness and $h$. Four cases are simulated ranging from fully wetted to fully covered and two intermediate regions in between. Scaling laws for slip length, slip velocity, roughness function and $DR$ are obtained for different penetration depths and are compared to past work for structured geometry. $DR$ is shown to depend on a competing effect between slip velocity and turbulent losses due to the Reynolds shear stress contribution. Presence of trapped air in the cavities significantly alters near-wall flow physics where we examine near-wall structures and propose a physical mechanism for their behaviour. The fully wetted roughness increases the peak value of turbulent intensities, whereas the presence of the interface suppresses them. The pressure fluctuations have competing contributions between turbulent pressure fluctuations and stagnation due to asperities, the near-wall structure is altered and breaks down with increasing slip. Overall, there exists a competing effect between the interface and the asperities, the interface suppresses turbulence whereas the asperities enhance them. The present work demonstrates DNS over a realistic multiphase SHS for the first time, to the best of our knowledge.

Cycle Slip Detection and Repair for Dual-Frequency LEO Satellite GPS Carrier Phase Observations with Orbit Dynamic Model Information

Cycle slip detection and repair are crucial for precise GPS-derived orbit determination of the low-Earth orbit (LEO) satellites. We present a new approach to detect and repair cycle slips for dual-frequency LEO satellite GPS observations. According to Newton’s equation of motion, the second-order time difference of the LEO satellite’s position (STP) is only related to the sampling interval and the satellite’s acceleration, which can be precisely obtained from the known orbit dynamic models. Then, several kinds of second-order time-difference geometry-free (STG) phase combinations, taking full advantage of the correlation between the satellite orbit variations and the dynamic model, with different level of ionospheric residuals, are proposed and adopted together to detect and fix cycle slips. The STG approach is tested with some LEO satellite GPS datasets. Results show that it is an effective cycle slip detection and repair method for LEO satellite GPS observations. This method also has some important features. Firstly, the STG combination is almost independent of the pseudorange. Secondly, this method is effective for LEO satellites, even in real-time application. Thirdly, this method is suitable for ground-based GPS receivers if we know the acceleration of the receivers.