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Highlights • Flow topology was built on the boattail surfaces. • Three distinctive flow regimes were observed. • The separation bubble develops in size with growth of the angles. • The length of separation bubble increases linearly with the angles. Abstract The surface flow pattern over a conical boattail on an axisymmetric body was investigated experimentally under low-speed and turbulent-boundary-layer conditions. Seven conical boattails with the same length but different angles from 10° to 22° were tested at a Reynolds number around 4.3 × 104, based on the model diameter. The study used the global luminescent oil-film (GLOF) skin friction measurement technique. The skin friction fields were measured and the corresponding flow topologies were extracted from the GLOF measurements. The effect of oil-film thickness on the separation position was also evaluated. Experimental results showed three different flow types on the boattail surface: (1) flow without separation, (2) flow with a separation bubble, and (3) fully separated flow. The critical angles for the transitions are discussed and compared with classic results for similar boattail models. The separation bubble generated at moderate boattail angles was observed for what we believe to be the first time under low-speed conditions, and the flow topology was clearly shown by the GLOF results. The azimuthally-averaged skin friction projected on the centerline showed different trends inside and behind the reattachment position when the boattail angle increased. [ABSTRACT FROM AUTHOR]

The dynamics of large-scale coherent structures (LSCS) are investigated in a rough-bed flow over the whole water column and longitudinal and transverse turbulent fluxes of momentum are compared. The relation between LSCS and fluctuations of skin friction and near-bed surface pressure is examined in order to understand the effects of LSCS on the bed. The study was carried out in an open channel with a non-moving permeable bed of screeded loose-mixed gravel (D50 = 1.5 cm) at Reynolds numbers ranging from 3.19 × 104 to 1.08 × 105. Quasi-instantaneous full-depth three-dimensional (3D) velocity profiles with high spatial and temporal resolution measured with an acoustic Doppler velocity profiler (ADVP) are combined with the signals of a hot-film sensor mounted on the top of the gravel and a piezoresistive pressure transducer placed in the top layer of the bed particles at the centre of the channel. Vertical turbulent momentum flux is clearly dominated by ejections and sweeps over most of the water column. Near the bed, contributions of sweeps are larger than those of ejections. This is reversed at the mid-depth (z/h = 0.5). No preferential quadrant orientation is seen for the transverse momentum flux. A correlation between LSCS dynamics in the water column and data from the sensors on the bed is made evident. Sweeps correlate with positive fluctuations of bottom pressure and skin friction while ejections correlate with negative ones. [ABSTRACT FROM AUTHOR]

The influence of distributed cylinder roughness elements on boundary layer transition for NACA0012 airfoil at Ma = 0.6 has been investigated by subsonic/transonic/supersonic wind tunnel experiment with oil-film interferometry. Three different heights and two different distances of cylinder roughness elements on the airfoil model were used, and the skin friction coefficient was measured by the oil-film interferometry. The experimental results show that higher roughness elements promote the transition earlier. In addition, narrower distance of roughness elements can delay the transition compared with the case of wider distance. [ABSTRACT FROM AUTHOR]

Abstract An elevated raft technique is proposed for cast-in-place bored piles, where loose sediments and excavation debris at the pile tip can be compressed by the applied load from the superstructure. The axial load-settlement (i.e., P - S curve) behaviour of bored piles with debris is evaluated by a series of model-scale laboratory experiments. Numerical simulation is conducted using a two-dimensional axisymmetric model for comparison. An analytical model is then proposed to capture the whole P - S curve in three segments: (a) the contribution of skin friction, (b) increased settlement associated with the compression of debris, and (c) the development of tip resistance. [ABSTRACT FROM AUTHOR]

Interaction between turbulence and particles is investigated in a channel flow. The fluid motion is calculated using direct numerical simulation (DNS) with a lattice Boltzmann (LB) method, and particles are tracked in a Lagrangian framework through the action of force imposed by the fluid. The particle diameter is smaller than the Kolmogorov length scale, and the point force is used to represent the feedback force of particles on the turbulence. The effects of particles on the turbulence and skin friction coefficient are examined with different particle inertias and mass loadings. Inertial particles suppress intensities of the spanwise and wall-normal components of velocity, and the Reynolds shear stress. It is also found that, relative to the reference particle-free flow, the overall mean skin-friction coefficient is reduced by particles. Changes of near wall turbulent structures such as longer and more regular streamwise low-speed streaks and less ejections and sweeps are the manifestation of drag reduction. [ABSTRACT FROM AUTHOR]

This paper presents a study of skin friction change of cement pastes used for micropiles as a geotechnical foundation when grout surface is affected by sulfate attack. In this research skin friction coefficient between cement paste – cement paste was measured, studying change in friction depending on the initial roughness of samples. Tests were performed after 3 and 6 months of immersion in a 15% Na 2 SO 4 solution. Results show that sulfate attack increases skin friction coefficient for both surface finishes, concluding that this attack does not have a negative effect on friction in the experimental conditions of this work. [ABSTRACT FROM AUTHOR]

Abstract This article shows the mathematical method to determine the lateral stress on the shaft and toe resistance of pile using the new approach. The method was originally invented by Meyer and Kowalow for the static load test. The approximation curve was used for the estimation of both settlement curve and toe resistance curve of the pile. The load applied at the head of the pile is balanced by the sum of two components: the resistance under the toe of the pile and the skin friction. Therefore, the settlement curve is compilation of two factors: the skin friction curve and the resistance under toe curve. The analysis was based on the verification of the methods using laboratory experiments, that is, static load tests. The results of the research allowed to determine the relationship between parameters of the Meyer–Kowalow curve. On the basis of the relationships, it was possible to determine the skin friction and the toe resistance of the pile. Mathematical analysis of curve parameters allowed to determine the influence of the toe resistance on the settlement. [ABSTRACT FROM AUTHOR]

The theory for skin friction and heat transfer prediction to a compressible turbulent boundary layer including hydrogen/air combustion, proposed by Stalker, is improved in this paper. The original theory is modified through two aspects. One is to enhance the accuracy of the relation between hydrogen surface mass fraction and streamwise Reynolds number by directly integrating the Kármán momentum integral relation without local similarity hypothesis. Consequently, a new skin friction formula is established. The other is to change the Prandtl number from constant value of unity in the original theory to a variable computed based on the molecular kinetic theory. The performance of the improved theory is evaluated by an experiment with a flow on a flat plate and a designed two-dimensional numerical experiment. The skin friction and heat transfer predicted by the improved theory are found to be better consistent with the experimental or numerical experimental data than the original theory. [ABSTRACT FROM AUTHOR]

In the present study, a numerical simulation is conducted to predict the influence of convergent-divergent nozzle geometry and NPR on the skin friction and shockwave location. Various shapes of nozzles are numerically simulated using the Computational Fluid Dynamics code. The shock position is examined to demonstrate the impact of nozzle shape on its location. Skin friction is shown to be smoothly decreasing at the divergent part of the nozzle for all NPRs lower than 2.0. However, an inverse behavioural trend was observed at NPR equal to 2. This could be attributed to the fact that the large disturbance of fluid near the wall is the major factor behind such an oddity. The results also show that the shock position is reliant on the nozzle geometry at certain NPR. [ABSTRACT FROM AUTHOR]

A series of pullout tests were conducted on a model pile in a soil with different initial water contents and different pile surface roughnesses to study their influences on pile skin friction. To measure axial strains and skin friction of the pile, fiber Bragg grating (FBG) strain sensors were used. Data were collected during the pullout tests and are analyzed in this paper. Test results indicate that the skin friction and axial strain of the model piles decrease with the initial water content of the soil but increase with the degree of surface roughness of the piles. The axial strain induced in the upper segment of the tested pile is considerably greater than that in the bottom segment of the pile. In addition, the skin friction is distributed nonlinearly along the length of the model piles. [ABSTRACT FROM AUTHOR]

The present paper approaches fluid-structure interaction by means of a 4-equation model. Experimental data collected from a straight copper pipe-rig lying directly on the lab floor is used for the model validation in terms of wave shape, timing and damping. The main focus lies on the friction coupling modelling considering skin and dry friction. For skin friction three approaches are analysed: quasi-steady, Brunone’s and Trikha’s unsteady friction. For dry friction Coulomb’s model is added in the beam momentum conservation equation. Results present a good fitting between experimental and numerical data, showing the dissipative effect of dry friction phenomenon which complement that of skin friction, specially in the short term simulation. [ABSTRACT FROM AUTHOR]

In this article, two full-scale pile loading tests were conducted to observe the field performance of the super-long bored piles, and a simplified approach for nonlinear analysis of the load-displacement behavior of a single pile was presented. The field tests on piles indicates that, under the maximum test load, more than 70% of the pile top settlement is caused by the compression of pile shaft. For practical purposes, the pile top settlement can be reduced through improving the pile shaft strength. When the load reaches the maximum test load, the proportion of the load carried by the pile tip is approximately 30%. The super-long pile is functioning as an end-bearing friction pile. The skin friction at shallow depth is fully mobilized and decreases from a peak value with increasing load. However, the skin friction of deeper soil is not fully developed due to less relative displacement. Furthermore, a BoxLucas1 model is used to capture the relationship between unit skin friction and pile-soil relative displacement, whereas a hyperbolic model is used to describe the relationship between toe stress and pile base displacement. Based on the BoxLucas1 model and the hyperbolic model, a load transfer method is used to clarify the response of a single pile, and a computational flow chart is developed. The efficiency and accuracy of the present method is verified using the field tests on piles. The proposed simple analytical approach is economical and efficient, resulting in savings in time and cost. [ABSTRACT FROM PUBLISHER]

An experimental setup was designed and built to estimate changes in the skin friction of fouling control coatings (FCC) over an extended period of time in conditions simulating the vast majority of ship profiles (regarding speed and activity) in the present market. The setup consisted of two separate parts: one aged FCCs directly in seawater in a dynamic manner by simulating the exposure condition of a ship's hull, and a second, laboratory part measured the torque (drag) of aged coatings in a rotary setup. From the spring to the autumn of 2013 and 2014, four commercial FCCs were exposed for 53 weeks in Roskilde Fjord, Denmark, which is characterized by relatively cold seawater and a salinity of approximately 1.2 wt%. The in situ immersion seawater conditions consisted of five-week cycles divided into 2 weeks of static immersion and 3 weeks of dynamic immersion, during which time the cylinders were rotated at a tangential velocity of 8.1 knots. The skin friction was found to generally increase more during the static period, compared to the dynamic ones. Over the course of the entire exposure period, the skin friction of the investigated FCCs decreased in the following order: fluorinated fouling release coating (FRC) (highest skin friction), hydrogel-based FRC without biocides, silylated acrylate self-polishing copolymer coating, and hydrogel-based FRC with biocides (lowest skin friction). However, the differences in skin friction between the latter three coatings were minor and often within the range of experimental uncertainty. The average surface roughness of the FCCs in the newly applied and mechanically cleaned condition, determined as the Rt(50) and R parameters, was evaluated as poor predictors of skin friction. [ABSTRACT FROM AUTHOR]

Fluvial bank erosion rates are often quantified by assuming that the erosion rate is a function of the excess (above a critical threshold) boundary shear stress applied by the flow. Research has shown that the form roughness induced by natural topographic bank features, such as slumps, spurs and embayments, is the dominant component of the spatially-averaged total shear stress, meaning that form roughness provides an important control on bank erosion rates. However, measuring the relative components of the total shear stress for a natural system is not straightforward. In this work we use the method of Kean and Smith to partition the form and skin drag components of river bank roughness using a time series (2005-2011) of high-resolution topographic surveys of an eroding bank of the Cecina River in central Italy. This method approximates the form drag component of the roughness along a longitudinal bank profile as a series of user-defined Gaussian curves. The extracted metrics are used in conjunction with an estimate of the outer region flow velocity to partition the form and skin drag components of the total boundary shear stress according to the Kean and Smith analytical solution. The relative magnitude of the form and skin shear stress at each survey date is analysed alongside DEMs of difference to reveal that intense episodes of erosion are followed by periods of quiescence. We show that this is due to the protection offered by increased form drag roughness following erosion. We conceptualise the dynamic feedbacks that exist between river discharge, bank erosion processes and bank form roughness, into a simple model of the self-limiting nature of river bank erosion. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Mobilization of negative skin friction (NSF) along a pile shaft increases the axial load (dragload) and pile head settlement (downdrag) of the pile, which may exceed the structural capacity of the pile and violate its serviceability performance, respectively. Therefore, the dragload estimation is essential for a pile embedded into a settling ground, where NSF is very likely to occur. This paper presents the results of a series of fully coupled consolidation numerical parametric studies of a pile embedded into a consolidating ground. The investigation focuses on the influences of pile geometry, ground compressibility, and loading condition on pile responses. The soil compressibility and time to apply the pile head load are found to have little influence on the degree of skin-friction mobilization in the long term. The long-term neutral plane (NP) lies within the 60-70% range of the pile embedded length when the pile head load is absent. The presence of a head load shifts NP upward, and the amount of upshift increases with decreasing pile diameter and length. A simple design chart is proposed to estimate the skin-friction distribution in the long term in cases where the pile head load is absent. Finally, an illustrative example is provided to demonstrate how the chart is used and performs. The study offers practicing engineers a simple and quick approach to estimating the dragload of a pile subject to NSF. [ABSTRACT FROM AUTHOR]

Various corrections were previously proposed to account for wall roughness with the k-ω and shear stress transport (SST) models. A simplified analysis, based upon the wall region analysis, is proposed to characterize the behavior of these roughness corrections. As this analysis points out some deficiencies for each correction, two new corrections are proposed for the SST model, to reproduce different behaviors, mainly in the transition regime. The correction development is based upon a previously developed strategy. A large set of boundary layer experiments is used to compare the different roughness corrections, confirm the failures of previous proposals, and validate the present ones. Moreover, it assesses the proposed simplified analysis. It also evidences the difficulty to determine the equivalent sand grain roughness for a given surface. The Colebrook based correction is recommended while the Nikuradse based one can add information about the envelope of possible behaviors in the transition regime. [ABSTRACT FROM AUTHOR]

This paper presents a field and laboratory investigation of pullout resistance of steel anchors in rock. The field testing involved pullout tests of six steel anchors installed in rock by gravity grouting and pressure grouting. The laboratory testing involved unconfined axial compression tests of six grout specimens prepared in custom-made molds using the two grouting methods to investigate the effect of grouting method on the mechanical properties of grout. The pressure-grouted specimens had a higher density and compressive strength, which was a result of their denser microstructure with significantly fewer and smaller voids from air bubble inclusions than the gravity-grouted specimens. The field pullout tests suggest a progressive failure mechanism, which was manifested through the progressive elongation of the bond zone. The pressure-grouted anchors yielded higher pullout resistance than did the gravity-grouted anchors. The increase in rock-grout bond strength from pressure grouting is likely due to a combination of several factors, including the resultant higher grout strength, higher rock-grout contact stresses, and grout injected under pressure into the fractures/fissures that existed in the bedrock. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a simple sensor for measuring shear stress in fluid flow adjacent to a wall. Experiments and numerical flowfield computations have shown no measurable disturbance of the flowfield and the sensor may thus be well suited to conditions in which the invasive character of known sensors causes premature transition to turbulence or flow separation. The sensing part occupies a minimum surface area on the wall. In principle, the sensor is capable of detecting individual vortices in near-wall turbulence. Accuracy and sensitivity are comparable to known sensors of the sublayer-fence type. [Copyright &y& Elsevier]

A finite volume-lattice Boltzmann algorithm is applied to study skin friction behaviour in different channel flows. For this purpose, cell centred scheme is adapted to discretize the Boltzmann equation and consistent boundary conditions are also addressed, which resulted in a wider domain of stability. A simulation of flow in a two dimensional channels with different geometries are carried out. The results are compared with previous valid results in which favourable agreement was observed. The results showed that skin friction in plane Poiseuille flow converged to 24/Re, but skin friction distribution in suddenly enlarged channels regain symmetry after some distance downstream of the expansion plane. [ABSTRACT FROM AUTHOR]

The current work emphasizes the modelling of the electroosmosis-modulated peristaltic flow of Jeffery liquid. Such flows emerge in understanding the movement of biological fluids in a microchannel, such as in targeted drug delivery and blood flow through micro arteries. The non-Newtonian fluid flows inside a non-uniform cross-section and an inclined microchannel. The effects of wall properties and variable fluid properties are considered. The long wavelength and small Re number approximations are assumed to simplify the governing equations. Debye-Hückel linearization is also utilized. The nonlinear governing equations are solved by utilizing the perturbation technique. MATLAB is used for the solution, velocity, temperature, skin friction, coefficient heat transport, concentration, shear wood number, and streamlines expressions. The obtained result in optimal electroosmotic velocity (or Helmholtz-Smoluchowski velocity) increases from −1 to 6; the axial circulation has substantial momentum. For larger optimal electroosmotic velocity, a subsequent boost in an axial electric field causes a significant deceleration. Further, the study helps biomedical engineers to create biomicrofluidics devices that may aid in carrying biological fluids. [ABSTRACT FROM AUTHOR]

The steady flow of an electrically conducting, incompressible micropolar fluid in a narrow gap between two concentric rotating vertical cylinders, with porous lining on inside of outer cylinder, under an imposed axial magnetic field is studied. Beavers and Joseph slip condition is taken at the porous lining boundary. The velocity profiles, coefficient of skin friction on the cylinders are calculated. The effects of Hartmann number, the porous lining thickness parameter, coupling number, couple stress parameters and Reynolds number on azimuthal velocity, micro-rotation component and coefficient of skin friction on cylinders are depicted through graphs. [ABSTRACT FROM AUTHOR]

This paper focuses on the analysis of unsteady pipe flows caused by a pump trip in a water pipeline system. Field experiments have been carried out to collect transient pressure and steady-state flow rate data in the Prado-Instituto Politécnico da Guarda (IPG) pumping system located in Guarda, Portugal. Observed transient pressures were compared with numerical results. Results obtained were excellent both in terms of damping and phase shift of the transient pressure signal, provided that unsteady friction effects were taken into account and an appropriate downstream end boundary condition was considered. The downstream end boundary condition was described by small tanks with variable level and with free discharge into the storage tank; the variation of water level in these small tanks results in the relief of extreme transient pressures (conversely to what is observed in the line-packing effect) and changes the shape of transient pressure waves. This analysis has shown that classical water hammer theory (neglecting unsteady friction) and the consideration of a constant-level reservoir at the downstream end do not accurately describe the observed transient behavior of pressurized pipe systems. [ABSTRACT FROM AUTHOR]

Traditional pile design methods of separately assessing the shaft and base capacities have been doubted by some model pile tests and numerical analyses. In this study, two case histories in China are investigated to look through the influence of the material strength beneath the pile base on the shaft resistance mobilized under working load and at failure. The case studies involve a series of full-scale load tests on long bored piles instrumented with strain gauges along the shafts. Comparative parameters include the formation of end-bearing soils and the craftwork of base-grouting, all attributing to one unified factor of material strength at the pile base. The field observations reveal that an increase in the material strength at the pile base puts significant and positive impact on mobilizing the shaft resistance. A simple analysis procedure based on soil mechanics principles is likely to capture the mechanism that the slip displacement over the whole shaft and the radial stress acting on the shaft at deep depth are essentially responsible for the phenomena. This study is an attempt to initiate design approaches with optimistic evaluation on the shaft capacity given that special techniques of enhancing the pile base strength have been made. [ABSTRACT FROM PUBLISHER]

Aim of the paper is to investigate the MHD effects on the unsteady boundary layer flow of an incompressible micropolar fluid over a stretching sheet when the sheet is stretched in its own plane. The stretching velocity is assumed to vary linearly with the distance along the sheet. Two equal and opposite forces are impulsively applied along x - axis so that the sheet is stretched, keeping the origin fixed in a micropolar fluid. The governing non-linear equations and their associated boundary conditions are first cast into dimensionless form by a local non-similarity transformation. The resulting equations are solved numerically using the Adams- Predictor Corrector method for the whole transient from the initial state to final steady-state flow. Numerical results are obtained and a representative set is diaplaced graphically to illustrate the influence of the various physical parameters on the velocity profiles, microrotation profiles as well as the Skin friction coefficient for various values of the material parameter K. It is found that there is a smooth transition from the small-time solution to the large-time solution. Results for the local skin friction coefficient are presented in table as well as in graph. [ABSTRACT FROM AUTHOR]

An exact solution to the problem of MHD transient free convection and mass transfer flow of a viscous, incompressible, and electrically conducting fluid past a suddenly started infinite vertical plate taking into account the thermal diffusion as well as the thermal radiation is presented. Assuming the medium to be nonscattered and the fluid to be nongray, emitting-absorbing, and optically thin radiation limit properties, the equations governing the flow and heat and mass transfer are solved by Laplace transform technique. The expressions for the velocity field, the concentration field, the skin friction at the plate in the direction of the flow, and the coefficient of heat transfer and mass transfer from the plate to the fluid have been obtained, and their numerical values for different values of the physical parameters involved in the problem have been demonstrated in graphs and tables, and these are physically interpreted. It is found that the thermal radiation retards the fluid flow whereas the Soret effect accelerates the flow. The viscous drag on the plate is increased under the Soret and magnetic field effects whereas the thermal radiation reduces the skin friction. Further, the rate of heat transfer at the plate increases under thermal radiation effect. Also, in the presence of radiation, the Soret effect results in a steady increase in the mass flux from the fluid to the plate. [ABSTRACT FROM AUTHOR]

Because of the importance of fundamental knowledge on turbulent heat transfer for further decreasing entropy production and improving efficiency in various thermofluid systems, we revisit a classical issue whether enhancing heat transfer is possible with skin friction reduced or at least not increased as much as heat transfer. The answer that numerous previous studies suggest is quite pessimistic because the analogy concept of momentum and heat transport holds well in a wide range of flows. Nevertheless, the recent progress in analyzing turbulence mechanics and designing turbulence control offers a chance to develop a scheme for dissimilar momentum and heat transport. By reexamining the governing equations and boundary conditions for convective heat transfer, the basic strategies for achieving dissimilar control in turbulent flow are generally classified into two groups, i.e., one for the averaged quantities and the other for the fluctuating turbulent components. As a result, two different approaches are discussed presently. First, under three typical heating conditions, the contribution of turbulent transport to wall friction and heat transfer is mathematically formulated, and it is shown that the difference in how the local turbulent transport of momentum and that of heat contribute to the friction and heat transfer coefficients is a key to answer whether the dissimilar control is feasible. Such control is likely to be achieved when the weight distributions for the stress and flux in the derived relationships are different. Second, we introduce a more general methodology, i.e., the optimal control theory. The Fréchet differentials obtained clearly show that the responses of velocity and scalar fields to a given control input are quite different due to the fact that the velocity is a divergence-free vector, while the temperature is a conservative scalar. By exploiting this inherent difference, the dissimilar control can be achieved even in flows where the averaged momentum and heat transport equations have the same form. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents the effects of thaumasite sulfate attack (TSA) on skin friction at the concrete/clay interface. Using clay-restrained conditions thaumasite formed attached to the concrete culminating in thaumasite layers of up to 24mm depending on interface pH and applied pressure. Thaumasite at the interface did not decrease the shear strength including skin friction and cohesion. Therefore it was concluded that TSA occurring at piles or foundation bases does not affect the stability of the superstructure regarding loss of friction and settlements. [Copyright &y& Elsevier]

In this study, the mixed convection of water at 4 °C along a wedge in a porous medium is investigated numerically using a finite difference method. To explore the effect of mixed convection, both forced and free convection-dominated regimes are considered. Non-similarity solutions are obtained for the variable walltemperature boundary condition. Velocity and temperature profiles as well as local dimensionless skin friction and the Nusselt number are obtained and compared with available numerical results for various values of different parameters. The wedge angle geometry parameter m and mixed convection parameter ξ ranged from 0 to 1 in both regimes, whereas different values of λ are considered for the purpose of comparison of heat transfer results. [ABSTRACT FROM AUTHOR]

Abstract: The problems of misalignment to flow direction and the need to drill a tapping hole on a measurement surface to obtain total and static pressures make the use of Preston probe in skin friction measurement in a turbulent flow a cumbersome task. The suitability of a multi-hole pressure probe in a non-nulling mode to overcome these problems was, therefore, investigated. The near-wall effect on multi-hole pressure probe readings was examined both experimentally and theoretically. The results indicate that the presence of the wall had negligible overall effect. Experiments were carried out in a pipe, on a flat plate and on a swept forward facing step to simulate one-, two-, and three-dimensional turbulent flows. The skin friction coefficient determined using the multi-hole pressure probe was found to have good agreement with published data. Since the technique is based on similarity principle with probe diameter as a characteristic parameter, it is expected that the method would be effective with probe geometries of different sizes provided that they are small enough to be submerged in the boundary layer and that their calibration coefficients are accurately determined. [Copyright &y& Elsevier]

The viscous, two-dimensional, free-surface flow induced by the propagation of nonlinear water waves over a rigid rippled bed was simulated numerically. The simulations were based on the numerical solution of the Navier-Stokes equations subject to fully nonlinear free-surface boundary conditions and appropriate bottom, inflow, and outflow boundary conditions. The equations were properly transformed so that the computational domain became time-independent. A hybrid scheme was used for the spatial discretization with finite differences in the streamwise direction and a pseudospectral approximation with Chebyshev polynomials in the vertical direction. A fractional time-step scheme was used for the temporal discretization. Over the rippled bed, the wave boundary layer thickness increased significantly, while vortex shedding at the ripple crest generated alternating circulation regions over the ripple trough. The velocity of the Eulerian drift profile was opposite to the direction of wave propagation far above the ripples, whereas close to the bed, its magnitude was influenced by the ripples up to a height of about six times the ripple height above the ripple crest. The amplitude of the wall shear stress on the ripples increased with increasing ripple steepness, whereas the amplitude of the corresponding friction drag force on a ripple was insensitive to this increase. The amplitude of the form drag force attributable to the dynamic pressure increased with increasing ripple steepness; therefore, the percentage of friction in the total drag force decreased with increasing ripple steepness. The period-averaged drag forces on a ripple were very weak, while the influence of form drag increased with increasing ripple steepness. [ABSTRACT FROM AUTHOR]

numerical solution of the first-order homogeneous chemical reaction in an unsteady free convective flow past a semi-infinite vertical plate is studied. The dimensionless governing equations are solved by an efficient, more accurate, unconditionally stable, and rapidly converging implicit finite-difference scheme. The effect of various parameters, such as the Prandtl number, Schmidt number, buoyancy ratio parameter, and chemical reaction parameter on flow velocity and temperature is determined. The velocity profiles are in excellent agreement with available results in the literature. The local and average values of skin friction and Nusselt and Sherwood numbers are calculated. The effects of the chemical reaction parameters on these values are discussed for both generative and destructive reactions. Owing to the presence of the first-order chemical reaction, the velocity is found to increase in the generative reaction and to decrease in the destructive reaction. [ABSTRACT FROM AUTHOR]

The method used in the classical paper by Zielke to estimate the unsteady component of shear stress in unsteady pipe flows is revisited. It is found that the method is undesirably sensitive to the size of the integration time step. The sensitivity is shown to be caused dominantly by the first term in the integration when inadequate allowance is made for the infinite value of the weighting function. A simple method of avoiding the error without requiring the use of small grid sizes is presented. [ABSTRACT FROM AUTHOR]

In this study, the mixed convection of water at 4°C along a wedge in a porous medium is investigated numerically using finite difference method. In order to explore the effect of mixed convection, both forced and free convection-dominated regimes are considered. Non-similarity solutions are obtained for the variable wall flux boundary condition. Velocity and temperature profiles as well as local dimensionless skin friction and Nusselt number are obtained and compared with the available numerical results for various values of different parameters. The wedge angle geometry parameter m and mixed convection parameter ξ are ranged from 0 to 1 in both regimes whereas different values of λ are considered for the purpose of comparison of heat transfer results. [ABSTRACT FROM AUTHOR]

Abstract: Design problems of single-lined pipe flow hydraulics are primarily concerned with determining pump power, pipe slope, discharge or pipe diameter or sometimes two of them. This study develops explicit approximate equations for the determination of pump power, discharge or pipe diameter, considering the gravity on a slope and pump power as two driving forces and skin friction and form drag as two balancing forces. A power law is introduced to represent the friction factor to account for the skin friction. When secondary losses are included in the governing equation, implicit equations are inevitably the result even if the power law of friction factor is used. By determining major non-dimensional physical numbers affecting the physical processes involved, approximate explicit equations are obtained from the computation results. Furthermore it is found that the ratios to account for the secondary losses are inversely or regularly proportional to the power of various non-dimensional physical numbers. [Copyright &y& Elsevier]

Abstract: The influence of temperature-dependent fluid properties on the hydro-magnetic flow and heat transfer over a stretching surface is studied. The stretching velocity and the transverse magnetic field are assumed to vary as a power of the distance from the origin. It is assumed that the fluid viscosity and the thermal conductivity vary as an inverse function and linear function of temperature, respectively. Using the similarity transformation, the governing coupled non-linear partial differential equations are transformed into coupled non-linear ordinary differential equations and are solved numerically by the Keller–Box method. The governing equations of the problem show that the flow and heat transfer characteristics depend on five parameters, namely the stretching parameter, viscosity parameter, magnetic parameter, variable thermal conductivity parameter, and the Prandtl number. The numerical values obtained for the velocity, temperature, skin friction, and the Nusselt number are presented through graphs and tables for several sets of values of the parameters. The effects of the parameters on the flow and heat transfer characteristics are discussed. [Copyright &y& Elsevier]

The influence of the installation method on the soil flow pattern, resulting external radial total stress changes, and final external shaft friction after consolidation has been investigated for caissons in soft clay by means of centrifuge model tests, large deformation finite-element (FE) analysis, and a simple cavity expansion approach. Both the centrifuge measurements and the FE results show that more soil is forced into the caisson under suction than under jacking. However, the difference in the resulting external radial total stress changes or penetration-induced excess pore-water pressure is much less significant, since the expansion-induced excess pore pressure is smaller for thin-walled caissons than for driven piles. After subsequent consolidation, the influence of the installation method reduces further, and the final shaft friction ratios are close for the two installation methods. Based on the magnitude of heave ratios derived from the centrifuge measurements and the FE analysis, a simple form of cavity expansion approach can reasonably estimate external radial stress changes during installation and after consolidation, and final shaft friction ratios for the caissons. An approach for estimating the external shaft friction ratios for vertical pullout of sealed caissons is proposed. [ABSTRACT FROM AUTHOR]

This study compares two methods for estimating static friction coefficients for skin. In the first method, referred to as the 'tilt method', a hand supporting a flat object is tilted until the object slides. The friction coefficient is estimated as the tangent of the angle of the object at the slip. The second method estimates the friction coefficient as the pull force required to begin moving a flat object over the surface of the hand, divided by object weight. Both methods were used to estimate friction coefficients for 12 subjects and three materials (cardboard, aluminium, rubber) against a flat hand and against fingertips. No differences in static friction coefficients were found between the two methods, except for that of rubber, where friction coefficient was 11% greater for the tilt method. As with previous studies, the friction coefficients varied with contact force and contact area. Static friction coefficient data are needed for analysis and design of objects that are grasped or manipulated with the hand. The tilt method described in this study can easily be used by ergonomic practitioners to estimate static friction coefficients in the field in a timely manner. [ABSTRACT FROM AUTHOR]

In this paper, water flow in a rib-roughened channel is investigated numerically by using Reynolds stress turbulence models (RSM) on a three-dimensional (3-D) domain. Computational results for mean streamwise velocity component and turbulent kinetic energy show good agreements with available experimental data. Five rib pitch-to-height ratios (p/h) of 1, 5, 10, 15 and 20 are analysed for six different Reynolds numbers (Re) of 3000, 7000, 12,000, 20,000, 40,000 and 65,000. Velocity vectors, streamlines and Reynolds stresses are showed for these ratios and Re numbers. Streamlines revealed that Reynolds numbers do not affect flowfield but play an important role in the Reynolds stresses. [ABSTRACT FROM AUTHOR]

A series of centrifuge tests were conducted on instrumented model suction caissons in normally consolidated, lightly overconsolidated, and sensitive clays, to investigate the uplift capacity and external radial stress changes for sealed caissons subjected to sustained loading and cyclic loading. The external shaft friction ratio during vertical pullout for these two types of loading was analyzed from the radial stress measured at failure, then the corresponding reverse end-bearing capacity factor was derived from the pullout capacity. Tests results were compared with those under monotonic undrained pullout. For caissons under sustained loading, the holding capacity was found to be 72–85% of that under monotonic undrained loading. Radial stress reduction around the caisson shaft reduced the external shaft friction ratio to 0.67–0.75, while dissipation of the “passive” suction at the caisson tip reduced the reverse end-bearing capacity factor to 7.5–9.4. Under cyclic loading, the uplift capacity of the caisson was found to be 72–86% of the monotonic capacity. Repeated loading reduced the external shaft friction ratio to 0.65–0.80, while the reverse end-bearing capacity factor reduced to 6.4–9.0. [ABSTRACT FROM AUTHOR]

Abstract: Microbubble and air film methods are believed to be applicable to skin friction reduction in ships. Small bubbles are dispersed into the turbulent boundary layer in the former case, and wide air layers cover the wall surface in the latter case. Previous studies did not specifically address the intermediate case between the microbubble and air film conditions. This study is concerned with the possibility and mechanism of drag reduction using relatively large air bubbles compared to the boundary layer thickness in a horizontal turbulent channel flow. The relationship between local skin friction and the bubble’s interfacial structure is investigated by synchronizing the measurement of wall-shear stress with the image acquisition of bubbles. The bubble sizes range from 2 to 90mm approximately. As a result, a negative correlation between the local skin friction and the local void fraction is confirmed by the time-resolved measurement. A new observation is the fact that the local skin friction decreases drastically in the rear part of individual large bubbles, and rapidly increases after the bubble’s rear interface passes. This characteristic underlies the bubble-size dependency of the average skin friction in the intermediate bubble size condition. [Copyright &y& Elsevier]

Conventional pile materials such as steel, concrete, and timber are prone to deterioration for many reasons. Fiber-reinforced polymer (FRP) concrete composites represent an alternative construction material for deep foundations that can eliminate many of the performance disadvantages of traditional piling materials. However, FRP composites present several difficulties related to constructability, and the lack of design tools for their implementation as a foundation element. This paper describes the results of an experimental study on frictional FRP/dense sand interface characteristics and the constructability of FRP–concrete composite piles. An innovative toe driving technique is developed to install the empty FRP shells in the soil and self-consolidating concrete is subsequently cast in them. The experimental program involves interface shear tests on small FRP samples and uplift load tests on large-scale model piles. Two different FRP pile materials with different roughness and a reference steel pile are examined. Static uplift load tests are conducted on different piles installed in soil samples subjected to different confining pressures in the pressure chamber. The results showed that the interface friction for FRP materials compared favorably with conventional steel material. It was shown that toe driving is suitable for installation of FRP piles in dense soils. [ABSTRACT FROM AUTHOR]

This paper reviews three relatively modern categories of skin-friction measurement techniques that are broadly classified as microelectromechanical systems (MEMS)-based sensors, oil-film interferometry, and liquid crystal coatings. The theory, development, limitations, uncertainties, and misconceptions of each of these techniques are presented. Current and future uses of the techniques are also discussed. From this review, it is evident that MEMS-based techniques possess great promise, but require further development to become reliable measurement tools. Oil-film techniques have enhanced capabilities and greater accuracy compared to conventional shear-stress measurement techniques (i.e., Preston tube, Clauser plot, etc.) and, as a result, are being employed with increasing frequency. Liquid crystal coatings are capable of making measurements of mean shear-stress vector distributions over a region of a model, but complex calibration and testing requirements limit their usefulness. [ABSTRACT FROM AUTHOR]

The unsteady hydromagnetic flow of electrically conducting liquids whose Prandtl numbers are different from unity has been considered when the flow takes place near an infinite vertical flat plate subject to uniform heat flux and accelerated motion. A unified exact solution has been derived for the boundary layer velocity and skin friction for the cases of magnetic field being fixed relative to the fluid or to the vertical plate. The solution has been presented in real forms for fluids whose Prandtl numbers are greater than or less than unity. The response of the boundary layer fluid velocity to the variations in magnetic and buoyancy forces has been discussed for two sample fluids corresponding to the different Prandtl number categories. The influence of these forces on the skin friction has also been shown. [ABSTRACT FROM AUTHOR]

This note introduces a rescaling approach that greatly simplifies the evaluation of flow and physical parameters such as skin friction and heat transfer rate in recent single phase nanofluids research for which the nanofluids begin to create a non-Newtonian fluid after 5-6% volumetric concentration of nanoparticles. Much of the task is hence reduced to a particular case for a chosen fluid. By the help of specified transformations, the nanofluid results can be obtained from known properties of regular fluid. Such rescaling is shown to work well for the rotating disk boundary layer flow in nanofluid, and thus sheds light upon the future studies of this kind when further physical mechanism are taken into account. [ABSTRACT FROM AUTHOR]