Behavior Of Skin Friction Research Articles

Friction and deformation behaviour of human skin

The skin friction and deformation behaviour both in-plane and perpendicular to the sliding direction have been measured simultaneously during in vivo reciprocating-sliding of a smooth steel ball probe against human forearm skin. In these experiments, the skin is compressed in front of the probe, forming a “bow-wave”, and stretched behind as a consequence of friction and lateral skin displacement. The effects of two probe diameters and two normal loads on the skin friction and deformation behaviours were investigated for nominally dry skin in a 22 °C, 50% relative humidity environment. Friction force “stick” and “slip” phases could be distinguished during the probe motion cycle and these could be related to the skin deformation behaviour, allowing the effective lateral skin stiffness to be estimated for the different experimental conditions. Comparison to measurements of the effective indentation stiffness of the skin showed differences in the effects of load and probe diameter that may be attributable to differences in the extent of the deformation field within the skin and the complex structure and anisotropic mechanical properties of the different skin layers. The different friction results observed for different loads and probe diameters could be interpreted using an adhesion friction model for smooth contact with hydrated skin, taking into account the skin effective elastic modulus. For the experimental conditions applied here, deformation friction appeared to play only a minor role in determining the total skin friction force.

Influence of surface microstructure on the sliding friction of plantar skin against hard substrates

This study investigated how the sliding friction coefficients of wet foot skin depend on the microscopic surface properties of contacted hard substrates. Fourteen subjects conducted repeated slip experiments with one foot on each of 28 different wet surfaces. The friction and normal forces transmitted to the substrates were measured using a tri-axial force plate, and coefficients of friction were determined over periods of stationary sliding. The surface structures of the substrates, characterised by an optical 3D profilometer, varied from roughness asperities in the range of micrometers to macroscopic surface elements in the range of millimetres. The analysis of the load dependence of the skin friction behaviour based on a two-term model provided information on the adhesion and deformation components of friction. Both adhesion and deformation were found to increase with the surface roughness of the substrates. Adhesion mechanisms seemed to predominate on smooth surfaces showing low friction coefficients (<0.3), while the contributions due to skin deformations were found to be up to 0.4 on rough surfaces with high friction coefficients (>0.5). Independent of friction measurements, the analysis of the surface microstructure of the substrates indicated that ploughing as a skin deformation mechanism could contribute around 50% to the deformation component of friction.

Friction of human skin against smooth and rough glass as a function of the contact pressure

The friction behaviour of human skin was studied by combining friction measurements using a tri-axial force plate with skin contact area measurements using a pressure sensitive film. Four subjects carried out friction measurement series, in which they rubbed the index finger pad and the edge of the hand against a smooth and a rough glass surface under dry and wet conditions. The normal loads were varied up to values of 50 N, leading to skin contact pressures of up to 120 kPa. The analysis of the pressure dependence of friction coefficients of skin for contrasting sliding conditions allowed to determine the involved friction mechanisms on the basis of theoretical concepts for the friction of elastomers. Adhesion was found to be involved in all investigated cases of friction between skin and glass. If adhesion mechanisms predominated (skin against smooth glass in the dry condition and skin against rough glass in the wet condition), the friction coefficients were generally high (typically >1) and decreased with increasing contact pressure according to power laws with typical exponents between −0.5 and −0.2. Contributions to the friction coefficient due to viscoelastic skin deformations were estimated to be relatively small (<0.2). In those cases where the deformation component of friction played an important role in connection with adhesion (skin against rough glass in the dry condition), the friction coefficients of skin were typically around 0.5 and their pressure dependence showed weak trends characterised by exponents ranging from −0.1 to +0.2. If hydrodynamic lubrication came into play (skin sliding on smooth glass in the wet condition), the friction coefficients were strongly reduced compared to dry friction (<1), and their decrease with increasing contact pressures was characterised by exponents of <−0.7.

Skin friction features of drilled CIP piles in sand from pile segment analysis

AbstractNumerical pile segment analysis is conducted in this study with an advanced soil model to investigate the skin friction behaviour of a drilled Cast‐In‐Place (CIP) pile installed in sand. Although the interface between the sand and pile is considered rough, thin elements adjacent to the pile are used to include effects of localized shear.Unit weights of fluid concrete and accompanied changes in stress are considered as the effects of pile installation. Changes in effective stresses are the most prominent effect due to pile installation with a change in direction of the major principal stress from the vertical to the radial direction.Shear behaviour of the sand at the interface during the early shear stage is related to the contractive tendency of the sand at small strain levels. Changes in the stress field around the pile with little changes in volumetric strain take place during the early shear stage. Stress redistributions during the early shear stage depend on the direction of the major principal stress before shear.Results of the pile segment analyses for drilled CIP piles show good agreement with design methods. Parametric studies are used to characterize the effects of sand density and pile diameter on the skin friction behaviour of drilled CIP piles. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Skin Friction and Heat Flux Measurements in Shock/Boundary Layer Interaction Flows

Modern nonintrusive techniques are used to make skin-friction and heat transfer measurements in two shockwave/turbulent boundary-layer interactions (SWTBLIs). The two-dimensional SWTBLI is generated by impingement of an incident oblique shock wave on a flat-plate boundary layer. The three-dimensional SWTBLI results from the interaction of the swept shock generated by a fin with a flat-plate boundary layer. The measurements are made using the global interferometry skin-friction technique for the skin friction and the quantitative infrared thermography technique for the heat transfer rate. The results show that, for the two- and three-dimensional interactions, there is a clear difference in the behavior of skin friction and heat transfer as the strength of the shock is changed. This observation suggests that the analogy between momentum and heat transfer, which is the basis of many simplified physical models, is not valid in SWTBLIs. These new data supplement the previous measurements that include boundary-layer properties, surface pressure distributions, and patterns of the limiting streamlines. Taken together, these data complete a data set that is suited for computational fluid dynamics validation.

Mean and Turbulence Characteristics of a Class of Three-Dimensional Wall Jets—Part 1: Mean Flow Characteristics

This paper reports experimental investigations on mean and turbulence characteristics of three-dimensional, incompressible, isothermal turbulent wall jets generated from orifices having the shapes of various segments of a circle. In Part 1, the mean flow characteristics are presented. The turbulence characteristics are presented in Part 2. The influence of the geometry on the characteristic decay region of the wall jet is brought out and the differences with other shapes are discussed. Mean velocity profiles both in the longitudinal and lateral planes are measured and compared with some of the theoretical profiles. Wall jet expansion rates and behavior of skin-friction are discussed. The influence of the geometry of the orifice on the various wall jet properties is presented and discussed. Particularly the differences between this class of geometry and rectangular geometries are critically discussed.

Pile Behavior Established from Model Tests

Experimental and analytical results are presented that were obtained from laboratory tests involving one-tenth scale pile models embedded in sand, that were conducted at 1 G for for the Iowa Department of Transportation. Scale model pile tests were conducted to experimenatlly investigate pile behavior for a vairety of geometric, loading, and soil density conditions. Soil resistance and displacement relationships for skin friction, end-bearing, and lateral behavior, obtained from measured pile strains, were mathematically represented by modified Ramberg-Osgood expressions whose characteristic parameters were established for finite element analyses. The details of the study are described, the results are presented, and the conclusions drawn from the results are discussed.

Motion and heat transfer in the Blasius flow containing a pulsating component

Studies are made of the unsteady flow and heat transfer characteristics in the laminar boundary layer about a flat plate when the oncoming free stream contains a pulsating component. The complete, unsteady, boundary layer equations are solved by using a recently-developed numerical solution procedure. A wide range of the two key external parameters in the free stream, i.e., the applitude of pulsation, A, and the frequency parameter α [ ωx ∗/U e ], is dealt with in the present numerical computations. Comprehensive and systematically organized numerical computational results have been acquired, providing descriptions of the details of unsteady flow and thermal fields. The present numerical results indicate consistently with the available data based on the previous linearized analytical predictions in the limits of α ⪡ 1 and α ⪡ 1. When the frequency parameter α is small, the flow in the entire depth of the boundary layer exhibits a phase lead over the oncoming free stream. The present computational results supply the details of the behavior of flow and skin friction in the range of intermediate values of α and for finite values of A. The thermal structure is scrutinized by using the computed results. The mean temperature field is found to be substantially unaffected by the presence of the pulsating component. For small values of α, both the skin friction and heat transfer are in phase with each other. However, as α increases, the conventional Reynolds analogy is shown to be inapplicable.

Low-Reynolds-number modelling of flows over a backward-facing step

The complex turbulent flow behind a backward-facing step is modelled using a full Reynolds stress closure. In order to develop a closure model that can resolve the complex near-wall flow in the recirculation region and in the recovery region downstream of the reattachment point, the performance of a low and a high Reynolds number version of the full Reynolds stress closure is examined and compared. Furthermore, the effects of redistribution modelling on the calculated flow is studied by comparing the performance of three redistribution models: one return-to-isotropy model and two with mean-strain effects. The results are grid independent and show that the flow downstream of the step is best described by a low-Reynolds-number model that does not depend on the conventional wall function assumption. However, the skin friction behavior is correctly predicted by the stipulation of a wall function. Of the three redistribution models examined, the return-to-isotropy model gives results that are in excellent agreement with measurements. Finally, the calculated results are adversely affected by refining the redistribution models to include meanstrain effects.