Skin Friction Data Research Articles

A Hybrid Approach of Buongiorno's Law and Darcy–Forchheimer Theory Using Artificial Neural Networks: Modeling Convective Transport in Al2O3‐EO Mono‐Nanofluid Around a Riga Wedge in Porous Medium

ABSTRACTThe inspiration for this study originates from a recognized research gap within the broader collection of studies on nanofluids, with a specific focus on their interactions with different surfaces and boundary conditions (BCs). The primary purpose of this research is to use an artificial neural network to examine the combination of Alumina‐Engine oil‐based nanofluid flow subject to electro‐magnetohydrodynamic effects, within a porous medium, and over a stretching surface with an impermeable structure under convective BCs. The flow model incorporates Thermophoresis and Brownian motion directly from Buongiorno's model. Accounting for the porous medium's effect, the model integrates the Forchheimer number (depicting local inertia) and the porosity factor developed in response to the presence of the porous medium. The conversion of governing equations into non‐linear ordinary differential systems is achieved by implementing transformations. A highly non‐linear ordinary differential system's final system is solved using a numerical scheme (Runge–Kutta fourth‐order). Findings indicate that the porosity factor positively impacts the skin friction and the momentum boundary layer. The influence suggests an increment in the frictional force and a decline in the velocity profile. The volume fraction, Prandtl number, and magnetic number significantly impact the flow profiles. The skin friction data is tabulated with some physical justifications.

Convective heat and zero-mass flux conditions in the time-dependent second-grade nanofluid flow by unsteady bidirectional surface movement

In this paper, a theoretical model is developed for the deliberation of time-dependent 3D magnetohydrodynamics second-grade (SG) nanofluid with radiation effects past an unsteady stretched surface. The work is carried out by involving the physical effects of thermophoresis and Brownian movement. To find the desired solutions, more practical boundary conditions are introduced familiar as the Robin and zero-mass diffusion conditions. The derived model of partial differential form is reframed into the structure of ordinary differential expressions by implementing the dimensionless variables which are reported numerically through the Runge-Kutta-Fehlberg (RKF) scheme with shooting procedure. Numerically determined skin-friction data and energy transportation rate are also addressed with appropriate analysis. Solutions are performed for nanofluid velocity, concentration of nanoparticles and nanofluid temperature. The analysis of dissimilar parametric values on liquid velocity, concentration of nanoparticles and temperature is reported using plots and tabulated data. The presence of Hartman and Biot numbers demonstrated the higher temperature curves. The nanoparticle concentration and temperature distributions reduced for larger unsteady parameter values. A comparison is also provided for limiting cases to authenticate our obtained results.

A Weibull Distribution: Flow and Heat Transfer of Nanofluids Containing Carbon Nanotubes with Radiation and Velocity Slip Effects

In this study, the Tiwari and Das model is numerically studied, in case of a moving plate containing both single-walled and multiwalled carbon nanotubes (SWCNTs and MWCNTs, respectively), in the presence of thermal radiation and the slip effect. Employing the similarity transformation, a set of 2nd-order partial differential equations (which are used to model the flow and heat transfer) are solved numerically using the boundary value problem with 4th-order accuracy (BVP4C) method. The effects of related parameters, such as the volume fraction of nanoparticles, moving, slip, and radiation parameter on the heat transfer performance are analysed and discussed. Results indicate that a unique solution was placed when the plate travels in assisting flow conditions. Additionally, as the nanoparticle volume fraction (φ) rises at φ = 0.2, the skin friction and heat transfer rate decrease. It is also observed that when the slip parameter (β) increases at β = 0.4, the skin friction decreases, whereas the heat transfer rate increases. Meanwhile, the heat transfer rate decreases when the thermal radiation (NR) increases to 0.7. Moreover, it is found that the SWCNTs are more efficient when the skin friction coefficient and the Nusselt number are considered. It is found that the Weibull distribution is more suitable in fitting the skin friction data.

Luminescent Oil Film Flow Tagging Skin Friction Meter Applied to FAITH Hill

The luminescent oil film flow tagging skin friction meter is presented and used to collect time-averaged spatially distributed skin friction data on the surface of NASA’s FAITH model. The technique estimates skin friction by monitoring the evolution of a luminescent oil film evolving under the influence of skin friction, pressure gradients, and gravity. The thickness distribution of the oil film, , and the velocity of the surface of the oil film, , are measured experimentally, and the dynamic viscosity of the oil, , is taken from the oil manufacturer or is determined through calibration. Then, skin friction is estimated as . The thickness distribution of the oil film is measured by ratioing the intensity of the luminescent signal from the fluorescent oil with the intensity of the incident light that is scattered from the surface of the model. The resulting quotient is independent of the intensity of the incident light and directly proportional to the thickness of the oil film. The velocity of the surface of the oil film is measured by tagging and following photochromic molecules in the oil film. The new luminescent oil film flow tagging skin friction data compare well with fringe image skin friction data. Also, an experimental study is conducted to determine the extent to which it is appropriate to neglect the effects of a pressure gradients and gravity when determining skin friction from the evolution of an oil film. Results are presented supporting that this approximation is appropriate for a sufficiently thin film of oil or for a sufficiently high skin friction magnitude.

An Integral Solution for Skin Friction in Turbulent Flow Over Aerodynamically Rough Surfaces With an Arbitrary Pressure Gradient

The combined law of the wall and wake, with the inclusion of the “roughness depression function” for the inner law in the “Log” region, is used as the inner coordinates' velocity profile in the integral form of the x momentum equation to solve for the local skin friction coefficient. The “equivalent sand grain roughness” concept is employed in the roughness depression function in the solution. Calculations are started at the beginning of roughness on a surface, as opposed to starting them using the measured experimental values at the first data point, when making comparisons of predictions with data sets. The dependence of the velocity wake strength on both pressure gradient and momentum thickness Reynolds number are taken into account. Comparisons of the prediction with experimental skin friction data, from the literature, have been made for some adverse, zero, and favorable (accelerating flows) pressure gradients. Predictions of the shape factor, roughness Reynolds number, and momentum thickness Reynolds number and comparisons with data are also made for some cases. In addition, some comparisons with the predictions of earlier investigators have also been made.

Three measuring techniques for assessing the mean wall skin friction in wall-bounded flows

The present paper aims at evaluating the mean wall skin friction data in laminar and turbulent boundary layer flows obtained from two optical and one thermal measuring techniques, namely, laser-Doppler anemometry (LDA), oil-film interferometry (OFI), and surface hot-film anemometry (SHFA), respectively. A comparison among the three techniques is presented, indicating close agreement in the mean wall skin friction data obtained, directly, from both the OFI and the LDA near-wall mean velocity profiles. On the other hand, the SHFA, markedly, over estimates the mean wall skin friction by 3.5–11.7% when compared with both the LDA and the OFI data, depending on the thermal conductivity of the substrate and glue material, probe calibration, probe contamination, temperature drift and Reynolds number. Satisfactory agreement, however, is observed among all three measuring techniques at higher Reynolds numbers, Rex>106, and within ±5% with empirical relations extracted from the literature. In addition, accurate velocity data within the inertial sublayer obtained using the LDA supports the applicability of the Clauser method to evaluate the wall skin friction when appropriate values for the constants of the logarithmic line are utilized.

A study on rotational augmentation using CFD analysis of flow in the inboard region of the MEXICO rotor blades

AbstractThis work presents an analysis of data from existing as well as new full‐rotor computational fluid dynamics computations on the MEXICO rotor, with focus on the flow around the inboard parts of the blades. The boundary layer separation characteristics on the airfoil sections in the inboard parts of the rotor are analysed using the pressure and the skin friction data at a range of angles of attack. These data are used to gain insight on the relative behaviour of separated boundary layers in 3D flow compared with 2D flow. It has been found that separation on airfoils in rotating flows is different from that in 2D flows in two respects: (i) there is a chord‐wise postponement (or delay) of the separation point, and (ii) the angle of attack at which separation is initiated is higher in 3D compared with 2D. Comments are made on the mechanism of stall delay, and the main differences between the skin friction and pressure distribution behaviours in 2D and 3D rotating flows are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.

Flow-induced degradation of drag-reducing polymer solutions within a high-Reynolds-number turbulent boundary layer

Polymer drag reduction, diffusion and degradation in a high-Reynolds-number turbulent boundary layer (TBL) flow were investigated. The TBL developed on a flat plate at free-stream speeds up to 20ms−1. Measurements were acquired up to 10.7m downstream of the leading edge, yielding downstream-distance-based Reynolds numbers up to 220 million. The test model surface was hydraulically smooth or fully rough. Flow diagnostics included local skin friction, near-wall polymer concentration, boundary layer sampling and rheological analysis of polymer solution samples. Skin-friction data revealed that the presence of surface roughness can produce a local increase in drag reduction near the injection location (compared with the flow over a smooth surface) because of enhanced mixing. However, the roughness ultimately led to a significant decrease in drag reduction with increasing speed and downstream distance. At the highest speed tested (20ms−1) no drag reduction was discernible at the first measurement location (0.56m downstream of injection), even at the highest polymer injection flux (10 times the flux of fluid in the near-wall region). Increased polymer degradation rates and polymer mixing were shown to be the contributing factors to the loss of drag reduction. Rheological analysis of liquid drawn from the TBL revealed that flow-induced polymer degradation by chain scission was often substantial. The inferred polymer molecular weight was successfully scaled with the local wall shear rate and residence time in the TBL. This scaling revealed an exponential decay that asymptotes to a finite (steady-state) molecular weight. The importance of the residence time to the scaling indicates that while individual polymer chains are stretched and ruptured on a relatively short time scale (~10−3s), because of the low percentage of individual chains stretched at any instant in time, a relatively long time period (~0.1s) is required to observe changes in the mean molecular weight. This scaling also indicates that most previous TBL studies would have observed minimal influence from degradation due to insufficient residence times.

Rotta skin friction law and Schoenherr formula

This paper examines the Rotta skin friction law (1950) and the Schoenherr empirical formula (1932) for flat plate turbulent boundary layers in the light of direct skin friction measurements conducted in the range of the momentum thickness Reynolds number up to Rθ = 2.13 × 105. First, we demonstrate that the skin friction data almost perfectly follow the Rotta skin friction law derived by assuming the existence of the logarithmic overlap region and by using the velocity profile constants determined by Nagib et al (2007 Phil. Trans. R. Soc. A 365 755–70). Next, we clarify the high Reynolds number asymptotic form of the Rotta law and derive the Schoenherr empirical formula from the asymptotic relation. The Schoenherr formula is shown to be a good approximation to the Rotta skin friction law for Rθ greater than 1.1×104. Above all, the present effort yields a new formula for the local skin friction, which is almost identical with the Rotta skin friction law in accuracy, with the relative deviation being less than 0.05% in the range of Cf less than 0.0037.

Refined cf relation for turbulent channels and consequences for high-Re experiments

There have been rising concerns regarding the accuracy of measurements in turbulent channel flows, in particular, measurements of the skin-friction results. In the present study, two different methods, namely, mean streamwise pressure gradient (PG) and oil film interferometry (OFI), are used to estimate the wall skin-friction relation, cf = f(Rem), for fully developed turbulent plane-channel flow over a wide range of Reynolds numbers. The channel skin-friction data are then fitted to the well-known logarithmic friction law, providing outstanding agreement with values for the constants of the logarithmic law of the mean velocity profile. A revised logarithmic skin-friction relation is developed, providing good agreement with our skin-friction results and data from the literature, when constants of the logarithmic friction relation adopted from the recent work of Zanoun et al (2003 Phys. Fluids 15 3079–89, 2005 4th Int. Conf. on Heat Transfer, Fluid Mechanics and Thermodynamics, HEAT2005, 19–22 September, Cairo, Egypt) are utilized. A new experimental channel facility is proposed, allowing measurements at high Reynolds numbers well beyond those achieved previously in laboratories, i.e. over five times the highest Rem reached in the present study, while maintaining sufficiently high spatial resolution.

Measurements of turbulent crossflow separation created by a curved body of revolution

Using the method pioneered by Gurzhienko (1934), the crossflow separation produced by a body of revolution in a steady turn is examined using a stationary deformed body placed in a wind tunnel. The body of revolution was deformed about a radius equal to three times the body's length. Surface pressure and skin-friction measurements revealed regions of separated flow occurring over the rear of the model. Extensive surface flow visualization showed the presence of separated flow bounded by a separation and reattachment line. This region of separated flow began just beyond the midpoint of the length of the body, which was consistent with the skin-friction data. Extensive turbulence measurements were performed at four cross-sections through the wake including two stations located beyond the length of the model. These measurements revealed the location of the off-body vortex, the levels of turbulent kinetic energy within the shear layer producing the off-body vorticity and the large values of 〈uw〉 stress within the wake. Velocity spectra measurements taken at several points in the wake show evidence of the inertial sublayer. Finally, surface flow topologies and outer-flow topologies are suggested based on the results of the surface flow visualization.

Skin Friction Measurements on the NASA Hump Model

The skin-friction distribution on a wall-mounted hump model has been obtained using oil-film interferometry. This effort is part of a larger study to provide validation cases for simulations of unsteady flows. The challenges of using oil-film interferometry on this model, including model curvature and close camera proximity, are discussed. Skin-friction measurements are obtained over most of the hump model, including especially high-quality measurements in the separated and reattachment regions. These results highlight the method’s ability to capture a wide range of skin friction including measurements in reverse-flow and high-gradient regions. The wall skin-friction data are shown to complement other experimental data, and the use of independent skin-friction measurements for scaling in wall-bounded flows is emphasized. A comparison with results from several computational simulations of the same flow is presented. The comparison indicates that, for the most part, the computations accurately predict the skin-friction ahead of separation, but fail to predict the reattachment point correctly, and thus the comparison in the separated and recovery regions of the flow is poor. The ability of the skin-friction measurements to pinpoint regions where the computation performs poorly in the near-wall region is also presented. From these results, it is evident that independent skin-friction measurements should be a part of all validation experiments conducted in wall-bounded flows.

Study of Direct-Measuring Skin-Friction Gauge with Rubber Sheet for Damping

This study concerns the direct measuring technique for skin-friction determination. Such a device measures the force on a small, movable surface element as a shear e ow passes over the surface. The typical direct-measuring skin-friction gauge uses a viscous liquid in the gap between the movable surface piece and the casing to dampen vibrations,tocreateanevensurface,tominimizetheeffectsofpressuregradients,andfortemperaturestabilization. In testing, the liquid slowly leaks out. Therefore, we have considered gauges with rubber to e ll all or some of the gap. This led to the development of a gauge with a thin rubber sheet to cover the face of the gauge instead of a previousdesign withrubbere lling theentireinternalvolume.First, ae niteelementmethodmodelwasemployed to fully understand the strain e eld involved and to e nalize the design. The resulting design consisted of a plastic skin frictiongaugewitha12.7-mm-diam e oatingelementonacantileverbeame exure,anapproximately0.51-mm-thick rubber sheet, a 1.6-mm-wide gap around the e oating element on a cantilever beam e exure, and semiconductor strain gauges at the beam base. Vibration tests were performed to determine if this design produced the required damping. These tests were successful. Supersonic wind-tunnel tests at Mach 2.4 with a total pressure of 3 atm and ambient total temperature demonstrated that the rubber sheet survived repeated tests and provided adequate damping. The skin-friction data obtained compared well with theory and other measurements.

Skin Friction Correlation in Open Channel Boundary Layers

Skin Friction Correlation in Open Channel Boundary Layers

Development of a Global Interferometer Skin-Friction Meter

An improvedmethodformeasuringskinfrictionintwo-dimensionale owsisdescribed.Theinstrument,termeda globalinterferometerskin-frictionmeter,isusedtomeasuretheshapeofathinoile lmplacedonatestmodelsubject to aerodynamic shear. The oil e lm shape is then related to the applied shear through lubrication theory. Through acquisition ofa singleimageoftheoile lm interferencepattern,theskin-friction distribution can beobtainedat any pointcovered by thee lm. Enhancementsto themethod arepresented that may extend theinstrument' sapplication to fully three-dimensional e ows with time-variant model temperatures. Results from tests performed on e at plate boundary layers along with numerical simulations show the instrument can accurately measure wall shear over relatively large areas in a single test. Results show the method provides numerous advantages over other shear measurement techniques. The instrument' s cone guration, the theoretical background, and the parameters that ine uence its accuracy are presented. CCURATE measurement of the wall shear stress distribution is important for understanding many types of e owe elds. Such data are crucial to understanding aerodynamic drag and for evalu- ating the performance of proposed drag reduction strategies. More- over,skin-frictiondataareextremelyvaluableforvalidationofcom- putationalsolutionsduetothedirectlinkbetweenthepredictedshear stress and the turbulence model. However, despite the importance of accurate skin-friction data, obtaining such, either experimentally or computationally, can be extremely dife cult, costly, and time con- suming. The level of dife culty increases considerably for measure- ments in complex, three-dimensional e ows with separation. Theprimarydife cultywithmeasuringwallshearliesinthelackof a universal measurementtechniquethat canprovideaccurate data in awide rangeof e ows.Winter 1 providesagood reviewof thevarious wallshear measurementtechniques. Nearly all of the existing meth- odsfor wall shear measurements, e.g., Preston tubes, Stanton tubes, sublayer fences, electrochemical techniques, and e oating element balances, have very limited ranges of application. In general, these methodsareintrusive,arerestrictedtocertaintypesofe ows,arelim- ited to e ows with modest pressure gradients, and require detailed calibration.Additionally,thesemethodsprovideonlypointwisedata and can be time consuming to implement on large surfaces. A more recent method of shear measurement that has shown promise is the laser interferometer skin-friction (LISF) meter. The LISFmeter wasoriginally invented byTanner and Blows 2 forusein low-speed e ows.The basic principle of operation of the LISF meter isto optically measure the time rate of thinning of an oil e lm placed on a test surface subject to aerodynamic shear. The rate of thinning of the oil e lm is determined using optical interference and can be related to the applied shear through lubrication theory. Although the LISF meter was originally used for low-speed aire ows, further development of the method 3-6 has resulted in an instrument that is capable of measurements over a broad range of e ow conditions, including supersonic e ows, 7-12 shock- wave/boundary-layer interactions with strong pressure gradients and e ow separation, 9-12 and more recently two-phase (air-water) and single-phase (water) pipe e ows. 13 Despite its relatively broad range of applicability, the LISF tech- niquesuffersfromseveral shortcomings. Foremost,themethod pro- videsonlypointwisedata.Hence,measuringsheardistributionsover

High-Lift Aerodynamic Computations with One- and Two-Equation Turbulence Models

The Spalart-Allmaras one-equation turbulence model and the Menter two-equation turbulence model are assessed for Navier-Stokes computations of high-lift aerodynamic flows, including cases with significant regions of separated flow. Cases considered include 1) separated flow over a single-element airfoil at high lift, 2) fully attached flows over the NLR 7301 airfoil and flap, and 3) separated flow over the GA(W)-1 airfoil and flap. For the single-element case, the Menter model provides better agreement with the experimental pressure distribution and velocity profiles. For the NLR 7301 airfoil and flap, both turbulence models provide excellent agreement with experimental pressures and skin-friction data. Velocity profiles are generally well predicted except in some parts of wakes. Turbulent shear stresses are not always well predicted. The predictions of the Spalart-Allmaras model are generally superior for this case, especially when boundary-layer confluence is important.

High-lift aerodynamic computations with one- and two-equation turbulence models

The Spalart-Allmaras one-equation turbulence model and the Menter two-equation turbulence model are assessed for Navier-Stokes computations of high-lift aerodynamic flows, including cases with significant regions of separated flow. Cases considered include 1) separated flow over a single-element airfoil at high lift, 2) fully attached flows over the NLR 7301 airfoil and flap, and 3) separated flow over the GA(W)-1 airfoil and flap. For the single-element case, the Menter model provides better agreement with the experimental pressure distribution and velocity profiles. For the NLR 7301 airfoil and flap, both turbulence models provide excellent agreement with experimental pressures and skin-friction data. Velocity profiles are generally well predicted except in some parts of wakes. Turbulent shear stresses are not always well predicted. The predictions of the Spalart-Allmaras model are generally superior for this case, especially when boundary-layer confluence is important.

Skin friction measurements using He-Ne laser

An experimental study of the skin friction measurement in a turbulent boundary-layer has been carried out. The skin friction measurements are made using the laser interferometer skin friction (LISF) meter, which optically detects the rate of thinning of an oil applied to the test surface. This technique produces reliable skin friction data over a wide range of flow situations up to 3-dimensional complicated flows with separation, where traditional skin friction measurement techniques are not applicable. The present measured data in a turbulent boundary-layer on a flat plate using the LISF technique shows a good comparison with the result from the previous velocity profile techniques, which proves the validity of the present technique. An extensive error analysis is carried out for the present technique yielding an uncertainty of about .+-.8%, which makes them suitable for CFD code validation purposes. Finally the measurements of the skin friction in a separated region after a surface-mounted obstacle are also presented.

Structure of a turbulent double-fin interaction at Mach 4

This paper examines the three-dimensional shock-wave/turbulent boundary-layer interaction in an inletlike geometry consisting of symmetrically placed 15-deg fins on a plate in an oncoming Mach 4 flow. Two different implicit numerical methods are employed to solve the full three-dimensional mean compressible Navier-Stokes equations: 1) a high resolution upwind scheme with the Baldwin-Lomax algebraic turbulence model and 2) the traditional Beam-Warming approach with a k-e model. The comparison with experiment is very good for surface pressures, pitot pressure surveys, and surface shear patterns, although discrepancies with skin friction data are evident in the trailing portion of the interaction. Both computational schemes, despite major differences in modeling, predict similar flow structures in agreement with previous results at Mach 8. The flow is described in terms of a separated, nonreattaching boundary layer, a vortex interaction, longitudinal centerline vortices, and entrainment. The computed and experimental shock patterns are related to the streamline structure to provide a unified understanding of the flowfield.

Enhanced Heat Transfer and Shear Stress Due to High Free-Stream Turbulence

Surface heat transfer and skin friction enhancements, as a result of free-stream turbulence levels between 10 percent < Tu > 20 percent, have been measured and compared in terms of correlations given throughout the literature. The results indicate that for this range of turbulence levels, the skin friction and heat transfer enhancements scale best using parameters that are a function of turbulence level and dissipation length scale. However, as turbulence levels approach Tu = 20 percent, the St′ parameter becomes more applicable and simpler to apply. As indicated by the measured rms velocity profiles, the maximum streamwise rms value in the near-wall region, which is needed for St′, is the same as that measured in the free stream at Tu = 20 percent. Analogous to St′, a new parameter, Cf′, was found to scale the skin friction data. Independent of all the correlations evaluated, the available data show that the heat transfer enhancement is greater than the enhancement of skin friction with increasing turbulence levels. At turbulence levels above Tu = 10 percent, the free-stream turbulence starts to penetrate the boundary layer and inactive motions begin replacing shear-stress producing motions that are associated with the fluid/wall interaction. Although inactive motions do not contribute to the shear stress, these motions are still active in removing heat.