Oil-film Interferometry Research Articles

Evidence of quasiequilibrium in pressure-gradient turbulent boundary layers

Two sets of measurements utilizing hot-wire anemometry and oil-film interferometry for flat-plate turbulent boundary layers, exposed to various controlled adverse and favourable pressure gradients, are used to evaluate history effects of the imposed and varying free-stream gradients. The results are from the NDF wind tunnel at Illinois Tech (IIT) and the MTL wind tunnel at KTH, over the range $800 < Re_\tau < 22\,000$ (where $Re_{\tau }$ is the friction Reynolds number). The streamwise pressure-gradient parameter $\beta \equiv (-\ell /\tau _{w}) \cdot (\partial P_{e}/\partial x)$ varied between $-2 < \beta < 7$ , where $\ell$ is an outer length scale for boundary layers equivalent to the half-height of channel flow and the radius of pipe flow, and is estimated for each boundary-layer profile; note that $\tau_w$ is the wall-shear stress and $P_e$ is the free-stream static pressure. Extracting from each profile the three parameters of the overlap region, following the recent work of Monkewitz & Nagib (J. Fluid Mech., vol. 967, 2023, p. A15) that led to an overlap region of combined logarithmic and linear parts, we find minimum history effects in the overlap region. Thus, the overlap region in this range of pressure-gradient boundary layers appears to be in ‘quasiequilibrium’.

Skin-friction measurements using oil-film interferometry in hypersonic transitional boundary layer flows

Skin-friction measurements in hypersonic transitional boundary layer (HTBL) flows on a flared cone at Mach 6 at zero angle of attack are obtained using oil-film interferometry (OFI). In addition, the amplitude evolution of instability waves is investigated using fast-response pressure sensors and Rayleigh-scattering flow visualization (RSFV). The connection between the skin-friction coefficient and near-wall dynamics during the whole transition process is illustrative and the underlying mechanism of the generation of the wall skin friction is revealed. There exist two peaks of skin-friction rise along the streamwise direction of the model. The first one is at the transitional region (denoted as FS) which is caused by the second-mode instability. The second one is at the region where the transition is completed (denoted as FT). The drastic growth of skin friction near the end of transition is mainly attributed to the rapid boundary-layer breakdown into small scales. It is found that the strong shear in the near-wall region in the quiet zone where dominant hairpin-shaped vortices evolved from the second modes can also cause a local skin-friction rise (denoted as FQ). Further investigations show that in the region between FS and FQ, there is a slight increase in the skin friction, which is associated with the growing low-frequency waves. The skin friction coefficient value decreases slowly after FT but does not reach the expected turbulent theoretical value by the end of the test domain, indicating that the development towards the completely turbulent boundary layer appears to be a gradual process.

Wall shear stress measurement using a zero-displacement floating-element balance

The floating-element (FE) principle, introduced nearly a century ago, remains one of the most versatile direct wall shear stress measurement methods. Yet, its intrinsic sources of systematic error, associated with the flow-exposed gap, off-axis load sensitivity, and calibration, have thus far limited its widespread application. In combination with the lack of standard designs and testing procedures, measurement reliability still hinges heavily on individual judgement and expertise. This paper presents a framework to curb these limitations, whereby the design and operation of a FE balance are leveraged by an analytical model that attempts to capture the behaviour and predict the relative contribution of the systematic sources of error. The design is based on a parallel-shift linkage and a zero-displacement force-feedback system. The FE has a surface area of 200×200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$200\ imes 200\\,$$\\end{document} mm, and measurement sensitivity is adjustable depending on the surface condition and the Reynolds number. It is thus suitable for application in a wide range of low-speed, boundary-layer wind tunnels, small or large scale. Measurements of the skin friction coefficient over a smooth wall show a remarkable agreement with oil-film interferometry, especially for Reθ>1.3×104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\rm Re}_{\ heta } > 1.3 \ imes 10^4$$\\end{document}. The discrepancy relative to the empirical Coles–Fernholz relation (κ=0.39\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\kappa = 0.39$$\\end{document} and C=4.352\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C = 4.352$$\\end{document}) is within 0.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.5\\,$$\\end{document}%, and the level of uncertainty is below 1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1\\,$$\\end{document}% for a confidence interval of 95\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$95\\,$$\\end{document}%.

Investigation of coherent structures in low-speed turbulent boundary layers controlled by AC-DBD plasma actuators

The dielectric barrier discharge (DBD) actuator has the advantages of being lightweight, having no moving parts, ease of use, and fast response, and has received widespread attention in flow control applications. Turbulence boundary layer drag reduction is one of many applications of DBD flow control, but the mechanism of DBD actuator turbulence drag reduction needs further investigation. The effect of DBD excitation on the skin-friction drag of a turbulent boundary layer on a flat plate at different flow speeds was investigated experimentally. The change in skin-friction drag was measured using oil film interferometry, and the velocity distribution within the boundary layer was obtained using a particle image velocimetry system. The results showed that under the action of the plasma actuator, the local skin-friction coefficient was measured to decrease by 49%. Through dynamic mode decomposition, plasma actuators can increase the thickness of the low-velocity region in the boundary layer, reduce the intensity of Q2 and Q4 events, and inhibit the development of coherent structures, thereby achieving drag reduction.

Improvement of extraction method of the Fizeau fringes spacing in the oil film interferometry and measurement of wall shear stress on an airfoil surface

Improvement of extraction method of the Fizeau fringes spacing in the oil film interferometry and measurement of wall shear stress on an airfoil surface

Measurement of wall shear stress on an airfoil surface by using the oil film interferometry with PIV analysis applied to Fizeau fringes

In this study, oil film interferometry (OFI) was applied to the flow around the suction surface of a two-dimensional airfoil under high lift conditions to measure the wall shear stress. However, existing OFI methods have difficulty measuring the wall shear stress in images affected by reverse and secondary flows. Therefore, the particle image velocimetry (PIV) method was applied to the Fizeau fringe images to determine the direction of progress from the calculated velocity vector. An airfoil with a wing section of NACA0012 was used, the Reynolds number was set to 8 × 104, and angle of attack was set at 11°. A direct spatial domain correlation was used for the PIV analysis method. At x/c = 0.75 to 0.83, the measured local skin friction coefficient was good agreeing with the large eddy simulation calculated results reported by Miyazawa et al. (Transaction of the JSME, Series B, Vol.72, No.721 (2006)). However, a large difference between the measured and calculated local skin friction coefficients occurred at x/c = 0.17 near the re-attachment point. The wall shear stress is determined by calculating the dominant frequencies using fast Fourier transform (FFT) analysis from the matrix data obtained by mean of the pixel intensities in the analysis region in the span direction. If the Fizeau fringe image is uniform in the span direction, the program can calculate the periodic waves. When the Fizeau fringes are tilted to the analysis range, the FFT analysis of the obtained matrix data results in an error because the dominant frequency cannot be calculated. Therefore, velocity vectors were detected near the re-attachment point by adapting PIV to the Fizeau fringe images. The local skin friction coefficients were calculated by OFI measurements with the Fizeau fringe images that were rotated by the angle of the velocity vector determined by PIV.

Measurement of Shear Stress Acting on Flat Plate Using Oil Film Interferometry

Measurement of Shear Stress Acting on Flat Plate Using Oil Film Interferometry

Effect of Swept Shockwave Boundary-Layer Interaction Strength on Surface Skin Friction

This investigation examines the flowfield topology beneath the single fin-generated shockwave boundary-layer interaction as a function of shock interaction strength and incoming boundary-layer thickness. A systematic application of oil film interferometry produced an experimental reconstruction of global skin friction maps for various Mach number and fin angle combinations. To determine the effect of boundary-layer thickness, data from the current study were compared against existing experimental data sets with thinner incoming boundary layers. Results for identically matching flow/geometric conditions show a good agreement between trends seen across the conical region of the interaction and exhibit skin friction coefficient peaks in the same position. Cases corresponding to a relatively thicker boundary layer showed higher skin friction coefficient values along the interaction periphery and upstream regions. An angular scaling was applied to align the inviscid shocks of data sets that matched interaction strength, and the results show similar behavior. The peak skin friction coefficient shows a monotonic increase with increasing shock strength. Conversely, the upstream normalized peak skin friction coefficient depends on boundary-layer thickness and shock strength. These findings indicate that boundary-layer effects are limited to the interaction periphery, whereas viscous effects driven by shockwave–boundary-layer interactions dominate the interior region.

A simplified photogrammetry procedure in oil-film interferometry for accurate skin-friction measurement over arbitrary geometries

A simplified photogrammetry procedure in oil-film interferometry for accurate skin-friction measurement over arbitrary geometries

Maximum Wall Stress on a Smooth Flat Plate Under Planar Jet Impingement

Abstract This paper investigates the maximum wall shear stress value τmax and its location xmax as measured on a smooth flat plate impinged upon by a normal planar jet. τmax and xmax are found to be closely related to the stagnation pressure Ps and the half-width of the mean wall pressure profile bpw. The measurements were made by two different techniques: a Stanton probe and oil film interferometry. The maximum wall shear stress location xmax is found to be independent of the jet Reynolds number. At a small nozzle-to-plate distance H≲6 Djet, xmax is related to the jet slot width as xmax≈1.1Djet. At a large nozzle-to-plate distance H≳6 Djet, the maximum wall shear stress location is related to the mean wall pressure half-width as xmax≈1.4 bpw. A new Reynolds number, referred to as the stagnation Reynolds number, is defined as Res=def2bpwPs/ρ/ν, where ρ is the fluid density and ν is the kinematic viscosity. The maximum wall shear stress is found to be strongly influenced by the stagnation Reynolds number, and the dependence as measured by Stanton probes is approximated by a power law of τmax/Ps≈0.38/Res0.38. The solution of the laminar flow equations in the Appendix gives an alternate relation for τmax, which is in better agreement with the oil film interferometry measurements. Dimensional analysis is performed to gain insight into the empirical findings.

Evaluation of maximum wall shear stress from air impingement to remove food deposits from stainless steel surfaces

The risk of cross-contamination during manufacturing of low water activity foods requires careful attention to cleaning and sanitation. Considering the challenges encountered in such manufacturing, air impingement can provide a unique approach to remove deposits from the food-contacted surface, particularly, the crevices or cracks. The objective of this study was to evaluate the shear stress-induced by air jet impingement for removal of food residues. The wall shear stress distribution on a surface below an air jet was measured using oil-film interferometry. The air impingement was operated at three level of air velocity (Reynolds number = 7756, 13,444, and 18,787) and three different nozzle height (24 (H/D = 6), 32 (H/D = 8), 40 mm (H/D = 10)). The shear stress was found to become higher along with the increase of air velocity and decrease of nozzle height. The observed distribution of wall shear stress on the surface was fitted using the Gaussian model and Polynomial model. The Gaussian model appeared to be preferable according to the statistical analysis, which aligned with previous literature. Based on the Gaussian model, the maximum wall shear stress was estimated. The maximum wall shear stress increased with Reynolds number ( Re ) and the ratio of jet height to jet nozzle diameter. The highest maximum shear stress (10.74 Pa) occurred at a Reynolds number of 18,787 and a height of 24 mm (H/D = 6). This trend was directly correlated with removal efficiency. The greatest removal of the treatment was achieved by 40% at maximum wall shear stress. A linear relationship between percent residue removed and maximum wall shear stress was obtained from the experimental results, and a threshold maximum wall shear stress of 2.73 Pa has been proposed for removal of peanut butter residue from a stainless-steel surface. The current study demonstrates the promising of the air impingement in the cleaning operation for low water activity food. • Wall shear stress increased with an increase in velocity and decrease in height. • Removal of food deposits revealed a similar trend to velocity and height. • Linear relation was found between maximum shear stress and food removal.

Comparison of the mean skin friction with the heat transfer and the flow development in turbulent planar wall-attaching jet flows.

Measurements of the mean skin friction below turbulent planar wall-attaching offset jet flows were performed using oil-film interferometry for jets initially offset (Hs) 0.2 to 1 times the jet height (Hj) from the wall. The results were compared with turbulent flow field measurements and with previous heat transfer measurements. The profiles of the mean skin friction coefficient below the offset or preattachment region were similar when compared relative to the distance to the reattachment location (Xr). The initial recovery length of the mean skin friction downstream of the reattachment location was proportional to the reattachment length similar to the decrease in the wall pressure fluctuations in this region. The longer recovery and initial decay of the mean skin friction coefficient appeared to collapse in terms of (x-Xr)/Hj; as did the initial change in the mean static pressure, the measures of the mean flow development, and the Stanton number downstream of the reattachment point. The decay in the maximum of the turbulent stresses associated with the attached shear layer also collapsed in terms of (x-Xr)/Hj, unlike the fluctuating pressure and initial recovery of the mean skin friction. The change in the location of the minimum and zero in the turbulent shear stress as the flows transitioned to the wall jet region collapse in terms of x-Xr/Hj similar to the longer recovery and initial decay of the mean skin friction. The mean skin friction for flows with Hs of Hj and jet Reynolds numbers (Rej) of 21800 to 54500, varied as Rej-0.2 below the attaching jet, in the reattachment region, and in the downstream wall jet region.

Measurement of Wall Shear Stress Acted on an Airfoil Suction Surface by the Oil Film Interferometry with the PIV Analysis

Measurement of Wall Shear Stress Acted on an Airfoil Suction Surface by the Oil Film Interferometry with the PIV Analysis

Wall shear stress measurements by white-light oil-film interferometry

Wall shear stress and wall streamline direction measurements were conducted on a generic delta wing model to demonstrate the potential of the white-light oil-film interferometry in three-dimensional flows. With the proposed algorithm, an automatic oil-film thickness calculation is possible and yields reproducible and plausible wall shear stress distributions on the leeward side of the delta wing for various angles of attack.Graphic abstract

Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers

The effect of spanwise heterogeneous surface geometry on turbulent boundary layer secondary flows and on skin friction is investigated experimentally. The surfaces consist of smooth streamwise-aligned ridges of different shapes and widths with spanwise wavelengths comparable to the boundary layer thickness ( ). Cross-stream stereoscopic particle image velocimetry combined with oil-film interferometry is used to investigate the flow field and assess the drag. Results show that the spanwise distribution of the skin friction varies as a consequence of the mean flow heterogeneity and is highly dependent on surface geometry. The swirling strength maps revealed remarkable changes in the secondary flow structures for different ridge shapes. For wide ridges, topological changes occur showing the appearance of tertiary vortices coexisting with the large-scale secondary structures. An imbalance in favour of these tertiary structures occurs over a certain width which take over the secondary structures, causing a swap in the locations of the low- and high-momentum pathways. Furthermore, the results indicate that the spanwise spacing alone is insufficient to characterise the surface heterogeneity. A new parameter ( ), which is based on the ratio of the perimeter over and below the mean surface height, is shown to adequately capture the changes in skin friction and streamwise circulation of the secondary motions. Triple decomposition allowed the quantification of the dispersive stresses for all these cases, which can contribute up to of the total shear stress when strong secondary motions occur.

Measurement of Flow on an Airfoil Surface by Using the Oil Film Interferometry and PIV analysis

Measurement of Flow on an Airfoil Surface by Using the Oil Film Interferometry and PIV analysis

Effect of Optical Photography Conditions on Fringe Velocity of Oil-Film Interferometry

Effect of Optical Photography Conditions on Fringe Velocity of Oil-Film Interferometry

Experimental realisation of near-equilibrium adverse-pressure-gradient turbulent boundary layers

A new experimental database of adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) obtained through hot-wire anemometry and oil-film interferometry covering a momentum–loss Reynolds number 450<Reθ<23450 and Clauser pressure-gradient-parameter range up to β≈2.4 is presented. Both increasing and approximately constant β distributions with the same upstream history are characterised. Turbulence statistics are compared among the different pressure-gradient distributions with additional numerical and experimental zero-pressure-gradient (ZPG) TBL data. Cases at approximately constant β, which can be considered as canonical representations of the boundary layer under a certain pressure-gradient magnitude, exhibit skin-friction and shape-factor curves consistent with the ones proposed by Vinuesa et al. (2017). These curves show a similar scaling behaviour as those proposed by Nagib et al. (2007) for ZPG TBLs. The pre-multiplied power-spectral density is employed to study the differences in the large-scale energy content throughout the boundary layer. Two different large-scale phenomena are identified, the first one related to the pressure gradient and the second one (also present in high-Re ZPG TBLs) due to the Reynolds number. Recently proposed scaling laws by Kitsios et al. (2016) and Maciel et al. (2018) are tested over a wider Reynolds-number range and for different β cases. The mean velocity and streamwise velocity fluctuation profiles are found to be dependent on the upstream development. The mean velocity profile is found to be self-similar only in the outer region, in agreement with classical theory. The mean and higher-order statistics of the new APG TBL database are made available under www.flow.kth.se.

Study on flow separation and transition of the airfoil in low Reynolds number

As typical flow characteristics in a low Reynolds number, laminar separation bubbles (LSBs) and transition to turbulence over airfoils have been extensively studied in recent years. In order to analyze their flow mechanism, numerical investigation using the finite volume method to solve the Reynolds averaged Navier-Stokes equations with a transition Shear Stress Transport (SST) four-equation transition model is performed in this work, combined with the experimental study facilitated by the oil film interferometry technique. Specifically, the transition SST four-equation transition model is solved to simulate the separation location and LSB structure at low Reynolds numbers on a Wortmann FX63-137 airfoil. Good agreement is obtained between the numerical simulation and experimental measurements regarding the separation, transition and reattachment location, aerodynamic coefficients, and overall flow structures. At higher Reynolds numbers of 200 000 and 300 000, similar bubble structures on the airfoil surface are observed, and the location of the bubble moves toward the leading edge of the airfoil by increasing the angle of attack. However, in Reynolds numbers ranging from 300 000 to 500 000, significant changes of the laminar flow separation structures emerge. The flow structure changes from the classical laminar separation bubble to the nonclassical separation flow structure that is composed of a major vortex 1(V1) and a minor vortex 2(V2). Due to the small distance between V1 and V2, it is difficult to distinguish the delicate structure of the two separation bubbles from the classical laminar separation bubble by the experimental method.

Application of oil-film interferometry image post-processing technology based on MATLAB

The image post-processing technology based on MATLAB was applied in oil-film interferometry to calculate the skin friction coefficient. This method can recognize the bright and dark fringes by the algorithm based on standard deviation, which is efficient and robust. Furthermore, it can process low-quality images and extract fringe spacing by converted grayscale image and the proper sliding window can catch the discrete objectives precisely by multiple selected windows. In addition, the calculation window size can adjust automatically, which improved the robustness of the algorithm greatly. By comparing the experimental results with the numerical results, it is proved that the method of post-processing based on MATLAB can accurately measure the skin friction in hypersonic flow.