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Skin friction measurement in zero and adverse pressure gradient boundary layers using oil film interferometry
| Content Provider | Semantic Scholar |
|---|---|
| Author | Madad, Reza Harun, Zambri Chauhan, Kapil Monty, Jason P. Marusic, Ivan |
| Copyright Year | 2010 |
| Abstract | Skin friction and velocity measurements were measured in zero and adverse pressure gradient boundary layers using oil-film interferometry and single hot-wire sensors. Oil-film interferograms were analyzed using the Empirical Mode Decomposition (EMD) and Hilbert transform techniques [9] that, in contrast to commonly used Fourier based techniques, are less subjective and superior for treating amplitude and frequency modulated data [4]. As an independent technique, the oil-film measurement has been used to assess the use of the Clauser-chart [5]. For strong pressure gradients, at the Reynolds numbers considered here, the Clauser-chart results were found to underestimate the wall-shear stress velocity as found by oil-film interferometry. Introduction The contribution of skin friction is a significant part of the total drag in almost all transportation systems. The measurement of skin friction is therefore important for applied problems, such as improving the performance of transportation vehicles and fundamental problems, such as characterizing surface flows. The assessment of the wall-shear stress, τ = μ ·∂U/∂z, has been the subject of many experimental and numerical studies. Here μ is the dynamic fluid viscosity, U the streamwise velocity, and z the distance from the wall. The knowledge of the mean wallshear stress is a necessary prerequisite to determine the friction velocity, uτ = (τ/ρ)1/2, as one of the fundamental turbulence scaling parameters. The temporal and spatial shear stress distribution is related to turbulent flow structures in the vicinity of the wall and is as such of major importance for the basic understanding of the development of near-wall turbulent events. It is not certain that the standard logarithmic law of the wall holds in adverse pressure gradient (APG) flows. In fact, such a pressure-gradient-dependence of the logarithmic region has been observed and reported in the literature, notably by Bourassa and Thomas [2], Spalart and Leonard [16], Nickels [12], Dixit and Ramesh [6], Chauhan et al. [3] and Nagib and Chauhan [11]. Skin friction estimation by the Clauser chart method relies on the universal logarithmic law and hence is not suitable for situations where this universality is known to fail. The same holds true for the Preston-tube method even though the details are different. In the case of strong pressure gradients a shift appears in the profile above or below the conventional log-law associated with a change in the profile’s shape, mentioned by Nagano et al.[10], Spalart et al. [17] and Fernholz et al. [8]. In contrast, Skare and Krogstad [18] and Bernard et al. [1] observed that the law of the wall is valid for higher Reynolds number APG flows and for the decelerating flow around an airfoil, respectively. Oil film interferometry is one of the few methods available for the direct and absolute measurement of skin friction. The technique consists of measuring the thinning rate of an oil film as it is being acted upon by the shear near the wall. This method allows very accurate measurements of the mean skin friction and does not require specific and expensive equipment. A simple camera and monochromatic light source are the only devices needed to perform oil-film interferometry measurements. The utilization of this technique in two-dimensional flows is demonstrated in a number of studies [7, 8, 15, 13, 19]. The aim of this paper is to assess the use of the Clauser-chart for strong pressure gradient flows by using oil-film interferometry as an independent technique. |
| Starting Page | 757 |
| Ending Page | 760 |
| Page Count | 4 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://people.eng.unimelb.edu.au/imarusic/publications/Conferences/Madad_AFMC_2010.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
