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Accurate measurements of local skin friction coefficient using hot-wire anemometry

โœ Scribed by Nick Hutchins; Kwing-So Choi


Publisher
Elsevier Science
Year
2002
Tongue
English
Weight
410 KB
Volume
38
Category
Article
ISSN
0376-0421

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โœฆ Synopsis


The practicality and accuracy of many existing methods of local skin friction measurement suffer when the boundary layer flow under consideration is non-canonical. Such shortcomings are exacerbated in three-dimensional flows, by the necessity to map local c f over a wider area in order to characterise fully the contribution to global skin friction. These problems have led the authors to seek novel experimental methods of c f measurement. The technique proposed herein utilises velocity measurements made using hot-wire anemometry combined with accurate positioning of the sensor element in respect to the test surface. In essence it is proposed that the local skin friction can be evaluated via a single velocity measurement made at a known wall-normal distance within the linear region of the viscous sublayer. This technique relies on accurate probe positioning, and two methods of achieving this are outlined. A study of the hot-wire characteristics in near-wall proximity has revealed a previously unnoticed feature corresponding to probe-wall contact. It is shown that this anomaly can be used as a positional flag to accurately locate the aerodynamic origin of the hot-wire sensor. A second technique using a laser triangulation displacement sensor is also outlined. Both positional techniques are shown to offer positioning to a sufficient level of accuracy for the proposed c f measurement technique. Single-point local c f measurement is tested experimentally, demonstrating the improved repeatability and standard error as predicted by initial error analysis. In this way it is shown that a single 90 s velocity sample coupled with accurate wall positioning can define local c f to a standard error of s c f E1:0%: Analysis of error contributions reveals that longer sampling periods can realise even greater accuracy. The proposed technique is also used to measure local c f in a threedimensional boundary layer where micro-vortex generators have introduced large-scale spanwise distortions. This is an example of an application in a non-canonical boundary layer, and initial results show that the method is capable of providing repeatable comparative spanwise-averaged skin friction results to within approximately 71%: The technique is immediately applicable to researchers using hot-wire anemometry in low Reynolds number flow over electrically nonconductive test surfaces.


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