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

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 sigma(cf) approximate to 1.0%. 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 three-dimensional 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 +/- 1%. The technique is immediately applicable to researchers using hot-wire anemometry in low Reynolds number flow over electrically non-conductive test surfaces. (C) 2002 Elsevier Science Ltd. All rights reserved.