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 $\text{90}\text{s}$ velocity sample coupled with accurate wall positioning can define local c_f to a standard error of σ_cf≈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.