Development of a novel dual-layer calibration-free hot-film sensor for dynamic skin friction measurements

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Skin friction is a major source of drag force on a passenger airplane, however, local skin friction distribution is hard to quantify. Up to now, none of the existing techniques has been applied to real vehicles due primarily to issues related to convenience, maintenance, non-unique calibration, and unwanted sensitivities to other parameters. Recently, the applicant's group developed a novel dual-layer hot-film sensor using two metal films separated by a polyimide membrane. The two metal films were connected in two separated Constant Temperature Anemometry (CTA) systems and operated at the same temperature so that the Joule heat generated by the upper film only transfers to the fluids and is measurable. Moreover, a theoretical model was used to calculate the skin friction with reasonable accuracy using the measured Joule heat and the temperature difference between the sensor and fluids. This technique does not require calibration and the results are not affected by the changes in fluid temperature. In a series of publications (see publication list in CCV [J4, J7, J8]), we had demonstrated that the calibration-free technique is reliable and convenient, the advantages over existing techniques are clear. Though promising, there are works to be done to make this technology ready for a broad range of real applications. First, current dual-film calibration technology can only measure time-average skin friction. Detailed theoretical analysis has to be done to pinpoint the critical frequency and the amplitude and phase relations beyond the critical frequency. Secondly, an insulation layer has to be applied over the sensor for underwater measurements to avoid electrochemical corrosion. However, with this insulation, mental-film temperature differs from the sensor-surface temperature where heat transfer occurs. A heat transfer model has to be developed for this insulation layer to allow the calibration-free measurement.   We propose to investigate the aforementioned issues using theoretical and experimental methods. In-house water-flow facility and air-flow flow wind tunnel will be built in the next five years. We expect the calibration-free dual-hot-film skin friction measurement technology will be in a matured state after this investigation. Fundamental fluid mechanics research and a broad range of industries such as aerospace, naval architecture, and ocean engineering will benefit from this technology.