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

壁面摩擦力是客机阻力的主要来源，但局部壁面摩擦力的分布很难量化。到目前为止，现有的技术还没有应用于真实的车辆，主要是因为与便利性、维护、不唯一的校准以及对其他参数的不想要的敏感性有关的问题。最近，申请人小组开发了一种新型的双层热膜传感器，该传感器使用由聚酰亚胺薄膜隔开的两个金属膜。这两种金属薄膜连接在两个独立的恒温测速(CTA)系统中，并在相同的温度下工作，因此上层薄膜产生的焦耳热只传递到流体中，并且是可测量的。此外，利用测量的焦耳热量和传感器与流体之间的温差，建立了理论模型，以合理的精度计算了壁面摩擦力。这项技术不需要校准，结果不受流体温度变化的影响。在一系列出版物中(见CCV[J4，J7，J8]中的出版物列表)，我们已经证明了免校准技术是可靠和方便的，相对于现有技术的优势是显而易见的。尽管前景看好，但要使这项技术为广泛的实际应用做好准备，还有很多工作要做。首先，目前的双膜标定技术只能测量时间平均摩阻。必须进行详细的理论分析，以确定临界频率以及超出临界频率的幅度和相位关系。其次，在水下测量时，必须在传感器上涂上绝缘层，以避免电化学腐蚀。然而，有了这种绝缘层，金属膜的温度就不同于发生热传递的传感器表面温度。必须为该绝缘层开发一个热传递模型，以便进行无需校准的测量。我们建议用理论和实验的方法来研究上述问题。未来五年内将建设室内水流设施和气流风洞。我们预计，在本次研究之后，无定标的双热膜表面摩擦力测量技术将处于成熟状态。基础流体力学研究以及航空航天、船舶建筑和海洋工程等广泛行业将受益于这项技术。

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.