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

蒙皮摩擦力是客机阻力的主要来源之一，但局部蒙皮摩擦力的分布难以量化。到目前为止，主要由于与便利性、维护、非唯一校准以及对其他参数的不必要的敏感性有关的问题，现有技术中没有一种应用于真实的车辆。最近，申请人的小组开发了一种新颖的双层热膜传感器，其使用由聚酰亚胺膜分隔的两个金属膜。这两个金属膜连接在两个独立的恒温风速仪（CTA）系统中，并在相同的温度下运行，使得上层膜产生的焦耳热仅传递给流体，并且是可测量的。此外，理论模型被用来计算皮肤摩擦力与合理的精度使用测量焦耳热和传感器和流体之间的温差。该技术不需要校准，并且结果不受流体温度变化的影响。在一系列出版物中（参见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，与现有技术相比的优势是显而易见的。虽然前景看好，但要使这项技术为广泛的真实的应用做好准备，还有很多工作要做。 首先，目前的双膜校准技术只能测量时间平均表面摩擦力。必须进行详细的理论分析，以确定临界频率和超过临界频率的幅度和相位关系。其次，必须在传感器上施加绝缘层以用于水下测量，以避免电化学腐蚀。然而，通过这种绝缘，金属膜温度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热传递模型，以允许免校准测量。 我们建议使用理论和实验方法来研究上述问题。在未来五年内，将建造内部水流设施和气流风洞。我们期待免标定的双热膜表面摩擦力测量技术在本研究之后能达到一个成熟的状态。基础流体力学研究和广泛的工业，如航空航天，海军建筑和海洋工程将受益于这项技术。

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.