Development of Dual Film Probe form Dynamic Measutemtn of Wall Shear Stress

为了改善粘贴式热膜探头测量时变壁面剪应力的性能，提出了一种新型探头结构--双膜探头，并对其静态和动态特性进行了测试。本研究分四步进行。双膜探针的试制：在聚酰亚胺薄片的两面，在同一位置上分别镀上两层镍膜。聚酰亚胺片的厚度分别为100、50和25 μ m，镍膜的宽度为0.1mm，长度为1mm，它们的温度分别由两套恒温风速计独立控制。上层薄膜用作壁面切应力传感器，下层薄膜用作保护层，以减少进入壁面的热传导损失.静态测试：在二维通道内表面粘贴测试膜，测定热膜输出电压与由纵向压力梯度求得的相应壁面切应力之间的关系。当管壁温度变化时，保护膜能有效减小输出漂移.动态实验：在二维通道内产生脉动流，比较了脉动流的相均壁面切应力和输出电流。研究发现，采用保护膜后，振幅退化现象得到了明显改善.数值模拟：为了优化膜层布置和传热特性，对膜层的耦合传热问题进行了数值分析。其原因是忽略了膜表面的温度分布。

In order to improve the characteristics of glue-on type hotfilm probe for time dependent wall shear stress measutement, a new type of probe arrangement, called dual film probe, have been proposed and its static and bynamic charactersitics have been tested. The inverstigations were performed in the following four step.1. Trial production of dual film probe: Two nickel films were doposited on both sides of a thin polyimid sheet at precisely the same position. The thickness of the polyimid sheet were 100,50 and 25 micrometers and the nickel films have the width of 0.1mm and the length of 1mm and their temperatures are independently controlled by employing two sets of constant temperature anemometers. Upper film is used as a wall shear stress sensor while the lower one is used as a guard to minimize the heat conduction loss into the wall.2. Static test: Test film was glued on the inner surface of the two-dimensinal channel and the telation between the hot-film output voltage and the corresponding wall shear stress evaluated from the longitudinal pressure gradient was determined. The guard film was found oeeficient to minimize the output drift when to wall temperature was changed.3. Dynamic test: The plusatile flow was produced in twodimensional channel and the phase averged wall shear stress and output coltage were compared. It was found that the amplitude degeneration is considerably improved by employing the guard film.4. Numerical simulation: To optimize the film arrangement and the meteral characteriastics, conjugated heat transfer problem was numerically analyzed. The agreement with the mesaured results were only wqualitative, the reason of which was attributed to the negligence of the temperature distribution on the film surface.