Development of the measuring technique for local wall shear stress by laser gradient meter

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The optical technique for measuring the local wall shear stress has been developed. At the first year of the project, the feasibility study of the method has been conducted by performing the measurement in a two-dimensional channel flow for which a number of experiments in the past to be referred as well as DNS database are available. The measurements of turbulence statistics have been undertaken under the varying Reynolds number.The test rig is a plane channel with the channel aspect ratio of 1 : 32 and 2m long. The width of the channel H is 20mm, and the measurements have been conducted at the location 160H distant from the channel inlet. The obtained mean skin friction coeffecient has well been corrilated with the existing experiments and DNS within 1%. The good agreements have been obtained also for the turbulence statistics such as the second-moment, skewness as well as the flatness factors.In the second and third year of the project, the effort has been paid for the development of the transmitting optics system that makes use of an optical fiber joint to improve the flexibility of the system. Because the precise manipulation of the thin fiber has required an extreme skill for preparing the suitable fiber arrangement, requiring the adjustment of the distance between two parallel flber core to be 20mum, most of the period has been used to develop the fiber joint mechanism. However, the therefrom achieved system has been proven to be more stable and reliable compared to the original system.It has also been discussed on the possibility of the use of analogue sensor that enables the detection of valid signal which may otherwise be rejected during the data validation algorithm in the statistic process. A V-type hot-wire sensor, mounted in a small hole on the wall, has found to be a potential candidate for the pilot analogue sensor system.