Diffractive optic fluid shear stress sensor

Recent advances in the design and fabrication of integrated diffractive optical elements has resulted in the development of a small and integrated micro-optical shear stress sensor. Miniaturization of these sensors has been achieved by integrating the collective function of a large number of refractive optical elements onto a single substrate without the loss of functionality and efficiency. This paper describes the fabrication of the micro-shear stress sensor and an experiment designed for the evaluation of its performance. The results show that, the micro-optical shear stress sensor accurately measures the wall velocity gradient for all laminar flows and for turbulent flows with moderate Reynolds numbers. Introduction Accurate measurement of the wall shear stress is needed for a number of aerodynamic studies. Shear forces generated by liquids or gases flowing over solid surfaces can significantly influence the performance of aircraft, ships, or surface-transport vehicles. Techniques that have been used for the measurement of shear stress include surface mounted force balance, surface mounted hot wire, surface mounted hot plate, MEMS hot-film, and oil film interferometery. The surface mounted force balance directly measures the shear force on the body. This technique requires a floating element, is usually not very small and has a limited frequency response. The hot wire, hot plate and MEMS hotfilm techniques all require calibration and are not generally suitable for liquid flows. The oil film technique has been used for aerodynamic applications, but has a limited utility in liquid flows. The optical shear stress sensor reported here was developed based on a technique first presented by Naqwi and Reynolds [1] using conventional optics. The concept was based on the use of a modified laser Doppler anemometry (LDA) with a set of diverging fringes located close to the wall and within the linear region of the boundary layer. The developed sensor provided an integrated diffractive optical element (doe) sensor that integrated the transmitter and receiver optics on the same substrate, as shown in Figure 1. The surface mounted sensor generates a pattern of diverging interference fringes, originated at the surface, and extended into the boundary layer region. As in the LDA technique, the scattered light from the particles passing through the fringes is collected through a window at the surface of the sensor. The intersection region of the transmission fringe pattern and the receiver field of view defined by the backscatter collection optics defined the overall measurement volume location and dimension.