A new optical measurement method for local wall shear stress and its signal processing techniques

A new optical measurement method for local wall shear stress is proposed. The method is an extension of laser Doppler technique. It has a possibility of detecting the both magnitude and angle of local wall shear stress. The principle was investigated for Doppler frequency and measurement volume. The Doppler frequency changes in a Doppler burst signal. The frequency variation depends on the magnitude and angle of local wall shear stress and the spanwise offset of scattering particle path. A signal processing technique is proposed to detect the magnitude and the angle of local wall shear stress. The technique consists of detecting Doppler frequency variation by time-frequency analysis and nonlinear least squares fit. A simulation was conducted to investigate the performance of the method. Simulated Doppler signals were generated by using the instantaneous velocity data of a fluid-flow simulation result. The generated signals with different noise levels were processed and the effect of the noise level was investigated. The local wall shear stress was estimated with moderate accuracy but its angle was not estimated with good accuracy. A further investigation is required to improve the accuracy of measurement for the method.