We simultaneously measured the flow field and wall shear stress in a straight pipe and behind an orifice by means of particle image velocimetry (PFV) and the limiting diffusion current technique (LDCT), respectively. The spatio-temporal correlation coefficients of the local vertical velocity and the shear stress fluctuations are presented, and the canonical correlations between the proper orthogonal decomposition (POD) spatial eigenmodes in the recirculation region and the wall shear stress are also discussed. A response time lag occurs between the wall shear stress and the velocity field. The large canonical correlation between the POD eigenmodes and wall shear stress suggests that large-scale flow structures are important for the shear stress fluctuation. These analyses are applied to the shear flow in channel and large scale motions are studied in relation with the shear stress fluctuations.
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