Wall-shear-stress measurements using volumetric µPTV

Abstract Accurately determining the wall-shear-stress, $$\tau _\textrm{w}$$ τ w , experimentally is challenging due to small spatial scales and large velocity gradients present in near-wall region of turbulent flows. To avoid these resolution requirements, several indirect iterative fitting methods, most notably Clauser chart method, exist for $$\overline{\tau }_{w}$$ ¯ w by mean profile further away from log-law layer. These methods often require proper selection constants, assumptions a canonical flow state, other empirical-based generalizations. reduce amount ambiguity, gradient assuming linear relationship between streamwise wall normal distance viscous sublayer can be used. However, this requires an accurate unbiased measurement below five units, which less than 50 µm high Reynolds number Therefore, study method volumetric defocusing microparticle tracking velocimetry presented that capable resolving boundary layer up $$U_{\textrm{e}}=44.7\,$$ U e = 44.7 m/s ( $$Re_{\theta }=27250$$ R e θ 27250 ). This allows through single optical access illumination imaging serves as excellent complement larger scale measurements information. The }_\textrm{w}$$ values determined approach were found good agreement obtained simultaneous parallax PTV measurement. Furthermore, analysis diagnostic plot cumulative distribution measured fluctuations region, showed both are accurately fluctuation profiles self-similar region.