Turbulent and transitional velocity measurements in a rectangular microchannel using microscopic particle image velocimetry

Microscopic particle image velocimetry (microPIV) experiments were performed on a polydimethylsiloxane (PDMS) microchannel with a cross-section measuring 320 μm × 330 μm for Reynolds numbers between 272 and 2853. Care was taken to ensure that the seed particle density was great enough that accurate instantaneous velocity vector fields could be obtained for all the Reynolds numbers investigated. Velocity fluctuations were calculated from ensembles of microPIV velocity fields. The 〈u′〉/umax fluctuation showed an increase at Re = 1535 and a further increase as Reynolds numbers were increased, suggesting that transition to turbulence began near Re = 1535, a Reynolds number lower than predicted by classical theory. The 〈u′〉/umax data also suggest the flow was fully-developed at a Reynolds number between 2630 and 2853, also lower than classical results. This finding was confirmed in plots of the mean velocity profile. For the fully developed flow, the measured 〈 u′ 〉/umax fluctuation agreed well with classical results for turbulent duct flow, but the 〈 v′ 〉/umax fluctuation was 25-40% lower than turbulent duct flow results. Finally, spatial correlations of velocity fluctuations were calculated to lend some insights into the characteristics of the large-scale turbulent structures observed in the turbulent microchannel flow.