Aspect Ratio Effects on Turbulent and Transitional Flow in Rectangular Microchannels as Measured With MicroPIV

Microscopic particle image velocimetry (microPIV) was used to measure velocities in rectangular microchannels with aspect ratios ranging from 0.97 to 5.69 for $200<Re<3267$⁠. Mean velocity profiles, velocity fluctuations, and Reynolds stresses were determined from the microPIV data. Transition to turbulence was observed at $Re=1765-2315$ for the five aspect ratios studied, agreeing very well with both recent microscale experiments and macroscale duct flow and indicating no evidence of early transition for any of the aspect ratios studied. The onset of fully turbulent flow was observed at $Re=2600-3200$⁠. For the fully turbulent flow, the $⟨u′⟩∕umax$ and $⟨v′⟩∕umax$ fluctuations at the channel centerline were 6% and 3%–3.5% and generally agreed well with macroscale results. As aspect ratio increased, the $⟨u⟩∕umax$ and $⟨u′⟩∕umax$ profiles became flatter, with nearly uniform values extending for some distance from the centerline of the channel. This region of uniform $⟨u⟩∕umax$ and $⟨u′⟩∕umax$ became larger with increasing aspect ratio. The Reynolds shear stress for fully turbulent flow also displayed a strong dependence on aspect ratio. For the $W∕H=0.97$ microchannel, $⟨u′v′⟩∕umax2$ steadily increased in value moving from the centerline to the wall, but for the higher aspect ratio microchannels, $⟨u′v′⟩∕umax2$ remained close to zero in the center region of the microchannel before increasing in value at locations close to the wall, and this region of near zero $⟨u′v′⟩∕umax2$ became larger with increasing aspect ratio. This behavior in the Reynolds shear stress is due to the region of uniform velocity and, hence, small mean shear, near the channel centerline of the high aspect ratio microchannels.