Structural characteristics of transition to turbulence in microscale capillaries

The maturation of flow in microscale capillaries (D=536 mu m) from a laminar to a turbulent state is studied via extensive microscopic particle image velocimetry measurements spanning the Reynolds-number range of 1900 <= Re <= 4500. Some previous studies of transition to turbulence in microscale passages have observed transition at anomalously low Re, leading to the suggestion that flow at these scales is fundamentally different from that at the macroscale. One possible culprit for these reports of early transition could be significant surface roughness in the microchannels employed. As such, care is taken in the present experiments to select microscale capillaries with minimal inner surface roughness in order to remove the possibility that roughness could trigger early transition. Consistent with transitional wall-bounded flows at the macroscale, transitional capillary flow is found to contain patches of increasingly disordered motion with increasing Re bounded by laminar flow behavior. The intensity and frequency of occurrence of these disordered motions grow with Re, and quadrant analysis supports a gradual maturation of the instantaneous Reynolds-shear-stress-producing events as the flow transitions toward a fully turbulent state. Proper orthogonal decomposition of the transitional data sets indicates that large-scale structures play a vital role in the transport of both kinetic energy as well as Reynolds-shear stress and visualization of these large-scale motions reveals spatial signatures consistent with hairpin vortex packets.