TABLE OF CONTENTS For macroscale flows, in situations where there is little or no free stream forcing, small disturbances initially grow in a linear fashion. As the disturbances become amplified, nonlinear effects become important. New modes generated by the flow nonlinearity fill the disturbance spectrum and the flow becomes locally turbulent. This is usually called natural transition. When the free stream disturbances, for example, generated by free stream turbulence, are high, the linear stage of growth may be bypassed in the transition process. This is usually referred to as bypass transition. The fluid dynamics of either the natural or the bypass transition is highly nonlinear where energy of certain dominant modes of fluctuations cascades to their higher harmonic modes and eventually transfers to fluctuations of other scales. Many recent experiments in fluidic microchannels have reported significant difference in the heat and momentum transfer coefficients compared with those at the macroscales. For example, while the friction factor varies inversely with the Reynolds number when the Reynolds number is small, the proportionality constant does not agree with the conventional correlation. The friction factors were also found to diverge from this inverse proportionality, which at macroscale indicates a change of flow characteristics from that of a laminar flow to a turbulent flow, at smaller Reynolds numbers than those commonly observed in the corresponding large channels. This early change of flow characteristics has been attributed to the effects of, for instance, gas rarefaction and other surface mechanisms, the large change of the flow Reynolds number...
