Laminar-turbulent transition process in pulsatile flow.

A controlled ex-vivo study of a simple, sinusoidally oscillating flow in a rigid, constant-area, smooth tube, has produced significant insight into the laminar-turbulent transition phenomenon. The development of turbulence was studied by analyzing the dynamic characteristics of the transition process; i.e., the velocity, growth rate, and intermittency which describe the generation and propagation of turbulent slugs. A new concept, the relaxation time, has been introduced to interpret the effect of a periodic flow component superposed on a mean flow. Classical stability concepts, such as the point of inflection criterion and the Reynolds number, which have been derived from steady-flow analysis, are shown to require modification when applied to an oscillatory flow. Neither the mean nor the instantaneous Reynolds number is a sufficient criterion for determining the transition of laminar to turbulent flow in a pulsatile system. Other necessary criteria are: (1) a source of disturbances, (2) the relaxation time, and (3) the distance from the fluid under observation to the source of disturbance. The concept of relaxation time indicates that slowly oscillating flows of large amplitude tend to suppress or destroy turbulence downstream from sources of disturbance. Qualitative observations are presented which indicate that systolic acceleration may be laminar regardless of the large value of the instantaneous Reynolds number, while diastolic deceleration probably produces disturbed, but not turbulent or highly dissipative, flow. ADDITIONAL