Written in: English
The addition of a small amount of polymers of high molecular weight can lead to a pressure drop decrease in turbulent flows. Over the years, numerous experimental and numerical studies have been conducted in attempts to make practical use of polymer-induced drag reduction, including long-distance transport of liquids, oil well operations, firefighting, transport of suspensions and slurries, and biomedical applications. The polymers successively stretch and coil by interacting with the turbulent structures, which changes the turbulent flow and further imposes a transient behaviour on the drag reduction (DR) as well as the subsequent mechanical polymer degradation. As a result, DR undergoes at least three stages over time: A, B, and C. In stage A, referred to as the developing time, DR departs from zero and assumes negative values due to a significant polymer stretching at the beginning of the process, which requires energy from the flow. After the minimum DR is reached, the polymers start their coil-stretch cycle and, in consequence, DR increases in response to the development of turbulent structures, achieving a maximum value, which makes for the beginning of stage B. However, during their coil-stretch cycle, polymers can be mechanically degraded as a result of an intense polymer stretching, which reduces their ability to act as energy exchange agents. Hence, when polymer degradation becomes pronounced, DR decreases until achieving a final value which indicates that the degradation has stopped and the molecular weight distribution has reached a steady state. The polymer degradation process characterizes the stage C. In the present work, numerical analyses are conducted aiming to investigate the stages A, B and C. The transient aspects of the polymer induced drag reduction phenomenon are explored with the aid of direct numerical simulations of turbulent plane Poiseulle and Couette flows of viscoelastic FENE-P fluids taking into account a wide range of Reynolds number, Weissenberg number and maximum polymer molecule extensibility. Stages A and B are carefully studied from tensor, statistical, energy budget and spectral perspectives. Additionally, a new and simple polymer scission model based on the molecule strain level is developed in order to numerically reproduce the stage C. It is found that the significant transient behaviour of DR within stage A is related to important exchanges of energy between the polymers, the mean flow and the turbulent structures, which is accentuated as the elasticity increases. In stage B, the dynamics of the flow is described by an autonomous regeneration cycle. The effects of polymers on such a cycle are attenuated by the molecules degradation during the stage C.