Transient aspects of drag reducing plane Couette flows

Abstract

The addition of a small amount of polymers of high molecular weight can lead to a decrease in the pressure drop in turbulent flows. Over the years, numerous studies have been conducted in attempts to make practical use of polymer-induced drag reduction. However, many aspects concerning its main mechanism are still unclear. One of those aspects is the development of the turbulent structures and polymer deformation over time in the beginning of the phenomenon. As an attempt to further understand the drag reduction over this developing time, we analyse a turbulent Couette flow of a FENE-P fluid with the aid of direct numerical simulations. We show that the initial interactions between the mean shear flow, turbulent structures, and polymer stretching are the key to understanding the step-by-step evolution of the drag reduction (DR). A few instants after the beginning of the simulation, the DR assumes a significantly negative value before starting to increase and reaches its maximum. When the DR is a minimum, the polymers experience their highest deformation state. The energy necessary to initially stretch them comes mainly from the mean shear flow, which causes a decrease of the DR until its minimum and negative value. After this point, the polymers start to strongly interact with the turbulent structures, which are partially suppressed, and the DR starts to increase. Part of the energy stored by the molecules is then released to the mean flow, increasing the DR to a maximum level while the polymer extension decreases. Lastly, DR reduces, reaching an asymptotic and positive value, which indicates the beginning of the statistical steady flow state.