Characteristics and origins of viscoelastic turbulence in a 3-stream contraction-expansion micro-channel
Date of Issue2016-03-04
School of Mechanical and Aerospace Engineering
At low Reynolds number (Re < 1), the flow of viscous liquids e.g. water, is laminar. An aqueous solution e.g. water becomes viscoelastic when a small amount of polymer additives (< 1 wt%) is added to it; its flow behavior can become drastically different and turbulent i.e. viscoelastic turbulence. This phenomenon has gained increasing attention because it violates the conventional school of thought i.e. high- Re criteria, for creating chaos and disorder in a fluid dynamics system. As the polymer molecules are invisible, the indirect deduction of molecular behavior via motion analyses of tracer additives has been adopted as the main investigative approach in viscoelastic turbulent flows. Based on this approach, reported works attribute viscoelastic turbulence to the release of elastic energy by the polymer molecules, which had been extended due to strong velocity gradients in the flow field. The release of energy occurs over a range of time scales which is dependent on the characteristic time scales of the molecules and bulk viscoelastic liquid. Although the reported characteristic time scales vary significantly, their effects on the structure of the viscoelastic turbulent flow field have not been investigated. Despite the significant number of investigations based on the conventional approach, the underlying mechanisms in viscoelastic turbulence remains elusive; several outstanding questions on viscoelastic flows remain unresolved e.g. the high Weissenberg number problem, and the drag reduction theory debate. “How do the polymer molecules change the flow field so drastically when they are only present in minute amounts?”. This fundamental question has yet to be clearly answered. Although fluorescent-tagged DNA molecules are commercially available, because they are costly and impractical for large-scale dynamic flow field investigations, their usage has been mostly limited to tracer additives or stress probes. Here, a viscoelastic turbulent flow field generated by a 3-stream flow through a contraction-expansion micro-channel was investigated. Based on the conventional approach i.e. tracer additives, the results show that the generated turbulence exhibits characteristics of 2-D turbulent flows, from which information on the molecular and bulk properties of the viscoelastic liquid could be extracted. In addition, a highspeed molecular imaging technique was developed to observe the polymer molecular conformation changes (i.e. extension and relaxation) in a turbulent flow. The technique is capable of image grabbing speeds of more than 1000 frames per second. The developed molecular fluorescein tagging technique has a much lower cost as compared to fluorescein-labeling of DNA, hence making it more feasible for large scale investigations. With the developed technique, the dynamic molecular conformation changes could be correlated with the statistical properties of the flow field obtained based on the conventional approach.
DRNTU::Engineering::Mechanical engineering::Fluid mechanics