Drag Reduction Performance and Turbulent Structure of Polymeric Solutions in Wall-bounded Turbulent Flows

• Author / Creator

  MOHAMMADTABAR,MOHAMMAD

• The higher drag of turbulent flows relative to laminar flows requires greater pumping power and results in a higher transportation cost. For this reason, reducing drag in turbulent pipe flows is of interest. Among various methods, the use of long-chain polymers has attracted more attention due to the small quantity of polymer required for drag reduction (DR). This technique was used in the Alaska oil pipeline, where the operators saved about $300 million per year through reduced pumping costs. Drag reducing polymers can have a flexible or a rigid molecular structure. Most biopolymers are categorized as rigid polymers. The general objective of this research is to investigate potential use of biopolymers instead of the commonly used synthetic polymers. To achieve this goal, the comparison of drag reduction performance and mechanisms of flexible and rigid polymers is necessary. Several techniques were used for the investigations including pressure loss measurements in flow loops to characterize drag reduction, rheometers for measuring shear and extensional viscosity, and a planar and stereoscopic particle image velocimetry (PIV) for characterization of turbulent structures in polymer solutions. It was observed that drag reduction was proportional to the relaxation time and Weiseenberg number. Polymer solutions with larger ratio of storage over loss modulus also led to a larger DR due to their stronger elastic behavior. In turbulent channel flow, the spatial correlation of the fluctuating velocity field shows that increasing polymer concentration increases the spatial coherence of streamwise fluctuations in the streamwise direction while they appear to have opposite sign in the wall-normal direction. The proper orthogonal decomposition (POD) of velocity fluctuations shows that the inclined shear layer structure of Newtonian wall flows becomes horizontal at the point of maximum drag reduction (MDR) and does not contribute to turbulence production. The investigations of rigid and flexible polymer solutions at a similar DR showed different profiles of streamwise Reynolds stress, while wall-normal Reynolds stresses and Reynolds shear stresses of polymer solutions were approximately the same. In addition, turbulent structures of one solution (SF polymers) at MDR were experimentally characterized. At MDR, the low and high speed streaks were elongated and thickened relative to those found in a turbulent Newtonian flow.
  The comparison of drag reduction performance and turbulent structures of rigid and flexible polymers (even at the same DR) showed the availability of different drag reduction mechanisms in polymer solutions with different molecular structures. To further investigate the effect of flexible and rigid polymers on turbulent structures, tomographic PIV can be used to characterize important parameters such as production and dissipation term of kinetic energy budget of turbulence. By using these measurements, drag reduction mechanisms existed in the literature can be effectively evaluated and a potential new drag reduction mechanism can be developed.

• Subjects / Keywords

  • Drag reduction
  • Turbulent flow
  • Polymeric solutions

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-qj4a-ds95

• License

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