Near-wall motion of inertial particles in a drag-reduced non-Newtonian turbulent flow

The kinematics of inertial particles suspended in the near-wall region of Newtonian and non-Newtonian turbulent channel flows was experimentally investigated. The non-Newtonian fluid was a homogeneous solution of 90 part per million of a polyacrylamide polymer in water with 66% drag reduction. All the experiments were performed at the same volumetric flow rate with Reynolds number of 34,300 based on bulk velocity, channel height, and the kinematic viscosity of water. The inertial particles were 250-μm glass beads with St of 35 (in water) at a volumetric concentration of 0.05%. A time-resolved two-dimensional particle tracking velocimetry was used to record particle images at acquisition frequency of 17.6 kHz and detect trajectory of flow tracers and the glass beads. The recorded data were processed using a two-dimensional particle tracking algorithm to obtain the Lagrangian kinematics of the beads. The comparison between laden flows of water and polymer solution showed reduction of number density of the beads and their momentum in the vicinity of the wall in the polymeric flow. The polymer solution remarkably reduced the wall-normal and shear Reynolds stresses of the beads, but had a negligible effect on their streamwise Reynolds stress. The wall-normal fluctuation of the beads reduced in the polymeric flow and their trajectories became parallel with the channel wall. Results also showed that the ejection and sweep motions were not the major mechanism for wall-normal distribution of the beads in the polymeric flow. Outcomes suggest that drag-reducing polymer solutions have the potential of reducing erosive wear in particle-laden pipelines. Graphic abstract:

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