Characterization of degraded drag-reducing polymer solution and its impact on the structure of turbulence

Abstract

Turbulence is often described in terms of either a random process or interacting coherent structures, which is really a choice of which parts of the turbulence to ignore. Focusing on coherent structures reducing the complexity of the problem, which is required to make flow control a possibility. The primary focus of the current study is to analyze how coherent structures within a turbulent boundary layer are modified with the addition of drag-reducing polymer solution (polyethylene oxide, PEO). The performance is known to be concentration dependent, so to mitigate the impact of concentration gradients within the boundary layer the study was performed within a homogeneous polymer ocean. However, this requires the PEO to be exposed to a pump that is known to cause mechanical degradation via chain scission, which significantly impact the polymers behavior. Thus mechanical degradation of dilute PEO solutions was investigated with a turbulent pipe flow setup. Comparative analysis between degraded and non-degraded PEO samples at the same mean molecular weight showed that deviations in the polymer performance scaled with the normalized difference between the initial and final molecular weights. Furthermore, based on literature as well as the current results it was shown that the polymer performance deviations are most likely related to changes in the molecular weight distribution. It was also shown that these deviations could be minimized by increasing the residence time. Limiting the turbulent boundary layer operation conditions to the range where it was shown that the degradation had negligible impact on the PEO performance, allowed a detailed study of the impact of the polymers on the turbulent boundary layer coherent structures. It was observed that the anisotropy of the flow scales increases in proportion to drag reduction, which is consistent with the anisotropy of the fluctuating velocity scales observed in the literature

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