Study on the characteristics of turbulent drag-reducing channel flow by particle image velocimetry combining with proper orthogonal decomposition analysis

Abstract

Turbulent drag reduction of 30 ppm cetyltrimethyl ammonium chloride (CTAC) solution flow in a channel was investigated with particle image velocimetry (PIV) combining with proper orthogonal decomposition (POD). Measurements were made at inlet fluid temperature of 304 K and at Reynolds number 2.5×104 (based on the channel height, bulk velocity, and solvent viscosity) for both water and CTAC solution flows with 70.0% drag reduction rate. The two-component velocity fields in the streamwise-wall normal plane were recorded by PIV. In order to study the characteristics of turbulent drag-reducing channel flow, POD was performed to identify the near-wall coherent structures based on PIV-measured data. POD is a powerful low-dimensional analysis tool that can be used to identify coherent structures embedded in the turbulent shear flow. We mainly studied a comparison between the first dominant POD eigenmodes of water and drag-reducing CTAC solution flows. Coherent structures were seen as the sum of several eigenmodes that possess a dominant energy of the flow, say 90%. It was obtained that the amount of eigenmodes required for capturing the coherent structures was 233 and 195 for water and CTAC solution flows, respectively, which means the decrease in the complexity in CTAC solution flow. Based on the analysis of POD eigenmodes of water and CTAC solution flows, we captured the processes that can reflect the ejection motion of low-speed fluid from the wall and sweep motion of high-speed fluid toward the wall associating with turbulent bursting events. The results showed that CTAC additives can inhibit the turbulent bursting processes (both strength and occurrence frequency), resulting in a great decrease in turbulent contribution to frictional drag and drag reduction, which is sufficient to understand deeply the mechanism of turbulent drag reduction.