添加剤を用いた抵抗低減流れのレイノルズ応力に寄与するコヒーレント構造に関する研究

Abstract

Dilute polymer solutions produce considerable drag reduction in turbulent pipe flows. The mechanisms of drag reduction have been widely studied by many researchers. Despite these many their efforts, it is still difficult to define the mechanisms. In order to understand the turbulent structures of dilute polymer solution flow and surfactant solution flow, we measured 2 components of velocity by 2D-LDV, estimated mean velocity and turbulent intensity, and calculated Reynolds shear stress from the velocity fluctuation data. Reynolds shear stress was analyzed by classifying the data into four quadrants. The characteristics of coherent structure (amplitude for each event, amplitude ratio, probability for each event) are discussed and compared with drag reducing flow and solvent (water) flow. It was found that the contributions to Reynolds shear stress from ejection(event of quadrant 2 ) and sweep(event of quadrant 4 ) are smaller near the wall in drag reducing flow than in solvent flow. We also found that the contributions to Reynolds shear stress from outward interaction(event of quadrant 1 ) and wallward interaction(event of quadrant 3 ) are important to reduce the drag. Contributions from each event in high drag reducing flow such as surfactant flow are equal, thus the flow has no coherent structure and it is a randomly mixing flow. Moreover, the relationships between eddy extension and Reynolds shear stress in drag reducing flows are discussed under the consideration that the drag reducing solutions are viscoelastic fluids. It is predicted that the eddy extension of drag reducing flow would be suppressed by the elasticity of the surfactant. The results obtained from this research are useful for the construction of a constitutive equation.