同心／偏心環状Poiseuille流の3次元間欠構造およびその流動様式

抄録

We conducted three-dimensional flow visualization of concentric/eccentric annular pipe flows in a transitional Reynolds-number regime with a single camera. One of our goals is to provide the first experimental evidence of helically-shaped intermittent turbulent structures in the concentric annular pipe flow, which has been reported only by a numerical simulation [Ishida et al., J. Fluid Mech. 794, R2 (2016)]. Another aim is to reveal the effect of the eccentricity on the intermittent structures that the gap instability caused in an eccentric annular pipe flow. A parametric study was executed for the radius ratios from 0.14 to 0.43 and the eccentricities from 0 to 0.8 (the ratio of the axes distance to a half of the hydraulic diameter). For the concentric cases with high radius ratios of 0.29 and 0.43, we confirmed the helical turbulence and helical puff. With intermediate eccentricities, such helical structures did not appear, instead weak turbulence and staggered vortices survived at much lower Reynolds numbers, i.e., the relaminarization was delayed because of the gap instability. For the highest eccentricity of 0.8, a transitional structure like a puff in a pipe flow without the inner pipe was observed due to the absence of the cross flow through the narrow gap between the cylinders.