Quadrant Analysis of Bubble Induced Velocity Fluctuations in a Transitional Boundary Layer

Our previous study showed that the frictional drag decreases with increasing void fraction at Re > 1300, while it increases at Re < 1000. Decomposition of the Reynolds shear stress also implied that bubbles induce isotropy of turbulence. In order to confirm our previous analysis and to further investigate flow fields in the vicinity of bubbles, we analyze velocity fluctuations on the quadrant space in the streamwise and transverse directions (u'–v'plane). Here, we focus on two specific Reynolds numbers (at Re ≈ 900 and ≈ 1410, which are close to the laminar-to-turbulent transition regime) and discuss bubble effects on sweep (u' > 0,v' < 0) and ejection (u' < 0,v' > 0) events as a function of the Reynolds number. We also illustrate velocity fluctuations in the vicinity of an individual bubble and a swarm of bubbles on the u'–v' coordinates. The results show that a bubble swarm suppresses the velocity fluctuations at Re ≈ 1410.