Experimental study on bubble induced velocity fluctuations in the boundary layer at transitional Reynolds numbers

Abstract

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 direction (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 region) and discuss bubble effects on the sweep (u' > 0, v' < 0) and ejection (u' < 0, v' > 0) events as a function of the Reynolds number. We also illustrate the velocity fluctuations in the vicinity of an individual bubble and a swarm of bubbles on the u' -- v' coordinates. The result shows that a bubble swarm suppresses the velocity fluctuations at Re ≈ 1410. We then rearrange the scatted PTV vectors and compute power spectra of the kinematic energy of each velocity component. It only indicates that the elongated bubble suppresses low frequency Evv at Re ≈ 1410.