Direct Numerical Simulation on Millimeter-sized Air Bubbles in Turbulent Channel Flow

Air injection has been utilized for frictional drag reduction in liquid transport in pipelines and ship surfaces in water over the past 40 years. Recent studies found that millimeter-sized bubbles can concurrently contribute to both frictional drag reduction and increase, but the mechanism remains unclear. Furthermore, visualizing the micro-scale film flow between the bubbles and the wall surface using Particle Image Velocimetry (PIV) is challenging. In response, we conducted our research in several stages, using OpenFOAM on the large-scale computer, Fugaku.

Millimeter-sized bubbles have high deformability, and influenced heavily by turbulent flow. We reproduced turbulent flow conditions through DNS. Also, the flow on the hull surface (Couette flow), which is driven differently from channel flow (Poiseuille flow), has yet to have its effect on millimeter-sized bubbles elucidated.

To investigate this, we generated Couette flow in turbulent channel flow. We validated our model by comparing results with experiments of a single millimeter-sized bubble in turbulent channel flow. Unique bubble characteristics (direction and deformation of the bubble) were found in Couette flow that differed from Poiseuille flow.

Using the validated model, we reproduced bubbly flow with multiple millimeter-sized bubbles. Our simulations aligned well with experimental observations. Additionally, we discerned differences according to the flow conditions.